Are there any species who, as a matter of normal behavior, eat members of their own species for nutrition? I don’t mean things like lions that kill other lions’ cubs, or praying mantis females that bite their mates’ heads off, or the odd cannibal tribe of humans, but species that are prey and predator to themselves.
Yes, many lower life forms do this. Obviously, it’s never a closed system, but it really helps weed out the competition and “reclaim” nutrients for the species. Heh.
Oh, sure. Tropical fish hobbyists sometimes grow guppies as food for the predator fish. There are special structures they have to put into the tank, so the guppy young’uns can hide from their own predatory parents. The same is true for several wild fish, too. Chickens will eat their own, in a brutal custom we call a pecking party.
You’re looking for the word cannibalism? I guess humans are the only ones enjoying cannabis 
Chimps seem to take a bite of each other sometimes, where cannibalism seems to be an inter-unit-group phenomenon and may be an extension of infanticide, which may function in changing “mother” into “female. (abstract )
Just like lion males will kill the cubs in pack he takes over, more here .
I have heard (from a not particularly citable source) that the final fate of nearly all sharks is to be cannibalized by other sharks.
Okay, the source was an Aquaman comic book. But it still makes perfect sense!
And I didn’t read the question… you may give me a wedgie.
I recall reading about a species of salamander, a certain percentage of which grows larger than the rest of the population and makes a habit of eating them.
I was going to mention rodents who eat their young, but I’m not sure that fits the OP.
But I do actually have an actual example to your question, there is one seaslug (wohoo) that is a predator on other seaslugs, including individuals of the same species .
I think the answer to this question depends on how nutritious pan fried semen is.
Because if that’s the case, my ex-GF was a flatout cannibal. ::rimshot:
On an serious note, dont some female spiders eat the male after sex, presumably for nutrition?
I think it does. At least as well as the fish noted earlier that routinely eat their own fry.
I got this from a PBS Nature-type special. In the US SW desert, there’s a toad species that has to go from hatched eggs to adults very fast during wet weather. The base food supply is fairy/brine shrimp things.
The eggs hatch into two types of tadpoles. Most are small little guys. A few are bigger. The little guys eat the shrimp. The big guys eat the little guys. Once all the little guys are gone, the big guys eat each other until maybe one or so is left per puddle. It morphs into an adult, digs deep into the mud and waits till next year.
I also saw a show about a “year in the life” of a intermittent river in SW Africa. As the river dries up into pools, the crocs and such get quite crowded. After all the lesser species are eaten, the big crocs eat the little crocs, etc. until there is only one left (called the Highlander!). Not especially designed for cannabilism like the above, but it’s eat or be eaten.
I’m surprised snakes haven’t been mentioned yet. IANAH (I am not a herpetologist), but I distinctly remember a Discovery channel broadcast which mentioned a few species of snakes that regularly eat their own species. Unfortunately, I don’t remember exactly which species they were.
Okay, the way the OP is worded the Black Widow spider might be out because she consumes the male after sex…but the Black Widow might still be in because her final act is being dinner for her young.
Well sharks do eat their own siblings in the womb.
Octopi* are generally considered extremely opportunistic in their feeding: if they can catch it, they’ll eat it. Including other members of their species.
*Yes, I know that there’s debate about the proper plural for octopus, but I’m just used to octopi, and that’s what I was taught, and that’s what I’m using.
Tiger salamander larvae have two morphs, one of which eats mostly invertebrates, and a larger, faster-developing “cannibalistic” one that eats mostly other salamander larvae. This kind of behavior is fairly widespread in a variety of organisms, such as the spadefoot toads mentioned previously,
Lots of fish species eat their siblings when they are young; particularly predatory types. Even herbivorous fish like grass carp will chow down on their brothers and sisters when they are a few weeks old, which is why so many species need to be graded so nobody’s big enough to swallow their neihbor whole. I once netted about 20 northern pike fry 1-2" long from a lake and held them in an aquarium. Within a month I had 1 big fat pike left.