Biology Reference
In-Depth Information
(a)
(b)
(c)
(d)
Fig. 10.12
Sex changers. Sex change may be from female to male, as in (a) the bluehead wrasse (terminal
male); or male to female, as in (b) the Clownfish
Amphiprion percula
; (c) the common slipper limpet (
Crepidula
fornicata
; photograph of a mating stack, where the largest individuals at the bottom are female and the
smaller individuals at the top are male; and (d) the Pandalid shrimp. Photo (a) © Kenneth Clifton; (b) © Peter
Buston; (c) © Rachel Collin; (d) © David Shale/naturepl.com
BOX 10.4
POPULATION SEX RATIOS, SEX CHANGE AND GONADS
As the examples in this chapter show, sex allocation theory is often able to
make clear predictions about when individuals should adjust their offspring
sex ratio conditionally in response to environmental conditions. In contrast,
when such facultative sex ratio adjustment occurs, theory has been much
less successful in predicting and explaining variation in the overall population
or breeding sex ratio. The reason for this is that the population sex ratio is
often predicted to depend upon biological details that are rarely known, such
as the details of male and female life histories, and whether other behaviours,
such as clutch size, are also adjusted conditionally (Frank 1987, 1990).
Indeed, a lack of an appreciation of this problem is one of the most common
misunderstandings in the field of sex allocation. Ric Charnov and Jim Bull
(1989) showed that the major exception to this problem is in sex changing
animals, where the sex ratio should be biased towards the sex that individuals
mature as, termed the
first sex
.
Consider the case of a protogynous species where individuals mature as
females and then change sex to males when older (bigger). In this case, the
relative fitness of males increases faster with age than it does for females. Males
and females must make an equal genetic contribution to the next generation,
because all offspring have two parents. Consequently, it must be true that:
Nm Wm
=
Nf Wf
(B10.4.1)
where
Nm
and
Nf
are the number of mature males and females, and
Wm
and
Wf
represent the mean fitness of males and females. Given that the fitness of a
male at the point of sex change will be equal to that of a female, and that male
