Demographic Analysis 
229 
survival of smaller juveniles. An hypothesis 
consistent with his data is that smaller 
juveniles, forced by larger juveniles into 
territories containing lower quality food, ' 
would be obliged to forage at times of the 
day (early and late) and on types of days 
(cloudy and cool) when known predators 
were likely to be actively feeding. Manipula- 
tion of predator density and supplementary 
feeding of lizards on “poorer” territories 
would provide a test of this hypothesis. In 
short, mark and recapture analysis, careful 
behavioral monitoring of individuals, and 
manipulation of important environmental 
variables are likely to provide insight into 
the workings of natural selection. 
In the present article we illustrate an al- 
ternative approach to a study of the effects 
of natural selection on the evolution of be- 
havioral or phenotypic traits related to social 
behavior. Finally, we present a model for dis- 
play behavior variability relative to the per- 
former’s visibility to conspecifics and preda- 
tors. 
HATCHLING SIZE, AGGRESSIVENESS, 
RESOURCE COMPETITION, AND 
FITNESS: SEASONAL CHANGES 
A number of studies of lizard life histories 
suggest that clutch size and egg size (rela- 
tive to female body size) differ between 
closely related species and between popula- 
tions within a species (Tinkle, 1969; Tinkle 
et. al., 1970; Ballinger and Schrank, 1972). 
Although these and other investigators have 
tried to relate these variations to natural 
selection, the adaptive significance of these 
differences is unknown (Tinkle et. al., 1970). 
Pianka (1970, 1972) suggested that larger 
egg size might increase the chances of hatch- 
ling survivorship, but he did not discuss how 
this advantage might occur, nor did he ex- 
plicitly suggest that hatchling size and ag- 
gressiveness might be intimately involved 
in this adaptation. 
In a number of temperate multibrooded 
lizards, there is a seasonal decrease in the 
size of successive clutches (Table 1). In 
several populations a seasonal decrease in 
clutch size has been shown to accompany an 
increase in egg size (Tinkle, 1967; Turner 
et. al., 1970; Ballinger and Schrank, 1972; 
Tinkle, 1972; Ballinger et. al., 1972; Derick- 
son, 1976). In only the last study were 
data sufficient to show that egg size corre- 
lates with hatchling size. This phenomenon 
poses an excellent opportunity to gain insight 
into the adaptive explanation for the ob- 
served clutch size and egg size differences be- 
tween populations and species. 
Research on the ecology, reproduction, and 
behavior of a population of the prairie swift 
lizard Sceloporm undulatus garmani con- 
ducted during the past 4 years has led to the 
following hypothesis: Seasonal decreases in 
availability of food during the hatching sea- 
son accompanied by increases in hatchling 
density causes increased resource competi- 
tion. The production by females late in the 
season of fewer but larger eggs that hatch 
into larger, more aggressive juveniles is 
adaptive because these juveniles are better 
able to aggressively defend the few optimal 
territories available at the time. Production 
of larger, more aggressive hatchlings early 
in the season when resources are relatively 
abundant, fails to result in higher juvenile 
survivorship and sacrifices offspring num- 
ber. The adaptive response is more but 
smaller eggs. In this section we will discuss 
the data from our studies that support the 
hypothesis. 
MATERIALS AND METHODS 
The prairie swift Scelopoms undulatus 
garmani is a small lizard of the family 
Iguanidae. Adults measure about 55 mm 
from the tip of the snout to the cloacal open- 
ing. They inhabit a variety of habitats in 
Kansas, ranging from sandy river bottoms 
in the south and central part of the State to 
sandstone and limestone outcroppings in the 
western prairies. This race of lizard is part 
of a geographically widespread species that 
occurs from New Jersey to Florida in the 
eastern United States, where it is primarily 
a forest-edge tree dweller, and to southern 
