Why is it that (on average and for most) fish, bird, and reptile species the females are larger than the males, but in mammal species the males are larger than the females? (Again, I am taking generally, on average.)

Question

I probably shouldn’t have included reptiles on this list. I got my fishes and reptiles mixed up haha

In: Biology

Answer 1

Well I don’t think they are in reptiles, but tend to be in birds and fish definitely, insects also I think. There can be lots of different reasons for sexual dimorphism, where the sexes differ in size or other traits. Humans are slightly below average in terms of dimorphism. But generally speaking one common reason is that in species that lay eggs, or have lots of children, it’s an advantage for the female to be nice and plump, with a body capable of producing and harbouring lots and lots of children. That’s much less often the case in mammals, who’s strategy tends to be fewer children and more care, so no advantage for plumpness. Us mammals love having a good scrap over the ladies, rather than having nice patterns on us or making a lovely nest, which is one of the many reasons why there’s the advantage for men of being larger.