J Comp Physiol B (2011) 181:423?435
DOI 10.1007/s00360-010-0521-7ORIGINAL PAPER
Is tissue maturation necessary for Xight? Changes in body 
composition during postnatal development in the big brown bat
Wendy R. Hood ? Olav T. Oftedal ? Thomas H. Kunz 
Received: 2 May 2010 / Revised: 13 September 2010 / Accepted: 22 September 2010 / Published online: 3 November 2010
?  Springer-Verlag 2010
Abstract Patterns of oVspring development reXect the
availability of energy and nutrients, limitations on an indi-
vidual?s capacity to use available resources, and tradeoVs
between the use of nutrients to support current metabolic
demands and tissue growth. To determine if the long period
of oVspring dependency in bats is associated with the need
for an advanced state of tissue maturation prior to Xight, we
examined body composition during postnatal growth in the
big brown bat, Eptesicus fuscus. Despite their large size at
birth (22% of maternal mass), newborn bats are relatively
immature, containing 82% body water in fat-free mass.
However, the total body water content of newborn bat pups
decreases to near-adult levels in advance of weaning, while
concentrations of total body fat and protein exceed adult
values. In contrast to many other mammals, postnatal
growth of bat pups was characterized by relatively stable
concentrations of calcium and phosphorus, but declining
concentrations of magnesium. These levels remained stable
or rebounded in late postnatal development. This casts
doubt on the hypothesis that low rates of mineral transfer
necessitate an extended lactation period in bats. However,
our Wnding of near-adult body composition at weaning is
consistent with the hypothesis that extended lactation in
bats is necessary for the young to achieve suYcient tissue
maturity to undertake the active Xight necessary for inde-
pendent feeding. In this respect, bats diVer from most other
mammals but resemble birds that must engage in active
Xight to achieve nutritional independence.
Keywords Body composition ? Calcium ? Chiroptera ? 
Eptesicus fuscus ? Flight ? Postnatal development
Introduction
Patterns of tissue maturation can provide insight into con-
straints on postnatal development such as availability of
energy and nutrients, limitations on an individual?s capacity
to use available resources, and tradeoVs at the tissue or cel-
lular level (Ricklefs 1979a; Ricklefs et al. 1998). This
should be particularly true during periods of maximal
growth, when tissue demand for structural materials is high.
Both poor nutrition and very high levels of activity can
redirect energetic substrates from muscle growth to catabolic
activity, reducing rates of tissue development (Ricklefs
1979a; Ricklefs et al. 1998).
In most mammals, nourishment is provided solely by the
mother in the form of milk (Clutton-Brock 1991). Mammalian
Communicated by I.D. Hume.
Electronic supplementary material The online version of this 
article (doi:10.1007/s00360-010-0521-7) contains supplementary 
material, which is available to authorized users.
W. R. Hood ? T. H. Kunz
Center for Ecology and Conservation Biology, 
Department of Biology, Boston University, 
Boston, MA 02215, USA
O. T. Oftedal
National Zoological Park, Smithsonian Institution, 
Washington, DC 20008, USA
Present Address:
W. R. Hood (&)
Department of Biological Sciences, Auburn University, 
101 Life Sciences Building, Auburn, AL 36849, USA
e-mail: wrhood@auburn.edu
Present Address:
O. T. Oftedal
Smithsonian Environmental Research Center, 
Edgewater, MD 21037, USA123
424 J Comp Physiol B (2011) 181:423?435milk is highly digestible and acts as an immediate buVer to
Xuctuations in food supply. There is considerable interest in
how species-speciWc milk composition relates to the nutri-
tional needs of the young, as well as to nutritional con-
straints on the mother (Oftedal 1984, 2000; Oftedal et al.
1993a). The quality of milk to some degree depends on
maternal condition. Milk composition and more notably
milk yield can vary with nutritional status of the mother
(Doreau et al. 1992; Hood et al. 2009; Rasmussen and War-
man 1983; Warman and Rasmussen 1983). Thus, changes
in tissue and body composition of young during the postna-
tal growth period may reXect a trade-oV between develop-
mental processes, maintenance requirements of developing
tissues, and the supply of nutrients available to the mother.
Because of their volant mode of locomotion and pro-
longed period of dependency, bats are a particularly inter-
esting group in which the transfer of nutrients from mothers
to oVspring through milk can be studied. Young bats do not
become nutritionally independent until they have achieved
about 70% of adult mass: nearly twice the average relative
mass (37%) of terrestrial species at weaning (Barclay 1994,
1995; Case 1978; Kunz 1987; Kunz and Hood 2000; Kurta
and Kunz 1987; Millar 1977). Because aerial insectivorous
species typically consume a low-calcium diet, it has been
postulated that low-calcium intake by mothers may con-
strain mineral deposition and thus the relative rates of skel-
etal maturation in dependent oVspring (Barclay 1994, 1995;
Studier and Sevick 1992), yet there is little direct evidence
to suggest that mineral intake in bats is suboptimal. For
example, Booher (2008) found no diVerence in the length
of forearm or body mass of litters born to big brown bats
(Eptesicus fuscus) fed a diet that mimicked the low-calcium
intake of free-ranging bats versus bats supplemented with
10? more calcium. Booher and Hood (2010) further exam-
ined the body composition of these young and found no
diVerences between the low- and high-calcium groups.
Based on the calcium content of the feces collected from
these animals, it was apparent that supplemented animals
did not absorb greater amounts of calcium than is typically
obtained from their normal insect diet (Booher and Hood
2010).
Alternatively, the long period of dependency in bats
may reXect a need for an advanced state of tissue matura-
tion, and particularly maturation of bone, prior to under-
taking sustained Xight. Survival at independence depends
on the ability of juvenile bats to forage eVectively.
Because the relative strain on the distal elements of the
skeleton is greater for Xying bats than species using other
forms of locomotion (Swartz and Middleton 2008), insec-
tivorous bats that must capture food on the wing may need
to approach or achieve tissue maturity before nutritional
independence (Papadimitriou et al. 1996; Swartz et al.
1992).
We studied the development of suckling big brown bats
(Eptesicus fuscus), an obligate, aerial-feeding insectivore
(Kurta and Baker 1990). Our goal was to test the hypothesis
that the extended period of suckling in bats yields a high
level of tissue maturation and mineral deposition prior to
nutritional independence from milk. Chemical maturity is
achieved when the concentrations of nutrients in the body
reach adult values (Moulton 1923). Because chemical
maturity reXects the developmental state of the tissues, the
chemical composition of the body can be used as an aggre-
gate indicator of tissue maturation. For this investigation,
we characterized changes in total body macronutrient and
macromineral composition of bat pups from birth until
weaning, and compared our results with data on adult
females (Hood et al. 2006). We assessed the prediction that
prolonged suckling in bats is associated with an advanced
level of tissue maturation by comparing our Wndings with
published data on body composition in non-volant mam-
mals and birds at nutritional independence.
