Biology Reference
In-Depth Information
that there is little competition within the flock while the birds are feeding. In the
savannah, birds cannot hide their nests and so they seek safety in protected sites,
such as spiny acacia trees. Nests are sometimes bulky to provide thermal
insulation against the heat of the sun. Because good breeding sites are few and
scattered, many birds nest together in the same tree. Within a colony, males
compete for nest sites and those that defend the best sites attract several females
while males in the poorer parts of the colony fail to breed. In addition, because
food is abundant, the female can feed the young by herself, so the male is
emancipated from parental care and can spend most of his time trying to attract
more females. This has favoured brighter plumage coloration in males and the
evolution of polygamy.
Supporting evidence for this interpretation comes from species with intermediate
ecology (Table 2.2). The grassland seed eaters have patchy food supplies, so group living is
favoured for efficient food finding. However, in grassland the nests are vulnerable, so
predation favours spacing out. The result is a compromise; these species have an
intermediate social organization, nesting in loose colonies and feeding in flocks.
These results suggest that food and predation are important in determining social
organization. They also reveal how several different traits, such as nests, feeding
behaviour, plumage colour and mating system, can all be considered together as a result
of the same ecological variables. Crook's work with the weaver birds inspired several
people to use the comparative method to study social organization in other groups.
David Lack (1968) extended the argument to include all bird species and Peter Jarman
(1974) used the same approach for the African ungulates.
Social organization in African ungulates
Jarman (1974) considered 74 species of African ungulates; all eat plant material but
differences in the precise type of food eaten are correlated with differences in movements,
mating systems and anti-predator behaviour (Fig. 2.3). The species were grouped into
five ecological categories (Table 2.3). Just as in the weaver birds, several adaptations
seem to go together.
The major correlate of diet and social organization is body size. Small species have a
higher metabolic requirement per unit weight and need to select high quality food, such
as berries and shoots. These tend to occur in the forest and are scattered in distribution,
so the small species are forced to live a solitary existence. The best way to avoid predators
in the forest is to hide. Because the females are dispersed, the males are also dispersed
and the commonest mating system is for a pair to occupy a territory together.
At the other extreme, the largest species eat poor quality food in bulk and graze less
selectively on the plains. It is not economical to defend such food supplies and these
species wander in herds, following the rains and fresh grazing. In these large herds there
is potential for the strongest males to monopolize several females by defence of a harem
or a dominance hierarchy of mating rights. When predators come along these species
cannot hide on the open plains, so either flee or rely on safety in numbers in the herd.
Ungulates of intermediate size show aspects of ecology and social organization in
between these two extremes (Table 2.3).
Body size, diet
and social
organization
