Biology Reference
In-Depth Information
Even distribution.
Little polygamy potential
Patchy distribution.
High polygamy potential
Fig. 9.2
The influence of the spatial distribution of resources (food, nest sites) or mates
on the ability of individuals to monopolize more than others. Dots are resources or mates
and circles are defended areas. With a patchy distribution of resources or mates there is
greater potential for some individuals to 'grab more than their fair share'.
compete for females either directly (A in Fig. 9.1) or indirectly (B) by anticipating how
resources will influence female dispersion and competing for resource rich sites.
The economics of female defence or resource defence by males will depend on their
distribution both in space and in time. When mates or resources are more patchily
distributed in space there will be greater opportunities for polygyny (Fig. 9.2). The key
factor for determining the temporal distribution of mates is the 'operational sex ratio'
(Emlen & Oring, 1977; Chapter 7), which is the ratio of receptive females to sexually
active males at any one time. If all the females bred in synchrony, then with a real sex ratio
of 1:1 in the population, the operational sex ratio at breeding would also be 1:1 and there
would be little opportunity for a male to mate with more than one female because by the
time he had mated once all the other females would have finished breeding. This applies,
for example, to common toads (
Bufo bufo
) which are 'explosive breeders': all the females
spawn within a few days, so a male has time to mate with one, or at most two, females
before the breeding season is ended. Bullfrogs,
Rana catesbeiana
, by contrast, are 'prolonged
breeders' with females arriving at the pond over several weeks. Males which can defend
the best spawning sites may mate with up to six females in a season (Wells, 1977).
We now examine experimental and comparative evidence for the scheme in Fig. 9.1.
Female dispersion
is influenced by
resources, male
dispersion is
influenced by
females
Mating systems
will be influenced
by spatial and
temporal
distribution of
mates
Experimental evidence: voles and wrasse
Grey-sided voles, Clethrionomys rufocanus
Rolf Anker Ims (1987) showed that female dispersion was influenced by food; when
food was provided in abundance at particular sites female ranges became smaller and
overlapped in the resource-rich areas. The males also homed in on these sites. Was the
change in male dispersion due to males following females or because they, independently,
followed changes in resource distribution? To test this, Ims (1988) introduced a small
population of voles on to a little wooded island in south-east Norway. In one experiment,
females were kept individually in small cages and their positions were moved each day