Methods
Big brown bat pups were collected with their mothers from
two maternity colonies located in Hollis and Milford, New
Hampshire, USA (42?74N 71?59W, 42?83N 71?68W,
respectively) during June and July 1999. All samples were
collected during the 5-week suckling period. At weekly
intervals, up to ten pups and their mothers were collected
from each colony between 0600 and 0900 h. Animals were
transported to Boston University where they were main-
tained in incubators at 30?C (within the zone of thermoneu-
trality for this species; Stack 1985). Pups were housed with
mothers (allowing them to be suckled by their mothers
overnight) and then separated the following morning for at
least 2 h prior to being killed, ensuring that the young had
minimal gastrointestinal contents. Bats were humanely
killed by prolonged exposure to gaseous chloroform (Gan-
non et al. 2007). Body composition analyses and care of the
mothers is described elsewhere (Hood et al. 2006). All pups
were weighed (?0.01 g), and lengths of forearm
(?0.1 mm) and total epiphyseal gap (?0.05 mm; forth right
metacarpal-phalangeal epiphysis; Kunz and Anthony 1982)
were measured post-mortem. These measurements were
used to estimate the age of pups (Burnett and Kunz 1982;
Hood et al. 2002). In 4 years of studying big brown bats in
Massachusetts and New Hampshire (1996, 1997, 1998 and
1999), we found no evidence that litter size deviated from
two (W.R. Hood, personal observation). Estimates of pup
age were slightly more accurate when accounting for the
relative age of the mother (Hood et al. 2002), but in the
present study we did not know the identity of all mothers.
Thus, maternal age was not considered as a variable in123
J Comp Physiol B (2011) 181:423?435 425estimates of pup age. Carcasses of euthanized bats were
frozen at -80?C until analyses were completed within
1 year. Protocols used in this study were approved by
Boston University?s Animal Care and Use Committee.
Analysis of body composition was based on a 4-com-
partment molecular model with the body divided into
water, fat, protein, and mineral (HeymsWeld et al. 1997;
Reynolds and Korine 2009; Speakman 2001). Carbohy-
drate storage as glycogen was assumed to be minimal
(<1% of body mass). Detailed descriptions of analytical
methods are described by Hood et al. (2006). BrieXy,
whole carcasses were thawed and homogenized in a Corn-
ing laboratory blender. The dry matter content of homoge-
nates was determined by drying samples to a constant
mass at 60?C in a forced convection oven. Fat composition
was determined based on the mass of fat extracted from
dried samples using petroleum ether in a Soxhlet apparatus
(0.315 ? 0.036 g samples). Nitrogen content was deter-
mined using a CHNS/O Elemental Analyzer (Perkin Elmer
PE2400 Series II, Shelton, Connecticut; 8.48 ? 1.80 mg
samples). Nitrogen content was multiplied by the standard
conversion factor for animal carcasses of 6.25 to estimate
crude protein (Robbins 1993). Samples for mineral analy-
sis (98.4 ? 10.8 mg) were digested in a microwave-accel-
erated reaction system (MARS5, CEM Corp., Mathews,
North Carolina) in 10 ml concentrated nitric acid. Calcium,
magnesium, sodium, and potassium were determined
using atomic absorption spectrophotometry (Smith-Hieftje
12, Thermo-Jarrell Ash, Franklin, MA). Phosphorus was
analyzed using the AOAC-modiWed Gomori method
(Horwitz 1980). Mineral recoveries of non-fat milk
standard reference material (#1549, National Institutes of
Standard and Technology, U.S. Department of Commerce,
Gaithersburg, MD) were >92.3% of guaranteed values
(Hood et al. 2006).
Statistical analyses were conducted using SAS (SAS
Institute Inc., Cary, NC). All proportional data were arc-
sine-transformed for analysis. Fat, protein, and mineral
composition are expressed in units that reveal functional
trends rather than the confounding eVects of change in
other constituents (Hood et al. 2006), with total body water
content expressed relative to fat-free mass (FFM), fat
expressed relative to dry mass (DM), and protein, calcium,
phosphorus, magnesium, potassium, and sodium expressed
relative to fat-free dry mass (FFDM). Although sodium and
potassium could be presented as molar concentrations to
reXect their electrolytic functions, a substantial proportion
of sodium is associated with bone and thus is insoluble
(Spray and Widdowson 1950). Because soluble and insolu-
ble sodium and potassium cannot be distinguished, we
present these elements relative to fat-free dry mass. All
nutrients are expressed as total body content to document
patterns of accretion.
Variation in nutrient accretion during postnatal growth
was evaluated relative to sex of pups and colony, using
multivariate analysis of covariance (MANCOVA, proc
GLM; SAS Institute Inc. 1990). Nutrient contents were set
as dependent variables, sex and colony as covariates, and
body mass of individual pups as the independent variable.
This analysis was limited to pups >10 days old because
few pups aged ?10 days were collected from one of the
colonies (Hollis, NH). Body mass, rather than age of pups,
was used for all comparisons presented here because body
composition is more strongly correlated with body mass
than age (Gardner et al. 1964) and exact pup age was not
always known. However, comparisons relative to pup age
are included in the supplementary materials to facilitate
comparison with other studies. The Wrst two and the Wnal
3 weeks of suckling appear to be distinct phases of postna-
tal development. Low activity but rapid increase in body
mass and development of the wings and pelage character-
ize the Wrst 2 weeks, whereas reduced rates of postnatal
growth, movement within the roost, wing Xapping, test
Xights, and the onset of independent foraging occur during
the Wnal weeks of dependency (Hood et al. 2002). To
account for the possibility that diVerences in activity were
correlated with a change in pattern of nutrient accretion,
we examined patterns of change in body composition with
both linear and segmented regressions (SAS proc NLIN)
and then presented the regression model that best charac-
terized the data. Because the F and P values should not be
compared between these two regression models directly
due to a change in degrees of freedom, we determined the
best Wt regression by comparing the least square error val-
ues between the statistical models. The regression with the
lowest least square error was considered to be the best Wt
model (Ryan and Porth 2007). In addition, we present data
that summarize body composition at birth and weaning
(Table 1). To facilitate comparison with other studies, all
regression equations are based on non-transformed data
and we included regression equations for changes in
body composition expressed relative to pup age in online
Supplement 1.
To provide context to our results, we compiled data on
the body composition of mammals and birds at birth/hatch-
ing and weaning/Xedging (Tables 3, 4; references, sample
sizes, ages, and body masses of animals are given in Sup-
plement 2). We converted all values to common units to
facilitate comparisons, and we were forced to exclude stud-
ies in which there were not enough data given for unit con-
version. Because data for several published studies are only
presented as graphs (Dunn 1975; Navarro 1992; Spray and
Widdowson 1950), we generated numeric values by digitiz-
ing these graphs using the programs DigiMATIC or ImageJ
(FEB software, ChesterWeld, VA; National Institutes of
Health, Bethesda, MD). We included the data in Spray and123
426 J Comp Physiol B (2011) 181:423?435Widdowson (1950) because of the wide variety of species,
completeness of mineral data, and its classic role in docu-
menting developmental changes in mammals. Estimates of
water content were obtained by diVerence rather than desic-
cation and thus may include error. Likewise, protein values
in Anderson and Alisauskas (2002) were also determined
by diVerence. Values for body composition from an unpub-
lished study on big brown bats (Stack 1985, Supplement 3)
were converted to units comparable to this study. Data on
the calcium and sodium composition of cave bat and Bra-
zilian free-tailed bat pups from Studier and Kunz (1995)
were also digitized for direct statistical comparison with big
brown bats. In the absence of data on water content and fat,
the units (mg/g live mass) used by Studier and Kunz (1995)
could not be converted to mg/g FFDM; therefore we con-
verted our values to a fresh mass basis for comparison
(Fig. 3). The calcium and sodium concentrations of big
brown bat, cave bat, and Brazilian free-tailed bat (Studier
and Kunz 1995) were compared using ANCOVA (proc
GLM; (SAS Institute Inc. 1990). Nutrient contents were
designated as dependent variables, species as independent
variables, and pup mass as a covariate.
