Abstract
Theory predicts the optimal timing of sex change will be the age or size at which half of an individual's expected fitness comes through reproduction as a male and half through reproduction as a female. In this way, sex allocation across the lifetime of a sequential hermaphrodite parallels the sex allocation of an outbreeding species exhibiting a 1∶1 ratio of sons to daughters. However, the expectation of a 1∶1 sex ratio is sensitive to variation in individual condition. If individuals within a population vary in condition, high-condition individuals are predicted to make increased allocations to the sex with the higher variance in reproductive success. An oft-cited example of this effect is seen in red deer, Cervus elaphus, in which mothers of high condition are more likely to produce sons, while those in low condition are more likely to produce daughters. Here, we show that individual condition is predicted to similarly affect the pattern of sex allocation, and thus the allocation of reproductive effort, in sequential hermaphrodites. High-condition sex-changers are expected to obtain more than half of their fitness in the high-payoff second sex and, as a result, are expected to reduce the allocation of reproductive effort in the initial sex. While the sex ratio in populations of sequential hermaphrodites is always skewed towards an excess of the initial sex, condition dependence is predicted to increase this effect.
Highlights
Sex change theory proposes that when size-specific reproduction increases at one rate for males and at another rate for females the functions describing these rates will inevitably cross, and sex change will be favored by natural selection [1,2,3,4,5]
Extending Fisher’s logic to sequential hermaphrodites, Leigh et al [4] showed that the optimal timing of sex change will be the age or size at which half of an individual’s expected reproductive success arrives through function as a female, and half through function as a male
The specifics of what endows an individual with ‘high condition’ are immaterial – what is important is that such individuals have higher than average fitness expectations [10]
Summary
Sex change theory proposes that when size-specific reproduction increases at one rate for males and at another rate for females the functions describing these rates will inevitably cross, and sex change will be favored by natural selection [1,2,3,4,5]. Recalling that the survival rate, and the lifespan, b, is a function of condition, we see from eq.16 that high-condition individuals are predicted to have lower allocations of reproductive effort at all ages, while having higher lifetime fitness, than lower condition individuals (eq.15; Fig. 3).
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