Biology Reference
In-Depth Information
Animals can be
expected to show
more variable
behaviour in more
variable
environments
Another factor which may affect how perfectly we should expect animals to behave is
the extent to which the environment varies. Allen Herre (1987) studied LMC in thirteen
species of pollinating fig wasps, which lay their eggs and develop in fig fruits. In these
species, the males are wingless and do not leave the fruit, while females disperse, so their
life history provides a close fit to that modelled by Hamilton. Herre found that although
individuals in all species adjusted their sex ratios, they did not do so equally. Specifically,
greater sex ratio adjustment was seen in species where the number of females laying
eggs in a fruit showed greater natural variation. This provides an elegant, demonstration
of the general point that a more variable environment selects for more variable behaviour.
Local resource enhancement
Relatives may not only compete, they may cooperate. In many cooperative breeding
vertebrates, offspring of one sex are more likely to remain in the group and help parents
rear further offspring. For example, females are more likely to help in the Seychelles warbler,
whereas males are more likely to help in African wild dogs and red-cockaded woodpeckers.
Many researchers argued that, in such species, the population sex ratios should be biased
towards the helping sex (Box 10.1). However, Ido Pen and Franjo Weissing (2000) showed
that this prediction would only hold in the simplest of cases, and that it is possible to predict
sex ratio biases in any direction, depending upon life history details. This illustrates the
advantage of producing mathematical models, which force the underlying assumptions to
be made explicit, and shows that verbal arguments can sometimes be misleading. Instead,
Pen and Weising showed that the clear prediction that could be made was that groups with
relatively few helpers should produce the helping sex, and groups that already have helpers
should produce the other sex. This prediction has been supported in a number of species.
For example, in African wild dogs, groups with relatively few helpers produced male-biased
litters with 63% sons, whereas groups with relatively many helpers produced female-
biased litters with 64% daughters (Creel
et al
. 1998).
The most striking example of sex ratio adjustment in a bird or mammal is provided by
the Seychelles warbler, where both LRE and LRC occur. In this species, breeding pairs
remain together on the same territory for up to nine years, producing one offspring a
year, with female offspring often staying behind on their natal territory to help, through
a range of behaviours, including territory defence, nest building, incubation and
feeding of young. Jan Komdeur and colleagues showed that the advantage of helping
depends strongly on territory quality. On high-quality territories, where there is a high
density of insect prey, having a helper is advantageous (local resource enhancement).
In contrast, on low-quality territories, where there is a low density of insect prey, the
increased competition for food means that having a helper is disadvantageous (local
resource competition). Amazingly, Komdeur
et al
. found that females adjust their
offspring sex ratios with precision in response to this, producing 90% females on high-
quality territories, and 80% sons on low-quality territories (Fig. 10.6).
In addition to their observational data, Komdeur
et al
. carried out an experiment in
which warblers were translocated to a new island, where they took high-quality
territories. Pairs moved from low-quality to high-quality territories switched from
producing 90% males to 85% females, while those moved from high-quality to high-
quality territories continued to produce 80% females. Furthermore, on high-quality
territories, while having one or two helpers is advantageous, the increased competition
Groups without
helpers should
overproduce the
helping sex
The benefit of
producing the
helping sex can
be reduced or
removed on low
quality territories
