Biology Reference
In-Depth Information
both sexes often invest heavily in parental care and it pays both to choose high quality
partners. In some cases, both sexes have similar ornaments (e.g. the head plumes of
some grebes). Darwin (1871) suggested these evolved through 'mutual mate choice'.
An alternative hypothesis is that female ornamentation arises simply as a non-adaptive
correlated response when males evolve ornaments. However, selection experiments
show that such correlated responses are weak and ornament expression is often limited
to one sex (Wiens, 2001). Furthermore, comparative studies reveal frequent changes in
female ornamentation during evolution (Kraaijeveld
et al
., 2007). There is now good
experimental evidence for mutual mate choice. For example, in the breeding season
both male and female crested auklets,
Aethia cristatella
, develop spectacular forehead
crests and experiments reveal that both sexes display most vigorously to members of the
opposite sex with longer crests (Jones & Hunter, 1993).
Female advertisement takes on an extreme form in some primates which live in
multimale groups, where a female has access to several males. Here, females develop
large sexual swellings, the size and colour varying during the menstrual cycle and
signalling changes in fecundity (Fig. 7.13b). In some species these swellings become so
large that the female finds it difficult to sit down comfortably, so the swellings must be as
much a handicap as a male peacock's tail. In baboons,
Papio cynocephalus anubis
,
individual differences in the size of the swellings are correlated with female quality
(ability to rear offspring; Domb & Pagel, 2001). Females may compete for matings with
the dominant male (who is best able to protect them or their offspring), or they may
compete to mate with several males in the group to give each a sufficient paternity
chance that they will desist from infanticide (Chapter 2). Similar sexual swellings occur
in alpine accentors,
Prunella collaris
, a montane songbird which also lives in multimale
groups. A male will help to feed a female's brood only if he has mated with her and so
has a chance of paternity. By sharing matings (and paternity) among several males a
female increases the chance that one or more males will help (Davies
et al
., 1996b).
Females compete for male attention by presenting bright red cloacas (Nakamura, 1990)
and by singing to attract males during their fertile period (Langmore
et al
., 1996).
Males may also be
choosy: mutual
mate choice
Sex role reversal
In some cases, female competition for males becomes so strong that there is reversal of
the usual sex roles. In the pipefish,
Syngnathus typhle
, it is the male who becomes
pregnant; he has a brood pouch in which a clutch of fertilized eggs are kept safe and
provided with nutrients and oxygen (Fig. 7.14). During his pregnancy, which lasts
several weeks, a female could produce several clutches of eggs. Therefore, males become
a limiting resource for female reproductive success and females compete for males, with
males preferring larger, more ornamental females who produce larger clutches
(Rosenqvist, 1990; Berglund
et al
., 2006).
Sometimes there may be seasonal variation in sexual competition. Darryl Gwynne
and Leigh Simmons (1990) have shown how seasonal variation in food availability leads
to changes in sex roles in
Kawanaphila
katydids in Australia. When food is scarce, the
male's large protein-rich spermatophore is costly to produce and also very valuable to
females (Fig. 7.15). Females compete for males and males are choosy, preferring larger
females who lay more eggs. However, when pollen-rich grass trees come into flower,
Females may
compete for
males
Seasonal changes
in which sex
competes the
most in katydids
…
