Biology Reference
In-Depth Information
themes of behavioural ecology is investigating how various
trade-offs are solved by natural selection.
Marcel Visser and Kate Lessells (2001) measured the effects of
these two extra trade-offs on great tit optimal clutch size by a
clever experimental design (first used by Heany & Monaghan
(1995) for studying clutch size in a seabird). In a nest-box
population of great tits in the Hoge Veluwe, a large national
park in The Netherlands, they had three experimental groups of
females, each raising two extra chicks:
Costs: adult
mortality
Benefits: number
of surviving
young produced
(i)
Free chicks
. Two extra nestlings were added to the nest, soon
after the female's own brood hatched. These females,
therefore, only had to raise two extra chicks.
(ii)
Free eggs
. Two extra eggs were added to the clutch on the
day the female began to incubate her own clutch. These
females, therefore, had to incubate two extra eggs as well as
raise the two extra chicks.
(iii)
Full costs
. The female was induced to lay two extra eggs by
removing the first four eggs of the clutch on the day they were
laid (previous experiments had shown that removal of four
led to two extra eggs being laid). These four removed eggs were
kept in a bed of moss and were returned to the clutch before
incubation began. So this third group had to lay the two extra
eggs, as well as incubate them, and raise the two extra chicks,
thus paying the full cost of an increased clutch size.
b
2
b
1
Clutch size
Fig. 1.7
The influence of adult mortality
on the optimal clutch size. The number of
young produced versus clutch size follows
a curve, as in Fig. 1.5, with b
1
being the
clutch size which maximizes the number
of young produced per brood. Increased
clutch size, however, has the cost of
increased adult mortality, shown here for
simplicity as a straight line. The clutch size
which maximizes lifetime reproductive
success is b
2
, where the distance between
the benefit and cost curves is a maximum.
This is less than the clutch size b
1
, which
maximizes reproductive success per brood.
From Charnov and Krebs (1974).
The results showed that the number of young produced who
survived to breeding age (recruits) did not differ between the
three treatments. Therefore, there was no support for the second
hypothesis;
full costs
females produced just as many surviving
young as those given free eggs or chicks. However, female survival
was
affected;
full costs
females had the lowest survival to the next breeding season, while
free chicks
females
survived the best, with
free eggs
females having intermediate survival. These results,
therefore, support the first hypothesis; there is a trade-off between increased reproductive
effort and adult survival. When female fitness was calculated,
full costs
females had
lower fitness than control females (who were left to raise the clutch size they initially
chose; Fig. 1.8). Therefore, when the costs of both egg production and incubation are
taken into account, the observed clutch size is optimal (at least in comparison with an
increase in clutch size of two eggs).
Brood size manipulations are most easily done with birds, but similar studies with
mice (König
et al
., 1988) and insects (Wilson, 1994) also suggest that reproductive rate
tends to maximize individual success, though the trade-offs involved vary from case to
case, and they are often tricky to measure.
Clutch size may vary from year to year and during the season depending on food
supplies, so individuals do show some variation. However, the variations are in relation
to their own selfish optima, not for the good of the group. A good example of individual
optimization is provided by Goran Högstedt's study (1980) of magpies,
Pica pica
,
A trade-off
between
reproductive
effort and adult
survival to
maximize lifetime
success
Individuals may
have different
optima
