Biology Reference
In-Depth Information
Komdeur
et al
. (2002) showed that just as extreme sex ratio biases occur in the second egg
as in the first. Given that this egg is laid only one day after the first egg, the sex ratio bias
could not result from post-ovulation mechanisms, such as sex-selective re-absorption of ova
or dumping eggs of the wrong sex. Consequently, this suggests that the control of offspring
sex involves some pre-ovulation control, such as biasing segregation towards the required
sex chromosome (Z or W; see Box 10.1). More generally, this stresses that we should always
test whether patterns of sex ratio bias can be explained by the alternate explanation of
differential mortality, such as individuals of one sex requiring more resources, so being
more likely to die during development, especially when conditions are relatively harsh.
There are also a number of species where the offspring sex ratios do not vary with the
number of helpers in a group, such as acorn woodpeckers and superb fairy wrens. How
can we explain this variation across species? Ashleigh Griffin and colleagues suggested
that weaker sex ratio adjustment might be seen in species where the benefits of adjusting
sex ratios were lower. If helpers provide little actual benefit, then there is little benefit in
groups without helpers preferentially producing the helping sex. They found support
for this hypothesis this with a meta-analysis (Box 10.3) across 11 species, with lower
levels of sex ratio adjustment in species where the actual benefit provided by helpers
was lower (Fig. 10.7; Griffin
et al
., 2005). In the species where helpers provide negligible
benefits, sex ratio adjustment is not observed.
In species where
helpers provide
less benefit, there
is reduced
selection for sex
ratio adjustment
0.75
11
10
0.5
8
7
0.25
6
9
5
4
0
3
-0.25
1
2
-0.25
0
0.25
0.5
0.75
Benefit of helpers
Fig. 10.7
The correlation between the extent to which sex ratios are adjusted and
the benefit provided by the presence of helpers. A more positive extent of sex ratio
adjustment signifies a greater tendency to produce offspring of the sex that helps more,
in patches where there is a lack of helpers. Across species, the significant positive
correlation indicates that sex ratio adjustment is greater in species where the presence
of helpers leads to greater fitness benefits. The data points represent (1) laughing
kookaburra, (2) sociable weaver, (3) Harris's hawk, (4) acorn woodpecker, (5) green
wood-hoopoe, (6) western bluebird, (7) alpine marmot, (8) redcockaded woodpecker,
(9) bell miner, (10) Seychelles warbler and (11) African wild dog. Griffin et al. (2005).
Reprinted with permission of the University of Chicago Press. Photo © Andrew Young.
