Usually, sexual selection is regarded to be responsible for sexual size
dimorphisms in animals. Sexual selection can result either from mate choice
(= intersexual selection) or from direct competition between individuals
of the same sex (= intrasexual selection). In L. callipterus, additionally
natural selection mechanisms may be responsible for the evolutionary origin
and stability of the extreme sexual size dimorphism, because of the peculiar
breeding substrate they use. In a study I conducted, I examined the influence of
both, sexual and natural selection mechanisms on
the evolution of large males and small females. Additionally, I used a
life history model of sexual size dimorphism to test whether intrasexual
selection mechanisms via male-male competition and natural selection mechanisms
via fecundity advantages of large females could explain the extreme sexual
size dimorphism in L. callipterus.
To test if intrasexual selection would influence male size I
performed lab experiments with direct male-male competition for territories
and for access to females. Larger males were more aggressive than their
smaller companions and they could hold their territories for longer time
periods. Therefore, larger males could spawn with more females and sired
more offspring. In the field there also was an advantage of large male
size due to superior competitive abilities. Large males were able to defend
nests with more and larger snail shells and therefore, had a higher reproductive
success (number of breeding females) than smaller males.
To test the influence of intersexual selection on male body size
I performed female choice experiments in the lab, and measured the numbers
of breeding females of males of different quality in the field. Neither
in the lab nor in the field females were found to choose particular males,
despite a great variation of available males in size and body condition.
I concluded that intersexual selection seems to be only of minor importance
for the evolution of sexual size dimorphism in L. callipterus, while
intrasexual selection mechanisms clearly favour bigger males. I discuss
that it is unlikely, however, that this mechanism is alone responsible
for the extreme sexual size dimorphism in L. callipterus.
Natural selection mechanisms are usually limiting the evolution
of extreme body sizes in one or the other of the two sexes. In contrast
in L. callipterus, natural selection may operate in the same direction
as sexual selection, i. e. increasing the size dimorphism of the
sexes, but in opposite directions in males and females. The clue to understanding
the extreme sexual size dimorphism in L. callipterus is probably
the particular breeding substrate. Males construct nests of empty snail
shells, that is they must be able to manipulate and carry these shells.
Females enter the nests and breed in these shells, that is they must be
small enough to fit into these shells.
I tested whether males must pass a minimum body size to be able to carry
shells efficiently. Experiments in which male sizes and the sizes of shells
were varied revealed that male minimum size is indeed limited by their
ability to collect empty snail shells. Only males ³
9.0 cm standard length were able to carry Neothauma shells at all,
and larger males were able to carry the shells more efficiently than smaller