In recent post I wrote about the new research regarding the thylacine’s size, I mentioned that maned wolves might violated the “costs of carnivory” rule, which states that predatory mammals that weigh more than 21 kilograms (46 pounds) must haunt larger prey sources to survive.
Maned wolves do exceed this size, but their diet does not consist of large prey. They are not a threat to ungulate livestock. They take only small prey, such as rabbits, rodents, and small birds. They could be a threat to chickens and other poultry, but they aren’t cattle killers.
On a superficial reading of their ecology and diet, one would assume they would break this 21 kilogram rule. The largest ones do get to around 23 kilograms, and if they are that large, then they surely break this “costs of carnivory” rule.
But they don’t.
The reason is they have a most unusual diet for a canid. Between 40 and 90 percent of their diet can consist of a single fruit called a lobeira or “wolf apple.” The average diet of a maned wolf is around 50 percent vegetable matter, which means they aren’t as bound by the rules of carnivorous diets as other mammalian predators are.
The maned wolf first appeared in the fossil record in what is today the Desert Southwest what is called the Blancan faunal age (late Pilocene to early Pleistocene).
It entered South America, along with a whole host of other canids, and it evolved to a specialist niche as a grassland predator. Many species of similar-sized dog were also diversifying in South America, it is likely that it evolved its unusual diet as a way of avoiding competition with more carnivorous canids.
So vegetarian are maned wolves that when fed a typical wild carnivoran diet in zoos, they often develop bladder stones. Their kidneys cannot absorb a particular amino acid called cystine, and the excess cystine turns into stones.
Most mid-sized canids are true generalists in their diets. The exceptions are the maned and Ethiopian wolves. The Ethiopian wolf runs between 14-19 kilograms, so its rodent specialized diet does not violate the rule.
But the maned wolf’s heavily frugivorous almost takes them out of the predator guild entirely. They are as almost omnivorous as most bears are, and all extant bear species exceed 21 kilograms at maturity.
So maned wolves don’t violate the costs of carnivory rule. They do so, because they are far less predatory than virtually any other dog species. They are certainly less predatory that other dogs of their size.