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

Nieuwoudt, Cara Julia (2001)

Thesis (MSc) -- Stellenbosch University, 2001.

Thesis

ENGLISH ABSTRACT: It has been suggested that rapid environmental change along the Western Cape coastal lowland has had a profound bearing on the evolution of cordylid populations in this region. To test a hypothesis on the evolution of melanism in the cordylid lizard, Cordylus polyzonus, the genetic relationship among melanistic, turquoise and brown morphotypes in this species was examined with allozyme electrophoresis. No polymorphic loci were found. No evidence was found that the melanistic population was genetically distinct from the other colour morphotypes. The results suggest that the studied melanistic population, is not a relict of a previously bigger melanistic population, but simply an ecotype within a larger gene pool. In the second section, some aspects of the ecology of the terrestrial lizard, Cordy Ius macropholis were investigated. Implications of a proposed hypothesis on habitat transition in this species, due to sea-level events, are discussed. Three complementary methods were used to investigate habitat selection in C. macropholis in a natural area of Strandveld Succulent Karoo vegetation. First, the sighting frequency of lizards in distinct plant categories was compared to the relative availability of the habitats. Except for one case, all .sightings of lizards were made in Euphorbia caput-medusae plants, the least available habitat category. Second, the habitat preference of C. macropholis in terms of shelter quality was examined. Given a choice of three shelter-types, both adult and juvenile lizards gave preference to E. caput-medusae, followed by rock crevices and plant debris. Prey availability in E. caput-medusae plants was also found to be consistent 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 compete for 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 was found 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 this species. In individual plants, segregation among adult males, and between adult males and juvenile males was significantly higher within the reproductive season than outside. Adult males also maintained a uniform spatial distribution within the reproductive season. Moreover, staged encounters indicated that adult males behaved aggressively among themselves and toward juvenile males, during the mating season. The data suggest that adult C. macropholis males are territorial and that less dominant males may be excluded from E. caput-medusae plants to habitats where their chances of survival may be lower. By implication, one might predict that grouping behaviour in C. macropholis is not well manifested and that movement of individuals among plants is high. This assumption was validated by analyzing the size and stability of groups, as well as movement patterns among plants. The size of aggregations within E. caput-medusae plants ranged from one to 14 individuals. Grouping was a year-round phenomenon, but aggregations exhibited a low degree of long-term social structure. Site fidelity was low, with females showing significantly higher site 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 that aggregation behaviour and high degree of movement in C. macropholis are the result of mutual competition for a limited plant resource.

AFRIKAANSE OPSOMMING: Daar is voorgestel dat snelle omgewingsveranderinge aan die Wes-Kaap se kuslaagland, 'n noemenswaardige effek op die evolusie van gordelakkedis-populasies in die gebied gehad het. In die eerste deel van die ondersoek, is die genetiese verwantskap tussen melanistiese, turkoois en bruin kleurmorfotipes in Cordylus polyzonus met behulp van ensiem-elektroforese bepaal. Die doel was om 'n hipotese vir die evolusie van melanisme in die spesie te toets. Geen polimorfiese lokusse is gevind nie. Daar was geen genetiese verskille tussen die melanistiese populasie wat bestudeer is, en die turkoois of bruin populasies nie. Dit suggereer dat die melanistiese populasie nie 'n oorblyfsel is van 'n eens groter melanistiese populasie nie, maar dat dit eerder 'n ekotipe is in 'n groter geenpoel. In die tweede gedeelte van die ondersoek, word enkele aspekte aangaande die ekologie van die terrestriële gordelakkedis, Cordylus macropholis, ondersoek. Die implikasies van 'n voorgestelde habitatsoorgang in die spesie as gevolg van omgewingsveranderinge, word bespreek. Habitatseleksie deur C. macropholis, in 'n natuurlike area binne die Sukkulente Strandveld Karoo-bioom, is ondersoek met behulp van drie metodes. Eerstens, is die frekwensie van akkedisse soos waargeneem in onderskeie plant-tipes, vergelyk met die relatiewe beskikbaarheid van die plant-tipes. Behalwe vir een geval, is alle akkedisse waargeneem in Euphorbia caputmedusae plante, die skaarsste van alle plant-tipes in die omgewing. Tweedens, is habitat keuse in terme van skuiling ondersoek. 'n Keuse van E. caput-medusae plante, sandsteenskeure en plantmateriaal is aan C. macropholis gestel. Beide volwasse en jong akkedisse het 'n voorkeur getoon vir E. caput-medusae plante, gevolg deur klipskeure en plantmateriaal. Derdens, is bepaal dat prooibeskikbaarheid in E. caput-medusae plante ooreenstem met die maaginhoude an C. macropholis gedurende twee verskillende tye. Dit is dus getoon dat C. macropholis 'n duidelike voorkeur het vir die relatiewe skaars skuiling, E. caput-meduae. Weens hierdie spesifieke voorkeur, is dit te wagte dat kompetisie vir potensiële maats, wat saamgroepeer in die plant, sterk mag wees. Om hierdie aanname te toets, is die geslagsverhouding en sosiale sisteem van C. macropholis, asook die ruimtelike verspreiding van mannetjies en hulle gedrag binne die teelseisoen, bestudeer. Herhaalde studies het 'n geslagsverhouding sterk ten gunste van wyfies getoon. Die geslagsverhouding het positief gekorreleer met populasie-digtheid, soos wat die geval is in baie poligame spesies. Samestellings van groepe akkedisse, het egter 'n monogame sosiale struktuur gesuggereer. Segregasie tussen volwasse mannetjies en tussen volwasse mannetjies en jong mannetjies binne plante, was aansienlik hoër gedurende die teelseisoen as buite die teelseisoen. Volwasse mannetjies het ook 'n uniforme ruimtelike verspreiding getoon gedurende die teelseisoen. 'n Laboratorium-eksperiment het verder aangetoon dat volwasse mannetjies, gedurende die teelseioen, nie net aggressief teenoor mekaar optree nie, maar ook teenoor onvolwasse mannetjies. Data stel dus voor dat volwasse mannetjies territoriaal is. Minder dominante mannetjies mag uitgesluit word van E. caput-medusae plante en verdryf word na areas waar hulle kanse op oorlewing minder mag wees. By implikasie, is dit te wagte dat groepgedrag in C. macropholis nie sterk gemanifesteer is nie en dat beweging van individue tussen plante aansienlik is. Die aanname is geëvalueer deur die grootte en stabiliteit van groepe in plante te monitor. Groepgrootte in E. caput-medusae plante het gewissel van een tot 14 individue. Groepgedrag is regdeur diejaar opgemerk, maar groepe het min tekens van langtermyn sosiale strukture getoon. Getrouheid aan spesifieke plantlokasies was laag, hoewel wyfies meer getrou was aan spesifieke plantlokasies as mannetjies. Beweging van individue in en uit die gemerkte populasie was hoog, maar resultate het geen differensiële bewegingspatrone vir die twee geslagte aangetoon nie. Die data bevestig dat groepgedrag en hoë mate van beweging in C. macropholis, die resultaat is van kompetisie vir 'n skaars hulpbron.

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