Why are females more powerful and larger than males in certain species while in others they are smaller and weaker?
In: Biology
There’s nothing inherently about being the male of a species that means you need to be larger than females, it’s just something that has become the norm in mammals possibly as a result of females nursing infants and taking on a more nurturing role vs a more protective role that males have adopted
Because evolutionary biology found reasons that males evolving to be smaller and/or females evolving to be larger, suited that species’ specific biological needs.
For example, in many species, the males’ ONLY social/biological role is to fertilize the eggs of the female. In some species, their biological role is to fertilize the eggs and then be EATEN to provide nutrition to sustain the female through laying/guarding eggs, etc. It is what it is.
In order to determine why any specific species evolved the way that it did, you’d need to pick a specific one, and then analyze that species for what biological advantages come from the size of the males and females within that species.
This might get flagged as “low effort”, but the honest answer is “why wouldn’t they be?”
It reminds me of that anecdote I heard on the show QI once where Stephen Fry (I believe) asked why the two best sports fishers at the time were women. “Why not?” was the answer.
It is a very ‘male centric’ way of looking at things when we feel the need for an explanation when women (or females in case of other species) are not bigger, better, faster, more, but the honest answer is while in the species “Homo sapiens” males tend to be physically bigger and stronger than the females, there is absolutely no reason why this would be the case in, say, spiders, parakeets or cuttlefish.
Because sperm is small. A larger female can bear more eggs, and therefore produce more offspring in her lifetime. The size of the male isn’t really relevant to how many sperm cells he can produce in his lifetime, so there is no benefit to being large.
There is actually a species of wasp that has been documented selecting the sex of the egg she lays based on the size of the (fruit? Beetle?) she is laying the egg in. Larger food sources get the female eggs, while smaller ones get male eggs.
In birds of prey, the female is often larger. She has greater energy expenditure, producing and laying eggs and all, so she’s bigger. With the male being smaller, the pair hunt for different strata/size of prey. So when they hunt to feed the chicks they are not directly competing for the resources, and can each bring back food.
As others have said, you have to get more specific, but to phrase it in a more specific way you can ask: “In humans why are males on average larger and stronger than females.”
And I’d say a likely reason is that a 9 month gestation period is quite long relative to other animals, so that would lead to a lot of sex role differentiation before modern civilization.
Caveat: This type of evolutionary biology can slip quickly into pseudoscience. By nature you’re taking a conclusion and working backwards, but there are I think more and less plausible guesses.
In species where males are bigger than females it’s because the male is used to competing with other males of his species for territory and status among males for females.
Because the female reproductive system takes a massive amount of space, especially for animals that lay a lot of eggs at once.
With sparrowhawks it is because the males hunt in summer when the females are on the nest, and all the leaves on the trees mean they have to be more maneuverable. In winter, the female hunts and has more success with speed over agility.
That’s just one example of how soecific, local and *adapted* the reason can be. Different reason in every place and for every situation.
You see this in a few mass egg laying creatures – a bigger female means more eggs and likely healthier eggs, so more offspring. The make sex cells are smaller and less energy intensive to produce, so there’s less benefit to them being bigger.
In these species if there isn’t an outside evolutionary pressure on the male to be bigger than the female, due to however the local ecosystem works, then the female of the species can wind up being larger.
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