eli5- Why do insects and arachnids like a praying mantis and spiders kill the one they mate with?
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Nature is not about survival of the INDIVIDUAL. It’s about survival of the SPECIES. And, to be really clear, it’s about whatever patterns didn’t sufficiently harm “survival of the species” to cause that species to be unsuccessful.
In these two cases, what has worked *sufficiently well to ensure further generations* is when the female eats the male of that species after mating. It gives the female a ton of super convenient instant calories and nutrients that can go into the development of her eggs.
The male? Well, yeah, he could have mated with another female later… but it was not necessary as a pattern for the entire species’ survival that a single male mates multiple times.
So it might be SUPER suboptimal for the male, and it might be SOMEWHAT suboptimal for the species, but it was not suboptimal enough for that species to be unsuccessful and go extinct.
Dudes becoming lunch just didn’t harm things enough to prevent dudes from becoming lunch.
Food. Growing and laying eggs takes a lot of energy. Most insects only mate once, and if the male dies afterwords it doesnt affect its ability to pass on its genes, so the evolved strategy is for the female to eat it and use that energy in making sure the offspring successfully develop and repeat the cycle.
These are insects that are solitary predators, they don’t have friends or family, there is only threats that are larger and prey that is smaller.
When the time is right they have a mating instinct that compels them to find a male and procreate. Once that’s finished, the mating instinct is over and the male better scoot because he’s smaller and might be prey when the hunting instinct turns back on.
There’s not much deep thought going on here.
The animals that do this don’t only do it after mating, they’re frequent cannibals whenever they catch their own juveniles or smaller adults.
It’s common in some insects because the females are much larger than the males. The females must carry the eggs and need to be more survivable to carry on the species. The males don’t do much except get in and out real fast, so it’s not detrimental to the species if most of them eventually get cannibalized after passing on their genes.
Many of these types of cannibalistic insects and spiders are opportunists who eat whatever they can catch and usually stay away from each other except to mate. Even baby spiders run from and eat each other upon hatching. Cannibalism actually isn’t that common in mating unless the female is starving.
For spiders, their life cycle and reproductive cycle come into play. Spiders like the Black Widow don’t mate throughout their lives like humans and other animals. They spend their whole lives hunting to survive and grow to maturity, and they reach sexual maturity at the end of their lives. During or shortly after mating, the male spider will die regardless of whether he’s eaten by his mate. If his mate is short on nutrients to reproduce, she may eat him.
Praying mantis can mate multiple times, and they do- unless a male mates with a starving female. Again, not common in the wild. It’s generally been observed in captivity when a scientist deliberately introduces sexually mature males to sexually mature, starved females.
I’ll explain this the way my Biology teacher did to me:
The male will do bugger all to raise the children. He’ll just run off and mate with a load of other females, and then run for the hills.
Laying eggs, however, takes a lot of energy.
So to maximise the number of eggs, and thus offspring that survive to reproductive age, the female eats the male, and uses the energy to lay more eggs. And this carries on because females who eat the males have more surviving offspring than those who don’t, so the next generation will have more females that eat the males.
Insects and arachnids engaging in such behavior could be rooted in evolutionary strategies to ensure successful reproduction, where sacrificing one partner might provide the other with valuable nutrients for producing healthy offspring.
This is known as [sexual cannibalism](https://en.wikipedia.org/wiki/Sexual_cannibalism) (which is a great band name).
Many hypotheses exist as to why it evolved. Two basic ones are that either:
1. The females are simply hungry and if the closest ready food source happens to be the male they just mated with, so be it.
2. The males are “willingly” (it’s hard to ascribe agency to instinctual behavior) giving their lives to provide nutrition to the female, which increases their offspring’s chances of surviving. After all, in lots of species the males simply die after mating, so why let those calories go to waste?
it’s not personal, you personalize it cos you’re human. they need energy and can eat them to survive and so they do. it’s more about efficiency of the species continuing. he’s done his bit, no reason to keep him around wasting resources.
Because the purpose of the genes is to procreate, and we evolved into a binary system a long time ago. The animal kingdom has evolved multiple binary strategies. Humans pairbond, the man gathers resources and the woman nurtures. Lions have multiple females in a pride and protects against other aggressive males. Certain insects capture the mate and consume it, since they are solitary animals the impregnation and consumption are all the male can do for the female.
So a praying mantis newlywed couple are in their hotel room about to do the deed when she asks her husband “Do you know what’s going to happen tonight?’
He says “No, I never met my father, and my mom doesn’t talk about it.”
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