why-animals-do-the-thing:

@tauntedoctopus asked if opportunistic carnivory means that herbivores that engage in it are actually omnivores. 

I would say no, because opportunistic carnivory is a behavior, where as specific diets are an evolutionary development. Just because an herbivore will occasionally eat a small animal that happens to be directly in front of it’s face or scavenge a carcass doesn’t mean that the nutritional needs or digestive processes of that animal has changed compared to another member of the same species whose diet is entirely meat-free. 

There’s no solid explanation for why so many herbivores have been seen eating eat meat, but here’s the one I’ve seen that I think makes the most sense:

“A herbivore that is willing to occasionally eat meat has the potential to either exploit an available niche as a predator, or to survive a brief period of starvation by eating meat when plants are scarce. For example, a prolonged drought may cause many herbivores to weaken and die, creating a scavenging opportunity for the survivors. Consuming meat from time to time may be a way for a species to maintain an option on carnivory, which may be the difference between extinction and survival in an emergency. (Source)”

Meat is also fairly energy-rich, and, assuming you can digest it at all, it’s worth grabbing a chunk of meat that’s held in front of your face. Herbivorous fish will happily scavenge from corpses, for example, even those that primarily feed on algae. 

I can’t think of any vertebrates, short of a few that eat only one plant species (i.e. koalas) that won’t nibble meat or insects if given an easy chance.