Hi everyone !

 

I would like to know why the sexual dimorphism is reduced when the species are monogamous and important when the species are polygamous ?

 

Many thanks in advance

 

Ps: It's not a homework question but a in case of question exam

I realize the anser was on wikipedia after reading carefully... Sorry for the disturbance and for the people interested by the answer:

 

A frequently studied type of sexual dimorphism is body size. Males typically have larger bodies than females. In some species, however, females have larger bodies than males. Sexual dimorphism in body size has been linked to mating behavior. In polygynous species, males compete for control over sexual access to females. Large males have an advantage in the competition for access to females, and they consequently pass their genes along to a greater number of offspring. This eventually leads to large differences in body size between females and males. Polygynous males are often 1.5 to 2.0 times larger in size than females. In monogamous species, on the other hand, females and males have more equal access to mates, so there is little or no sexual dimorphism in body size.

In a truely monogamous species there is much less sexual selection on males almost every male can find a mate.

The matter is (as usual) quite a bit more complicated. In birds, for instance many polygamous species are monomorphic and certain sexual dimorphisms correlate better e.g. with parental care (Owens and Hartley 1998 Proc. R. Soc. Lond. B 7 vol. 265 no. 1394 397-407).