246 
Rand & Rand 
We choose energy over two other impor- 
tant variables of any situation, time and 
risk, primarily because how energy is used 
explains much of the observed situation. 
Intuitively we feel that, for any animal that 
spends much of its time motionless as an 
iguana does, time is less important than risk. 
For practical and heuristic reasons we have 
concentrated on energy and invoke risk only 
when energy factors do not seem adequate 
to explain what we see. 
One of the virtues about using “energy” 
in building a model is that it can be meas- 
ured. Although the techniques developed by 
Bennett and Dawson, 1972, and Bennett and 
Licht, 1973, for measuring metabolism in 
active iguanas could, with some modification, 
have been applied in our studies, we did not 
use them. Instead, we ranked or ordered the 
various acts, particularly the displays, in 
terms of energy costs. This was done (Tables 
1 and 2) by watching the animals, classify- 
ing the acts, and applying some simple rules. 
Acts that were performed more vigorously 
than others were considered to take more 
energy. In assessing similar acts, the one 
which took longer or moved the animal 
further was considered to take more energy, 
and a combination of two acts was consid- 
ered to take more energy than either act 
alone. 
COMMUNAL NESTING OF IGUANAS 
Iguanas are large, herbivorous, arboreal 
lizards that are widespread in the neo- 
tropics. Usually iguanas nest singly, but 
occasionally (presumably when a site is par- 
ticularly favorable) they may nest com- 
munally. Slothia, a tiny islet off Barro 
Colorado Island (the Smithsonian Tropical 
Research Institute preserve in the Panama 
Canal), is such a communal nest site (Rand, 
1968). Here most nesting occurs in a 5 X 10 
meter clearing next to which we built a 
blind. From here the action was watched 
and photographed. 
During the nesting season, one commonly 
saw 10 to 12 iguanas in the clearing, and as 
many as 30 may be in sight at one time. We 
estimated that perhaps 150 to 200 females 
nest on Slothia every year during the dry 
season. No iguanas live on Slothia perma- 
nently. Females come here only to nest, but 
we do not know from how far. We radio- 
tracked one female which traveled about 
three miles after finishing her nesting (Mont- 
gomery et al., 1973). 
A newly arrived female must explore the 
island. On the ground and particularly in 
the open, she is very nervous at first and 
does much bobbing and tonguing. Females 
Table 1. Initial Approach by “Intruder,” Number of Times Observed (Expected), 
% Effective 
Slow approach Quick approach 
Low energy investment High energy investment X2(df) 
Hole depth 3.66 
Shallow 
121 
(113) 
23 
(31) 
Deep 
263 
(271) 
82 
(74) 
Resident's position 
23.30 
Out of hole 
200 
(221) 
23.5% 
81 
(60) 
59.3% 
Head in 
75 
( 66) 
12.0% 
9 
(18) 
22.2% 
Body in/head out 
39 
( 38) 
2.6% 
9 
(10) 
0 
All in 
70 
( 60) 
0 
6 
(16) 
0 
Resident’s occupation 
12.59 
Waiting 
149 
(163) 
23.5% 
65 
(51) 
50.8% 
Digging 
110 
( 96) 
8.2% 
17 
(31) 
31.3% 
