Effects of Digit Orientation on Gecko Adhesive Force Capacity: Synthetic and Behavioral Studies.

In this study we developed an analytical relationship between adhesive digit orientation and adhesive force capacity to describe the tendencies of climbing organisms that use adhesion for climbing to align their toes in the direction of loading, maximizing adhesive force capacity. We fabricated a multi-component adhesive device with multiple contact surfaces, or digits, to act as a model system mimicking the angular motion of a foot and found the synthetic experiments agree with the developed analytical relationship. In turn, we find that observations of gekkonid lizards climbing on vertical substrates correlate well with our analytical relationship; a reduction in toe spacing is seen on the forelimbs when the animals are facing up. Interestingly, the toes on the hindlimbs tend to have an increase in spacing, possibly a mechanism for stabilization rather than load-bearing.

A new metric for measuring condition in large predatory sharks.

A simple metric (span condition analysis; SCA) is presented for quantifying the condition of sharks based on four measurements of body girth relative to body length. Data on 104 live sharks from four species that vary in body form, behaviour and habitat use (Carcharhinus leucas, Carcharhinus limbatus, Ginglymostoma cirratum and Galeocerdo cuvier) are given. Condition shows similar levels of variability among individuals within each species. Carcharhinus leucas showed a positive relationship between condition and body size, whereas the other three species showed no relationship. There was little evidence for strong differences in condition between males and females, although more male sharks are needed for some species (e.g. G. cuvier) to verify this finding. SCA is potentially viable for other large marine or terrestrial animals that are captured live and then released.

What determines dewlap diversity in Anolis lizards? An among-island comparison.

Animal signalling systems are extremely diverse as they are under different, often conflicting, selective pressures. A classic textbook example of a diverse signal is the anoline dewlap. Both at the inter- and intraspecific levels, dewlap size, colour, shape and pattern vary extensively. Here, we attempt to elucidate the various factors explaining the diversity in dewlap size and pattern among seven Anolis sagrei populations from different islands in the Bahamas. The seven islands differ in the surface area, number and kind of predators, sexual size dimorphism and Anolis species composition. In addition, we investigate whether selective pressures acting on dewlap design differ between males and females. Whereas dewlap pattern appears to serve a role in species recognition in both sexes, our data suggest that relative dewlap size is under natural and/or sexual selection. We find evidence for the role of the dewlap as a pursuit-deterrence signal in both males and females as relative dewlap size is larger on islands where A. sagrei occurs sympatrically with predatory Leiocephalus lizards. Additionally, in males relatively large dewlaps seem to be selected for in a sexual context, whereas in females natural selection, for instance by other predators than Leiocephalus lizards, appears to constrain relative dewlap size.

Morphological convergence as a consequence of extreme functional demands: examples from the feeding system of natricine snakes.

Despite repeated acquisitions of aquatic or semi-aquatic lifestyles revolving around piscivory, snakes have not evolved suction feeding. Instead, snakes use frontally or laterally directed strikes to capture prey under water. If the aquatic medium constrains strike performance because of its physical properties, we predict morphological and functional convergence in snakes that use similar strike behaviours. Here we use natricine snakes to test for such patterns of convergence in morphology and function. Our data show that frontal strikers have converged on a similar morphology characterized by narrow elongate heads with a reduced projected frontal surface area. Moreover, simple computational fluid dynamics models show that the observed morphological differences are likely biologically relevant as they affect the flow of water around the head. In general, our data suggest that the direction of evolution may be predictable if constraints are strong and evolutionary solutions limited.

Frugivory in polychrotid lizards: effects of body size.

As more data have become available on lizard diets in the past few decades, researchers have stressed the importance of lizards as pollinators and seed dispersers. Whereas large body size has been traditionally put forward as a major biological factor "allowing" herbivory and frugivory in lizards, a recent review of frugivory and seed dispersal by lizards showed that frugivory might be considered to be a typical island phenomenon, independent of body size. Here we show that frugivory is correlated with lizard body size among a group of syntopic Anolis species in Jamaica, with larger species eating more fruit. Additionally, the size of the fruits consumed is significantly related to lizard body size. Multiple regression analyses show that this is largely a pure body size effect as head shape or residual bite force are uncorrelated to overall fruit size. Moreover, we demonstrate that among polychrotid (Anolis-like) lizards in general, those that consume fruit are on average larger than those that do not. Lizards from the mainland were not significantly different in body size from island species. We thus suggest that fruit consumption in polychrotid lizards is mediated by large body size whether living on islands or not.

Comparative and behavioral analyses of preferred speed: Anolis lizards as a model system.

