Biology Reference
In-Depth Information
Males
~W
m
Females
~W
f
t
Age
Fig. B10.4.1
Sex change from female to male. The relative fitness of males
increases more rapidly with age than that of females. Consequently, individuals are
selected to mature as females and then change sex to males at age
τ
. Note the
similarity to figure 10.8.
fitness increases faster with age (that is why sex change in this direction is
favoured!), this means that
Wm
>
Wf
(Fig. B10.4.1), so, in order for Equation
B10.4.1 to hold, then Nm < Nf. This means that there will be more females than
males and, hence, a female-biased sex ratio. The converse prediction for
protandrous (male first) species can equally be made, showing that a male-
biased sex ratio is predicted.
Support for this prediction has been provided with data from 121 sex changing
animal species, spanning a wide range of taxa, including fish, crustaceans,
mollusks, echinoderms and annelid worms (Fig. B10.4.2). The average
population sex ratio of female first (protogynous) species was significantly female
biased, while that of male first (protandrous) species was significantly male
biased. Furthermore, the difference between these two groups was significant
when considering species as independent data points or phylogenetically
controlled independent comparisons.
More recently, a study of 116 fish species by Philip Molloy and colleagues
(2007) has shown that the relative gonad size of males correlates with the
occurrence of sex change. Specifically, males in female first (protogynous)
species had relatively smaller testes than males of species where sex change
did not occur (Fig. B10.4.3). This covariation across strategies can be
explained because the mating system influences both sex change and sperm
competition. In species where males can monopolize females, this selects for
female first sex change, but also means that male-male competition during
spawning will be reduced, and hence the benefits from larger testes are
reduced (Chapter 7). In contrast, in species where males cannot monopolize
females, there will not be selection for sex change, and male-male sperm
competition will be maximized during spawning, leading to a greater benefit
from increased testes size.