Results
Body composition was not signiWcantly inXuenced by sex
of pups whether expressed as nutrient accretion or on a per-
centage basis (MANCOVA nutrient accretion: Wilk?s
 = 0.661, F8,16 = 1.03, P > 0.445, percent composition:
Wilk?s  = 0.609, F8,16 = 1.28, P > 0.318). There was also
no signiWcant diVerence in nutrient accretion by colony
(MANCOVA: nutrient accretion: Wilk?s  = 0.505,
F8,16 = 1.96, P > 0.119); however, there was a marginally
signiWcant inter-colony diVerence in the magnesium con-
centration of pups (MANCOVA: percent composition:
Wilk?s  = 0.433, F8,16 = 2.62, P < 0.048), with those from
the Hollis colony averaging 1.75 ? 0.5 mg/g Mg and those
from the Milford colony averaging 1.62 ? 0.05 mg/g Mg.
Because this diVerence is small and no other nutrients
Table 1 Body mass, 
macronutrient mass, and 
composition of big brown 
bats (Eptesicus fuscus) 
at birth and weaning
Birth Weaning Gain during postnatal 
growth relative to 
uterine development (%)
N 3 5 ?
Age (days) 0?1 30?35 ?
Mass (g) 3.40 ? 0.14 11.3 ? 0.4 231
% Adult mass 20?23 65?74 ?
Macronutrients
Dry matter (g) 0.66 ? 0.04 3.43 ? 0.10 417
Water (g) 2.74 ? 0.13 7.68 ? 0.20 180
Protein (g) 0.46 ? 0.03 2.29 ? 0.12 396
Fat (g) 0.07 ? 0.01 0.64 ? 0.10 855
Dry matter (% BM) 19.5 ? 1.0 30.1 ? 0.5 ?
Water (% BM) 80.5 ? 1.0 69.8 ? 0.3 ?
Water (% FFM) 82.1 ? 0.8 73.4 ? 0.5 ?
Protein (% BM) 13.6 ? 0.6 20.6 ? 0.6 ?
Protein (% FFDM) 77.5 ? 0.6 82.2 ? 0.4 ?
Fat (% BM) 1.99 ? 0.32 5.80 ? 0.96 ?
Fat (% DM) 10.1 ? 1.3 18.7 ? 2.9 ?
Macrominerals
Calcium (mg) 28.6 ? 2.0 105.9 ? 4.2 270
Phosphorus (mg) 19.4 ? 1.2 79.5 ? 2.7 310
Magnesium (mg) 1.12 ? 0.5 4.88 ? 0.29 336
Potassium (mg) 9.69 ? 0.29 32.0 ? 1.1 230
Sodium (mg) 4.70 ? 0.16 16.3 ? 1.5 247
Calcium (mg/g FFDM) 48.2 ? 3.8 38.1 ? 1.1 ?
Phosphorus (mg/g FFDM) 32.7 ? 2.2 28.7 ? 1.3 ?
Magnesium (mg/g FFDM) 1.88 ? 0.08 1.73 ? 0.05 ?
Potassium (mg/g FFDM) 16.4 ? 0.8 10.5 ? 0.7 ?
Sodium (mg/g FFDM) 7.93 ? 0.40 5.84 ? 0.42 ?
Nutrient gain during postnatal 
growth is also presented as 
cumulative gain relative to gain 
in utero. Means 
presented ? standard error
BM body mass; FFM fat free 
mass; FFDM fat free dry mass; 
DM dry mass123
J Comp Physiol B (2011) 181:423?435 427diVered among colonies, we did not consider colony as a
variable in the remaining analyses.
The assayed constituents accounted for 97.8 ? 0.1% of
total body mass in big brown bat pups. The remaining 2%
of the body was primarily attributable to polar lipids (which
are not extracted using petroleum ether) and glycogen,
although chloride, trace constituents, inaccuracy in the
standard protein conversion factor, and minor analytical
error may also have contributed.
The macronutrient and mineral composition of big
brown bat pups at birth and weaning are summarized in
Table 1, and changes in mass of macronutrients and miner-
als and the relative concentrations of macronutrients and
minerals as bat pups increased in body mass are presented
in Figs. 1 and 2. Macronutrient and mineral accretion in
pups during lactation was typically 200?400% of nutrient
accretion during intra-uterine growth, although fat accre-
tion was even greater, 855% (Table 1).
We found signiWcant accretion for all nutrients during
postnatal growth (Fig. 1; Table 2). Accretion was linear
throughout postnatal development for all nutrients except
calcium, phosphorus, and magnesium. Rates of calcium,
phosphorus, and magnesium accretion increased during late
postnatal growth (Fig. 1; Table 2). When nutrients were
expressed on a proportional basis, linear regression best
explained changes in nutrient concentration for all nutrients
except magnesium (Table 2). Protein (% FFDM) and fat
(%DM) concentrations increased during postnatal growth
but concentrations of water (% FFM), phosphorus, and
sodium (mg/g FFDM) decreased, while calcium and phos-
phorus remained unchanged (Fig. 2; Table 2). Magnesium
decreased during early postnatal growth, but then recovered
prior to weaning (Fig. 2; Table 2).
Discussion
Chemical and tissue maturation during the postnatal period
in big brown bats occurs at an accelerated rate relative to
intra-uterine development. Accretion for all macronutrients
and macrominerals was two to eight times greater during
5 weeks of suckling relative to 8 weeks in utero. Dispropor-
tionate development during postnatal growth would be
expected to be pronounced in species that are small at birth,
Fig. 1 Accretion of dry matter, 
protein, fat, and macrominerals 
in big brown bat pups during the 
postnatal growth period 
expressed as total mass of each 
nutrient. Regression lines 
indicate signiWcant changes in 
composition. Calcium, phospho-
rus, and magnesium were best 
explained using a segmented 
regression. The breakpoint (bp, 
g body mass) is indicated in each 
graph. In graphs that present 
more than one mineral, solid 
circles correspond with the 
upper label (calcium or potas-
sium) and the white circles 
correspond with the lower label 
(phosphorus or sodium)
0
50
100
150
200
250
0.0
0.5
1.0
1.5
0
2
4
6
8
2 4 6 8 10 12 14 2 4 6 8 10 12 14
0
10
20
30
40
50
Body Mass (g)
Dry Matter
Protein
Fat
Calcium
Phosphorus
Magnesium
Potassium
Sodium
2 4 6 8 10 12 142 4 6 8 10 12 14
0
1
2
3
4
2 4 6 8 10 12 142 4 6 8 10 12 14
0
1
2
3
4
5
To
ta
l n
ut
rie
nt
 in
 b
od
y 
(m
g)
To
ta
l n
ut
rie
nt
 in
 b
od
y 
(g)
bp = 8.0 g
bp = 7.6 g
bp = 7.5 g123
428 J Comp Physiol B (2011) 181:423?435but bat pups are both large at birth (12?43% of maternal
mass) and large at weaning (Kunz et al. 2009; Kunz and
Hood 2000; Kurta and Kunz 1987). Thus, young bats
experience high nutrient gain in utero and this gain intensi-
Wes during suckling as pups approach adult size prior to
independence.
Body water, protein, and electrolytes
At birth bat pups were at a relatively immature state of
development having a body water content of 82.1% of fat-
free mass (FFM). By comparison, altricial mammals such
as rats, mice, and terrestrial carnivores contain 81?87%
water and altricial birds such as passerines contain 88?91%
water on a FFM basis; precocial mammals such as seals,
ruminants, and horses contain 72?76% water and precocial
birds such as ducks and galliforms contain 79?81% water
(Table 3). Although the discrepancy in body water content
according to developmental state at birth or hatching is well
known (Spray and Widdowson 1950; Starck and Ricklefs
1998), the relative chemical immaturity of newborn big
brown bats is surprising, given their large relative size at
birth (22% of maternal mass as compared with 0.4?13% in
the newborn eutherian mammals and newly hatched birds
listed in Table 3).
