The expression 'dog eat dog' set me wondering; Does dog eat dog? How does the expression 'it's a dog-eat-dog world' fit with that? What animals are cannibalistic?
Dog eat dog is an expression used to describe a situation in which people will do anything to be successful, even if what they do harms other people. Some would say this is the same as 'survival of the fittest'; I don't agree that success and survival are equivalent.
Dogs do eat dog, particularly when (i) there's a severe shortage of food or (ii) there's a surfeit of puppies and related issues [1] (iii) when fed dog meat.
Dog meat is meat of dog, not meat specifically for dogs. wikipedia indicates It was estimated in 2014 that worldwide, 25 million dogs are eaten each year by humans. It is uncommon but not illegal to eat dog in the UK, while it is a commonplace in SE Asia (I did, while there). There are issues with that meat [5]. Not least, euthanised cats and dogs are in the food chain as dog meat, which means that so are the chemicals used to put them away. Of course, feeding an animal its own species short-circuits all sorts of biological safety measures and increases the chances of perpetration of disease.Clearly there can be issues when an animal is fed its own kind. This is how we developed mad cow disease, after all. This might be an essay topic for biologists as the issue is not at all clear-cut.
Animals prone to cannibalism [6] include chimps, rabbits, chickens and hamsters, but it is far more widespread than you might think. Of other types of animal [6], sharks, snakes, salamanders, frogs, earwigs, mantises and spiders. I read the reasons for rabbits and hamsters to be cannibalistic as much the same reasons as those given for dogs. I read the activity of sharks as much the same as for frogs; when there is localised crowding, even the siblings are food.
Wikipedia [7] writes a good deal more sanely on this than [6], pointing to records of this occurring widely, especially where there is a drastic food shortage. It is common in aquatic systems and in no way is cannibalism restricted to one class such as, say, carnivores.
You might be interested in the classification of cannibalism, so I'll give you the headings and point you to the wikipedia article at [7] again: sexual cannibalism, size-structured cannibalism, filial cannibalism, infanticide, intrauterine cannibalism.
As for human cannibalism, I discover there are two forms, endo- and exo-, depending upon whether the person being eaten is from within or without (outwith) the community. So eating the visiting missionary is exocannibalism, while eating the revered village elder is endocannibalism.
Example of bad taste in a PC world: One (red) Indian asks another' "Did Cannibal cross the scalps?" But if that offends you then the whole topic of cannibalism probably does too. I am personally indirectly aware of the famine during the cultural revolution (sorry, Cultural Revolution, 文化大革命), which produced the expression "Sell baby, make food", a mark of extreme desperation. For many Chinese, just recognising that such things occurred is not allowed, in which case, they shouldn't read about the Guangxi Massacre. But then I doubt such people recognise the Nanjing Massacre either, which was quite different.
Biologists might read further on prion diseases, which seem to be perpetuated (passed on, so to speak) by cannibalism.
DJS 20210618
top pic advertises the eopnymous 2016 film
[1] https://dogcare.dailypuppy.com/can-dogs-mastitis-treatment-3662.html
[2] https://www.quora.com/Can-dogs-eat-dog-meat It's Quora, so entries are sometimes unsupported opinion.
[3] https://en.wikipedia.org/wiki/Dog_meat#cite_note-Cz-10
[4] https://litigation-essentials.lexisnexis.com/webcd/app?action=DocumentDisplay&crawlid=1&doctype=cite&docid=21+Animal+L.+29&srctype=smi&srcid=3B15&key=d9029f1ccf66735779b5e3defa01b270 wouldn't load for me.
[6] https://animalogic.ca/blog/11-animals-that-are-prone-to-cannibalism
[7] https://en.wikipedia.org/wiki/Cannibalism but do also read the Talk section.
[8] https://en.wikipedia.org/wiki/Human_cannibalism