Biology Reference
In-Depth Information
16
14
No kingfisher
With kingfisher
12
10
8
6
4
2
0
2
20
40
Prey density
Fig. 3.11
Hungry sticklebacks normally prefer to attack high density areas of prey but
after a model kingfisher was flown over the tank they preferred to attack low density
areas. From Milinski and Heller (1978). Reprinted with permission from the Nature
Publishing Group.
quickly, so it prefers the low density of prey. The balance of costs and benefits shifts from
feeding to vigilance as the stickleback becomes less hungry.
Consistent with this hypothesis, Milinski and Heller found that predation risk
influences choice of feeding rate. When they flew a model kingfisher (
Alecedo atthis
)
(a predator on sticklebacks) over a tank containing hungry fish they found that the
sticklebacks preferred to attack low rather than high prey densities (Fig. 3.11). This is to
be expected if the hungry fish, in spite of its high chance of starvation, places a very
high premium on vigilance when a predator is in the vicinity.
An important difference between Milinski and Heller's analysis of foraging and those
described earlier is that the cost-benefit calculations include the animal's hunger state.
An optimization model in which the animal's state changes as a result of its behaviour
(the fish becomes less hungry as a result of feeding) is referred to as a dynamic, as
opposed to static, model. In fact, the traditional view that an animal's internal state
controls its behaviour can be turned on its head and the animal can be seen as using its
behavioural repertoire to control the internal state in an optimal way. The influence of
the kingfisher on the stickleback is to alter the optimal allocation of time to feeding and
vigilance, so that the fish decreases its hunger at a slower rate.
The idea of a dynamic feedback between foraging, body reserves and danger of
predation has been used by Jim Gilliam (1982) to predict how an individual should shift
Hungry
sticklebacks
risk danger of
predation to
obtain high
intake rates
