An analysis of the Cordylus Polyzonus complex (Reptilia : Cordylidae) in the South-Western Cape

[摘要] Mouton and Oelofsen (1988) suggested that melanistic cordylid populations in the southwesternCape represent relict, cold-adapted populations which evolved under adverseclimatic conditions during the Last Glacial Period, 18000-16000 BP. The first section ofthis study was undertaken to test their model for the evolution of melanism in the cordylidspecies, Cordylus polyzonus. For this purpose, geographic character variation amongpopulations of Cordylus polyzonus in the region south of 32°15' latitude and west of 19015'longitude was investigated. Variation in 122 external morphological characters was analysedin 306 specimens from 93 localities. Although no inter-locality variation was observed inmost of the characters, a high degree of concordancy was observed in the geographicalvariation of three headshield characters. These three headshield characters are considerednon-adaptive and can accordingly be regarded as good indicators of genealogicalrelationship. Since they varied together with other adaptive characters such as melanism andbody size, it can be concluded that geographically isolated melanistic populations ofCordylus polyzonus represent relicts of a once larger melanistic population. The dataunderscore the Mouton-Oelofsen hypothesis for the evolution of melanistic cordylid taxa inthe south-western Cape.A further corrolary of the Mouton-Oelofsen hypothesis, is that melanistic cordylids arepresently restricted to cool enclaves. This assumption was evaluated in the second section ofthis study by analysing prevailing climatic conditions associated with the occurrence ofextant melanistic populations of the genera Cordylus and Pseudocordylus in the southwesternCape. Climatic data were obtained from 123 weather stations in the study-area. Aclose correspondence was found between the distribution of melanistic populations along thewestcoast and the primary upwelling zones of the southern Benguela Current. Lower meandaily temperatures and a high incidence of advective sea fog on the adjacent coastal regionsare direct effects of these cold upwelled waters. Likewise, montane melanistic populations,occurring at relatively high altitudes along the extreme western borders of the Cape FoldMountains, also experience lower mean daily temperatures and a high incidence oforographic fog and cloud cover. Melanistic cordylid populations therefore generally have to4.contend with relatively exposed environmental conditions of low temperature and limitedsolar radiation due to the filtering effect of fog and cloud cover. The fact that all themelanistic taxa in this region occur as small isolated populations limited to cool enclaves,suggests that they are presently in a contracted state under strong environmental pressure. Itis postulated that palaeoclimatic conditions very similar to climatic conditions presentlyprevailing in these enclaves were experienced over the entire western coastal region, at leastas far as the Orange River, during the Last Glacial Maximum when melanism probablyevolved. Our results therefore corroborate the Mouton-Oelofsen hypothesis that isolatedmelanistic cordylid populations represent cold-adapted relicts.One would accordingly expect melanistic populations to posses some enhanced ability toabsorb infrared radiant heat to cope with these conditions of limited sunshine and lowertemperatures. In section three of this study, differences in dorsal skin reflectivity betweenmelanistic and turquoise ~. polyzonus populations were quantified. Furthermore, thephysiological ability of this species to change body colour was investigated, as well asontogenetic colour change. Significant differences in dorsal skin reflectivity existed betweenmelanistic and turquoise specimens in the 500-1300 mjl spectral range, demonstrating thegreater heat-absorbing capacity of melanistic populations. The data underscore the MoutonOelofsenmodel that melanistic populations are cold-adapted relicts. Furthermore,ontogenetic colour change is a real phenomenon in the melanistic variation of C. polyzonus.In this respect it is unique among the melanistic cordylid taxa in that ontogenetic colourchange seemingly does not occur in the other forms. Experiments to investigate short termcolour change in response to different temperatures regimes, produced no conclusive results.On the other hand, seasonal colour changes could be demonstrated for both melanistic andturquoise specimens, indicating that this species has the physiological capacity to changecolour. It is, however, believed that geographic colour variation in ~. polyzonus cannot beattributed to this capacity, but is rather the result of selection over time. This view isunderscored by the results of section one demonstrating that, apart from colour and bodysize, melanistic and turquoise forms also differ in certain non-adaptive traits; suggesting thatthey do not belong to the same primary gene pool.

[发布机构] Stellenbosch University