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Ferguson & Bohlen 
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promise to provide insight into the workings 
of natural selection. Kettlewell’s (1961) 
studies on the evolution of industrial melan- 
ism consider these factors and may serve 
as a model for students of evolutionary 
biology. It is our purpose to illustrate how 
studies on lizard social behavior are particu- 
larly suitable for throwing light on the 
processes of natural selection. 
There are four main attributes of lizards 
(particularly many species of iguanids and 
agamids) that make them excellent subjects 
for such studies: (1) Many lizards, particu- 
larly diurnal heliothermic species, are con- 
spicuous in their habitat. They can be easily 
discovered by an investigator and observed 
for long periods of time. (2) Individuals 
often have restricted lifetime movements so 
that they can be repeatedly discovered, ob- 
served and photographed for over a period of 
days, weeks, months, or years. (3) Individ- 
uals often can be easily caught or trapped 
for marking; and (4) Restricted removal of 
toes renders individuals easily identified by 
an investigator for the life of the animal. 
Small distinctive paint markings render in- 
dividuals easily identified by an investigator 
from a distance and without recapturing the 
animal. Such markings, however, are only 
retained until the animal undergoes ecdysis 
(usually to a few weeks). 
Due to these factors, the number of studies 
on the life history and demography of lizard 
populations has steadily increased since the 
original work by Fitch on Sceloporus oc- 
cidentalis and Stebbins on Sceloportcs graci- 
oitsus (Fitch, 1940; Stebbins, 1948; Stebbins 
and Robinson, 1946; see also Blair, 1960; 
Harris, 1964; Tinkle, 1967, 1969; Tinkle et. 
al., 1970; Turner et. al., 1970; Jenssen, 
19706; Tinkle and Ballinger 1972; Vinegar, 
1975a, 1975c ). Accurate schedules of age- 
specific mortality and fecundity are now 
known for a number of populations of a num- 
ber of species. Mortality and fecundity, so 
easily observed in these populations by mark 
and recapture techniques, are the major 
measures of fitness (Williams, 1966). 
In addition to demographic studies, studies 
of quantitative variations of both display and 
nondisplay behavior have been conducted in 
recent years in both a natural and laboratory 
setting (Ferguson, 1970, 1971, 1973; Jenssen, 
1971, 1975, this volume, for studies on varia- 
tion of stereotyped displays; Harris, 1964; 
Tinkle, 1967 ; Rand, 1967 ; Yedlin and 
Ferguson, 1973; Fox, 1973; Ruibal and 
Philibosian, 1974; Philibosian, 1975; Simon, 
1975; Vinegar, 19756, for studies of varia- 
tions in aggressiveness or territorial be- 
havior.) These studies reveal the existence 
of easily quantifiable variations of social be- 
havioral traits within a population. Thus, 
since the fitness of individuals can be de- 
termined through monitoring of survival and 
reproductive success, the fitness of individ- 
uals bearing variants of behavioral traits 
can also be recorded. In short, a means 
exists for observing the outcome of natural 
selection. 
There remains the difficulty of identifying 
what environmental factors are responsible 
for the differences in fitness between in- 
dividuals bearing different variants or de- 
grees of a trait and how these factors oper- 
ate. An established ecological method of 
dealing with this problem is the manipulation 
in a natural setting of the variables con- 
sidered most likely to affect fitness and/or 
the close observation of the interactions be- 
tween organisms and specific environmental 
factors. Thus, for example, Connell (1961) 
demonstrated the effect of competition on 
survival of a barnacle species by removing 
another closely related species; Paine (1971) 
demonstrated the effect of predatory starfish 
on the competition between prey species in 
a coral reef community by removing starfish. 
In the side-blotched lizard Uta stansburiana, 
Turner et. al. (1973), demonstrated an effect 
of winter rainfall and predators on repro- 
duction and survival by watering the en- 
vironment in experimental field plots in 
winter and by removing predatory leopard 
lizards in an enclosed natural plot. In the 
same species. Fox (1973) determined 
through mark and recapture analysis and 
careful observation that certain patterns of 
activity, as well as the consumption of poorer 
quality food, were correlated with lower 
