instead of living geographically apart as described above for subspecies. For example,
the lesser snow goose has two color morphs, blue and white, which occur together
in the same population. This species is polymorphic for color. The common guille-
mot or murre (Uria aalge) has a “bridled” morph with a ring of white feathers around
the eye. The normal morph has no white marks. Along the coast of Europe the
frequency of the bridled morph increases with latitude and humidity from 0.5% to
over 50% (Southern 1951).
Where the frequencies of the morphs in these populations have remained relatively
constant, as in the bridled guillemot, the state is called a stable polymorphism. There
are also cases where one morph displaces another, perhaps because of changing con-
ditions or because two races, originally separate, have recently become sympatric.
Such may have happened to the lesser snow geese where the two color morphs used
to spend the winter in separate areas but now share their wintering grounds in the
central USA (Cooke 1988). The temporary state in which one morph is replacing
another is called a transient polymorphism.
The selective advantages accruing to the various morphs are usually unknown.
Mechanisms that could maintain the polymorphism include:
1 Heterozygote advantage. The heterozygote has a selective advantage over both
homozygotes. Often the rare allele is a genetic dominant, lethal or disadvantageous
in homozygous form.
2 Frequency-dependent selection. The rarest morph has a selective advantage over the
others. This could occur, for example, where predators have a search imagefor the
common morph and so overlook the rare one.
3 Alternating selection. Different morphs are advantageous under different environ-
mental conditions. Some morphs may be adapted to wet and cool conditions, others
to hot and dry conditions, such that they are advantaged seasonally.Why do we need to know about adaptation from the point of view of wildlife man-
agement? Many species are becoming rare through loss of their habitat. To rectify
this by preserving habitats we need to know their physiological and behavioral
adaptations and constraints. For example, to improve the breeding ponds for ducks
in the Canadian prairies we need to know the tolerances the ducks have for levels
of alkalinity and salinity. Recently hatched ducklings of mallard (Anas platyrhynchos)
and blue-winged teal (A.discors) require fresh water for survival because their salt
glands are not completely functional until the ducklings are 6 days old. Growth of
mallards in the first month of life is slowed when they live in moderately saline water.
Although dabbling ducks often nest on islands in lakes of high salt content, female
mallards lead their ducklings to freshwater lakes, and gadwall (A.strepera) ducklings
use freshwater seepage zones (Swanson et al. 1984).
The social organization of animals is an adaptation to habitat, food supply, avoid-
ance of predators, and courtship. Jarman (1974) compares the social organization
of the African antelopes (see Section 4.8). At one extreme is the tiny dikdik
(Rhynchotragus kirkii) that lives in pairs jointly defending a territory in thick scrub.
They avoid predators by staying very still when predators are around, and they run
only at the last moment. They rely on concealment. Their food is high protein shoots,
buds, and flowers on bushes. Being small (5 kg), dikdik do not eat large amounts of
food. However, the food is sparsely distributed and so dikdik are also dispersed –
they cannot live in large groups. Equally, this sparse food supply should be defendedANIMALS AS INDIVIDUALS 33
3.8 Applied aspects
3.8.1Adaptation