Biology Reference
In-Depth Information
the period when brood production is dependent upon the body reserves of the queen. At
this point, each queen no longer benefits from the presence of the others, and she can
gain an enormous advantage from monopolizing reproduction. This leads to females
who have previously shown no aggression to each other, fighting to the death.
… but later fight
to the death
Group augmentation in meerkats
By-product benefits can also be important in many cooperatively breeding vertebrates.
Tim Clutton-Brock and colleagues carried out a long-term study on meerkats, a small
(
1 kg) mongoose that is found in arid regions of southern Africa. Meerkats live in
groups of up to 20 adults, accompanied by their dependent young. Each group is
comprised of a dominant pair, subordinates of both sexes that were born in that group
and, in some cases, subordinate males who have immigrated into the group. Successful
breeding by meerkats is completely reliant on help from the subordinate members of
the group, who help feed and guard the young at the burrow, while the rest of the
group spends the day foraging elsewhere. This contrasts with species such as long-
tailed tits, where helpers are a bonus that only occurs at some nests and not a
requirement. The helping behaviours of meerkats are extremely costly - over the
12-hour period following the start of a full day babysitting, babysitters lost 1% of
their body weight, while foraging members increased their body weight by 5.9%
(Clutton-Brock
et al
., 1998). Over the total time taken for a breeding attempt, the top
babysitters lost up to 11% of their body weight.
Through increasing group size, the combined effects of which are sometimes called
'group augmentation', this helping behaviour can provide a future benefit to the babysitters
in at least two ways. Firstly, a larger group size can be beneficial to all the members of the
group, because large groups do better. For example, larger groups can watch out for
predators more efficiently, spend a larger proportion of time foraging and are more likely
to win territorial conflicts with other groups (Fig. 12.5a). This leads to mortality rates
being lower in larger groups (Fig. 12.5b). Secondly, subordinate females, and immigrant
subordinate males may inherit the dominant, breeding position; by helping they ensure
that they will have helpers present in the future. One way of testing this idea is by examining
whether there are sex differences in helping rates because future fitness benefits of helping
would be greatest for the sex that is most likely to remain and breed in the natal group.
Consistent with this, females are the most helping sex in meerkats, and the helping rate of
males drops when they become adults and are about to leave the group (Fig. 12.5c).
The data from meerkats also illustrate how it can be difficult to separate the direct and
indirect (kin selected) benefits of cooperation. The above discussion makes it clear that
helping can provide direct fitness benefits to helpers. However, most members of a meerkat
group are related to both the pups that they help raise, and the rest of the group (Fig.
B11.3a). This means they gain indirect benefits through: (a) helping raise relatives and (b)
subordinate relatives also gaining the benefits of increased group size. An implication of
this is that the relative importance of various factors that favour helping may vary with
both sex and age. For example, natal males may help due primarily to the indirect benefits
of helping relatives, and the direct survival benefits of being in a larger group, whereas
immigrant males may help due to the survival benefits of being in a larger group and to
produce subordinates who would help them if they obtain dominance in that group.
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Helping by
meerkats is costly
in the short term
…
… but can provide
benefits in the
future
Males and
females help at
different rates
Both direct and
indirect fitness
benefits can
contribute to the
evolution of
helping
behaviours
