Biology Reference
In-Depth Information
work? The problem is that of cheating; a partner may be tempted to do less than its fair
share of work by relying on the compensatory reactions of the other partner.
This conflict over parental investment was first modelled as an evolutionary game, in
which each partner independently plays a fixed effort (a 'blind bid') and the optimal
effort for each parent is then resolved over evolutionary time (Chase, 1980; Houston &
Davies, 1985). At the ESS, each parent will invest a fixed level of effort that maximizes
its own fitness, given the effort invested by its mate. Consider a pair. The male will have
a 'best response', in terms of parental effort, to a given effort put in by the female.
Likewise, the female will have a 'best response' to any given effort by the male. If brood
productivity is an increasing but decelerating function of total parental effort, and the
costs of increased effort for a parent are non-decelerating (as in Fig. 8.2), then it can be
shown that the 'best responses' for each parent will involve incomplete compensation.
That is to say, if one parent reduces its effort, the other will increase its effort but not
sufficiently to compensate for the loss (Figs 8.5a, 8.5b). Such incomplete compensation
leads to stable biparental care, as explained in Fig. 8.5c.
If conditions led to a partner fully compensating, or even over-compensating for a
reduction in effort by the other partner, then biparental care would be unstable and the
ESS is for uniparental care (as explained in Fig. 8.5d).
This basic framework has been extended to incorporate behavioural negotiation
between the parents, where each parent may adjust its own effort in response to that of
its partner. Here, it is the 'response rules' that evolve rather than the effort levels. The
mathematics now become complex and the evolutionary outcome of the negotiation
game differ in detail from the 'blind bid' analysis. Nevertheless, the model still predicts
incomplete compensation, this time on a behavioural time scale (McNamara
et al
., 1999).
The key theoretical prediction, therefore, is that in cases of biparental care, parents
should respond to reduced partner effort with incomplete compensation. A partner
who cheats, by reducing its effort, will suffer reduced fitness because its young will get
less well fed. This has been tested in numerous experiments with birds (e.g. starlings
Sturnus vulgaris
), where the effort of one partner has been reduced by various
techniques, including temporary removals, feather cutting, tail weighting and
testosterone implants. Overall, the mean response of the other partner was indeed to
increase its effort, but with partial compensation (Harrison
et al
., 2009). Nevertheless,
in some studies the unmanipulated partner showed no response, or full compensation.
And a careful study by Camilla Hinde (2006) produced a surprising result. She tricked
parent great tits,
Parus major
, into increasing parental effort by augmenting the begging
calls of their brood with playback of extra calls through a little loudspeaker placed next
to the nest (Fig. 8.6). When one parent only (either male or female) was exposed to the
playback, it increased its provisioning rate (the expected response to an apparently
more hungry brood). However, the other parent also increased its provisioning
(Table 8.4), even though it did not experience increased chick begging (the playback
had no effect on the chicks themselves). The unmanipulated adult must, therefore, have
responded directly to its partner, increasing its own effort in response to the partner's
increase.
This matching increase in response is not what our model predicts. How might we
explain it? The most likely explanation is that the model is too simple. When one parent
changes its level of investment in parental care, this may influence the behaviour of the
other parent for two reasons. The first, addressed by the model, is that a change in
Conflicts over
how much care
each member of a
pair should
provide
In theory,
incomplete
compensation
stabilizes
biparental care
An experiment
with great tits:
partners match
each other's
response