Bat pups accumulated dry mass linearly from birth until
weaning. During this time, the percent of body water
decreased while the percent body protein increased, with
both water and protein reaching adult levels at or just after
mid-dependency. Continued accretion beyond mid-depen-
dency reXects further growth after attaining tissue maturity.
Comparable changes in body water have been reported for
this and other species of bats during the postnatal period
(Reynolds and Kunz 2000; Stack 1985; Supplement 3). We
determined the crude protein content of carcasses based on
the nitrogen content multiplied by a standard conversion
factor, 6.25. Although this conversion factor has been in
common use for nearly a century, its validity has not been
broadly tested (Rafecas et al. 1994). All studies described
in Tables 3 and 4 that directly measure protein use this con-
version factor. We recognize that high rates of transcription
and translation in developing tissues may entail changes in
Fig. 2 Body composition of big 
brown bat pups during the post-
natal growth period, expressed 
as percent of total body compo-
sition. DM, FFM, and FFDM 
indicate that values are 
expressed relative to the dry 
mass, fat-free mass, and fat-free 
dry mass of the pups respec-
tively. Regression lines indicate 
signiWcant changes in composi-
tion. Magnesium was best ex-
plained using a segmented 
regression. The breakpoint (bp) 
for this regression is indicated in 
the graph. Dotted horizontal 
lines indicate the average body 
composition of lactating mothers 
(Hood et al. 2006). In graphs 
that present more than one min-
eral, black circles and the upper 
dashed line correspond with the 
upper label (calcium or potas-
sium) and the white circles and 
lower dashed line correspond 
with the lower label (phosphorus 
or sodium)
60
70
80
90
100
0
20
40
60
80
2 4 6 8 10 12 142 4 6 8 10 12 14
60
70
80
90
100
0
10
20
30
40
2 4 6 8 10 12 14 2 4 6 8 10 12 14
0
1
2
3
Co
nc
en
tra
tio
n 
(m
g/g
 FF
DM
)
Co
m
po
sit
io
n 
(%
 FF
M)
Co
m
po
sit
io
n 
(%
 FF
DM
)
Body Mass (g)
Water
Protein
Fat
Calcium
Phosphorus
Magnesium
Potassium
Sodium
Co
m
po
sit
io
n 
(%
 D
M)
bp = 7.6 g
2 4 6 8 10 12 14 2 4 6 8 10 12 14
0
10
20
30123
J Comp Physiol B (2011) 181:423?435 429total nucleic acid: amino acid ratios, but the possible eVect
on the conversion factor is not clear. Additional research is
needed to determine the accuracy of this factor across
species and at diVerent developmental stages.
It is well known that sodium is more concentrated in the
extracellular solution than within cells (Randall et al.
2001); thus, the postnatal decrease in the concentration of
sodium in the body lends support to the assertion that much
of the loss of body water is from the extracellular space as
tissues mature (Spray and Widdowson 1950). Concentra-
tion of potassium was expected to increase with tissue mat-
uration (Spray and Widdowson 1950) rather than decrease
as described in the present study. When potassium was
expressed relative to body water content (mmol/l of body
water), the negative relationship between concentration of
total body potassium and body mass was no longer appar-
ent when characterized with linear and segmented regres-
sion (P > 0.45 and P > 0.13, respectively). Findings were
similar for sodium when expressed relative to mmol/l body
water (linear P > 0.76, segmented regression failed to con-
verge on a breakpoint). In some species, up to 50% of the
body?s sodium is stored in bone (Harrison et al. 1936;
Spray and Widdowson 1950). Given that sodium and potas-
sium do not run counter to one another, it is possible that
bats also deposit a portion of their body?s sodium in bone.
Concentration of both of these electrolytes approach adult
values by weaning.
Trends in total body water during postnatal development
have been described for a limited number of birds and
mammals. At maturity, the total body water of birds and
mammals is approximately 73% of adult fat-free mass
(Widdowson and Dickerson 1964), although this value can
be somewhat lower in species with proportionately larger
amounts of low-water constituents such as bone, fur, and
feathers. In general, total body water at independence in
bats was more similar to the body water of Xedging birds
than weaning mammals (Table 3). Only three species of
mammals in Table 3 have lower body water at weaning
than bats, and these species include two species of phocid
seals and the horse. Some species of seals are born at near-
adult concentrations of body water and body water changes
little, relative to fat-free mass, during the postnatal period
Table 2 Best-Wt regressions describing changes in the accretion and concentration of macronutrients and minerals in Eptesicus fuscus pups during
suckling expressed as a function of body mass
Regression models are linear unless a break point is given. The breakpoint describes the point of intersection between the two regression lines that
best characterize the pattern of change in the variable. Means and standard errors are given when change is not signiWcant. Statistical analyses are
based on arcsine-transformed values to normalize proportional data but provided regression equations are based on original data to facilitate the
use of the equations to calculate body composition
FFM fat free mass; FFDM fat free dry mass; DM dry mass
Break point Regression F df P R2
Nutrient accretion as a function of mass
Dry matter (g) ? (0.306 ? mass) ? 0.747 1,448 1, 44 <0.001 0.971
Protein (g) ? (0.242 ? mass) ? 0.411 2,108 1, 44 <0.001 0.980
Fat (g) ? (0.095 ? mass) ? 0.318 94.3 1, 44 <0.001 0.682
Ca (mg) <8.0 (6.13 ? mass) + 10.0 77.9 3, 42 <0.001 NA
>8.0 (19.0 ? mass) ? 92.5
P (mg) <7.6 (6.09 ? mass) ? 0.21 21.5 3, 42 <0.001 NA
>7.6 (11.4 ? mass) ? 40.8
Mg (mg) <7.5 (0.311 ? mass) + 0.154 376 3, 42 <0.001 NA
>7.5 (0.765 ? mass) ? 3.48
K (mg) ? (3.22 ? mass) ? 0.40 380 1, 44 <0.001 0.896
Na (mg) ? (1.50 ? mass) + 0.19 345 1, 44 <0.001 0.887
Change in composition as a function of mass
Water (% FFM) ? (?1.04 ? mass) + 84.6 272 1, 44 <0.001 0.860
Protein (%FFDM) ? (0.63 ? mass) + 75.5 45.0 1, 44 <0.001 0.506
Fat (% DM) ? (1.35 ? mass) + 6.02 25.2 1, 44 <0.001 0.364
Ca (mg/g FFDM) ? NS, x = 40.8 ? 1.1 0.37 1, 44 0.546 0.008
P (mg/g FFDM) ? NS, x = 30.3 ? 0.5 0.07 1, 44 0.800 0.002
Mg (mg/g FFDM) <7.6 (?0.11 ? mass) + 2.29 21.4 3, 42 <0.001 NA
>7.6 (0.094 ? mass) + 0.75
K (mg/g FFDM) ? (?0.46 ? mass) + 17.9 18.1 1, 44 <0.001 0.292
Na (mg/g FFDM) ? (?0.28 ? mass) + 9.09 39.4 1, 44 <0.001 0.473123
430 J Comp Physiol B (2011) 181:423?435(Oftedal et al. 1993b). The horse also achieves adult con-
centrations of body water by weaning, but this follows a
long period of reliance on non-milk forage; complete wean-
ing does not occur until approximately 1 year of age. The
relationship between body water and the life history of ani-
mals is undoubtedly complex. Our observation that near-
adult concentrations of body water is common to animals
that Xy at independence warrants further investigation, as it
suggests that tissue maturity is a prerequisite for Xight.
Body fat
Bat pups deposit on average eight times more fat during
postnatal development than during prenatal development.
