Analysis of the gliding behavior of Ptychozoon Lionatum (Reptilia: Gekkonidae)

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 negatively 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 Philippine Islands. The five species are characterized by extensive digital webbing; small skin flaps on the head, neck, and limbs; a strongly depressed tail with scalloped lateral 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. Remarks on the functional significance of these structures have been largely speculative due to the scarcity of reliable field observation and experimentation. Cantor (1847) suggested a parachutelike 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 concealing 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 sumHERPETOLOGICA 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 concealment 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 Zoological 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 recorded 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 hindfeet 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 calculated. Ventral surface area estimates ranged from 20.69-33.38 cm. Ten gliding trials were conducted, four on 26 May and two each on 28 May, 4 June and 26 June. A trial consisted of releasing 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 afternoon (1530-1800 h) at air temperatures of 24.5°-28.0°C. Clear and relatively windless conditions prevailed during the trials. The animals were released in a consistent manner by being held at arm's length away from the building. Each specimen 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 horizontal distance from contact point to a point directly below the release point. The calculated straight-line distance from release point to contact point was used as an estimate of glide distance for computation of glide rate. Calculated glide rates probably 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.