An analysis of the Cordylus Polyzonus complex (Reptilia : Cordylidae) in the South-Western Cape
Date
1990-03
Authors
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Journal ISSN
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Publisher
Stellenbosch : University of Stellenbosch
Abstract
Mouton and Oelofsen (1988) suggested that melanistic cordylid populations in the southwestern
Cape represent relict, cold-adapted populations which evolved under adverse
climatic conditions during the Last Glacial Period, 18000-16000 BP. The first section of
this study was undertaken to test their model for the evolution of melanism in the cordylid
species, Cordylus polyzonus. For this purpose, geographic character variation among
populations 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 analysed
in 306 specimens from 93 localities. Although no inter-locality variation was observed in
most of the characters, a high degree of concordancy was observed in the geographical
variation of three headshield characters. These three headshield characters are considered
non-adaptive and can accordingly be regarded as good indicators of genealogical
relationship. Since they varied together with other adaptive characters such as melanism and
body size, it can be concluded that geographically isolated melanistic populations of
Cordylus polyzonus represent relicts of a once larger melanistic population. The data
underscore the Mouton-Oelofsen hypothesis for the evolution of melanistic cordylid taxa in
the south-western Cape.
A further corrolary of the Mouton-Oelofsen hypothesis, is that melanistic cordylids are
presently restricted to cool enclaves. This assumption was evaluated in the second section of
this study by analysing prevailing climatic conditions associated with the occurrence of
extant melanistic populations of the genera Cordylus and Pseudocordylus in the southwestern
Cape. Climatic data were obtained from 123 weather stations in the study-area. A
close correspondence was found between the distribution of melanistic populations along the
westcoast and the primary upwelling zones of the southern Benguela Current. Lower mean
daily temperatures and a high incidence of advective sea fog on the adjacent coastal regions
are direct effects of these cold upwelled waters. Likewise, montane melanistic populations,
occurring at relatively high altitudes along the extreme western borders of the Cape Fold
Mountains, also experience lower mean daily temperatures and a high incidence of
orographic fog and cloud cover. Melanistic cordylid populations therefore generally have to
4.
contend with relatively exposed environmental conditions of low temperature and limited
solar radiation due to the filtering effect of fog and cloud cover. The fact that all the
melanistic 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. It
is postulated that palaeoclimatic conditions very similar to climatic conditions presently
prevailing in these enclaves were experienced over the entire western coastal region, at least
as far as the Orange River, during the Last Glacial Maximum when melanism probably
evolved. Our results therefore corroborate the Mouton-Oelofsen hypothesis that isolated
melanistic cordylid populations represent cold-adapted relicts.
One would accordingly expect melanistic populations to posses some enhanced ability to
absorb infrared radiant heat to cope with these conditions of limited sunshine and lower
temperatures. In section three of this study, differences in dorsal skin reflectivity between
melanistic and turquoise ~. polyzonus populations were quantified. Furthermore, the
physiological ability of this species to change body colour was investigated, as well as
ontogenetic colour change. Significant differences in dorsal skin reflectivity existed between
melanistic and turquoise specimens in the 500-1300 mjl spectral range, demonstrating the
greater heat-absorbing capacity of melanistic populations. The data underscore the MoutonOelofsen
model 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 colour
change seemingly does not occur in the other forms. Experiments to investigate short term
colour change in response to different temperatures regimes, produced no conclusive results.
On the other hand, seasonal colour changes could be demonstrated for both melanistic and
turquoise specimens, indicating that this species has the physiological capacity to change
colour. It is, however, believed that geographic colour variation in ~. polyzonus cannot be
attributed to this capacity, but is rather the result of selection over time. This view is
underscored by the results of section one demonstrating that, apart from colour and body
size, melanistic and turquoise forms also differ in certain non-adaptive traits; suggesting that
they do not belong to the same primary gene pool.
Description
Thesis (MSc (Botany and Zoology))--University of Stellenbosch, 1990.
Keywords
Cordylidae -- Evolution, Melanism, Dissertations -- Zoology, Theses -- Zoology