State-dependent male mating tactics in the
grey seal: the importance of body size
Damian C. Lidgard,a,b Daryl J. Boness,a W. Don Bowen,c and Jim I. McMillanc
aConservation and Research Center, National Zoological Park, Smithsonian Institution, 3001
Connecticut Avenue NW, Washington, DC 20008-2598, USA, bLaboratoire de Biologie et
Environnement Marins, Universite? de La Rochelle, La Rochelle 17000, France, cMarine Fish
Division, Bedford Institute of Oceanography, Department of Fisheries and Oceans, Dartmouth,
Nova Scotia B2Y 4A2, Canada
The purpose of this study was to examine the importance of body size and body composition as determinants of conditional
mating tactics exhibited in male grey seals. We combined behavioral observations with measures of energy expenditure and
success on 42 known-age individuals during the breeding seasons of 1997?2001 at Sable Island, Canada. Males with a large body
mass arrived at the breeding grounds with more body fat and body energy and catabolized less body protein than smaller males.
Males consumed 1.9 6 0.2 MJ day
1, and those with a smaller percentage of body fat had higher rates of food energy intake. The
amount of body energy on arrival was positively correlated with the duration of the breeding period. Males that exhibited the
primary mating tactic of consortship were heavier, had absolutely more body fat and body energy, and sustained breeding longer
than those males that did not exhibit the primary tactic. Amongst all males, body mass showed a quadratic relationship with the
number of female consorts mated and the estimated number of pups sired. Thus, intermediate-sized males mated with the most
consorts and achieved the highest success. Intermediate body size may be optimal during breeding due to greater agility in male
combat. Body size was an important determinant of mating tactics used by male grey seals. A large body size provided an
energetic advantage of greater endurance while an intermediate body size may provide greater competitive ability in acquiring
consortships. Key words: body size, conditional mating tactics, endurance rivalry, energetics, Halichoerus grypus, pinnipeds,
reproductive behavior. [Behav Ecol 16:541?549 (2005)]
Much of the variation in some polygynous mating systemsmay be due to the differential ability of males to defend
females (Clutton-Brock, 1989). Males will differ in their
competitive ability due to differences in their state. Variations
in state among individuals may occur due to genetic variation,
ontogeny, environmental fluctuations during growth, and
hormonal action (Dunbar, 1982; Festa-Bianchet et al., 2004;
Lindstro?m, 1999; Moore, 1991; Moore et al., 1998). Condi-
tional reproductive strategies comprise a primary tactic that
yields average high success and several alternative tactics that
yield lower average success. State is thought to be an
important determinant of tactics such that individuals exhibit
the tactic or tactics that provide the highest success according
to their state (Dunbar, 1982; Gross, 1996).
There are many traits that represent an individual?s state,
and all will contribute toward shaping the life history
(McNamara and Houston, 1996). In some polygynous mating
systems, body size and body composition are likely to be
important components of state in breeding adult males. Here,
the primary mating tactic typically involves an extended
period of attendance that is often correlated with success
(Boness, 1991; Halliday and Tejedo, 1995; Thornhill, 1981;
Wiley, 1974). Such tactics can be costly due to the risks of
being injured or killed during male combat (Bartsh et al.,
1992; Clutton-Brock et al., 1979; Thornhill, 1981) and high
energy expenditure (Deutsch et al., 1990; Judge and Brooks,
2001; Vehrencamp et al., 1989; Yoccoz et al., 2002). In many
species, large body size can provide a male that engages in the
primary mating tactic with an advantage in male-male
competition (Andersson, 1994; Enders, 1993; Howard, 1978;
McElligott et al., 2001). Among mammals that store energy
for breeding, stored fat becomes a greater fraction of body
mass as size increases (Lindstedt and Boyce, 1985). Large
individuals may therefore have both absolutely and relatively
more stored energy for breeding (e.g., Coltman et al., 1998)
and an advantage in endurance rivalry (Andersson, 1994;
Arnould and Duck, 1997; Judge and Brooks, 2001; Murphy,
1998). We may therefore expect individuals of a large body
size and with large energy stores to exhibit the primary mating
tactic.
Small individuals or those in poor condition (i.e., low body fat
per unit body mass) might either fail to become reproductive
and not appear at the breeding grounds or might engage in
alternative tactics (Dunbar, 1982; Gross, 1996). Several molec-
ular studies have shown that alternative mating tactics yield
success (e.g., Coltman et al., 1999, 2002). Nevertheless, few
studies have examined the consequences of expressing
alternative tactics on energy expenditure. In addition to their
lower ability to compete, individuals exhibiting alternative
tactics may also have high energy expenditure, which pre-
sumably could further reduce their endurance ability at the
breeding grounds. Coltman et al. (1998) and Judge and Brooks
(2001) have demonstrated energetic constraints in male harbor
seals (Phoca vitulina) and male bullfrogs (Rana catesbeiana),
respectively.
The grey seal (Halichoerus grypus) is a sexually dimorphic
pinniped with a polygynous mating system (Anderson et al.,
1975; Boness and James, 1979). Males arrive at the breeding
colony with large quantities of stored energy and compete for
access to females. The primary mating tactic is prolonged
female defense whereby a male defends a position among
Address correspondence to Damian C. Lidgard, who is now at
Department of Biology, Dalhousie University, Halifax, Nova Scotia
B3H 4J1, Canada. E-mail: damian.lidgard@dal.ca.
