Behavior and Neurology of Lizards 
N. Greenberg and P. D. MacLean, eds. 
NIMH, 1978. 
Demographic Analysis: A Tool for the Study of Natural 
Selection of Behavioral Traits 
Gary W. Ferguson 
Department of Biology 
Texas Christian University 
and 
Charles H. Bohlen 
East Central Junior College 
SUMMARY. While there has been considerable literature on lizard displays and social behavior, 
few investigators have realized the potential of iguanid lizards for the quantitative study of 
the effects of natural selection on variations of behavior traits. The combined ease of mark and 
recapture analysis, of behavioral observation and cinematography in the laboratory and field, 
and of experimental manipulation, render lizards ideally suited for such a study. 
The studies of prairie swifts (Sceloporus undulatus garmani) described here indicate how vary- 
ing egg size, hatchling size, and degree of aggressiveness in juveniles may influence fitness. As 
food supplies for hatchlings dwindle in late summer and lizard densities increase, resource com- 
petition increases. The increased competition manifests itself as an increase in dispersion and 
density dependent mortality toward the end of the hatching season. As an adaptive response to 
greater late seasonal competition of hatchlings, females apportion more of their available energy 
into each egg. They produce larger eggs which hatch into larger and more aggressive hatchlings. 
Survival of larger late hatchlings to the next breeding season (fitness) is significantly greater 
than that of smaller late hatchlings. Among early hatchlings, survival of larger hatchlings is 
not significantly different from that of smaller hatchlings. 
Signature displays of lizards have been shown to vary geographically. The adaptive significance 
of this variation has been the source of considerable speculation. Signature display complexity in 
the side-blotched lizard (Uta stanshuriana) has been shown to be positively correlated with vege- 
tation density. This phenomenon is the basis of a model that generates predictions testable 
using observational and demographic techniques and relates the display complexity to the con- 
spicuousness of displaying lizards to predators, and their conspicuousness to conspecific lizards. 
INTRODUCTION 
Within the past two decades, the descrip- 
tion of reptilian display behavior has ad- 
vanced from the anecdotal qualitative level 
to a relatively sophisticated level of quantita- 
tive analysis of variation within and between 
populations (Carpenter, in press; Ferguson, 
in press, for reviews; Jenssen, this volume). 
Also, nondisplay aspects of lizard social be- 
havior such as territoriality, dominance, and 
aggressiveness have been extensively studied 
(Fox, 1973; Brattstrom, 1974; Ruibal and 
Philibosian, 1974; Simon, 1975; Vinegar, 
19756). 
While a number of investigators have 
focused on the evolution of social behavior, 
few have realized the potential of monitoring 
behavioral variation or variation of traits 
with some signal value in lizards to reveal 
natural selection within populations. Studies 
that show how individual “fitness” ^ is cor- 
related with trait variants and how environ- 
mental variable are likely to infiuence fitness 
^ Fitness is defined as the ability of an individual 
to contribute to the next breeding generation. 
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