Biology Reference
In-Depth Information
defend small territories to which females are attracted for mating. However 16% of males
have white ruffs and tufts and their behaviour is very different; they do not fight but instead
act as satellites on the edge of territories and attempt to steal copulations while the fighters
are busy defending their territories (Hogan-Warburg, 1966; van Rhijn, 1973).
David Lank and colleagues (1995) reared chicks from eggs collected in the wild and
assigned paternity using DNA profiles. They also bred ruffs in captivity. These pedigree
data showed that male plumage and behaviour differences were genetically controlled
and consistent with a single locus, two allele autosomal genetic polymorphism. A third
male morph has recently been discovered (Jukema & Piersma, 2006); this has no ruff or
tufts and looks just like a female (though it is a little larger). Indeed, it is sometimes
mounted by fighter and satellite males. These female mimics occur at a low frequency
(1% or less of males). They have enormous testes (2.5 times the volume of those of
fighter males) and are likely to behave as sneakers.
Assessing the success of these three male strategies is a challenging task; satellite
mating success varies between leks and with the season. Furthermore, satellites may visit
several leks and a female may copulate with both a fighter and a satellite male, leading to
multiple paternity in half the clutches (Lank
et al
., 2002). So it is not yet known whether
the three morphs are maintained by frequency dependent advantage, or whether the
strategies persist because each does best in different places or at different times.
Three male
morphs in a
shorebird
A marine isopod with three male morphs
Stephen Shuster and colleagues studied a marine isopod crustacean,
Paracerceis sculpta
,
in the northern part of the Gulf of California. This breeds in the body cavities of an
intertidal sponge. Males come in three discrete morphs (Fig. 5.18b). These are
determined by a single autosomal locus with three alleles which influence male growth
and maturation rate (Shuster & Sassaman, 1997).
(i)
Large 'alpha' males
position themselves at the entrance of the sponge, with their
head pointing down into the cavity (the spongocoel) and their large abdominal
spines (uropods) sticking out as a defence. Females are attracted to spongocoels
with an alpha male and the male grabs them and feels them carefully before
allowing them to enter. An alpha male may accumulate harems of up to 19 females
in a single spongocoel and defends them against other alpha males who come and
try to evict him. The male mates with a female when she moults. The female then
broods her eggs and the young leave the spongocoel to feed on intertidal algae.
(ii)
Small 'beta' males
lack spines and resemble sexually mature females. They mimic
female behaviour and so trick the alpha male into allowing them to enter the
spongocoel. Once inside, they have a chance to mate with the females.
(iii)
Tiny 'gamma' males
invade spongocoels by stealth, using their small size and rapid
movements to rush through the entrance. Alpha males try to block their entrance or
to grab them and fling them away. However, gamma males sometimes sneak through
and once inside the sponge they, too, have a chance to gain matings with the females.
Each male morph
of a marine
isopod gains
equal success
In laboratory experiments with artificial spongocoels made from a synthetic polymer,
Shuster (1989) varied the number of females and males per spongocoel and then