Received 21 July 2003; revised 3 December 2004; accepted 15
December 2004.
Behavioral Ecology
doi:10.1093/beheco/ari023
Advance Access publication 2 February 2005

 The Author 2005. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: journals.permissions@oupjournals.org
a group of females that may change in membership over time
(Anderson et al., 1975; Boness and James, 1979). Studies have
demonstrated a positive correlation between male size and
mating success (Anderson and Fedak, 1985) and length of
stay and mating success (Anderson et al., 1975; Boness and
James, 1979; Tinker et al., 1995; Twiss et al., 1994). Three
alternative mating tactics have been defined (Lidgard, 2003):
the first involves a male mating with a female that has weaned
her pup and is departing the colony; the second involves
a male that is in defense of females mating with a neighboring
lactating female; and the third involves a male that is not in
defense of females mating with a lactating female that is either
alone or is a consort of another male. Males that exhibit
alternative tactics have been shown to achieve some success
(Ambs et al., 1999; Amos et al., 1993; Lidgard et al., 2004;
Worthington Wilmer et al., 1999). Lidgard (2003) categorized
males according to whether they exhibited the primary tactic
of consortship (consort males) or only expressed alternative
tactics (nonconsort males). Nonconsort males were mostly
young, made more moves and spent shorter periods of time at
a site, were less competitive, and had a lower body condition
index (body mass/standard length) than consort males.
These differences remained after controlling for age, suggest-
ing that other components of state such as body size may be
important determinants of mating tactics.
We combined detailed behavioral observations with mea-
sures of energy expenditure and success to test the hypothesis
that body size and body composition are important determi-
nants of the mating tactics exhibited by male grey seals. We
predicted that energy expenditure would be the same for
males of all body sizes but that breeding endurance would be
positively correlated with total body energy (TBE) of the male
on arrival. We also predicted that only large males would
exhibit the primary tactic of consortship and that the number
of consorts mated would increase with body size.
METHODS
Study site and study animals
The study was conducted during the breeding seasons of 1997
to 2001 at Sable Island (43 559 N, 60 009 W) situated 288 km
ESE of Halifax, Nova Scotia. The island is approximately 54 km
long and 1.2 km wide, with vegetated sand dunes along its
length and both wide and narrow beaches around its perimeter.
The breeding season extends from mid December to early
February, and the estimated pup production in 1997 was 25,400;
the annual rate of increase is 13% (Bowen et al., 2003). A
proportion of both male and female grey seals are branded and
are of known age. We studied 42 branded males whose age
range (10?31 years) approximates the reproductive life span of
this species on Sable Island (Bowen WD, Boness DJ, Iverson SJ,
and McMillan JI, unpublished data). A wide range of body
masses and lengths were represented. The study design was
cross-sectional such that each male was studied in only 1 year.
Field procedures
Males were captured at the start and toward the end of the
breeding season using a nylon net fastened between two
aluminum poles. At each capture males were injected
intramuscularly with a preweighed dose of tritium oxide
(HTO; 18.5 MBq ml
1; approximately 0.02 g kg
1 body mass).
The isotope was delivered using a 10-ml syringe fitted with a
three-way tap valve and another 10-ml syringe filled with ;5 ml
of distilled water. This configuration allowed us to remove
any residual tritium and ensured complete administration
(Beck et al., 2003). Males were then weighed using tandem
300-kg (61 kg) Salter spring balances and thereafter held in
the net to allow the tritium to equilibrate. Ninety minutes
after the HTO was administered, approximately 10 ml of
blood was taken from the extradural vein; this was repeated 15
min later to determine if the isotope was equilibrated in body
fluids. To determine water flux, a blood sample was taken at
the final capture prior to HTO administration to measure the
residual isotope level. Blood samples were collected in 10-ml
Vacutainers (with no additives) and centrifuged for 15?20 min.
Serum was removed and stored frozen at 
20C in cryovials.
Prior to release, each male was fitted with a radio transmitter
(164?165 MHz, Advanced Telemetry Systems, http://www.
atstrack.com) to relocate them on the island and a timed-depth
recorder (TDR; Wildlife Computers, http://www.wildlifecom-
puters.com) to record overall activity and diving behavior and
a length was taken (Lidgard et al., 2003). In 1997 and 1998, both
radio transmitter and TDR were removed at final capture late in
the season. From 1999 to 2001, males wore the radio transmitter
until they left the breeding colony (N? 27) or until the research
team departed the island (N ? 4).
Estimation of body composition, water flux, and energy
expenditure
The specific activity of 3H in each serum sample was measured
in triplicate and distilled using the evaporated-freeze-capture
method of Ortiz et al. (1978). For each replicate, 50 ll of
serum was transferred to a preweighed (60.1 mg) scintillation
vial, distilled, reweighed to obtain the weight of the distillate,
and 10 ml of Scintiverse II was added. A Beckman LS 5000CE
scintillation counter was used to measure the activity of 3H.
