Sometimes stated along the lines of "I haven't seen any stockpile of contacts with paw prints on them stating that animals have signed on to any contract for their rights". The assertion is that animals don't have the ability to assert their own rights and aren't in any agreement for their rights and therefore can't have any.
There was no point in time when anyone signed any contract to not be murdered. And it's not as though if you didn't sign such a contract that murdering you would be suddenly and magically ethical.
People are born with severe mental handicaps all the time. They have no ability to comprehend or sign such a contract. I don't see many carnists jumping to eat them, even though when cooked they are just as delicious as any other animal. Or what about North Sentinel Islanders? They will attack and try to murder you if you approach their island in the Indian Ocean. So in fact, not only did they never sign a reciprocal-morality contract, they would do quite the opposite.
So then, if you subscribe to this theory that only those who sign a contract somehow honoring your morality are allowed to live, you should start picking up some heinous habits regarding the killing of other people.
If you ask most carnists they will tell you that factory farming is wrong. But this would indicate that farmed animals have, then, some rights, namely to not be factory farmed. But what contract did they sign to get that right?
Furthermore, they would never hurt a dolphin, puppy, or other "cute" animal. That would be incompatible with this claim, because if animals have no rights without signing a contract, then baby dolphins would have no right to not get their fins cut off and be thrown back into the water to suffer and die, and stray dogs with no owner have no right to not be slow-roasted on a spit and slathered in barbecue sauce. So you go ahead and make this argument, and I'll warm up my baby-kitten shredder.
There's no situation in which you need to express your right to not be victimized by some perpetrator. Do you need to hang around your neck "Do not punch in face or any other location"? Do you need to put a sign on everything you own saying "Do not steal"? Maybe the idea is to claim everything you own by urinating on it like an animal because, after all, that's what lions do.