Biology Reference
In-Depth Information
ground at the time of weaning, when they are three to four weeks old. Soon after this
the juvenile males disperse while the juvenile females tend to remain near their natal
area. This means that males seldom, if ever, interact with close relatives whereas females
spend their whole lives surrounded by close female kin.
Sherman found that closely related females (mother and daughters, sisters) seldom
fought for nest burrows and seldom chased each other from their territories. Indeed,
they cooperated to defend each other's young against infanticidal conspecifics. Of all
young born, 8% were dragged from their burrows and killed by other ground squirrels.
The killers were not close relatives of the victims and were either young males wandering
in search of an easy meal or immigrant adult females searching for new nesting
burrows. These females attempted to take over occupied burrows, killing any young
they found as a way of clearing the territory of potential competitors (Sherman, 1981a,
1981b). Such cooperation among close relatives, in contrast to the conflict among
unrelated individuals, is exactly what would be predicted from the theory of kin
selection.
Individuals also gave alarm calls whenever a predator, such as a coyote or weasel,
approached. Callers probably suffered a cost from giving the alarm because they were
more likely to be attacked by the predator, perhaps because the calls made them more
conspicuous. Others, however, benefited from the early warning and were more likely
to escape. Sherman (1977) found that females were much more likely to give alarm
calls than males and, furthermore, females with close relatives nearby were more likely
to give calls than females without. Although in most cases the beneficiaries of the
alarm calling were likely to be offspring, individuals also gave alarm calls when only
parents or non-descendant relatives were nearby. For example, young females who had
yet to produce their own young gave alarm calls to warn their mother and sisters of
the approach of a predator. In another species of ground squirrel,
Spermophilus
tereticaudus,
males were much more likely to give alarm calls before leaving their
mother's home area, when they had relatives nearby, but when they dispersed and left
their close relatives they were more likely to remain silent when a predator came along
(Dunford, 1977).
These data on alarm calls show clearly that individuals are more likely to incur
the cost of calling when relatives are nearby to gain a benefit. Warning offspring
(i.e. parental care) and warning sisters are, of course, just different ways of increasing
gene propagation to future generations. However, it is still interesting to ask whether
alarm calling could evolve mainly because of benefit to non-descendant relatives. The
best evidence for this comes from a long-term study by John Hoogland (1983, 1995) of
another colonial rodent, the black-tailed prairie dog,
Cynomys ludovicianus
.
Black-tailed prairie dogs live in social groups called coteries, typically one adult male
with three to four adult females and their offspring. Young females remain in their natal
coterie all their lives while young males disperse in their second year. All the females and
yearling males within a coterie are, therefore, usually close genetic relatives. Hoogland
studied alarm calling responses by presenting a stuffed specimen of a natural predator,
the badger
Taxidea taxus
(Fig. 11.1). This enabled him to get more data than could be
obtained by waiting for natural predator attacks and also allowed him to control for
proximity of the predator to the prairie dogs. Figure 11.1 summarizes the results of over
700 experiments. The data show that individuals gave alarm calls just as frequently
when there were only non-descendant kin in their home coterie as when there were
Unrelated ground
squirrels kill each
other's offspring,
relatives do not
Alarms are given
when relatives
are nearby
Prairie dog alarms
are mainly given
in the presence of
offspring or other
relatives…