The activity of the standard from which the injectant was
taken was measured in triplicate at the same time as the serum
samples. Mean specific activity was calculated from triplicates
where the coefficient of variation (CV%) among replicates
was ,2% (most samples). Otherwise, the two closest samples
were used. Dilution space was calculated according to the
equation given in Bowen et al. (1999). We used the equation
in Bowen and Iverson (1998) to calculate total body water
(TBW) and the equations given in Reilly and Fedak (1990) to
calculate total body protein (TBP), total body fat (TPF), and
TBE. Daily water flux was calculated according to Oftedal and
Iverson (1987).
To estimate food intake, we used the following winter diet
estimated from the recovery of hard parts from grey seal
feces on Sable Island; northern sandlance (Ammodytes dubius)
60%, capelin (Mallotus villosus) 6%, winter flounder
(Pseudopleuronectes americanus) 14%, cod (Gadus morhua)
11%, and silver hake (Merluccius bilinearus) 8% (Bowen WD,
unpublished data). The mean composition of this diet was
77.7% TBW, 15.7% TBP, and 5.6% TBF (Budge SM, Iverson
SJ, and Bowen WD, unpublished data). We used the equation
in Bowen et al. (2001) to estimate daily food intake and
assumed an assimilation efficiency of 93% (Lawson et al.,
1997; Ronald et al., 1984). Daily food intake was converted
into daily food energy intake (FEI) by assuming a calorific
density of fat of 39.3 MJ kg
1 and a calorific density of protein
of 23.6 MJ kg
1 (Schmidt-Nielsen, 1990). Daily FEI was
converted to daily metabolizable energy intake (MEI)
assuming that 15% of the ingested food energy was lost as
fecal and urinary matter (Lawson et al., 1997; Ronald et al.,
1984). Daily energy expenditure (DEE) was given as the sum
of daily TBE expended and daily MEI gained.
Behavior on land and at sea
Each male was located between one and three times a day
using a R2000 radio receiver (164?165 MHz, Advanced
542 Behavioral Ecology
Telemetry Systems) and dipole and yaggi antennae according
to the protocol described in Lidgard et al. (2001). At each
sighting, a description of the location and a global positioning
system (GPS) reading (Garmin 45 & 48, http://www.garmin.
com) was recorded. To account for the error associated with
GPS locations prior to the removal of selective availability
(May 2000), a move from one site to another was recorded
when the distance was 100 m. To minimize the influence of
capture on behavioral data, the first location used in the
analysis was the first land location the day after capture or
final capture. TDR records were used to estimate deep-diving
effort (see Lidgard et al., 2003, for the definition and the dive
effort for each male). We used the operational sex ratio (OSR;
Kvarnemo and Ahnesjo, 1996) as a proxy for the level of
competition to test if the level of male-male competition
around a male influenced daily expenditure. The OSR has
a range of 0?1.0 as the number of males relative to receptive
females increases. The number of males and the number of
females in estrus within a 10 m radius of the male was
recorded at each location. We assumed that females came into
estrus when their pup was at least 14 days of age (Boness and
James, 1979; see Kovacs and Lavigne, 1986, for the age-
classification scheme).
Measures of success
We defined four mating tactics. Males were deemed as
exhibiting the primary tactic of consortship (referred to as
consort males) if they defended at least one estrous female
for 2 days. A period of 2 days was chosen because a visit to
a male on any one day may occur when the consort male is
temporarily away from the consort female; thus, a visit on
a second day increases the chance of observing the male in
defense of the consort female. Three alternative tactics have
been described (Lidgard, 2003) and are (1) male mating with
a female that has weaned her pup and is departing the colony,
(2) male that is in defense of females mating with
a neighboring lactating female, and (3) male that is not in
defense of females mating with a lactating female that is either
alone or is a consort of another male.
Extraconsort fertilizations occur in grey seals (Ambs et al.,
1999), thus we were unable to use the number of females
mated as a measure of success. Rather, we used rates of
fertilization success for the primary tactic of consortship
(0.27) and the alternative tactic of mating with departing
females (0.10; Lidgard et al., 2004) and the number of
females mated to estimate the number of pups sired. In
Lidgard et al. (2004), the rate of fertilization success was
estimated by grouping paternity data from a sample of focal
males and by estimating the proportion of offspring sired for
the primary tactic and the alternative tactic of males mating
with departing females. To provide an estimate of the number
of pups sired in this study, the number of females mated
through exhibiting the primary tactic or an alternative tactic
was multiplied by the appropriate rate of fertilization success.
Lidgard et al. (2004) only provided a measure of fertilization
success for one of the three alternative tactics. However, given
that the remaining two alternative tactics also do not involve
a period of defense, we have assumed that the rate of
fertilization success of these two tactics was similar to that of
the departing female tactic.
Data analysis
We assumed that capture mass was a good estimate of mass on
arrival at the breeding grounds based on the following
observations. Twenty-five of the 42 males in this study were
tagged with a radio transmitter in the months previous to the
study season, and the median number of days between their
first sighting on the island and capture was two. Eight other
males were sighted prior to capture, and the median number
of days between their first sighting and capture was also two.
Although the remaining nine males were first sighted on the
day of capture, given that the entire colony was surveyed
for brands at least twice before the date of the last capture
(7 January), only a few days could have passed between their
first sighting and their arrival.