Table 3 Percent adult mass, 
body water, fat, and protein of 
birds and mammals at birth/
hatching (B/H) and weaning/
Xedging (W/F)
% Adult mass Water (%FFM) Protein (%FFDM) Fat (%DM)
B/H W/F B/H W/F B/H W/F B/H W/F
Chiroptera
Big brown bat 22 70 82.1 73.0 77.5 82.3 10.1 14.5
Mammalia
Rodentia
House mousea,h 5.9 30 85.1 79.6 85.8 85.8 12.6 20.2
Norway rata 1.5 17 86.4 77.6 80.3 81.2 9.16 21.6
Guinea piga 10 14 78.9 79.3 78.1 81.4 34.7 7.69
Lagomorpha
Domestic rabbita 3 24 86.7 78.5 81.8 80.1 15.0 23.7
Carnivora
American mink 1 27 84.8 79.6 87.9 89.4 12.4 40.3
Domestic cata 2 42 82.1 76.2 88.6 82.7 11.3 26.4
Domestic dog 2 ? 81.1 ? 77.5 ? 4.59 ?
American black bear 0.4 ? 84.5 ? 78.0 ? 5.64 ?
Harp seal 7 28 72.0 71.8 78.7 77.2 9.93 76.0
Hooded seal 13 24 71.7 70.9 81.4 73.3 36.7 73.5
Cetartiodactyla
Domestic pig 2 11 85.7 79.4 76.0 79.2 7.10 32.6
Domestic sheep 8 19 76.3 76.6 80.4 86.1 10.8 26.3
Dairy cattle 9 40 76.0 73.6 80.4 ? 14.8 29.8
Perissodactyla
Domestic horse 10 73 74.6 69.5 69.2 74.1 9.52 39.1
Aves
Anseriformes
King eidera 3.1 ? 78.9 ? ? ? 35.0 ?
Galliformes
Japanese quaila,e 5 42.7 81.1 73.2 ? 84.8 26.7 30.7
Domestic chickenc 2 24 78.7 72.8 84.9 86.6 19.7 21.3
Wild turkey 0.4 2 79.0 75.5 82.4 81.8 29.6 6.44
Pelicaniformes
Double-crested cormorantd 2 88 87.2 68.5 ? ? 1.5 17.1
Cape gannetb 2 117 77.8 70.9 ? ? 33.3 46.4
Charadriiformes
Common terna 13 ? 80.2 ? ? ? 8.9 ?
Passeriformes
Common blackbirdf 4 63 91.1 79.0 71.0 69.4 12.0 8.75
Song thrushg 8 79 91.1 78.4 99.4 72.2 12.0 8.33
European starlinga 7 ? 88.0 ? ? ? 13.3 ?
Units were converted from the 
original published values to 
match those in this study where 
necessary. All data were not 
available for all species. Body 
composition was determined by 
chemical analysis of whole body 
composition minus gut contents 
(or in post-absorptive individu-
als) and fur and feathers were 
included in the chemical analy-
sis unless otherwise noted. Ref-
erences given in Supplement 2
FFM fat free mass; FFDM fat 
free dry mass; DM dry mass
a Unclear if gut contents 
removed
b Feathers and stomach contents 
removed
c Eviscerated
d Adult body mass based on age 
at 24 weeks, includes gut con-
tents and feathers
e Includes gut contents and 
feathers, Edwards and Denman 
(1975)
f Adult body mass based on 
Dunning (2008)
g Adult body mass based on 
Hedenstrom and Rosen (2001)
h Adult mass based on the aver-
age given in National Research 
Council (1995)123
J Comp Physiol B (2011) 181:423?435 431Most of this fat appears to be gained during late postnatal
growth, although individual variation in fat deposition is
extremely high. Fat deposition occurs when dietary intake
of energy exceeds requirements for metabolism and lean
mass gains. Milk energy output of mothers is highest dur-
ing late lactation (Hood 2001), coinciding with peak
demand of the pups as they become larger, more agile, and
begin to Xy. High individual variation in fat deposition by
bat pups likely reXects a combination of variation in milk
energy intake and variation in energy expenditure of pups,
and thus the extent to which energy intake exceeds growth
and maintenance demands.
A >800% gain in body fat during postnatal development
is as much a reXection of low fat deposition in utero as it is
the large gain in body mass during the postnatal period. The
percentage of fat in the body at weaning was lower than in
most mammalian species and some birds. In addition, the
fat content in big brown bat pups at weaning in this study
was lower than values reported for this species by Stack
(1985), indicating that fat deposition may vary among years
and that the diVering methods of lipid extraction (ether
extraction vs. ether-alcohol extraction) may produce diVer-
ences in recovered fat (see Hood et al. 2006 for a discussion
of methodological diVerences). Interestingly, in our study
the body fat content of many pups exceeded that of their
mothers at mid dependency. A high concentration of body
fat relative to adults at weaning may function as an ?insur-
ance policy? for suckling mammals, increasing the proba-
bility of survival as weaned young learn to forage under
variable environmental conditions (Muelbert and Bowen
1993; Muelbert et al. 2003). Insectivorous bats do not have
a particularly abrupt weaning period (Kunz and Hood
2000), but their food resources (mostly Xying insects) may
be subject to periods of low availability due to rainfall and
low temperatures (Taylor 1963; Williams 1961). Thus,
inter-year variation in fat reserves might reXect inter-year
variation in food availability to mothers and could be
important for the survival of pups if they face temporary
food shortages when they are learning to Xy and forage.
Body calcium, phosphorus, and magnesium
While total body calcium, phosphorus, and magnesium
increased throughout the postnatal growth period, concen-
trations of calcium and phosphorus remained constant
while the concentration of magnesium decreased and then
subsequently rebounded. Concentrations of calcium and
phosphorus in big brown bats are near or below adult levels
Table 4 Concentration of calcium, phosphorus, magnesium, potassium, and sodium (mg/g fat free dry mass) for birds and mammals at birth/
hatching (B/H) and weaning/Xedging (W/F)
As necessary, units were converted from the original published values. All data were not available for all species. Body composition was deter-
mined by chemical analysis of whole body composition minus gut contents (or in post-absorptive individuals) and fur and feathers were included
in the chemical analysis unless otherwise noted. References given in Supplement 2; see Table 3 footnotes for comments on sample preparation
Calcium Phosphorus Magnesium Potassium Sodium
B/H W/F B/H W/F B/H W/F B/H W/F B/H W/F
Big brown bat 48.2 36.6 32.7 26.5 1.88 1.73 16.4 10.5 7.93 5.53
Mammalia
Rodentia
House mouse 22.8 29.4 23.0 23.8 2.25 1.59 18.4 13.8 15.2 7.55
Norway rat 24.7 38.8 23.7 27.8 1.80 2.24 15.8 15.8 18.0 8.55
Guinea pig 53.5 51.1 35.1 30.2 1.96 1.99 14.2 12.4 8.19 8.96
Lagomorpha
Domestic rabbit 36.4 ? 26.7 30.5 1.76 2.04 16.3 14.6 18.5 8.27
Carnivora
Domestic cat 38.1 42.3 25.6 26.5 1.49 1.74 13.0 14.1 13.7 7.72
Cetartiodactyla
Domestic pig 69.0 49.1 39.0 31.1 2.05 1.97 15.0 15.7 16.9 9.26
Dairy cattle 60.9 59.5 34.8 33.8 2.39 ? 9.13 ? 10.4 ?
Perissodactyla
Domestic horse ? 69.3 ? 33.4 ? 1.70 ? 7.50 ? 6.80
Aves
Passeriformes
C. blackbird 7.96 24.4 20.46 18.63 1.48 1.33 14.3 10.1 30.7 7.19
Song thrush 6.82 20.9 18.86 16.73 1.32 1.36 20.2 9.82 25.2 7.55123
432 J Comp Physiol B (2011) 181:423?435from birth until weaning. Our comparison of mineral
composition of pups with adults in Fig. 2 is based on an
average of females at the onset of lactation (Hood et al.
