ANALYSIS OF THE GLIDING BEHAVIOR OF PTYCHOZOON LIONATUM (REPTILIA: GEKKONIDAE) DALE L. MARCELLINI AND THOMAS E. KEEFER ABSTRACT: The glides of the lizard, Ptychozoon lionatum, with lateral cutaneous expansions tied and untied were measured and timed. Analysis demonstrated that untied animals traveled farther and at lower speeds than animals with expansions tied. Glide distances for untied lizards were nega- tively correlated with weight/surface area and with weight/snout-vent length. It is suggested that the primary function of the lateral cutaneous expansions is to facilitate gliding. PTYCHOZOON is a small genus of arboreal geckos occurring in southeast Asia, the Indo-Australian Archipelago, and the Phil- ippine Islands. The five species are charac- terized by extensive digital webbing; small skin flaps on the head, neck, and limbs; a strongly depressed tail with scalloped lat- eral membrane; and large lateral cutaneous folds or expansions from axilla to groin (Fig. 1). The latter lie close to the body during rest or normal locomotor activity, are apparently not under muscular control, and lack skeletal or muscular support. Re- marks on the functional significance of these structures have been largely specu- lative due to the scarcity of reliable field observation and experimentation. Cantor (1847) suggested a parachute- like function for the lateral expansions. Boulenger (1890) figured Ptychozoon homalocephalum [= P. kuhli] in "flight." However, Annandale (1905) and Barbour (1912) dismissed the "flight" hypothesis and maintained a cryptic function for the expansions. De Rooij (1915) made no mention of flight but referred to a conceal- ing purpose. Smith (1935) suggested that the lateral expansions, though not under muscular control, might be raised by wind resistance in "flight." Behavioral observations in nature have rarely been reported. Boulenger (1908) referred to a P. homalocephalum reportedly caught in flight between two trees, and Taylor (1922) recorded the capture of the type specimen of P. intermedia when it was disturbed in a tree and jumped to the ground. Taylor (1963) reported a P. lionatum that was discovered near the sum- HERPETOLOGICA 32:362-366, December 1976 mit of a small dead tree and, after prodding with a bamboo pole, took "flight" and jumped a horizontal gap of about 2.5 m to another tree where it landed on the trunk about 5 m lower than its point of departure. Casual experiments by several workers (Tweedie, 1950; Tweedie, 1954; Heyer and Pongsapipatana, 1970), each conducted with a single experimental animal and with few replications, have established the gliding ability of Ptychozoon. Tiwari (1961) restated the assumption (Tweedie, 1950) that the webbed feet and frilled tail of Ptychozoon are adaptations for conceal- ment and gliding, while lateral cutaneous expansions function solely in gliding. In this paper we quantify the gliding ability of Ptychozoon lionatum and establish the function of the lateral expansions. MATERIALS AND METHODS Eight Ptychozoon lionatum (6 9 9, 2 S $ ) in the United States National Zoolog- ical Park collection were housed, singly or in groups of two or three, in glass terraria at 25?-32?C, and maintained on a diet of living insects. All specimens appeared healthy throughout the study period (26 May-26 June) except one animal which died of an injury not associated with the study. Specimens were individually marked with spots of nontoxic paint on the dorsum. Weights (6.1-12.5 g) and linear measurements (SVL 73-85 mm) were re- corded on 26 May and 26 June. Ventral surface area of each specimen was estimated by summing estimates of tail, feet and body ventral surface areas. Tail surface area was defined as length X width December 1976] HERPETOLOGICA 363 FIG. 1.?Ventral aspect of living Ptychozoon lionatum photographed through glass. at midpoint vent to tip. The maximum straight-line distance across the expanded front foot was taken as the diameter of a circle; each