Given our small sample in each year and the fact that the
age structure of the males studied differed by year, the effects
of year were not addressed in this study. All analyses were
performed using SPSS version 11.5 for Windows, and the
probability level for significance was a ? 0.05. Energetic and
behavioral variables were transformed as necessary to meet
assumptions of parametric statistics. Standard errors are given
with means.
RESULTS
Of the 42 males captured at the start of the season, only 40
were reweighed near the end of the breeding season because
in 1999 two males departed the breeding colony early (see
Appendix). At the beginning of the season, TBW was
measured in 37 of the 42 males. Near the end of the breeding
season, 8 of those 37 males (including the two males that
departed the breeding colony early) did not receive final TBW
measurements, and the tritium apparently did not equilibrate
in three of those males. Thus, 40 males were used to estimate
mass loss, 37 males were used to estimate initial body
composition, and 26 males were used to estimate changes in
body composition, water flux, and food intake. All males had
complete behavioral records on land, and 30 males had
complete TDR records. The mean number of days between
the initial and final capture was 22 6 1 days. From a subset of
males (N ? 27) that were followed from their arrival to their
departure from the breeding colony, the duration of the
breeding season was 29 6 1 days and the final capture
occurred on day 21 6 1.
Body size, body composition, and changes over the
breeding season
The mean mass of males at capture was 290.6 6 5.4 kg, and
their mean length was 2.1 6 0.02 m (N ? 42; Table 1).
Capture mass comprised an average of 53.0 6 0.5% TBW,
17.5 6 0.2% TBP, 27.2 6 0.8% TBF, and 4396 6 129 MJ TBE
(N ? 37). There was little variation among individuals in
percentage TBP and TBF at capture (CV ? 7.6% and 17.1%,
respectively). Smaller males had absolutely less TBF and TBE
at capture than larger males (Pearson r ? .734, N ? 37, p ,
.0001 and r ? .848, p , .0001, respectively). There was
a negative relationship between percentage TBF at capture
and age (partial [controlling for mass] r ? 
.449, df ? 34, p ?
.006). When controlling for age, larger males had a greater
percentage TBF at capture than smaller males (partial r ?
.448, df ? 34, p ? .005), suggesting that large males arrived at
the breeding colony in better condition.
During the sampling period, males lost 18.1 6 0.5% (N ?
40) of their capture mass, which was equivalent to an average
loss of 2.4 6 0.1 kg day
1 (CV ? 15.6%) or a loss of 0.8 6
0.02% of their capture mass per day. Although all males lost
a similar proportion of their capture mass per day, larger
males lost absolutely more of their capture mass per day than
smaller males (partial [controlling for number of days] r ?
.69, N ? 40, p , .001). Protein accounted for 10.8 6 1.6%
(CV ?73.1%; N ? 26) of mass loss and body fat accounted for
50.7 6 5.5% (CV ? 55.0%). Males with a greater percentage
Lidgard et al. ? Grey seal male mating tactics 543
TBF on capture expended less TBP per day (Pearson r?
.735,
N ? 26, p , .0001; Figure 1). All males lost TBE throughout
the sampling period with a mean loss of 56.3 6 5.0 MJ day
1
(Table 2). Assuming an energy density of 39.3 MJ kg
1 for fat
and 23.6 MJ kg
1 for protein (Schmidt-Nielsen, 1990), fat and
protein catabolism accounted for 79.9 6 4.2% and 18.2 6
4.1%, respectively, of the daily change in TBE. All of the 26
males with body composition data also had estimates of water
flux (Table 2). All males apart from one visited the sea and
fed. The mean daily food intake was 0.4 6 0.04 kg day
1 (N ?
26), which gave a mean daily MEI of 1.9 6 0.2 MJ day
1 (CV ?
49.5%). Mean DEE was 58.2 6 4.9 MJ day
1. The percentage
DEE that was gained from food varied between 0% and 21%
(4.5 6 0.8%). There was no relationship between DEE and
capture mass (Pearson r ? .159, N ? 26, p ? .439). There was
a strong negative relationship between percentage TBF at
capture and percentage DEE gained from feeding (arcsine
transformed data, Pearson r ? 
.661, p , .001; Figure 2).
Body size, body composition, and male mating tactics
Twenty-nine of the 42 males exhibited the primary tactic of
consortship and mated with consort females. The number of
consorts mated per male varied from 0 to 12 (see Appendix).
Males that exhibited the primary tactic were heavier at capture
than males that did not (ANOVA, F1,40 ? 15.3, p , .0001;
Table 1) and had absolutely more of each body component
(MANOVA, F3,33 ? 5.74, p ? .003) and TBE (ANOVA, F1,35 ?
15.2, p , .001). However, the proportion of each body
component did not differ between consort and nonconsort
males (MANOVA, F3,22 ? 0.611, p ? .615), indicating that they
arrived in similar relative condition. Mass-specific mass loss
per day and DEE also did not differ between consort and
nonconsort males (ANOVA, F1,38 ? 0.565, p ? .457 and F1,24 ?
0.187, p ? .669, respectively).
