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[lalaloo73]

I'm taking an online nutrition class.  our current topic of the week is "are you a vegetarian?".  someone in the class posted a response with a really crappy attitude and incorrect information.  (read below)

"Humans are predators.  That is how our bodies are built, from the shape of our teeth to the placement of our eyes.  I for one completely

Comments

[tonights]

Well... I don't know that I would say "humans are predators," but humans are biologically opportunistic omnivores. We have teeth & a long intestine built to let us deal with meat along with plant matter, and we lack the digestive apparatuses common to ruminants and other strictly herbivorous organisms - so I'm also not sure what you mean about her information being wrong. Humans are classically omnivorous in all relevant anatomical areas. We do have the choice whether or not we want to consume animal protein, but we are certainly evolutionarily equipped to do so.

I don't think there's anything wrong with her attitude, either. She isn't forcing you to eat meat, just stating facts.

[tonights]

Oh, and your eye placement argument is incorrect. Predatory animals have binocular vision for stereopsis/increased perception/ease of chasing things down. Prey animals have eyes on either sides of their head to increase their field of vision. If what you're saying is true, humans would have eyes where their ears are.

[lalaloo73]

hmm. good point.

[shaydlip]

No, it is not incorrect.

ALL primates have forward facing eyes- and guess what, most are not meat eaters.

There's a considerable amount of debate on why stereoscopic vision evolved in the first primate groups, some thinking it's so that primates can navigate the 3D arboreal environment, so you would need it for depth perception for jumping etc. There's other people thinking that the first primate was a visual predator, a very very small animal like the tarsier who ate insects and small things like that.

So if we go with the visual predator hypothesis, then why do all primates still have stereoscopic vision? Wouldn't it then be maladaptive? Well, yes, but primates started being social so you could have more eyes for predators but keeping your stereoscopic vision.

[shaydlip]

First, yes all primates will opportunistically eat insects, but it is not their primary mode of getting the food they need. Once you get above a certain body weight, I think it's 500g or so, you need to add in other things to the diet. The same goes for going from fruit to leaves, there's a body size threshold there as well. You are more likely to evolve traits for your needs than your opportunistic snatches here and there.

Yes, chimps have been found to hunt, but it has not really been shown that they get a lot of their nutritional content. I suspect that they have more to do with community building and such. Does anyone know if bonobos hunt? I doubt it, but I could be wrong.

[shaydlip]

I suppose the question is the dependency factor. I just checked out on google scholar and it does look like you're right, at least with frugivorous primates. However, maybe its the locomotor person that I am, but I really doubt that stereoscopic vision, while at first it was for insectivory, it also became useful for other things. After all, koalas are not visual predators but they still have forward facing eyes.

[yipeeskipp]

Chimpanzees (but not bonobos) are known to hunt the red colobus monkey, but red colobus meat makes up 10% or less of their diet. Only the males hunt, and their reasons for doing so are still uncertain... possibly a "male bonding" event, possibly to offer sexy meat to females, probably something we haven't thought of yet ;)

Your argument about the evolution of stereoscopic vision in primates is sound. From what I've learned, it seems primates evolved convergent and frontated orbits to aid them in navigating forests, catching insects, and/or having the depth perception to accurately see and reach a piece of fruit against a background of leaves.

[irisheye]

"Only the males hunt, and their reasons for doing so are still uncertain... possibly a "male bonding" event, possibly to offer sexy meat to females, probably something we haven't thought of yet ;)"

Not so. See the recent discovery of tool use by chimps to hunt (article here). It's mostly females and juveniles who hunt this way.