Biology Reference
In-Depth Information
from size than females. Jennie McCabe and Ali Dunn (1997) tested this hypothesis by
examining the consequences of size under field conditions. They found that whilst
larger females produce more eggs, larger males are more successful at obtaining
mates and obtain larger mates, which therefore produce more eggs (Fig. 10.11).
When all of these factors are combined, they found that size had much greater fitness
consequences for males.
Sex change
Sex change, where individuals mature as one sex, and later change to the other, occurs
in a variety of fish, invertebrates and plants (Fig. 10.12). Ghiselin (1969) argued that
sex change would be favoured if the fitness of an individual varies with age or size, and
the relationship is different for males and females. In this case, natural selection favours
individuals who mature as the sex whose fitness increases more slowly with age (first
sex) and then change to the other sex (second sex) when older. The idea here is very
similar to the classic Trivers and Willard (1973) argument shown in Fig. 10.8, except
that age or size replaces maternal condition as the factor on the x axis that influences
offspring fitness. A special feature of sex change, discussed in Box 10.4, is that it is one
of the few applications of the Trivers and Willard hypothesis where useful predictions
can also be made about the population sex ratio.
In many coral reef fish, female first (protogynous) sex change occurs. Robert Warner
and colleagues have argued that this occur in species where the mating system leads to
male mating success being monopolized by the oldest, largest individuals. For example,
in the bluehead wrasse
Thalassoma bifasciatum
, males set up territories where the
females come to spawn (Warner
et al
., 1975). Females choose to mate with the largest
terminal phase males. This leads to a huge size advantage, with large males spawning
more than 40 times a day and small ones less than twice a day (Warner
et al
., 1975).
Furthermore, individuals are able to time their sex change with remarkable precision in
response to the social conditions. If the largest males on a reef are removed, the next
largest individuals (females) will change sex and become brightly coloured males. The
cues involved in stimulating such social sex change are the matter of debate, and could
be behavioural, visual or chemical.
Sex change can also be favoured in the other direction, from male to female
(protandry), if male size has little effect on breeding success. In this case, an individual
may then reproduce best as a male when small because it is able to spawn with some of
the large, most fecund females. An example of a fish that changes from male to female
is the anemonefish, or clownfish
Amphiprion akallopisos,
which lives on coral reefs in
the Indian Ocean. It lives in close symbiosis with sea anemones and because there is
usually only enough space for two fish to inhabit the same anemone this species lives
in pairs. In effect, the habitat forces them to be monogamous. The reproductive success
of a pair is limited more by the female's ability to produce eggs than by the male's
ability to produce sperm, so each individual does better if the larger one is female. Like
the wrasses, sex change is socially controlled. If the female is removed, the male is then
joined by a smaller individual, so he changes sex and lays the eggs while the newcomer
functions as a male (Fricke & Fricke, 1977).
If being old and
big provides a
greater benefit to
one sex, then sex
change can be
favoured
When old large
males can
monopolize the
mating with
females, then
individuals may
be selected to
mature as
females, and then
change sex later
in life to males
When males are
not able to
monopolize
matings, then sex
change can be
favoured in the
other direction
from male to
female