For all males, the estimated number of pups sired per male
through exhibiting the primary and alternative tactics varied
between 0 and 3.2 (see Appendix). The estimated number of
pups sired was correlated with the maximum duration of stay
at a site (Pearson r ? .606, N ? 42, p , .001; Figure 3). To
determine the relative importance of body size on the
maximum duration of stay, a stepwise linear regression model
was developed with body mass at capture, length, and age as
predictor variables. The model with length and age excluded
explained the greatest variation in duration of stay (F2,39 ?
5.01, p ? .031). Body mass at capture also showed a quadratic
relationship with the number of consorts mated (F2,39 ? 4.91,
p ? .013) and the estimated number of pups sired (F2,39 ?
5.40, p ? .009; Figure 4). To determine whether these
relationships were driven by age with smaller, younger, and
less experienced males being constrained by their low
competitive ability and older, larger males suffering from
the effects of senescence, stepwise linear regression models
were developed with the linear and quadratic terms for body
mass and age included. The original quadratic models with
Table 2
Daily water flux, energy intake, and expenditure for breeding
grey seal males (N ? 26) on Sable Island, 1999?2001
Mean 6 SE CV%
Fractional daily water flux (k) 0.04 6 0.003 25.0
Total body water flux (l day
1) 5.2 6 0.4 35.6
Metabolizable food energy intake
(MEI) (MJ day
1)
1.9 6 0.2 49.5
Body energy expenditure (TBE)
(MJ day
1)
56.3 6 5.0 44.8
Daily energy expenditure (DEE)
(MJ day
1)
58.2 6 4.9 42.7
Percentage of DEE from MEI 4.5 6 0.8 95.1
Table 1
Body mass, length, mass loss, and body composition for breeding grey seal males on arrival at Sable Island, 1997?2001: for all males
and by male type
All males Consort males Nonconsort males
pa (ANOVA)Mean 6 SE CV% Mean 6 SE CV% Mean 6 SE CV%
Body mass (kg) 290.6 6 5.4 (42) 12.1 302.9 6 5.5 (29) 9.7 263.2 6 9.0 (13) 12.4 ,.001
Length (m) 2.1 6 0.02 (42) 4.7 2.2 6 0.02 (29) 4.7 2.1 6 0.03 (13) 5.3 .077
Loss (kg day
1) 2.4 6 0.1 (40) 15.6 2.5 6 0.08 (29) 16.8 2.3 6 0.06 (11) 8.4 .064
Body water (kg) 153.4 6 3.0 (37) 11.7 161.9 6 3.9 (18) 10.2 146.6 6 6.8 (8) 13.2 .006
Body protein (kg) 50.7 6 1.0 (37) 12.1 53.4 6 1.4 (18) 10.8 48.8 6 2.3 (8) 13.3 .016
Body fat (kg) 79.4 6 3.0 (37) 23.3 84.1 6 3.7 (18) 18.6 68.4 6 6.2 (8) 25.7 .002
Body energy (MJ) 4396 6 129 (37) 17.8 4651 6 153 (18) 13.9 3909 6 276 (8) 20.0 ,.001
a p-value refers to comparison between consort and nonconsort males.
Numbers in parentheses are sample sizes.
Figure 1
Daily loss of total body protein kg day
1 and the percentage total body
fat at capture for male grey seals (N ? 26) on Sable Island, Nova
Scotia, 1997?2001.
544 Behavioral Ecology
age excluded explained the greatest variation in success.
Thus, the maximum duration of stay at a site increased with
body mass, but intermediate-sized males mated with more
consorts and achieved the highest success. Figure 4 also
suggests there is a threshold body size for achieving success;
males below 260 kg achieved zero or low success.
The DEE was not correlated with the maximum duration of
stay at a site (partial [controlling for mass] r ? 
.031, df ? 23,
p ? .882) or with the OSR (partial r ? 
.068, df ? 23, p ?
.745). Thus, there appear to be no differential costs associated
with components of a mating tactic. Of the 30 males that had
complete TDR records, 27 males went to sea during the
sampling period. The mean proportion of time spent at sea
was 0.23 6 0.04, and the mean deep-dive effort was 1.2 6 0.2 h
day
1. Deep-dive effort was strongly correlated with the
maximum duration of stay (partial r ? 
0.559, df ? 27, p ?
.002) and the MEI (partial r ? .731, df ? 14, p ? .001). Thus,
males that had short periods of stay at a site and therefore
mated with few consorts spent more time at sea deep diving,
and males that dove deep acquired more energy from food.
For the subset of males that was studied until they left the
breeding colony (N ? 27), the amount of TBE at capture was
positively correlated with the duration of the breeding period
(Pearson r ? .625, N ? 22, p ? .002; Figure 5), and males that
had a longer breeding period lost a greater proportion of
their mass (Pearson r ? .472, N ? 25, p ? .017). Consort males
had a significantly longer breeding period (32 6 1.4 days,
N ? 17) than nonconsort males (23 6 2.2 days, N ? 10;
ANOVA, F1,25 ? 13.5, p ? .001). The duration of the breeding
period was strongly correlated with the estimated number of
pups sired (Pearson r ? .464, N ? 27, p ? .015), but there was
no relationship when capture mass was controlled for (partial
r ? .012, df ? 24, p ? .953).
