Saturday, November 27, 2010

Predator Defense in Three Quiet Different Gecko Clades

Geckos tend to be small and rather defenseless lizards. They have the ability to shed their tail, or tear their skin to escape a predator. And, while a few large species may deliver a nasty bite, smaller species cannot. Geckos also seem to lack chemical defenses to repel potential predators and therefore they rely on avoiding predators by detecting their odor, or through cryptic coloration or habits, or by distracting potential predators with their tails. Several recent papers describe ways in which some geckos avoid being eaten.

The Australia, nocturnal, rock-dwelling Velvet Gecko, Oedura lesueurii, lives in the same habitat with centipedes. Pike et al. used a combination of field data and laboratory experiments to determine whether a predatory and venomous centipede (Scolopendridae: Cormocephalus sp.) influences habitat selection by this gecko. In the field, they found centipedes and geckos were syntopic, used crevices beneath rocks with similar physical dimensions, thermal regimes and degree of sun exposure. Not surprisingly, geckos rarely shared rocks with centipedes in the field. In laboratory trials, both geckos and centipedes selected shelters with narrow rather than wide crevices and the presence of a centipede caused juvenile geckos to avoid those hiding places. However, adult geckos continued to select narrow crevices even if these contained centipedes. When the experimenters added centipedes to narrow crevices beneath small and large rocks occupied by geckos, both juvenile and adult geckos exited the crevice, especially if it was under a small rock rather than large rock. Thus, centipedes influence habitat selection by velvet geckos, and anti-predator behaviors of geckos are both size- and context-dependent. In a related project, Webb et al. (2010) tested the responses of the Velvet Gecko to the odors of two snake species. One species, the Broad-headed Snake, Hoplocephalus bungaroides, feeds on geckos (therefore is dangerous), the other, the Small-eyed Snake, Cryptophis nigrescens, feeds on skinks, but not geckos (less dangerous). The authors tested whether predator avoidance by prey was modulated by thermal costs associated with retreat-site selection and in both the presence and absence of thermal costs, Velvet Geckos avoided crevices scented by both dangerous and less dangerous snake species. Their results suggest that Velvet Geckos treat both snake predators as equally dangerous. They repeated the experiments in the field and obtained similar results, the geckos avoided crevices that were temperature - friendly but scented with the odor of snakes- regardless of whether or not the snakes were considered dangerous by the researchers. Suggesting, the Velvet Gecko treats both snakes as dangerous and avoids them.

Alonso et al. (2010) examined the behavior of the Yellow-headed Gecko, Gonatodes albogularis, towards humans (the predators) in three habitats near Bogot√° in Colombia. They collected data on the tail display as an observer approached the lizard. During transects geckos were approached by the observer in a standardized way, and details of their tail-waving displays were recorded. In control recordings animals were watched from a distant site without approaching them. Their results showed sexual differences in tail-waving. Male geckos waved their tails more frequently than females. But they found no significant differences between male and female flight distances, nor height above the substrate when the lizards were initially located. Males displayed more frequently than females when approached than when the simulated predator remained stationary. The authors suggest the display functions as a pursuit-deterrent signal to potential predators; but note that some tail displays were performed in the presence of conspecifics, therefore the display may also have a social function.

In southwestern Africa, Higham and Russell (2010) examined how two related species of Namib Day Geckos of the genus Rhoptropus runaway from predators. Rhoptropus afer and Rhoptropus bradfieldi are found in the coastal desert regions of Namibia, and while R. afer commonly runs on sandy substrates and moves between isolated sheets of rock, R. bradfieldi lives and runs on isolated boulders. The morphology of the two species is quiet divergent. The authors recorded the inclination of the substrates, quantified the maximum speed and acceleration of each species on a level track, and measured their seed and acceleration during escapes in the field. They found that R. bradfieldi occupies steeper surfaces than those occupied by R. afer. On the track and in nature, R. afer runs faster than R. bradfieldi. Rhoptropus afer commonly runs for more than 2 or 3 m to escape a predator, whereas R. bradfieldi commonly runs shorter distances, often less than 50 cm during an escape sprint. Higham and Russell concluded that: (1) R. afer attains higher maximum speeds when escaping under controlled and field conditions, although the magnitude differs between conditions; and (2) hindlimb morphology correlates with maximum running speed in R. afer, but not in R. bradfieldi. They propose that these two gecko species represent distinct and highly divergent ecomorphs.

Literature

Alonso, M. L. B., J. M. Cotrina, D. A. Pardo, E. Font and M. Molina-Borja. 2010. Sex differences in antipredator tail-waving displays of the diurnal yellow-headed gecko Gonatodes albogularis from tropical forests of Colombia. Journal of Ethology, 28:305-311.  DOI: 10.1007/s10164-009-0186-4

Higham, T. and A. P. Russell. 2010. Divergence in locomotor performance, ecology, and morphology between two sympatric sister species of desert-dwelling gecko. Biological Journal of the Linnean Society, 101:860–869.

Pike, D. A., B. M. Croak, J. K. Webb, and R. Shine. 2010. Context-dependent avoidance of predatory centipedes by nocturnal geckos (Oedura lesueurii). Behavior, 147:397-412.

Webb, J. K., D. A. Pike and R. Shine. 2010. Olfactory recognition of predators by nocturnal lizards: safety outweighs thermal benefits. Behavioral Ecology, 21:72-77.

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