front foot was thus % of this circle and the computed area of the circle was an estimate of the surface area of the two front feet. The area of the two hind- feet was similarly estimated. Ventral body surface area was computed by visualizing the animal's trunk as a rectangle, the width being maximum width of the expanded lateral folds and the length being from the vent to a line across the gular region. Head and limb surface areas were not calcu- lated. Ventral surface area estimates ranged from 20.69-33.38 cm2. Ten gliding trials were conducted, four on 26 May and two each on 28 May, 4 June and 26 June. A trial consisted of re- leasing each animal from the edge of the roof of a building 8.25 m above the ground. In five trials the lateral cutaneous folds of the animals were restrained in a natural resting position by tying two pieces of fine thread around the trunk, one posterior to the axilla and the other anterior to the groin; in the other five trials the folds were not tied. Trials were run in the late after- noon (1530-1800 h) at air temperatures of 24.5?-28.0?C. Clear and relatively wind- less conditions prevailed during the trials. The animals were released in a con- sistent manner by being held at arm's length away from the building. Each speci- men was restrained immediately behind its head, the longitudinal axis of its body was held perpendicular to the wall of the building with the animal's snout directed outward. A person on the ground below timed the descent with a stopwatch, marked the contact point and measured the hori- zontal distance from contact point to a point directly below the release point. The calculated straight-line distance from re- lease point to contact point was used as an estimate of glide distance for computation of glide rate. Calculated glide rates prob- ably represent underestimates as the glide paths were curved and not straight. Film records of some of the descents were made with a movie camera operated at 54 frames/second. RESULTS Table 1 and Fig. 2 show data for drops of P. lionatum with lateral folds tied and untied. The greatest horizontal distance attained was 9.35 m for individual 3 with 364 HERPETOLOGICA [Vol. 32, No. 4 t i -8- UNTIED TIED UNTIED TIED DISTANCE RATE FIG. 2.?Horizontal distances traveled and glide rates for Ptychozoon lionatum with lateral cuta- neous folds tied and untied. Vertical lines are ranges, horizontal lines are means. Rectangles represent 95% confidence limits of means and solid black rectangles indicate SE of means. N = 25 for each condition. folds untied, and the shortest distance was 0.91 m for individual 5 with folds tied. The mean horizontal distance for untied ani- mals was 5.24 m and the mean for tied animals 3.20 m. The means were compared using a f-test and showed a significant dif- ference at the p < .01 level. A comparison of glide rates (Fig. 2) also disclosed a sig- nificant difference between the means at the p < .01 level. Geckos with folds tied generally travelled shorter distances at a higher rate than those with folds untied (Fig. 2). It is clear that animals with folds tied retained considerable gliding ability; individual 5 reached a horizontal distance of 6.30 m with folds tied (Table 1), well above the mean distance of untied descents. Generally, horizontal distance was posi- tively correlated with time. However, a glide with a very short horizontal distance (first untied descent of individual 4) oc- casionally took as long as one with 6 X as great a horizontal component (untied de- scent of individual 5). This appeared to be the result of differing behavior during the descent. It was hypothesized that horizontal dis- tance would be correlated with such phys- ical parameters as weight, SVL, and ven- tral surface area. Correlation coefficients were calculated between horizontal dis- tance of untied glides and (1) weight/ SVL, and (2) weight/estimated ventral surface area. Highly significant negative correlations of r = -.57 (p < .01) and r = -.51 (p < .05), respectively, were found. Filmed descents showed that the be- havior and subsequent glide path of the geckos, with or without tied lateral expan- sions, was similar. Three distinct types of descent were observed. In all types the animals appeared to control their body position and there were no cases of tum- bling. First, and least common, was a nearly direct vertical drop without strong posturing (3 of 50 descents; 2 tied and 1 untied). Second was a nearly vertical drop with posturing during the last 5 to 7 m of the descent (13 of 50; 9 tied and 4 untied). Third, and most common, was a descent consisting of an initial vertical drop of 1.2- 3.0 m with strong posturing followed by a descending arc of varying distance (34 of 50; 14 tied and 20 untied). The arc was steep at first and leveled out during the last few meters with a slight rise just before landing. Type 3 descents varied in steep- ness, distance and rate. The posture assumed in all three types of descents was similar but differed in in- tensity. The basic posture observed in all descents was one with limbs and tail extended and belly downwards. This pos- ture was further enhanced by stronger ex- tension of the limbs and tail and expan- sion of the webbed digits. The stronger posture increased the effective ventral sur- face area as the appendages were spread in the same plane as the body. If not tied around the trunk, the lateral cutaneous ex- pansions were widely spread by wind re- sistance, further increasing surface area. The time it took a gecko to reach the ground was a function of the type of de- scent and the posture assumed. Thus, as discussed previously, a descent may have December 1976] HERPETOLOGICA 365 TABLE 1.?Data for 50 drops of eight Ptychozoon lionatum with lateral cutaneous expansions untied and tied. Rates calculated with descent distance. Values are: horizontal distance (m), descent dis- tance (m), time (s), rate (m/s). Individual Untied descents Individual Tied descents l 8.03-11.51-2.5-4.6 I 4.27- 9.29-1.8-5.2 6.65-10.60-2.3-4.6 4.27- 9.29-1.8-5.2 4.62- 9.46-1.9-5.0 4.62- 9.46-1.5-6.3 2 3.71- 9.05-1.7-5.3 2.74- 8.69-1.6-5.4 5.84-10.11-2.4-4.2 4.65- 9.47-1.6-5.9 6.35-10.41-2.4-4.3 2 1.07- 8.32-1.5-5.6 3.05- 8.80-1.8-4.9 4.01- 9.17-1.4-6.6 .3 5.11- 9.70-2.1-4.6 1.42- 8.37-1.6-5.2 9.35-12.47-2.5-5.0 5.13- 9.71-1.8-5.4 2.08- 8.51-1.6-5.3 3 3.96- 9.15-1.6-5.7 4 0.97- 8.31-1.7-4.9 4 1.88- 8.46-1.6-5.3 1.52- 8.39-1.5-5.6 5 6.30-10.38-1.8-5.8 5 6.32-10.39-1.7-6.1 0.91- 8.30-1.4-5.9 6 8.33-11.72-2.6-4.5 6 3.63- 9.01-1.7-5.3 6.30-10.38-1.9-5.5 1.55- 8.39-1.4-6.0 7.87-11.40-2.3-5.0 2.84- 8.72-1.6-5.4 6.91-10.76-2.1-5.1 3.66- 9.03-1.7-5.3 2.34- 8.58-2.0-4.3 7 3.40- 8.92-1.6-5.6 7 7.47-11.13-2.2-5.1 1.88- 8.46-1.5-5.6 4.72- 9.50-1.7-5.6 2.69- 8.68-1.5-5.8 6.93-10.77-1.9-5.7 1.88- 8.46-2.7-3.1 8 5.92-10.15-2.4-4.2 3.05- 8.80-1.4-6.3 3.73- 9.05-2.1-4.3 8 3.38- 8.92-1.8-5.0 1.42- 8.37-1.7-4.9 1.60- 8.40-1.8-^.7 5.38- 9.85-1.9-5.2 5.23- 9.77-1.8-5.4 been short in duration but with a consider- able horizontal component, while another may have been of similar duration but with a very small horizontal component. The horizontal direction of the descents appeared to be at random, although no quantitative records were kept. The wall of the building precluded horizontal move- ments in that direction, and 30 of 80 trials were terminated when animals struck the side of the building. The higher number of successful trials was probably because the animals were held with their heads pointing away from the wall prior to release. DISCUSSION A function of the lateral abdominal ex- pansions of P. lionatum is clearly to aid in gliding flight by increasing ventral surface area. The hypothesis that they function in crypsis cannot be rejected, but appears questionable. The folds lie closely curled around the animal's trunk when it is resting or crawling; thus situated, they do not break up the visual outline of the animal's body. The frilled