DISCUSSION
Mating systems are typically conditional, and individuals are
expected to exhibit the tactic that yields the greatest success
relative to their state (Gross, 1996; McNamara and Houston,
Figure 2
Percentage daily energy expenditure gained from food and
percentage total body fat at capture for male grey seals (N ? 26)
on Sable Island, Nova Scotia, 1997?2001.
Figure 3
Estimated number of pups sired and the maximum duration of stay
at a site for male grey seals (N ? 42) on Sable Island, Nova Scotia,
1997?2001.
Figure 5
Number of days between capture and departure and total body
energy at capture for male grey seals (N ? 27) on Sable Island, Nova
Scotia, 1997?2001.
Figure 4
Estimated number of pups sired and the mass of males at capture for
male grey seals (N ? 42) on Sable Island, Nova Scotia, 1997?2001.
Lidgard et al. ? Grey seal male mating tactics 545
1996). This study has shown that body size on arrival at the
breeding grounds was an important determinant of the
mating tactics exhibited by male grey seals and of their
success. Males that exhibited the primary mating tactic of
consortship were heavier, arrived with absolutely more body
fat and energy reserves, and sustained breeding longer than
those males that did not exhibit the primary tactic. Amongst
all males, body fat, energy reserves, and the duration of the
breeding period increased with body mass; however, in-
termediate-sized males mated with the most consorts and
achieved the highest success. Many other studies have
demonstrated relationships between size and success across
several taxa (Enders, 1993; Fleming and Gross, 1994; Howard,
1978; McElligott et al., 2001).
Given that extraconsort fertilizations occur in grey seals
(Ambs et al., 1999), we used the rates of fertilization success
given in Lidgard et al. (2004) to estimate the number of pups
sired by males. However, without a measure of success for the
other alternative tactics and a better understanding of the
frequency by which males exhibit alternative tactics, our
measures of success should be treated as minimum estimates.
Further, we have also assumed that the tactic of consortship
yields the highest per capita success and is therefore the
primary tactic (Gross, 1996), but we acknowledge that the two
alternative tactics of which we have no estimates of fertiliza-
tion success may achieve a higher success. However, we
consider this unlikely. The order and timing of mating relative
to ovulation has been shown to be important in determining
fertilization success in some mammals (Huck et al. 1985, 1989;
Preston et al., 2003). Thus, males that defend females and
copulate on several occasions likely increase their chances of
siring the offspring. Because alternative tactics do not involve
a period of defense and may involve a single copulation, we
consider it unlikely that their per capita success is higher than
that of the primary tactic.
Large body size confers two advantages to a male: greater
competitive ability in male combat and greater endurance
(Andersson, 1994; Murphy, 1998). This study has shown that
although the duration of stay at a site was correlated with body
mass and success, intermediate-sized males achieved the
highest success. Lidgard (2003) suggested that grey seal males
on Sable Island might suffer from high male-male competi-
tion and high male turnover due to strongly male-biased
OSRs. Thus, the ability of a male to engage in an extended
period of consortship at a site might depend more on the
male?s competitive ability and less on the amount of body
energy available for an extended period of stay. However, this
will differ according to local variations in the degree of male-
male competition.
Several previous studies on the grey seal have failed to find
a relationship between size and success (Godsell, 1991; Tinker
et al., 1995; Twiss, 1991). This might be due to excluding
males that failed to establish consortships or a small sample
size. In this study, the number of consortships varied among
males, and body mass at capture was both a determinant of
the primary tactic of consortship and a predictor of the
number of consortships and success. However, the relation-
ship between body mass and success was not linear but rather
quadratic, suggesting that intermediate-sized males achieved
the highest success. We found no evidence to suggest that this
relationship was driven by male age, thus body mass rather
than age appears to be an important determinant for
exhibiting the primary tactic of consortship and achieving
success in male grey seals. In ungulates, male combat typically
involves clashes, butts, and pushing behaviors (e.g., Clutton-
Brock et al., 1979); thus, larger, heavier males with larger
horns or antlers achieve the highest success (Coltman et al.,
2002; McElligott et al., 2001; Preston et al., 2003). Similar
forms of male combat occur in elephant seals (Mirounga spp.;
Le Boeuf and Petersen, 1969), where male size determines
rank, which, in turn, determines success (Fabiani et al., 2004;
Haley et al., 1994). However, the behavior of male grey seals
during combat is quite different. Although pushing and
clashing behaviors do occur, wrestling appears to be more
important, and the winner in aggressive encounters is often
the male that bites the opponents? hind flippers, which
requires a certain degree of agility (Twiss, 1991). Males also
need to move quickly to chase opponents away from the area
of defense. Thus, large males may simply be too large and lack
the required agility to be successful in male combat and
defend multiple females. This study also suggests there is
a threshold body mass for exhibiting the primary tactic of
consortship and for achieving success. Body mass is likely
important during male combat in pushing and clashing
events. An intermediate body size may therefore provide the
necessary strength and agility to be successful in defending
multiple females from male competitors. This suggests that
stabilizing selection may be operating on body size in male
grey seals.
Thirteen males did not exhibit the primary tactic of
consortship and achieved low success. Of these, seven were
14 years old or less and are thus likely to increase in body size
with concomitant increases in success (McLaren, 1993). While
young and small, these males may express alternative mating
tactics as a means to maximize their age-specific reproductive
output (Dunbar, 1982). However, the remaining older males
are likely to be at a disadvantage as a result of their smaller
body size for the remainder of their reproductive life.
