Biology Reference
In-Depth Information
Fig. 9.13
Sex role reversal in birds. This male African jacana performs all the parental
duties. Females are larger than males and compete for males by defending large
territories. Photo © Tony Heald/naturepl.com.
larger and more brightly coloured and females competing for males to incubate their
clutches for them. In phalaropes (
Phalaropus
spp.) a female defends one male, lays a
clutch for him to incubate and then goes off to find a second male (sequential polyandry;
Reynolds, 1987). In spotted sandpipers (
Actitis macularia
) and jacanas (Fig. 9.13) a
female competes to defend a large territory in which she may have several males
simultaneously incubating her clutches or caring for her chicks (resource defence
polyandry).
Why should some shorebirds show such sex role reversal? A likely explanation is that
shorebirds are characterized by a small clutch size, which never normally exceeds four
eggs. The four eggs are large and fit snugly together and experiments suggest that
they represent an incubation limit: adding an extra egg reduces hatching success. If
shorebirds are indeed 'stuck' with a maximum clutch of four, selection may particularly
favour female desertion because, with a fixed clutch size, the only way females can
increase their reproductive output if conditions become more favourable is to lay more
clutches. In the spotted sandpiper, productivity on the breeding grounds can be so high
that the female becomes rather like an egg factory, laying up to five clutches in 40 days,
a total of 20 eggs, which represents four times her own body weight. Her reproductive
success is no longer limited by her ability to form reserves for the eggs but rather by the
number of males she can find to incubate them. This has led to the evolution of sex role
reversal with females being 25% larger than males and females competing to gain as
many mates as they can (Lank
et al
., 1985).
In jacanas, too, the female is larger than the male and her territory may overlap the
smaller territories of up to four males (Butchart
et al
., 1999a; Emlen & Wrege, 2004).
Some shorebirds
show sex role
reversal: females
compete for
males