2006). The mineral content of bone diVers between males
and females in some mammals. For example, bone mineral
content is greater in adult females than in male Norway
rats (Rattus norvegicus), lower in adult females than males
in humans (Homo sapiens), but does not diVer by sex in
the rabbit (Lenchik et al. 2004; Spray and Widdowson
1950). The mineral content of adult male big brown bats is
unknown. The mineral content of bone in females
increases prior to lactation in some species, including the
Norway rat and house mouse, Mus musculus (Ellinger
et al. 1952; Horst et al. 1997; Sharpe et al. 2003; Wysol-
merski 2002), and thus it is possible that the onset of lacta-
tion value in our comparison is inXated relative to the bone
mineral contents of adult males and non-reproductive
females.
The high concentration of calcium in newborn big brown
bat pups appears to reXect a higher level of skeletal miner-
alization than in many other species (Table 4). Histological
data for three additional insectivorous bat species support
this result; the level of ossiWcation observed in several skel-
etal elements of the little brown myotis (Myotis lucifugus),
southeastern myotis (Myotis austroriparius), and Brazilian
free-tailed bat (Tadarida brasiliensis) are greater at birth
than the house mouse and Norway rat at or near weaning
(Adams 1992; Hermanson and Wilkins 2008; Patton and
Kaufman 1995). Hermanson and Wilkins (2008) described
considerable variation in the degree of ossiWcation for
diVerent bones in late embryological and early neonatal
development in the southeastern myotis and Brazilian free-
tailed bat. Thus, a relatively high total body concentration
of calcium need not indicate high levels of mineralization
for all bones.
Although total body calcium concentration may not have
reached adult levels by weaning, it is probable that stable
concentrations of total body calcium during development
reXect intake of adequate sources of calcium. A decline in
the concentration of calcium in bone following birth has
been described in several species including domestic cats
(Felis catus), domestic dogs (Canis familiaris), domestic
rabbits (Oryctolagus cuniculus), Norway rats, and domestic
pigs (Sus scrofa) (Bauer et al. 1929; Slater and Widdowson
1962), and in the diaphysis of the human femur. Although
the decline in bone calcium can be prevented by supple-
mental calcium, at least in young cats (Slater and Widdow-
son 1962), this decrease should be considered a normal
developmental process that is reversed later in life. The
mineral concentration of bone in domestic cats returns to
parturition levels by weaning (Dickerson 1962; Spray and
Widdowson 1950), whereas human femurs reach mineral
concentrations comparable to the time of parturition by age
12 (Dickerson 1962). The early postnatal decline is thought
to be associated with the movement of calcium stored in the
trabeculae (i.e. cancellous bone) during gestation into the
matrix of the growing cortical bone (Widdowson and Dick-
erson 1964). In addition, a proteinaceous organic matrix is
laid down prior to mineralization. During periods of rapid
development, the organic matrix can be deposited at a
greater rate than mineral and this contributes to a decrease
in bone mineral concentration (Teitelbaum 1990). In big
brown bats, the pups of mothers supplemented with cal-
cium showed no diVerence in total body calcium content,
compared with the young of unsupplemented mothers
(Booher and Hood 2010). Again, these results suggest that
intake of calcium is unlikely to aVect bone development, at
least in big brown bats consuming calcium concentrations
typical of free-ranging individuals.
Only one previous study has examined the total body
macromineral concentrations of bats during postnatal
growth. Studier and Kunz (1995) described the body com-
position of the cave myotis (Myotis velifer) and the Brazil-
ian free-tailed bat, but calcium and sodium were the only
elements common to their study and ours (Fig. 3). During
early lactation, the calcium content of cave myotis and big
brown bat pups were similar, while the calcium content of
Brazilian free-tailed bats was signiWcantly higher. During
late lactation, however, calcium concentration of big brown
bat pups was signiWcantly lower than in the cave myotis
and the Brazilian free-tailed bat. The two latter species both
produce one oVspring, relative to two produced by big
brown bats. Thus, it is tempting to conclude that lower total
body calcium during late lactation in big brown bat pups
relative to cave myotis and Brazilian free-tailed bats may
be associated with greater nutritional stress on the big
brown bat mothers that are nursing two rather than one
young, as in the cave myotis and Brazilian free-tailed bat.
However, without data on total body fat and total body
water, both of which contribute substantially to variation in
total body mass, it is possible that the observed diVerence
among species can be attributed to diVerences in fat or
water, or to inter-laboratory analytical variation. Sodium
concentration diVered among all three species, but the bio-
logical signiWcance of these diVerences is diYcult to inter-
pret.
The relative mass of the skeleton increases with increas-
ing body mass in bats and non-volant mammals, so that the
skeletal mass makes a greater contribution to total body
mass in larger versus small species (Prange et al. 1979).
This diVerence in the proportion of the body that is skeleton
complicates interspeciWc comparisons. However, it is
apparent that concentrations of calcium and phosphorus,
but not magnesium, may be greater in big brown bats at
birth and weaning than in many larger mammal species
(Table 4). Because bone strength is correlated with mineral123
J Comp Physiol B (2011) 181:423?435 433content (Currey 1969), it appears that bats are born with
and maintain stronger skeletal elements than other small
mammals of comparable body size. With an increase in
wing dimensions and no change in calcium or phosphorus
concentration within the body, it seems that high accretion
rates are important for supporting growth of wing elements
that support the mechanical stresses associated with pow-
ered Xight (Papadimitriou et al. 1996; Swartz and Middle-
ton 2008).
Conclusions
The results of this investigation suggest that the prolonged
suckling period of bats allows the young to reach or exceed
adult body water, protein, and fat concentrations by wean-
ing. Given the large relative mass of muscle compared with
other tissues, adult levels of water and protein suggest that
muscle maturation is at or near completion at weaning.
Concentrations of total body calcium, phosphorus, and
magnesium were slightly below adult values at weaning,
but relative to other species bats appear to achieve a rela-
tively high degree of bone mineralization by weaning.
Thus, Wndings of this study conWrm that the prolonged
lactation period in bats allows pups to reach an advanced
state of maturation relative to other mammalian species. It
is likely that this advanced state is a prerequisite for the
active Xight which pups must undertake to become nutri-
tionally independent.
Acknowledgments We thank the A. Husk and C. Zahn families for
access to their barns for bat collection. K. Jung assisted in the Weld and
D. Hellinga, K. Flickinger, L. Nelson and M. Jakubasz assisted in the
Nutrition Laboratory. Regina Eisert, Christina Booher, GeoV Hill, par-
ticipants in the Hill/Hood labs, and three anonymous reviewers made
valuable comments contributing to the Wnal form of this manuscript.
A National Institutes of Child Health and Development Grant
(5-T32HD078387) to I.P. Callard helped support this project in the Wnal
year of analysis. W. Hood thanks the Smithsonian OYce of Fellowships
for a Graduate Student Research Fellowship. Additional support was
provided by Boston University?s Center for Ecology and Conservation
Biology.
References
Adams RA (1992) Comparative skeletogenesis of the forearm of the
little brown bat (Myotis lucifugus) and the Norway rat (Rattus
norvegicus). J Morphol 214:251?260
Agricultural Research Council (1980) The nutrient requirements of
ruminant livestock. Commonwealth Agricultural Bureau, Slough,
England
Anderson VR, Alisauskas RT (2002) Composition and growth of king
eider ducklings in relation to egg size. Auk 119:62?70
Barclay RMR (1994) Constraints on reproduction by Xying verte-
brates?energy and calcium. Am Nat 144:1021?1031
Barclay RMR (1995) Does energy or calcium availability constrain
reproduction by bats? In: Racey PA, Swift SM (eds) Ecology,
evolution and behaviour of bats (Symposia of the Zoological
Society of London). Oxford University Press, London, pp 245?
