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.)

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

In: Biology
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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.

Some animals employ a quantity over quality offspring strategy. To produce more offspring, the females need to be larger to hold the offspring (usually in the form of eggs).

Mammals typically employ a quality over quantity offspring strategy. Nursing an offspring with milk requires the mother to be present. Since each female member of the species doesn’t produce many offspring, the best strategy for a male is to have many breeding females. Mammal species tend to have larger males because the toughest, strongest males are able to claim and protect a larger number of breeding females.

Generally speaking being big is harder. Needs more food, more energy etc. So you need a benefit.

If a female is mass producing via lots of eggs, then the bigger the better because the bigger she is, the more eggs you lay, the better the chances your eggs reach adulthood and mass produce enough eggs for one of those to reproduce. (People think of it as trying to have kids, you really need to be trying to have grandkids).

Males, in that scenario don’t need to be big, they just need to get someone pregnant. So why be big if you just need to walk up and put on netflix…. unless… unless you’re there with your netflix log in and your bottle of wine and fucking Brian is there with his fucking netflix code. Then you have to beat up Brian…. and the bigger you are, the better chance you’ll beat up Brian.

So, in mammals, you’re not laying eggs, so you’re not having more kids if you’re bigger. So you don’t really get the same drive to large females, and so when the male competition drive still exists, you get bigger males.

Now its complicated, because there are multiple drivers at work. So you have say, reptiles like crocs which lay more eggs if they’re bigger, but generally males are larger, because a larger male will breed with more females.