of smell, use a “cloud” of dorsal gland 
scent that surrounds each herd as a 
cue to judge their distance from the 
herd’s center. Vision is certainly less 
important. Peccaries have notoriously 
poor eyesight and, if the wind is right, 
will blunder to within a few feet of 
an unconcealed, but motionless, per- 
son before taking flight. 
Hearing may be the most important 
orienting mechanism. On several oc- 
casions I was very close to feeding 
herds as they drifted past me, and 
each time I was impressed by the 
sounds that envelop a feeding herd: 
the click of hoofs on rocks, the crunch- 
ing and smacking sounds of chewing, 
and the periodic low grunting all herd 
members make. The low grunt is a 
repetitive flat sound, similar to the 
grunts made by pigs. Lost peccaries 
emit the sound almost constantly and 
appear to listen for a return call; they 
quickly approach animals in the vi- 
cinity that respond. Thus, group co- 
hesion seems to be promoted primarily 
through hearing and olfaction. 
Why do collared peccaries stay 
close together and why is vision not 
an important means of doing so? The 
species probably evolved from other 
peccaries in the Pliocene rain forests 
of South America. Predation by large 
cats is today, and no doubt was then, 
a serious threat, constituting a selec- 
tive pressure strong enough to cause 
individual peccaries to bunch to- 
gether. In the dense rain forest un- 
derstory, these low-slung beasts would 
have had trouble using vision to keep 
in contact with each other, but they 
could have used sounds and smells 
(to which vegetation is more “trans- 
parent”) to do so. Bird watchers are 
familiar with this phenomenon. In 
thick vegetation, far more birds can 
be heard than seen. 
The tendency for peccaries to stay 
close together is so strong that most 
individuals probably never leave the 
herd into which they are born. As 
a result, herds most likely comprise 
a group of closely related animals. 
This, in turn, has probably been re- 
sponsible for the evolution of the 
highly cooperative behavior that herd 
members extend toward each other. 
(Chacoan and white-lipped peccaries 
also live in herds, and white-lipped 
groups can exceed 200 individuals. Es- 
sentially nothing is known about the 
constancy of herd composition or so- 
cial behavior in these two species.) 
Cooperative behavior among pec- 
caries can be crucial in winter. In 
central Arizona, w inter nights are usu- 
ally chilly and sometimes downright 
cold. On one memorable morning at 
the 3-Bar, the temperature before the 
sun rose was 17°F, and there was 
a light covering of snow that capped 
the saguaros and made them seem 
to shiver. Peccaries are typical tropical 
mammals. Because they have no heat- 
retaining wooly undercoat, peccaries 
quickly begin to expend extra energy 
to maintain a constant body tempera- 
ture when they are exposed to cold. 
As a result, in the winter they are 
diurnal, moving and feeding during 
the day. Huddling together on winter 
nights is essential if the animals are 
to stay alive. (As summer nears, pec- 
caries become increasingly nocturnal, 
moving and feeding at night and rest- 
ing in a shaded bed-ground through- 
out the day.) 
One typical winter morning, I ar- 
rived at a vantage point above a herd’s 
bed-ground before sunrise and before 
the animals were up. Rustling amid 
the turbinella oaks began as the sun 
appeared, and soon stretching, yawn- 
ing, and shaking animals drifted into 
view. There was a concentrated bout 
of mutual rubbing between herd mem- 
bers, then the animals began to move 
in a rough single file from the bed- 
ground. They traveled only a short 
distance until, reaching a sunny spot, 
they began to feed. 
During the winter much of the 
highly nutritious food on which pec- 
caries usually depend, such as acorns, 
bulbs, mesquite beans, and cactus 
fruit, is unavailable, and for several 
months the animals may subsist al- 
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