Group-living in the Australian skink, Egernia stokesii

Author: Elvira Lanham

Lanham, Elvira, 2001 Group-living in the Australian skink, Egernia stokesii, Flinders University, School of Biological Sciences

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Until recently, reptiles have been thought to exhibit little more than rudimentary sociality. Males of most species studied so far are either solitary or defend territories, containing one or more females. Some species have developed complex visual displays to maintain their territory and tolerance of other males is limited to non breeding juveniles.

Much of the ideas about lizard social behaviour have come from the study of visually-orientated species such as the iguanids and agamids. Little is known about the more fossorial species, but as their behaviour receives more attention there appears to be more diversity among these taxa than first thought.

Egernia stokesii displays unusually stable, long-term group fidelity, with groups in one well studied population ranging from 2-17 individuals (Duffield and Bull, 2001 ). Group members share the same home range and overnighting refuges and can stay together for at least five years. Individuals within these groups spend significantly more time interacting with one another than with lizards from neighbouring groups and they are significantly more related to each other than to the rest of the population.

The question that this thesis addresses is why this unusual behaviour occurs. What are the benefits to E. stokesii of year round group-living when other closely related species that share the same habitat do not show this type of group fidelity.

Initially, grouping behaviour was investigated in other populations than the one from which this behaviour was first described. This was to ensure that group fidelity was not just a phenomena of one population and also to address the hypothesis that group size will vary depending on environmental conditions. A survey of four regions within the South Australian distribution of this species found groups of up to nine individuals occurred throughout the South Australian range of this lizard, and group size fluctuated depending on habitat.

The mechanisms behind the formation of these groups was then investigated by examining the possibility that grouping occurred only as a result of a limit in suitable refuge sites. Groups of different compositions were provided with opportunities to disperse by allowing them access to more crevices than there were indiv.iduals. In general, lizards still remained aggregated when compared to a random distribution, especially related juveniles.

If grouping behaviour could not just be accounted for by a lack of suitable habitat, then there should be benefits to the individual. The physical benefits of grouping to the thermal biology of the lizards were examined. Lizards formed larger aggregations at cooler temperatures and they were able to maintain a higher temperature after sunset when they were in groups, compared to when they were alone.

Antipredator advantages of grouping behaviour were also investigated. Juvenile lizards in a group came out from their shelter for longer period than those living alone. Both adults and juveniles in a group showed reduced vigilance behaviour (measured as eyes open per minute of basking time) compared to lizards living alone, although the vigilance of the group as a whole was not reduced. In fact, in a separate experiment, lizards in a group responded sooner to a potential threat than when they were by themselves.

Individual lizards derive both thermal and antipredator benefits from joining a group, but these may not be equally shared among group members. In the final data chapter of this thesis, experiments are reported that show that subordinate lizards spent more time in refuges and less time basking when they were grouped, compared to when they were alone. This suggests they were curtailing activities such as thermoregulation as a response to the presence of other lizards.

The results reported in this thesis suggest a sophisticated sociality in E. stokesii that has not previously been documented in lizards.

Keywords: Australian skink, Egernia stokesii, lizard social behaviour, animal ecology

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2001
School: School of Biological Sciences
Supervisor: Mike Bull