248 
Rand & Rand 
different. Once the hole has been filled, the 
female works out from it, surface filling. She 
scratches at the surface, shifts debris, and 
fills any other holes or depressions in the 
area. 
For a solitary female, the surface filling 
conceals the precise location of her nest from 
any predator in the general area. In the 
dense aggregation on Slothia, such surface 
filling causes complications. When females 
interfere with one another, one occasionally 
sees quite unusual displays such as tail lash- 
ing. (This is much more common in an anti- 
predator situation in which it can be quite 
effective. The “filling” female usually seems 
to win out, without, however, preventing the 
other one from completing her digging. 
At the height of the season, the clearing 
on Slothia is full of iguanas. They gather to 
nest, presumably because the advantages of 
doing so outweigh the disadvantages (Sex- 
ton, 1975). We have identified some of the 
advantages : easier digging, sunlight for 
spotting predators, and, perhaps most im- 
portant, the location on a small island and 
fewer egg predators (Rand and Robinson, 
1969). The costs of nesting on Slothia in- 
clude: a greater distance to travel, the need 
to defend the burrow against other iguanas, 
the conspicuousness of the site to predators, 
and, perhaps most significantly, the loss of 
eggs dug up by other iguanas. 
IGUANA DISPUTES AND 
THEIR ENERGETICS 
We have outlined the nesting behavior of 
female iguanas on Slothia and the advan- 
tages and disadvantages of nesting in this 
reptile rookery. We describe next the kinds 
of disputes that occur between females and 
the displays involved. We feel that the key 
to understanding this situation is the 
iguanas’ “concern” with conserving energy. 
The importance that we place on energy is 
consistent with what is known about lizard 
energetics from the work of Bartholomew 
and Tucker (1964), Moberly (1968a, b), 
Bennett and Dawson (1972), and Bennett 
and Licht (1973). As mentioned during the 
film, energy available to a fasting iguana is 
limited at two levels. First, the energy avail- 
able to a female for the whole nesting season 
is the energy that she has stored in her body 
at the beginning of the season and must use 
to move to the breeding site, dig and defend 
a burrow, lay eggs, fill the hole, move back 
to a feeding area, and still retain a reserve 
to escape the attack of a predator. Energy 
is limited because of: (1) the small fat re- 
serves of the females examined on their way 
to the nest site; (2) the inability of the 
iguanas to replenish energy by feeding dur- 
ing the nesting; and (3) the emaciation of 
the female after nesting. It is this energy 
limitation that puts a premium on a female’s 
gaining a partially completed burrow and 
defending it once she has it. Energy is 
also limited at a second level by the amount 
available during a single bout of activity. 
Lizards (except monitors) do not sustain 
activity by aerobic metabolism. Instead they 
build up an oxygen debt during activity 
Table 2 Energy and Effectiveness of Displays 
Displays In order of 
increasing energy 
Times 
observed 
% times displayer 
wins immediately 
% times displayer wins 
dispute, immediately or 
eventually 
Mouth open 
24 
29.2 
62.5 
Head swing 
59 
57.6 
78.0 
Huff 
146 
62.3 
93.8 
Head swing/huff 
57 
63.2 
96.5 
Lunge 
79 
79.7 
91.1 
Lunge/huff 
30 
93.3 
100.0 
Bite 
35 
17.1 
100.0 
