Biology Reference
In-Depth Information
between habitats as it grows up. His analysis applies to the bluegill sunfish (
Lepomis
macrochirus
). In experimental ponds in Michigan, USA, Earl Werner found that these
fish could obtain a higher rate of food intake by foraging on benthic invertebrates such
as chironomid larvae than they could by foraging either on the plankton or near the
emergent vegetation at the edge of the pond. As might be expected the fish spend most
of their time (more than 75%) foraging on the benthos. However, when predators in the
form of largemouth bass (
Micropterus salmoides
) were added to the pond, a significant
change in habitat use by the sunfish was seen. The bass could eat only the smallest
sunfish (the others were too big) and these fish now spent more than half their foraging
time in the reeds feeding on plankton where they were relatively safe, even though as a
result their food intake was reduced by about one-third and their seasonal growth rate
by 27%. The bigger sunfish continued to forage with equanimity on the benthos
(Werner
et al
., 1983). The little fish thus face a trade-off: is it better to stay in the relative
safety of the reeds and grow slowly, prolonging the period of vulnerability to predators,
or is it better to gamble on rapid growth to a safe size in the benthos? Gilliam was able
to show that the best thing (to maximize its total chance of survival) for the fish to do is
to stay in the safety of the reeds until a certain size is reached and then to go for the
benthic prey. This accords with observation: the young fish in the presence of predators
tend to feed in safe places, and as they get bigger they shift to the better feeding areas.
Bluegill sunfish:
age changes in
habitat choice
Social learning
While individuals often sample alternatives themselves to determine which is the most
profitable option (Krebs
et al
., 1978; Lima, 1984), sometimes they use the behaviour of
others as a source of information. Social learning is likely to be particularly beneficial
when individual learning is costly, for example because it is time consuming or
dangerous.
This is illustrated well by the contrast in sampling behaviour of two species of
sticklebacks (Fig. 3.12). Three-spined sticklebacks
Gasterosteus aculeatus
have large
spines and armoured body plates that protect them from predators and allow them
to sample alternative food patches directly, in relative safety. By contrast, nine-
spined sticklebacks
Pungitius pungitius
have weaker defences and spend much of
their time hiding in weedy vegetation. Experiments show that nine-spined
sticklebacks exploit 'public information' by watching conspecific or heterospecific
demonstrator fish foraging and then choosing to approach the patch with better
feeding rates. However, three-spined sticklebacks ignore public information and rely
on their own experience to select rich over poor patches (Coolen
et al
., 2003). These
different strategies might reflect a trade-off between reliable but more costly self-
acquired information and potentially less reliable but cheaper socially-transmitted
information (Laland, 2008).
Social learning may involve several different learning mechanisms (Laland, 2008).
For example, a naïve individual might simply have its attention drawn to a task by the
action of others, and then learn by itself how to perform the task. Alternatively, naïve
individuals might learn by copying the action of a demonstrator. Or naïve individuals
might be taught by experienced individuals.
Social versus
individual
learning in
sticklebacks
