The effects of coastal lowland instability : melanism in Cordylus polyzonus and terrestriality in Cordylus macropholis

[摘要] ENGLISH ABSTRACT: It has been suggested that rapid environmental change along the Western Cape coastal lowlandhas had a profound bearing on the evolution of cordylid populations in this region. To test ahypothesis on the evolution of melanism in the cordylid lizard, Cordylus polyzonus, the geneticrelationship among melanistic, turquoise and brown morphotypes in this species was examinedwith allozyme electrophoresis. No polymorphic loci were found. No evidence was found thatthe melanistic population was genetically distinct from the other colour morphotypes. Theresults suggest that the studied melanistic population, is not a relict of a previously biggermelanistic population, but simply an ecotype within a larger gene pool.In the second section, some aspects of the ecology of the terrestrial lizard, Cordy Iusmacropholis were investigated. Implications of a proposed hypothesis on habitat transition inthis species, due to sea-level events, are discussed. Three complementary methods were used toinvestigate habitat selection in C. macropholis in a natural area of Strandveld Succulent Karoovegetation. First, the sighting frequency of lizards in distinct plant categories was compared tothe relative availability of the habitats. Except for one case, all .sightings of lizards were made inEuphorbia caput-medusae plants, the least available habitat category. Second, the habitatpreference of C. macropholis in terms of shelter quality was examined. Given a choice of threeshelter-types, both adult and juvenile lizards gave preference to E. caput-medusae, followed byrock crevices and plant debris. Prey availability in E. caput-medusae plants was also found to beconsistent with prey items in the stomachs of C. macropholis at two independent sampling times.Results indicated that C. macropholis shows a distinct preference for the relatively scarce refuge,E. caput-medusae.Given the preference for such a scarce resource, one might expect C. macropholis to competefor potential mates that aggregate in these plants. To test this assumption, sex ratio, social structure, male spatial patterns and male social behaviour during the mating season were studied.Repetitive sampling of two populations revealed highly female-biased sex ratios. Sex ratio wasfound to relate positively to population density, as is the case in many polygynie lizard species.However, the composition of aggregations pointed to a monogamous mating structure for thisspecies. In individual plants, segregation among adult males, and between adult males andjuvenile males was significantly higher within the reproductive season than outside. Adult malesalso maintained a uniform spatial distribution within the reproductive season. Moreover, stagedencounters indicated that adult males behaved aggressively among themselves and towardjuvenile males, during the mating season. The data suggest that adult C. macropholis males areterritorial and that less dominant males may be excluded from E. caput-medusae plants tohabitats where their chances of survival may be lower.By implication, one might predict that grouping behaviour in C. macropholis is not wellmanifested and that movement of individuals among plants is high. This assumption wasvalidated by analyzing the size and stability of groups, as well as movement patterns amongplants. The size of aggregations within E. caput-medusae plants ranged from one to 14individuals. Grouping was a year-round phenomenon, but aggregations exhibited a low degreeof long-term social structure. Site fidelity was low, with females showing significantly highersite fidelity than males. Movement of individuals in and out of a marked population was high.Results suggested no differential movement patterns between the sexes. The data confirmed thataggregation behaviour and high degree of movement in C. macropholis are the result of mutualcompetition for a limited plant resource.

[发布机构] Stellenbosch University