Differences in size at adulthood among males may be partly
attributed to genetic variation, maternal effects on weaning
mass (Mellish et al., 1999), and environmental influences on
growth (Festa-Bianchet et al., 2004; Lindstro?m, 1999). If small
males are never able to exhibit the primary mating tactic and
experience a lower lifetime reproductive success as a result,
they may exhibit alternative mating tactics as means to ?make
the best of a bad job? (Dunbar, 1982). Although alternative
tactics have a lower probability of fertilization success than the
primary tactic (Lidgard et al., 2004), within a single breeding
season small males may be able to achieve at least some
success through exhibiting alternative tactics. Indeed, if male
grey seals mate at sea, as suggested by Worthington Wilmer
et al. (1999), small size may provide an advantage through
greater agility in aquatic male combat (Andersson, 1994).
Further, other components of state, in addition to body mass,
such as experience, and sex and stress hormones are likely to
influence success. Alexander and Irvine (1998) and Abott et
al. (2003) have shown that in horses and some species of
primate, subdominant individuals have higher basal levels of
the stress hormone cortisol. High levels of cortisol contribute
to lower levels of testosterone and inhibition of dominant or
territorial behaviors (Boonstra and Singleton, 1993; Moore,
1991; Sapolsky, 1985).
Large males arrived with relatively and absolutely more
body fat and body energy than smaller males and were able to
sustain breeding for longer. Judge and Brooks (2001) have
demonstrated that male bullfrogs that arrive at the breeding
grounds in better condition (i.e., greater body mass per unit
length) have longer periods of tenure. In this study, large
males were also able to minimize protein catabolism, whereas
smaller males with absolutely less fat expended more of their
body protein as energy and acquired more energy through
food intake. In penguins where both the male and female
sustain long periods of fasting during the breeding season,
critically low fat stores (20?30% of initial fat stores expended)
may be part of a stimulus that initiates feeding trips
(Groscolas and Robin, 2001). A similar refeeding stimulus
546 Behavioral Ecology
may occur in those male grey seals that remain at a site in
defense of females for extended periods of time. In this study,
one male stayed at the same location for a period of 24 days.
At day 21, percentage TBF was 12%, and he had expended
32% of initial fat reserves. Two days after leaving this site he
went to sea for 2 days. Deutsch et al. (1994) suggested that
a similar stimulus for feeding may operate in northern
elephant seals (Mirounga angustirostris).
Coltman et al. (1998) have shown that small harbor seal
males forage prior to and during the availability of estrous
females to overcome the energetic constraints of small size.
Judge and Brooks (2001) have demonstrated energetic
constraints on the breeding behavior of male bullfrogs. We
found that males that had short maximum periods of stay, and
therefore more likely to exhibit only alternative tactics, spent
more time engaged in deep diving, and deep-dive effort was
strongly correlated with food intake. However, we found no
evidence to suggest that spending short periods of time at
a site was more energetically demanding than remaining at
a site for long periods. Thus, we are unable to conclude
whether small grey seal males suffer from energetic con-
straints. Further, although the size of body fat reserves and the
extent of protein catabolism may be involved in stimulating
feeding, trips to sea may also occur in response to failing to
establish consortship, losing consortship, or simply taking
a break in between periods of consortship. Male wood bison
(Bison bison athabascae) have been shown to take breaks in be-
tween periods of defending female groups, and this is thought
to allow them to regain condition (Komers et al., 1992).
Despite differences in mating tactic, consort and non-
consort males arrived at the breeding grounds in similar
condition and expended the same relative amount of energy.
Initial percentage TBF in four species of pinniped represent-
ing different mating systems is very similar to the initial
percentage TBF reported in this study (22?29%; Boyd and
Duck, 1991; Coltman et al., 1998; Kovacs et al., 1996; Crocker
DE, Webb PM, and Houser DS, unpublished data). If there
are costs associated with energy storage (Beck et al., 2003;
Jo?nsson, 1997), this comparison suggests that males may
optimize the size of their stored energy but ultimately are
limited by their body size. Coltman et al. (1998) found no
significant differences in the daily specific energy loss among
species of pinniped representing aquatic and terrestrial
mating systems. If there are reproductive costs associated
with energy expenditure (Clutton-Brock, 1984), males may
optimize the trade-off between the energy expended in one
season and the subsequent reproductive costs such that males
of all body sizes, regardless of the mating system, expend the
same relative amount of body energy during the breeding
season. This highlights the advantages of a large body size for
endurance rivalry in breeding male pinnipeds.