I quantified the movement patterns of eight morphologically and ecologically diverse Caribbean Anolis lizard species in the field to address the following questions: (1) Do these eight species move at preferred speeds, and if so, what are these speeds? (2) What proportion of their maximum sprinting capacities do the anole species use when moving undisturbed? (3) What percentage of the time do lizards spend moving, and how far do they typically travel on a daily basis? (4) Have the preferred speeds of anoles coevolved with structural habitat use? Most of the distributions of speeds were highly skewed, with a preponderance of slow-speed locomotion (<20% of maximum capacity). Median speeds varied almost eightfold among species, from a low of 4.9 cm/s (3.0% of maximum) to a high of 38.0 cm/s (22.4% of maximum). For all eight species, at least 75% of their locomotor movements took place between 0% and 40% of maximum capacity. The eight species varied almost 15-fold in the percentage of time they spent moving, indicating that not all anole species are equally sedentary. Through usage of modern comparative methods, I showed that Anolis species that move slowly through their environments also tend to use narrow perch diameters and have large habitat breadths. These findings show how evolutionary approaches can be profitably integrated with physiological data to understand how species use their habitats.

Size matters: ontogenetic variation in the three-dimensional kinematics of steady-speed locomotion in the lizard Dipsosaurus dorsalis.

Although many studies have investigated how locomotor capacities change with size, few studies have examined whether different-sized individuals within a species have similar kinematics during locomotion. We quantified the skeletal limb morphology and the three-dimensional kinematics of the hindlimb of four sizes (4-66 g) of the lizard Dipsosaurus dorsalis moving steadily at both the walk-run transition (50 % duty factor) and at a moderately fast speed of 250 cm s(-)(1). We used analyses of variance to test whether limb movements changed with size and to determine whether size and speed had interactive effects on kinematics. The disproportionately long hindlimbs of smaller lizards partly contributed to their relatively greater (i.e. adjusted by snout-vent length) values of linear kinematic variables. Both relative linear and angular kinematics changed significantly with both size and speed, both of which had widespread interactive effects. By having more extension of the knee and ankle joints, and thus a relatively higher hip height during stance, the slow-speed movements of small lizards displayed some of the characteristics of the fast-speed movements in larger lizards. Further, approximately one-fifth and two-fifths of the strides of the two smallest size classes were digitigrade at the lower and higher speeds, respectively, whereas the two largest size classes always had a plantigrade foot posture. Some of the most striking effects of size on kinematics were most evident at the lower of the two speeds. Unlike interspecific studies, which show that the limbs often become more crouched with decreased size, the more extended limbs of smaller lizards in this study suggest that variation in size alone cannot be the causal reason for differences in limb posture.

A field study of the effects of incline on the escape locomotion of a bipedal lizard, Callisaurus draconoides.

We analyzed footprints on the surface of a sand dune to estimate maximal running speeds and the incidence of bipedality in nature, as well as to investigate the effects of incline on the escape locomotion of the lizard Callisaurus draconoides. Previous laboratory tests predicted that inclines would negatively affect sprinting performance in C. draconoides. Although physiologists commonly assume that escape locomotion will be near maximal capacity, we found that only 11% of all strides were greater than 90% of maximal speed of C. draconoides. Escape paths averaged 10 m in length and were generally straight. Approximately 30% of the strides taken by C. draconoides were bipedal, and this value was three times greater than previously found for the closely related species Uma scoparia. The modal value of bipedal stride lengths was greater than that for quadrupedal strides. Inclines negatively affected velocity of only the first meter of C. draconoides escape paths. The location of nearest cover had better predictive value for the initial orientation of C. draconoides escapes than incline. On steep slopes (>15 degrees), C. draconoides avoided running directly downhill and uphill and primarily ran horizontally, whereas on shallow slopes, lizards exhibited approximately equal amounts of horizontal, direct uphill, and direct downhill running.

Effects of incline on speed, acceleration, body posture and hindlimb kinematics in two species of lizard Callisaurus draconoides and Uma scoparia.

We examined the effects of incline on locomotor performance and kinematics in two closely related species of iguanian lizards that co-occur in sandy desert habitats. Callisaurus draconoides differs from Uma scoparia of equal snout-vent length by being less massive and having greater limb and tail lengths. We analyzed high-speed video tapes of lizards sprinting from a standstill on a sand-covered racetrack which was level or inclined 30 degrees uphill. C. draconoides sprinted significantly faster than U. scoparia on both level and uphill sand surfaces, although U. scoparia is considered to be more specialized for sandy habitats. Initial accelerations (over the first 50 ms) did not differ significantly either between species or between inclines within species. Overall, the effects of incline were more pronounced for C. draconoides than for U. scoparia. For example, the incline caused a significant decrease in the maximum stride length of C. draconoides but not in that of U. scoparia. For C. draconoides, uphill stride durations were significantly shorter than on the level surface, and this partially compensated for the effects of shorter uphill stride lengths on velocity. C. draconoides ran bipedally more often than did U. scoparia on both the level and uphill surfaces.