Biology Reference
In-Depth Information
0.55
Fig. 10.2 The sex
ratio at birth in
primate species
where either
females, males and
females or males
are the dispersing
sex. The sex ratio is
biased towards the
dispersing sex. From
Silk and Brown
(2008). The photo
shows chimpanzee
by Joan Silk.
0.50
0.45
0.40
female
male and female
Dispersal
male
If relatives of one
sex compete for
resources, then
the sex ratio may
be biased towards
the other sex
resource competition (LRC) among females reduces their value as offspring: in the
extreme case only one daughter might be able to survive on the food available near
home, and so investment in other daughters would be wasted. The extent to which such
adaptive adjustment of the offspring sex ratio occurs in primates has proved extremely
contentious, because the data is so variable, and because it has been argued that
chromosomal (genetic) sex determination acts as a constraint that prevents control of
offspring sex ratios in taxa such as birds and mammals.
In a comprehensive survey of data from 102 primate species, Joan Silk and Gillian
Brown (2008) showed that there was support for this prediction, but with relatively
weak effects, showing only a slight bias to approximately 53% males in species where
they are dispersing sex, and 55% females in species where they are the dispersing sex
(Fig. 10.2). Much greater biases are seen in species where LRC is likely to be more
extreme. For example, in the army ant Eciton burchelli , new colonies are only formed by
the division of older colonies into two swarms, one of which is headed by the old queen
and the other headed by one of her daughters. The resultant competition between sisters
to head the swarm can explain why colonies produce six females and 3000 males, giving
a sex ratio of about 99.8% males amongst the reproductives (Franks & Holldobler, 1987).
Local resource competition can also explain sex ratio variation within species or
populations. William Brown and Laurent Keller (2000) found that in the narrow-headed
ant Formica exsecta, colonies tended to produce only male or only female reproductives
(Fig.  10.3), and that such 'split sex ratios' could not be explained by the more common
explanation of variation in relatedness asymmetry that we shall meet in chapter 13. They
suggested that the explanation was variation in the extent of LRC across colonies, due to
variation in the number of queens. In this species, the dispersal of queens is often limited,
with newly mated queens being recruited back into their parental colony, from which
they  may eventually disperse with workers to initiate new colonies nearby. In colonies
where  there are more queens, or lower resource availability, then there will be greater
LRC,  so the  relative benefit of producing new queens will be reduced. Brown, Keller and
colleagues  tested this prediction by examining how the sex ratio of reproductives in a
colony varied queen number and a number of ecological variables. In support of their
hypothesis, they found that colonies produced a higher proportion of males when they
contained a greater number of queens (Fig. 10.3), and when there was a lower availability
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