Nesting Iguanas 
247 
frequently follow one another. They spend 
much time resting and sunning. 
The females maintain short individual 
distances and show only occasional aggres- 
sion. In selection of a site for the nest, a de- 
pression is desirable and a partially com- 
pleted burrow even better. The female digs 
by scratching the dirt out with several 
strokes of a front foot and then kicking it 
back with the hind foot on the same side. She 
alternates digging with the left and right 
legs. Digging is a precarious occupation, par- 
ticularly in the early stages ; since the female 
must put her head down into the hole, she 
cannot watch for an approaching predator, 
whereas her body, outside, is fully exposed 
to any attack. Once the hole is deep enough, 
she turns around inside and emerges head 
first, pushing dirt before her. 
The iguanas, by their digging, help main- 
tain the clearing from year to year. The soil, 
because it has been burrowed into repeated- 
ly, is relatively free of roots and surface 
vegetation. Even so, digging the nest bur- 
row is probably the hardest work that an 
iguana ever does. It usually takes several 
days to complete a nest. A female rests many 
times between her bouts of digging. Pre- 
sumably she builds up an oxygen debt as she 
digs and defends herself and then pays it 
back as she rests. The level of oxygen debt 
she will tolerate determines how long she 
can work and, as we shall see later, how 
vigorously she defends her burrow. When 
she is ready to resume digging, she may find 
the burrow occupied by another iguana 
actively digging and defending it. She must 
then either displace this new resident or 
build a nest elsewhere. Sometimes an im- 
patient female attempts to force her way 
into an occupied burrow, but she is seldom 
successful. She may have difficulty in getting 
out again when the resident bites and holds 
on. 
Although iguana bites are obviously un- 
pleasant, it is unlikely that they produce the 
serious kinds of injury inflicted by predators. 
One female observed in 2 successive years 
was missing her left front leg; her injury 
was probably from a predator. Iguanas, par- 
ticularly young ones, are sought after by 
many kinds of predators. One of the most 
conspicuous is the crocodile, which also 
nests on Slothia and is frequently around 
during the nesting season. One observer saw 
a crocodile catch and eat an adult iguana in 
the water just off Slothia. Predation on 
adult iguanas, however, is relatively low, and 
many females survive for more than one 
breeding season. 
Once the female has completed her bur- 
row, she lays her eggs in a chamber at the 
end. Iguanas lay large clutches; the precise 
number of eggs varies with the size of the 
female. In our study, the number ranged 
from two dozen eggs for a 31 cm female to 
five dozen for a 44 cm female Iguanas lay one 
clutch a year. Hatchlings mature in two or, 
perhaps more commonly, in 3 years. 
In the course of digging her own nest, an 
iguana may encounter one made earlier, and, 
when she does, she kicks out the eggs as if 
they were pebbles. Eggs on the surface are 
quickly eaten by vultures. The digging-up of 
earlier nests seems to be a major way in 
which nesting iguanas on Slothia interfere 
with one another. After the eggs have been 
laid, the female emerges from the nest and 
fills the burrow. She pushes and kicks dirt 
into the nest with her feet and tamps it down 
with her snout. Because of the large excava- 
tion, she must scratch dirt toward the nest 
from a considerable distance. 
After passage of the eggs, the shrunken 
sides of the spent female are conspicuous. 
There are also hollows at the tail base and 
the temples. Iguanas do not feed during 
nesting. When they reach the nest site, there 
is little, if any, food in the gut and very small 
fat deposits. During the laying operation an 
iguana expends a considerable fraction of 
the energy available to her. This is one of 
the energy constraints on a nesting female 
and one of the reasons why we conclude that 
energy limitations are important. The 
energy content of the eggs themselves is 
only a part of the cost of reproduction. 
The limb movements used in filling a bur- 
row look identical to those used in digging. 
Only the reverse orientation of the body is 
