Nesting Iguanas 
249 
bouts that they discharge during rest 
periods (Bennett and Dawson, 1972; Ben- 
nett and Licht, 1973). High levels of lactate 
can be tolerated, but an oxygen debt must 
eventually be paid for either aerobically dur- 
ing a rest period or possibly anaerobically 
at a much higher energy cost. It is this 
energy restraint that we suggest forces a 
female to periodically interrupt her digging 
and rest. 
An additional point about energy must be 
made. Ectotherms such as lizards have a very 
low resting or basal metabolism. Any act, 
even one using a small absolute amount of 
energy, involves a much larger relative in- 
crease in energy expenditure for a reptile 
than it does for an endothermic bird or mam- 
mal with its much higher basal metabolism. 
For an endotherm, at least some of the ener- 
getic cost of activity can be charged against 
the metabolic activity required to maintain 
body temperature. We believe that a display 
that might be a trivial energetic cost to a 
mammal or bird would be significant to a 
reptile operating on a much lower energy 
budget. 
These considerations suggest to us that the 
iguanas are “making their decisions” to 
dispute on the basis of three aspects of the 
situation : The value of the nest hole in terms 
of energy invested in its digging, the prob- 
ability of winning or retaining it, and the 
cost of the dispute in terms of energy ex- 
pended. From our rankings of displays in 
order of energy cost, we are at the very least 
able to evaluate the cost of a dispute point 
by noting which acts are more or less ex- 
pensive in terms of energy; from the depth 
of the nest burrowed we are able to gauge 
the worth of the prize; and from examina- 
tion of some 580 encounters, we are able to 
estimate the probabilities of winning and 
losing associated with different situations 
and courses of action. (For an extensive 
presentation of these data, see Rand and 
Rand, 1976.) We will present several aspects 
of these analyses to illustrate how energy 
considerations help to explain the observed 
situation. 
First, we would predict that a female 
looking for a place to dig would invest more 
energy in winning a deep hole than in win- 
ning a shallow one. Also she is likely to in- 
vest more energy in disputing for a hole that 
she will probably win (i.e., one to which the 
resident seems less than fully committed). 
On the basis of the data given in Table 1, it 
is evident that both the position of the resi- 
dent at the hole and what the resident is 
doing influence the outcome. This table also 
shows correlations between the types of ap- 
proach used with respect to both the resi- 
dent’s position and nature of activity at the 
hole. An iguana is more likely to make the 
high energy approach under conditions in- 
dicating that a resident is likely to be dis- 
placed. The correlation between approach 
and depth of hole, though in the predicted 
direction, is not nearly as strong as expected. 
A possible explanation is that the approach- 
ing iguana frequently cannot tell from out- 
side how deep the hole actually is. 
Let us consider next the resident’s re- 
sponse to an intruder. We contend that 
whether a resident leaves the hole, relin- 
quishes it, or defends it, depends on the 
energy immediately available to her. If she 
has very little energy (i.e., already has a 
fairly high oxygen debt) or at least not 
enough energy both to defend and dig 
further, she leaves. If sufficient energy is 
available for both activities, she stays, de- 
fends, and continues to dig. Sometimes she 
miscalculates, defends and wins, and then 
leaves presumably because not enough energy 
remains to dig further. 
Even if she had not been challenged, she 
eventually would have had to interrupt her 
digging to depart and rest awhile. In one 
sense we are saying that a resident’s re- 
sponse to a challenge depends on how close 
she is to the required moment to rest. 
As we have described, when a resident 
is approached she may respond with one of 
the displays listed in Table 2. Displays differ 
both in the amount of energy expended in 
their execution and in their effectiveness in 
driving off the intruder. The correlation be- 
tween these two aspects of the display is very 
high. The more energy an iguana puts into a 
