Biology Reference
In-Depth Information
CHAPTER 10
Sex Allocation
Photo © David Shuker and Stuart West
All sexually reproducing organisms must decide how to allocate resources to male and
female reproduction. This encompasses a range of related questions across the
different types of breeding systems. In dioecious species, where individuals are either
male or female for their entire lifetime, such as birds and mammals, the problem is
whether to produce male or female offspring? In sequential hermaphrodites, or sex
changers, where individuals function as one sex early in their life, and then switch to
the other, such as many reef fish, the problem is what sex to be first, and when to
change sex? In species where the offspring sex is determined by the environment
(environmental sex determination), such as some shrimps and fish, what cue should
determine sex, and how?
It used to be the conventional wisdom that whether interesting things happened with
sex allocation depended upon the method of sex determination (Box 10.1). In species
with genetic sex determination, such as birds or mammals, it was assumed that sex
determination was random, and could not be controlled by parents. This idea had been
supported by selection experiments on domestic animals such as chickens (where a
female biased sex ratio would bring huge economic benefits), which had failed to
shift  offspring sex ratios from equal numbers of males and females. In contrast, sex
determination mechanisms such as haplodiploidy and environmental sex determination
allowed greater control over offspring sex, so interesting patterns of sex allocation could
occur, with biases towards sons or daughters. For example, in haplodiploid species, such
as ants, bees and wasps, a female can adjust the sex of her offspring, in response to local
conditions, by whether or not she fertilizes an egg (Box 10.1).
However, over the last 40 years, this picture has changed dramatically, with the
discovery that a huge variety of organisms, even birds and mammals, are manipulating
the sex of their offspring in ways that increase their fitness. Indeed, research on sex
allocation has provided one of the most productive and successful areas of behavioural
ecology, illustrating several general points about how natural selection shapes
Sex allocation is
the allocation of
resources to male
versus female
reproduction in
sexual species
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