Biology Reference
In-Depth Information
40
(a)
(b)
19.5
30
19.0
20
18.5
10
18.0
17.5
0
12
14
16
18
20
22
0
2
468 012
14
Weight (g)
Number in brood
Fig. 1.6
Experimental manipulation of brood sizes in great tits. (a) In larger broods of
great tits the young weigh less at fledging because the parents cannot feed them so
efficiently. (b) The weight of a nestling at fledging determines its chances of survival;
heavier chicks survive better. From Perrins (1965).
optimum to maximize the number of surviving young per brood from a selfish
individual's point of view (Fig. 1.5b, curve). The most commonly observed clutch size is
close to the predicted optimum but slightly lower. Why is this?
One hypothesis is that the optimum in Fig. 1.5b (curve) is the one which maximizes
the number of surviving young
per brood
whereas, at least in stable populations, we
would expect natural selection to design animals to maximize their
lifetime
reproductive
output. If increased brood sizes are costly to adult survival, and hence chances of
further reproduction, then the clutch size which maximizes lifetime breeding success
will be slightly less than that which maximizes success per breeding attempt (Fig. 1.7).
Box 1.1 gives a more general model for the optimal trade-off between current and future
reproductive effort.
A second hypothesis for the lower than predicted clutch size is that when great tits are
experimentally given extra eggs or chicks they may well be able to rear some extra young
efficiently, but we have ignored the costs of egg production and incubation (Monaghan &
Nager, 1997). A fairer test would be to somehow manipulate birds into laying extra
eggs, rather than giving them extra eggs or chicks for free. If females were forced to pay
the 'full cost' of laying and incubating the extra eggs, then this may reduce the predicted
optimal brood size to maximize the number of surviving chicks per brood.
Note that both hypotheses involve measuring further trade-offs. David Lack's
predicted optimum (Fig. 1.5b, curve) involved the trade-off between offspring number
and quality. Our first hypothesis for the mis-match between his prediction and the
observed clutch size is that we need to consider, in addition, the trade-off between
adult reproductive effort and adult mortality. The second hypothesis concerns another
trade-off, that between investment in egg production and incubation versus chick
care. As we shall see throughout this topic, resources are limited and one of the main
Two hypotheses
for the mismatch
between
observed and
predicted …
…. involve
considering
further trade-offs