258
Bauer W, Aub J, Albright F (1929) Studies of calcium and phosphorus
metabolism. V. A study of the bone trabeculae as a readily avail-
able reserve supply of calcium. J Exp Med 49:145?162
Bilby LW, Widdowson EM (1971) Chemical composition of growth in
nestling blackbirds and thrushes. Br J Nutr 25:127?134
Booher CM (2008) EVects of calcium availability on reproductive out-
put of big brown bats. J Zool 274:38?43
Booher CM, Hood WR (2010) Calcium utilization during reproduction
in big brown bats (Eptesicus fuscus). J Mammal 91:952?959
Burnett CD, Kunz TH (1982) Growth rates and age estimation in
Eptesicus fuscus and comparison with Mytois lucifugus. J Mammal
63:33?41
Case TJ (1978) On the evolution and adaptive signiWcance of postna-
tal-growth rates in terrestrial vertebrates. Q Rev Biol 53:243?282
Clutton-Brock TH (1991) The evolution of parental care. Princeton
University Press, Princeton, NJ
Currey JD (1969) Mechanical consequences of variation in mineral
content of bone. J Biomech 2:1?11
Dickerson JWT (1962) Changes in composition of human femur dur-
ing growth. Biochem J 82:56?61
Doreau M, Boulot S, Bauchart D, Barlet JP, Martinrosset W (1992)
Voluntary intake, milk-production and plasma metabolites in
nursing mares fed 2 diVerent diets. J Nutr 122:992?999
Dunn EH (1975) Growth, body components and energy content of
nestling double-crested cormorants. Condor 77:431?438
Dunning JB (2008) CRC handbook of avian body masses. CRC Press,
Boca Raton
Fig. 3 Calcium and sodium composition of big brown bat pups (this
study), cave myotis, and Brazilian free-tailed bat pups (Studier and
Kunz 1995) between birth and weaning. Data are expressed in mg/g
live mass, as presented in Studier and Kunz (1995). Linear regression
lines are presented to emphasize trends for each species
0 7 14 21 28 35 42
0
1
2
5
10
15
20
Co
m
po
si
tio
n 
(m
g/g
 liv
e m
as
s)
Calcium
Sodium
Big brown bat
Cave myotis
Brazilian free-tailed bat
Age (days)123
434 J Comp Physiol B (2011) 181:423?435Edwards HM (1981) Carcass composition studies 3. InXuences of age,
sex and calorie-protein content of the diet on carcass composition
of Japanese quail. Poult Sci 60:2506?2512
Edwards HM, Denman F (1975) Carcass Composition Studies. 2.
InXuences of breed, sex and diet on gross composition of carcass
and fatty-acid composition of adipose-tissue. Poult Sci 54:1230?
1238
Ellenberger HB, Newlander JA, Jones CH (1950) Composition of the
bodies of dairy cattle. Vt Exp Stn Bull, pp 1?66
Ellinger GM, Duckworth J, Dalgarno AC, Quenouille MH (1952)
Skeletal changes during pregnancy and lactation in the rat: eVect
of diVerent levels of dietary calcium. Br J Nutr 6:235?253
Gannon WL, Sikes RS, Comm ACU (2007) Guidelines of the
American society of mammalogists for the use of wild mammals
in research. J Mammal 88:809?823
Gardner RW, Bensadou A, Hogue DE (1964) Body composition and
eYciency of growth of suckling lambs as aVected by level of feed
intake. J Anim Sci 23:943?952
Harrison HE, Darrow DC, Yannet H (1936) The total electrolyte con-
tent of animals and its probable relation to the distribution of body
water. J Biol Chem 113:515?529
Hedenstrom A, Rosen M (2001) Predator versus prey: on aerial hunt-
ing and escape strategies in birds. Behav Ecol 12:150?156
Hermanson JW, Wilkins KT (2008) Growth and development of two
species of bats in a shared maternity roost. Cells Tissues Organs
187:24?34
HeymsWeld SB, Wang ZM, Baumgartner RN, Ross R (1997) Human
body composition: advances in models and methods. Annu Rev
Nutr 17:527?558
Hood WR (2001) Nutritional limitations on lactation and postnatal
growth in the big brown bat, Eptesicus fuscus. Boston University,
Boston, MA
Hood WR, Bloss J, Kunz TH (2002) Intrinsic and extrinsic sources of
variation in size at birth and rates of postnatal growth in the big
brown bat Eptesicus fuscus (Chiroptera: Vespertilionidae). J Zool
258:355?363
Hood WR, Oftedal OT, Kunz TH (2006) Variation in body composi-
tion of female big brown bats (Eptesicus fuscus) during lactation.
J Comp Physiol B 176:807?819
Hood WR, Voltura MB, Oftedal OT (2009) Methods of measuring
milk composition and yield in small mammals. In: Kunz TH,
Parsons S (eds) Ecological and behavioral methods for the study
of bats. John Hopkins University Press, Baltimore, MD,
pp 529?553
Horst RL, GoV JP, Reinhardt TA (1997) Calcium and vitamin D
metabolism during lactation. J Mammary Gland Biol Neoplasia
2:253?263
Horwitz WE (1980) OYcial methods of analysis on the association of
oYcial analytical chemists, Washington, DC
Jagusch KT, Norton BW, Walker DM (1970) Body composition stud-
ies with milk-fed lamb. 1. Chemical composition and caloriWc
content of body and organs of newly-born lambs. J Agric Sci
75:273?277
Kunz TH (1987) Post-natal growth and energetics of suckling bats. In:
Fenton MB, Racey PA, Rayner RMV (eds) Recent advances in
the study of bats. Cambridge University Press, Cambridge,
pp 395?420
Kunz TH, Anthony ELP (1982) Age estimation and postnatal-growth
in the bat Myotis lucifugus. J Mammal 63:23?32
Kunz TH, Hood WR (2000) Parental care and postnatal growth in the
Chiroptera. In: Krutzsch PH, Creichton EG (eds) Reproductive
biology of bats. Academic Press, New York, pp 415?468
Kunz TH, Adams RA, Hood WR (2009) Methods of assessing size at
birth and postnatal growth and development in bat. In: Kunz TH,
Parsons S (eds) Ecological and behavioral methods for the study
of bats. John Hopkins University Press, Baltimore, pp 273?314
Kurta A, Baker RH (1990) Eptesicus fuscus. MammalSpecies
356:1?10
Kurta A, Kunz TH (1987) Size of bats at birth and maternal investment
during pregnancy. In: Loudon ASI, Racey PA (eds) Reproductive
energetics in mammals (Symposia of the Zoological Society of
London). Oxford University Press, London, pp 79?106
Lenchik L, Vatti S, Register TC (2004) Interpretation of bone mineral
density as it related to bone health and fracture risk. In: Holick
MF, Dawson-Hughes B (eds) Nutrition and bone health. Humana
Press, Inc., Totowa, pp 63?84
Meyer H, Ahleswede L (1978) The intrauterine growth and body com-
position of foals and the nutrient requirements of pregnant mares.
Anim Res Dev 8:86?112
Millar JS (1977) Adaptive features of mammalian reproduction. Evo-
lution 31:370?386
Moulton CR (1923) Age and chemical development in mammals.
