Laboratory Study of Escape Tactics of Parthenogenetic and Gonochoristic Cnemidophorus from Southern Texas

Abstract

Obligate parthenogenesis is rare in animals, especially vertebrates (Bell, 1982). Nonetheless, the existence of parthenogenesis in a world dominated by sexually reproducing species has motivated evolutionary biologists to expend considerable effort trying to explain why parthenogenesis exists at all, and given that it does exist, why is it not more common (Michod and Levin, 1988; Vrijenhoek, 1989; Ladle, 1992). This effort has produced a large body of theory, much of it focusing on the hypothetical advantages and disadvantages derived from the parthenogenetic mode of reproduction by an allfemale species compared with sexual reproduction by gonochoristic (= bisexual) species (Michod and Levin, 1988; Price et al., 1993, and references therein). The lizard genus Cnemidophorus provides an excellent opportunity to conduct comparative studies because nearly one-third of the species are obligate parthenogens, all apparently originating from hybrids between two or more gonochoristic species (Wright, 1993). Furthermore, most of these parthenogens coexist with gonochoristic congeners (Case, 1990; Paulissen et al., 1992, and references therein), making it possible to compare species within the same conditions. Some recent studies have attempted to identify a correlation between reproductive mode and lizard antipredator behavior. For example, in a study conducted in New Mexico, Price (1992) found that the parthenogenetic species C. tesselatus was more approachable, less cautious, and presumably more vulnerable to predators that the gonochoristic species C. tigris marmoratus (= C. marmoratus). However, in a similar study conducted in southern Texas, Paulissen (1995) found no significant differences in approachability or frequency of various escape behaviors used by two parthenogens of the C. laredoensis complex and the gonochoristic species C. gularis and concluded that all these species were equally vulnerable to predators. These contrasting results suggest there is no consistent correlation between reproductive mode and vulnerability to predators. However, both studies measured only one component of lizard antipredator behavior, namely wariness (Snell et al., 1988). Locomotion, including speed, stamina, and directness of the route taken to a refuge (i.e., straight line or zig-zag), is also a key component of a lizard's antipredator repertoire (Snell et al., 1988). This may be particularly important for Cnemidophorus lizards in southern Texas because their most