tail and the small flaps of skin on head, neck and limbs appear to make Ptychozoon less conspicuous to a human observer and probably serve a dual func- tion in gliding and camouflage. The work of Tweedie with P. kuhli (1950, 1954) and that of Heyer and Pongsapi- patana (1970) with P. lionatum, although limited, gave results similar to those pre- sented in this paper. Both horizontal dis- tances and glide rates were greater, but release points were higher. Tweedie (1954) reported orientation to wind direction dur- ing descent, and other investigators (Bou- lenger, 1908; Taylor, 1963) mention glides from one tree to another, implying volun- tary orientation not apparent in the present study. Oliver (1951) made an arbitrary distinc- tion between gliding and parachuting. He 366 HERPETOLOGICA [Vol. 32, No. 4 defined gliding as a retarded descent along a path deviating > 45? from the vertical, and parachuting as a retarded descent along a path deviating < 45?. In the pres- ent study only two of the untied and none of the tied descents were "glides" in Oliver's sense if angle of descent is de- rived from a right triangle with vertices at the release point, a point on the ground directly below the release point, and the landing point. Since all animals fell ver- tically one to several meters before any horizontal deflection occurred, a more ac- curate estimate of angle of descent was ob- tained by assuming the glide path to have started 1.25 m below the release point. Even then only five of the untied and none of the tied descents qualified as "glides"; thus the definition may be inadequate. It is apparent that P. lionatum can perform a retarded vertical descent ("parachute") as well as cover considerable horizontal distances ("glide"). Separate definition of these behaviors is arbitrary, as one merges into the other. It is suggested that be- havioral descriptions are more useful than definitions, which should be avoided. Acknowledgments.?We express our apprecia- tion to Michael Davenport, Curator, and to the keepers of the Reptile Unit of the National Zoolog- ical Park for their cooperation during this study. LITERATURE CITED ANNANDALE, N. 1905. Notes on some Oriental geckos in the Indian Museum, Calcutta, with descriptions of new forms. Ann. Mag. Nat. Hist., (Ser. 7) 15:26-32. BARBOUR, T. 1912. A contribution to the zoo- geography of the East Indian islands. Mem. Mus. Comp. Zool. 44:1-203. BOULENGER, G. A. 1890. Reptilia and batrachia in: The fauna of British India. Taylor and Francis, London. . 1908. Fishes, batrachians and reptiles. J. Fed. Malay State Mus. 3:64. CANTOR, T. 1847. Catalogue of reptiles inhabit- ing the Malayan Peninsula and islands, col- lected or observed by Theodore Cantor, Esq., M.D. J. Asiatic Soc. Bengal 16:607-656. DE ROOIJ, N. 1915. The reptiles of the Indo- Australian Archipelago. Vol. I. E. J. Brill, Leiden. HEYER, W. R., AND S. PONGSAPIPATANA. 1970. Gliding speeds of Ptychozoon lionatum (Rep- tilia: Gekkonidae) and Chrysopelea ornata (Reptilia: Colubridae). Herpetologica 26:317- 319. OLIVER, J. A. 1951. "Gliding" in amphibians and reptiles, with a remark on an arboreal adaptation in the lizard, Anolis carolinensis carolinensis Voight. Am. Nat. 85:171-176. SMITH, M. A. 1935. The fauna of British In- dia. Vol. II. Taylor and Francis, London. TAYLOR, E. II. 1922. The lizards of the Phil- ippine Islands. Bur. Sci., Dept. Agri. Nat. Res., Manila. Publ. No. 17. . 1963. The lizards of Thailand. Univ. Kansas Sci. Bull. 44:687-1077. TIWARI, K. K. 1961. The eggs and flight of the gecko Ptychozoon kuhli Stejneger from Car Nicobar. J. Bombay Nat. His. Soc. 58: 523-527. TWEEDIE, M. W. F. 1950. The flying gecko, Ptychozoon kuhli Stejn. Proc. Zool. Soc. London 120:13. . 1954. Notes on Malayan reptiles, No. 3. Bull. Raffles Mus., Singapore (25) :107-117. Received: 25 August 1975 Accepted: 18 November 1975 Department of Herpetology, National Zoological Park, Smithsonian Institution, Washington, DC. 20009, USA