J Biol Chem 57:79?97
Muelbert MMC, Bowen WD (1993) Duration of lactation and post
weaning changes in mass and body composition of harbour seal,
Phoca vitulina, pups. Can J Zool 71:1405?1414
Muelbert MMC, Bowen WD, Iverson SJ (2003) Weaning mass aVects
changes in body composition and food intake in harbor seal pups
during the Wrst month of independence. Physiol Biochem Zool
76:418?427
National Research Council (1995) Nutrient requirements of laboratory
animals. National Academy of Sciences, Washington, DC
Navarro RA (1992) Body composition, fat reserves, and fasting capa-
bility of Cape gannet chicks. Wilson Bull 104:644?655
Oftedal OT (1981) Milk, protein and energy intakes of suckling mam-
malian young: a comparative study. Cornell University, Ithaca
Oftedal OT (1984) Milk composition, milk yield and energy output at
peak lactation: a comparative review. In: Peaker M, Vernon RG,
Knight CH (eds) Physiological strategies in lactation (Symposia
of the Zoological Society of London). Oxford University Press,
London, pp 33?85
Oftedal OT (2000) Use of maternal reserves as a lactation strategy in
large mammals. Proc Nutr Soc 59:99?106
Oftedal OT, Alt GL, Widdowson EM, Jakubasz MR (1993a) Nutrition
and growth of suckling black bears (Ursus americanus) during
their mothers winter fast. Br J Nutr 70:59?79
Oftedal OT, Bowen WD, Boness DJ (1993b) Energy transfer by lactat-
ing hooded seals and nutrient deposition in their pups during the
4 days from birth to weaning. Physiol Zool 66:412?436
Oftedal OT, Bowen WD, Boness DJ (1996) Lactation performance and
nutrient deposition in pups of the harp seal, Phoca groenlandica,
on ice Xoes oV southeast Labrador. Physiol Zool 69:635?657
Oju EM, Waibel PE, Noll SL (1988) Early protein undernutrition and
subsequent realimentation in turkeys. 1. EVect of performance
and body composition. Poult Sci 67:1750?1759
Papadimitriou HM, Swartz SM, Kunz TH (1996) Ontogenetic and ana-
tomic variation in mineralization of the wing skeleton of the Mex-
ican free-tailed bat, Tadarida brasiliensis. J Zool 240:411?426
Patton JT, Kaufman MH (1995) The timing of ossiWcation of the limb
bones, and growth-rates of various long bones of the fore and hind
limbs of the prenatal and early postnatal laboratory mouse. J Anat
186:175?185
Prange HD, Anderson JF, Rahn H (1979) Scaling of skeletal mass to
body-mass in birds and mammals. Am Nat 113:103?122
Rafecas I, Esteve M, Fernandezlopez JA, Remesar X, Alemany M
(1994) Whole-rat protein-content estimation?applicability of the
N-X-6.25 factor. Br J Nutr 72:199?209
Randall D, Burggren W, French K (2001) Eckert animal physiology.
W.H. Freeman and Co., New York
Rasmussen KM, Warman NL (1983) EVect of maternal malnutrition
during the reproductive cycle on growth and nutritional status of
suckling rat pups. Am J Clin Nutr 38:77?83123
J Comp Physiol B (2011) 181:423?435 435Reynolds DS, Korine C (2009) Body composition analysis of bats. In:
Kunz TH, Parsons S (eds) Ecological and behavioral methods for
the study of bats. John Hopkins University Press, Baltimore,
pp 674?691
Reynolds DS, Kunz TH (2000) Changes in body composition during
reproduction and postnatal growth in the little brown bat, Myotis
lucifugus (Chiroptera : Vespertilionidae). Ecoscience 7:10?17
Ricklefs RE (1979a) Adaptation, constraint, and compromise in avian
postnatal development. Biol Rev Camb Philos Soc 54:269?290
Ricklefs RE (1979b) Patterns of growth in birds. 5. Comparative-study
of development in the starling, common tern, and Japanese quail.
Auk 96:10?30
Ricklefs RE, Starck JM, Kornarzewski M (1998) Internal constraints
on growth in birds. In: Starck JM, Ricklefs RE (eds) Avian growth
and development. Oxford University Press, New York, pp 266?
287
Robbins CT (1993) Wildlife feeding and nutrition. Academic Press,
New York, New York
Robbins KR, Ballew JE (1984) Relationship of sex and body growth-
rate with daily accretion rates of fat, protein and ash in chickens.
Growth 48:44?58
Ryan SE, Porth LS (2007) A tutorial on the piecewise regression
approach applied to bedload transport data. General technical
report RMRS-GTR-189, Fort Collins, CO US., p 41
SAS Institute Inc. (1990) SAS/STAT? user?s guide, version 6. SAS
Institute Inc., Cary, NC
Schryver HF, Hintz HF, Lowe JE, Hintz RL, Harper RB, Reid JT
(1974) Mineral composition of whole body, liver and bone of
young horses. J Nutr 104:126?132
Sharpe CJ, Fudge NJ, Kovacs CS (2003) A rapid 35% Xux in bone
mass occurs during pregnancy and lactation cycles in mice. Bone
32:S227
Slater JE, Widdowson EM (1962) Skeletal development of suckling
kittens with and without supplementary calcium phosphate. Br J
Nutr 16:39?48
Speakman JR (2001) Introduction. In: Speakman JR (ed) Body compo-
sition analysis of animals. A handbook of non-destructive meth-
ods. Cambridge University Press, Cambridge, pp 1?7
Spray CM, Widdowson EM (1950) The eVect of growth and develop-
ment on the composition of mammals. Br J Nutr 4:332?353
Stack MH (1985) Energetics of reproduction in the big brown bat,
Eptesicus fuscus. Boston University, Boston, MA
Starck JM, Ricklefs RE (1998) Patterns of development: the altricial-
precocial spectrum. In: Starck JM, Ricklefs RE (eds) Avian
growth and development. Oxford University Press, New York,
pp 3?30
Studier EH, Kunz TH (1995) Accretion of nitrogen and minerals in
suckling bats, Myotis velifer and Tadarida brasiliensis.
J Mammal 76:32?42
Studier EH, Sevick SH (1992) Live mass, water content, nitrogen and
mineral levels in some insects from south-central lower Michi-
gan. Comp Biochem Physiol A 103:579?595
Swartz SM, Middleton KM (2008) Biomechanics of the bat limb skel-
eton: scaling, material properties and mechanics. Cells Tissues
Organs 187:59?84
Swartz SM, Bennett MB, Carrier DR (1992) Wing bone stresses in free
Xying bats and the evolution of skeletal design for Xight. Nature
359:726?729
Taylor LR (1963) Analysis of the eVect of temperature on insects in
Xight. J Anim Ecol 32:99?117
Teitelbaum SL (1990) Skeletal growth and development. In: Favus MJ
(ed) Primer on the metabolic bone diseases of disorders of mineral
metabolism. American Society for Bone and Mineral Research,
Kelseyville, CA, pp 7?10
Warman NL, Rasmussen KM (1983) EVects of malnutrition during the
reproductive cycle on nutritional status and lactational perfor-
mance of rat dams. Nutr Res 3:527?545
Widdowson EM, Dickerson JWT (1964) Chemical composition of
the body. In: Comer CL, Bronner F (eds) Mineral metabolism, an
advanced treatise. Academic Press, New York, pp 1?247
Williams CB (1961) Studies in the eVect of weather conditions on the
activity and abundance of insect populations. Philos Trans Roy
Soc Lond Ser B Biol Sci 244:331?378
Wysolmerski JJ (2002) The evolutionary origins of maternal calcium
and bone metabolism during lactation. J Mammary Gland Biol
Neoplasia 7:267?276123