Biology Reference
In-Depth Information
to simulate movement about a home range. When females were spaced out, free-ranging
males (tracked by radiotelemetry) became dispersed, overlapping their ranges with the
female ranges. When females were clumped, by placing cages close together, the males
aggregated on the female clumps. By contrast, when males were then kept in individual
cages the dispersion of free-ranging females was not affected by experimental changes
in male dispersion. This study shows that the causal links are from resources to female
dispersion and then from female dispersion to male dispersion, as in Fig. 9.1.
Blue-headed wrasse, Thalassoma bifasciatum
Robert Warner (1987, 1990) studied these coral reef fish in the Caribbean. Females
spawned in favourite sites on the downcurrent edges of a reef where the pelagic eggs
were swept quickly into the open sea, so avoiding predation from other reef fish.
Individual females returned almost every day to particular sites where they laid a few
eggs. Males competed to defend territories at these preferred sites, with the largest males
defending the best sites and so gaining most mates.
To assess the roles of the two sexes in determining spawning sites, Warner removed
either all the breeding males or all the females from local isolated populations and
replaced them with fish from other populations. When males were replaced, most of the
spawning sites remained the same as before. By contrast, when females were replaced
there were marked changes in sites used, even though some males initially continued to
display at and defend the original sites. This neat experiment shows that females choose
the spawning sites and males simply compete to defend sites which females prefer.
In both voles and
wrasse, females
follow resources,
males follow
females
Comparative evidence: mammalian mating systems
The influence of variation in resource and female dispersion on mating systems is well
illustrated by mammals, where the economics of female monopolization by males is
influenced by three main factors: female group size, female range size and the seasonality
of breeding (Fig. 9.3). The following comparative survey is based on the review by
Clutton-Brock (1989).
Females solitary: range defensible by male
In over 60% of mammalian species females are solitary and a male defends a territory
which overlaps one or more female ranges. If female ranges are small relative to the area
which a male can defend then the male can be polygynous. If female ranges are larger, then
the male may only be able to defend one female, hence monogamy (e.g. most rodents and
nocturnal prosimians; Kleiman, 1977). Usually the male simply mates with the female
then leaves her to care for the offspring alone. More rarely (3% of mammalian species) the
male may help defend the young against predators (e.g. klipspringer,
Oreotragus oreotragus
)
or carry them (e.g. siamangs, marmosets, tamarins) or help to feed them (e.g. jackals, wild
dogs). Obligate monogamy occurs where female ranges are small enough for a male to
defend but where a male is unable to defend a large enough area to have more than one
female (Rutberg, 1983). In these cases a male may then maximize his reproductive success
by providing parental care. Species with obligate monogamy tend to have large litter sizes.
For example, this mating system is common in canids, which have large litters, and rare in
