Abstract
Thick-tailed geckos (Underwoodisaurus milii) are unusual among Australian lizards (and indeed, among lizards in general) in their tendency to aggregate in the wild. There are many anecdotal reports of several individuals found clustered together in a single rock crevice. My project was designed to quantify patterns of aggregation in the field, and to use controlled trials in the laboratory to identify physical and social cues for aggregation. My ultimate aim was to understand why the lizards display this unusual grouping behaviour.
My field data showed that thick-tailed geckos do aggregate; the lizards were found in groups more frequently than expected under the null hypothesis of independent assortment. Retreat-sites used in the field were a non-random subset in terms of several abiotic variables, and the lizards also displayed significant selectivity for such cues in the laboratory. Lizards aggregated in the laboratory even when suitable vacant shelter-sites were available, indicating that aggregation in the field is not simply a response to limited availability of appropriate shelter. The tendency to form aggregations was affected in only minor ways by a lizard’s sex or age group, or by its familiarity with the other individuals used in the trials. However, the aggregative response was eliminated when the opportunity for physical interaction was removed. Lizards in physical contact with conspecifics heated and cooled more slowly than did solitary animals, reflecting the greater effective mass (and thus, thermal inertia) of the aggregated group. Additionally, exposure to rapidly cooling conditions stimulated geckos to huddle together more closely.
Based on my results, I propose that aggregation behaviour in thick-tailed geckos has evolved for biophysical advantages rather than ‘social’ interactions. More specifically aggregation may enhance fitness by permitting the lizards to control their rates of heat exchange behaviourally. Compared to related species, thick-tailed geckos are unusual in their large body size, cold-climate distribution and use of thermally fluctuating rock crevices as retreat-sites. Because they are exposed to profound diel and seasonal cycles in temperature, the control of thermal exchange rates achieved via huddling may be of significant benefit in terms of maintaining suitable body temperatures. For example, geckos may be able to maintain relatively high temperatures late into the evening before leaving rocks to forage, and may avoid dangerously high day-time maxima and dangerously low night-time minima by retarding heating and cooling rates.
Research conducted during Honours year under supervision of Prof Rick Shine, at the University of Sydney, Australia.
Thesis:
Shah B. (2002). Why do thick-tailed geckos (Underwoodisaurus milii) aggregate? BSc Honours Thesis, University of Sydney, Sydney, New South Wales, Australia.
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Thick-tailed gecko (Underwoodisaurus milii) in the wild, rock lifted
Thick-tailed gecko (Underwoodisaurus milii)
in the lab, shelter lifted
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