Biology Reference
In-Depth Information
come from other areas. Ben Sheldon and
colleagues showed that the same logic
can explain sex ratio variation in response
to the quality or attractiveness of a mate.
In blue tits, males have an ultraviolet
patch on the top of their head, which
appears to act as a reliable signal of
quality (Chapters 7 and 14). Sheldon
et al
. (1999) showed that females mated
to males with a brighter UV patch laid a
higher proportion of sons (Fig. 10.10).
They argued that females should adjust
their offspring sex ratio in this way,
because higher quality mates would lead
to higher quality offspring, either through
passing on good genes or through higher
quality paternal care, and that sons
would benefit more from being higher
quality. The idea here is very similar to
the classic Trivers and Willard (1973)
argument shown in Fig. 10.8, except that
mate quality replaces maternal condition
as the factor on the x axis that influences
offspring fitness. Sheldon
et al
. also
confirmed their result experimentally,
showing that when the UV signal was
blocked out with sunblock, this led to a
higher proportion of female offspring.
Whilst these results were initially controversial, both the observational and experimental
patterns have since been replicated in three different European populations.
Sons
Daughters
Sons
Daughters
τ
Maternal quality
Fig. 10.8
The Trivers and Willard
hypothesis. The relative fitness of sons
increases more rapidly with maternal
quality than that of daughters.
Consequently, females in relatively
good condition (>
τ
) would do best by
producing sons and females in
relatively poor condition (<
τ
) would
do best by producing daughters. From
Trivers and Willard (1973). Reprinted
with permission from AAAS.
12
10
8
6
4
2
0
-2.0
-1.6
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
1.6
2.0
Subordinate
Maternal dominance
Dominant
Fig. 10.9
In red deer, the lifetime reproductive success (LRS) of sons (filled circles and
solid line) increases more rapidly with their mother's social rank than daughters (open
circles and dashed line). From Clutton-Brock
et al
. (1984). Photo © Alison Morris.