We are very grateful to Suzanne Ambs, Debbie Austin, Carrie Beck,
Suzanne Budge, Dave Coltman, Margi Cooper, Tom Hubbard, Steven
Insley, Sara Iverson, Shelley Lang, Tyler Schulz, and Strahan Tucker
for assistance in the field. Special thanks to Sara Iverson for making
arrangements for the tritium and providing training and laboratory
facilities for conducting the isotope analyses. We are also grateful for
infrastructure support provided on Sable Island by Gerry Forbes. Two
anonymous reviewers provided valuable comments on the manuscript
and improved its quality. The study was supported by a Smithsonian
Institution Graduate and Predoctorate Fellowship awarded to D.C.L.
and funds from the Friends of the National Zoo, the Smithsonian
APPENDIX
Table A1
Phenotypic characteristics, initial body composition, record duration, maximum duration of stay, and mating and reproductive success for
breeding grey seal males on Sable Island, 1997?2001 (see text for explanation of abbreviations)
Male
Age
(years)
Initial
mass
(kg)
Length
(m)
Initial body composition
Record
(days)
Max.
duration of
stay (days)
Number of
females mated
Number of
pups sireda
TBW
(kg)
TBF
(kg)
TBP
(kg)
TBE
(MJ)
Consort
matings
Nonconsort
matings
1997
6 11 252.0 2.0 10.0 19.9 2 0 0.5
2 23 262.0 1.9 20.0 27.9 7 0 1.9
7 12 289.0 2.0 10.0 19.8 1 0 0.3
1 23 351.5 2.1 10.0 22.8 3 4 1.2
1998
8 12 264.0 1.8 121.7 98.6 38.6 4868 3.0 20.0 0 0 0
4 24 288.0 2.2 141.1 95.2 45.6 4906 7.0 25.0 3 0 0.8
9 12 288.0 2.0 140.8 95.7 45.5 4920 14.1 23.1 6 0 1.6
5 25 293.0 2.2 155.0 80.1 51.2 4437 13.0 24.0 4 1 1.2
3 20 302.5 2.1 149.1 98.7 48.3 5109 11.9 23.3 6 0 1.6
11 11 318.0 2.2 10.0 19.1 4 2 1.3
10 11 337.0 2.3 165.1 111.4 53.3 5740 3.8 21.9 2 0 0.5
1999
16 13 212.0 2.0 111.9 58.3 36.9 3220 2.7 15.2 0 0 0
13 12 230.0 2.1 127.9 53.7 42.8 3181 1.8 15.0 0 0 0
12 10 239.0 2.1 137.4 49.2 46.4 3086 5.3 16.9 0 0 0
14 12 256.5 2.1 139.3 64.8 46.3 3710 3.1 17.0 0 4 0.4
21 29 277.0 2.2 161.2 54.1 54.6 3482 2.7 20.0 0 1 0.1
20 29 279.0 2.0 149.5 73.4 49.6 4130 5.2 17.1 0 4 0.4
19 26 291.5 2.1 154.2 79.7 50.9 4414 11.2 17.9 2 2 0.7
15 14 296.0 2.0 156.9 80.5 51.8 4468 5.8 19.0 2 1 0.6
18 21 305.0 2.1 155.2 92.3 50.7 4914 2.9 15.1 7 0 1.9
17 21 316.0 2.1 164.0 91.0 53.9 4938 4.2 19.8 1 2 0.5
Lidgard et al. ? Grey seal male mating tactics 547
Institution, the Christensen Fund, the Canadian Department of
Fisheries and Oceans, and the National Science and Engineering
Research Council of Canada.
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Table A1, Continued
Male
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mass
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Initial body composition
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duration of
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128 22 275.0 2.2 157.9 56.9 53.2 3563 1.0 13.9 0 0 0
124 27 278.5 2.2 159.4 58.3 53.7 3630 5.0 17.8 3 2 1.0
115 22 281.0 2.2 157.1 64.4 52.6 3845 8.8 27.0 1 1 0.4
122 26 302.0 2.2 160.2 81.9 52.9 4551 4.1 19.9 1 0 0.3
117 13 303.5 2.2 151.1 96.8 49.1 5052 2.8 26.3 0 10 1.0
114 15 311.5 2.3 162.4 88.6 53.4 4830 22.2 30.2 2 0 0.5
118 13 315.0 2.2 152.6 106.7 49.2 5448 7.1 27.1 1 0 0.3
119 14 316.0 2.2 161.6 94.6 52.8 5056 21.2 22.1 9 0 2.4
120 26 366.0 2.3 204.4 84.1 68.5 5017 7.9 21.2 2 4 0.9
2001
197 16 235.5 2.1 135.1 48.9 45.6 3055 3.2 21.2 0 1 0.1
195 28 250.0 2.2 135.2 64.0 44.9 3644 5.0 19.0 0 2 0.2
188 23 274.0 2.1 157.6 56.3 53.2 3537 16.8 31.0 12 0 3.2
194 16 275.0 2.1 140.9 81.9 46.1 4386 3.9 25.9 2 1 0.6
189 28 283.0 2.2 146.2 82.6 47.9 4457 11.0 27.0 6 1 1.7
198 23 287.0 2.3 158.8 68.2 53.1 4008 9.0 19.1 2 3 0.8
206 31 287.5 2.2 156.1 72.8 51.9 4161 12.0 24.9 2 1 0.6
191 15 302.5 2.1 144.0 106.2 46.1 5356 9.0 31.1 2 0 0.5
192 31 335.5 2.2 177.3 92.0 58.5 5090 10.8 28.8 0 2 0.2
196 23 335.5 2.2 182.5 83.9 60.8 4835 3.9 17.0 1 6 0.9
208 28 380.5 2.3 195.7 112.2 64.1 6033 10.0 22.8 2 1 0.6
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