A vegan diet is commonly thought of as being animal-friendly.

How, then, could a vegan diet wipe out an entire species of large mammals? The answer lies in who is practicing that diet.

Sure, when humans eat only plants, its good news for other animals. Plus, we humans have the distinct evolutionary advantage of a pre-frontal cortex and opposable thumbs, which, when combined, allow us to conceptualize and construct alternatives like meatless, bleeding burgers. But when other mammals begin to rely solely on plants to survive, it may not be the best thing for their species.

According to a recent study published in the Journal of Quaternary Science, a now-extinct bear called the cave bear, a close relative of the brown bear, once adhered to a very €œstrict€ vegan diet and that ended up being its undoing. By looking at the collagen composition of 25,000-year-old bear bones, the authors of the study were able to establish that cave bears ate no fish, mammals, or insects.

€œWe were particularly interested in what exactly the cave bears ate, and whether there is a connection between their diet and their extinction,€ Dr. Hervé Bocherens, one of the authors of the study, told PhysOrg. €œAccording to our newest findings, these extinct relatives of the brown bear lived on a strictly vegan diet.€

This clue about the cave bears food intake gave the team of scientists a possible explanation for their extinction. Unlike its distant cousin the brown bear, who are omnivorous and eat berries, mushrooms, and the occasional human, cave bears may have been a little too selective for their own good. Still, cave bears, who apparently didnt even live in caves, would have inhabited Europe some 400,000 years ago and survived on this rigid diet until 25,000 years ago, when they couldnt keep up with a changing environment.

€œSimilar to todays giant panda, the cave bears were therefore extremely inflexible in regard to their food,€ researchers wrote. €œWe assume that this unbalanced diet, in combination with the reduced supply of plants during the last ice age, ultimately led to the cave bears extinction.€

Their assumption is not a stretch. Were no experts on mammalian diets but it seems like a diet made up of only plant-based foods might not be substantial enough for a half-ton bear€”its not enough for some humans.