I know insects do this, and some species of birds, but for mammals is there anything that a mammal can eat (that occurs naturally) that will render their flesh distasteful or dangerous to eat, or is a mammals body incapable of accumulating toxins or poisons like that?
I don’t know if this is what you are looking for but Polar Bear livers have toxic levels of Vitamin A.. It gets passed up the food chain from marine algae. I suppose you could feed one another diet that makes it not toxic.
Distasteful is easy. Almost anything with volatile compunds will make flesh distasteful, at least in the short term. Eucalyptus oil and onion/garlic are two obvious sources of contamination that taints flesh. Of course whether you view these things as distasteful is a personal prefence, but many people do. Seafood is another problem contaminant. In that case the oils impart a distinctive flavour to the flesh, especially of non-ruminants like pigs and chickens. Once again this depends on whether you would find faintly tuna flavoured bacon and eggs distasteful, but it has been a problem with commerical egg and pork production in places.
In terms of toxicity the easiest way to ensure contaminition is to have the animal graze over land with high heavy metal levels. The metals gradually accumulate in the flesh. It’s slow, but it will produce a poisonous mammal, and because heavy metals tend to bioaccumulate the dose can easily be fatal to regular predators while being harmless to the grazer.
There are naturally occuring organic compounds that can make an animal poisonous, but AFAIK they would all be fatal to the animal that accumulated them in short order, so they don’t really count.
I’m not sure that polar bear livers should count. Any carnivore that preys on other carnivores can accumulate poisonous levels of retinol. It’s not exclusive to polar bears but also applies to cats and dogs and presumably any other carnivorous mammal. I’ve seen it suggested that humans could accumulate concentrations that would toxic to the humans. So it’s really more of an overdose issue than a true toxicity.
Many years ago a neighbor had to destroy a couple of beef cows he was raising, they had eaten an unknown amount of tansy ragwort. Besides making the cows sick, it made their meat inedible. For years the state promoted tansy elimination and pointed out it was the propery owners obligation to destroy it if found or you could be fined. You see the stuff everywhere now days.
I have heard that if you eat enough garlic, mosquitoes won’t bite you because you don’t taste good. I’ve never tried it, though.
…just a little bit of plutonium…