Various creatures have different dietary requirements and different tolerances for various substances. For example, felines need taurine and react poorly to citrus, while humans need citrus (some source of vitamin C, anyway). Onions, grapes, chocolate and alcohol are bad for various animals. Birds aren't affected by capsaicin.
This makes me sad.
Let's say I am a brilliant geneticist / wizard / supreme being. Without resorting to permanent changes to the laws of physics / ongoing magic, and without making changes to their normal diets, would it be possible to change all mammals so that they can safely eat any food that humans can eat? Does biology somehow necessitate that an animal eating a certain diet, or just certain animals, must be susceptible to certain substances that are (mostly) non-toxic to humans?
- If it helps, you can ignore bats and anything that lives mainly in the water.
- I'm not talking about eating a human diet in entirety; e.g. cats would still be obligate carnivores and I don't mind if they still need taurine, horses would still mainly eat gress. I just don't want my cats/horses to die if they consume a modest amount of chocolate.
- They don't need to eat human quantities of food; that would be silly. Similar average lethal doses (mg/kG) is fine.
- This isn't about chewing or digesting human food. Yes, carnivores may not want to eat much grain because their gut won't be able to break it down well enough, but they should be able to enjoy an occasional chocolate martini.
- They shouldn't be subject to cumulative poisoning from any substance that is not a cumulative poison to humans.
Edit (thanks to John for a fascinating discussion!): I can live with not accounting for non-"linear" scaling effects being a problem for really small animals. I'm also meddling in sizes, such that my "mice" are closer to maybe 20 kg and my "elephants" are closer to 500 kg. Don't think too hard about the additional effects from this, it's just to say that I can mostly ignore the toxicity problems to the extent that raw size is the main cause.