Biology Reference
In-Depth Information
support for the Trivers and Willard hypothesis (Sheldon & West, 2004). As well as
asking whether consistent patterns occur, meta-analysis provides a method for
examining whether variation across species in strength of a relationship can be
explained. For example, could variation across species in whether the Trivers and
Willard hypothesis is supported be explained because it doesn't always apply (for an
analogous analysis see Fig. 10.7)? Furthermore, methods have recently been
developed which allow such 'comparative meta-analyses' to be carried out in ways
that allow for the fact that, as discussed in chapter 2, species are not independent
data points (Hadfield & Nakagawa, 2010).
Sex allocation in variable environments
Maternal condition
Robert Trivers and Dan Willard (1973) suggested that individuals should adjust the sex of
their offspring in response to environmental conditions. They envisaged a mammal
population in which three assumptions held: (1) females in better condition have more
resources for reproduction and produce better quality offspring; (2) higher quality offspring
become higher quality adults; and (3) sons gain a greater fitness benefit from being higher
quality adults. They assumed that this last assumption would hold when competition for
mates among males is intense, with the highest quality mates gaining a disproportionate
share of the matings, as is the case in many polygynous mammals (Chapter 7). The
consequence of these assumptions is that if offspring fitness is plotted against maternal
quality, then fitness increases more rapidly for sons than it does for daughters (Fig. 10.8).
In this case, Trivers and Willard argued that relatively low quality mothers would be
selected to produce daughters, and relatively high quality mothers to produce sons.
Tim Clutton-Brock and colleagues (Clutton Brock
et al
., 1984) tested this prediction
with red deer, where female condition is determined by their rank in a dominance
hierarchy. They found that females shifted their offspring sex ratio in the predicted
direction, from 47% males in low-ranking females to 61% males in high-ranking
females. Furthermore, they also showed that the assumptions made by Trivers and
Willard's held: (1) higher ranking females produced heavier (better quality) offspring;
(2) heavier offspring became bigger and higher quality adults; and (3) sons gained a
greater benefit from increased adult body size. Increased quality is more important for
males because red deer are polygynous, with males fighting to defend harems of females
during the rutting season, leading to a much higher mating success for larger, better
condition males. Overall, they were able to examine the combined consequences of
these effects, showing that a mother's rank had a significantly greater effect on the
lifetime reproductive success of her sons than that of her daughters (Fig. 10.9).
Females in better
condition can be
selected to
preferentially
produce sons
Red deer adjust
their offspring sex
ratios in response
to maternal
condition, with
mothers in better
condition being
more likely to
produce sons
Mate attractiveness
The application of the Trivers and Willard hypothesis to ungulates has proved
controversial, with mixed empirical support. However, much clearer support has
