This is an argument against veganism that, thankfully, seems to have waned in popularity in recent decades, but is still sometimes advanced by those dogmatically speciesist and vehemently anti-vegan types who are not afraid to sacrifice accuracy or forfeit respectability in order to defend their ideology. What lies at the root of this line of reasoning is the absurd assumption that someone's intelligence is somehow a legitimate criterion for moral consideration and that those who fall below a certain intelligence threshold are ipso facto unworthy to be included in the moral universe of those who have declared by fiat that they are smarter than the rest of the animal kingdom.
Despite my best efforts to comprehend this position, I have never understood why anyone would think that intelligence is a legitimate criterion for moral consideration. It strikes me as a complete non sequitur. Where the right to avoid unnecessary suffering is concerned, one's degree of intelligence is almost as irrelevant and arbitrary a criterion as one's means of locomotion, sleeping habits, or lung capacity. If intelligence is the sine quo non for moral consideration, then what status should we confer upon those millions of developmentally disabled Homo sapiens whose intelligence will never surpass that of a full-grown elephant, chimpanzee, or dolphin? If one accepts that intelligence is the magic ticket into the world of moral consideration, then cognitively impaired human beings should be fair game for anyone who is inclined to objectify, commodify, exploit, or enslave them. And yet, this obvious implication of the “dumb animals” argument is never conceded by those who claim that an animal's intelligence should determine his or her legitimacy as an object of moral consideration.
Another obvious flaw in this assertion is the inarguable fact that many non-human animals are not dumb at all. Ravens and crows are remarkably clever creatures, as are many parrots, cephalopods, cetaceous mammals, and great apes. And with respect to intelligence, most people don't give so-called "farm" animals nearly enough credit either. A common myth concerning domesticated turkeys is that when it rains, they will look up in utter fascination, beaks agape, and drown as a result. There's not a shred of truth to this filthy slander against turkeys and the myth is thoroughly debunked by the sensible folks at Snopes.
And the intelligence of pigs and chickens is also greatly under-rated. An adult pig is smarter than a three year-old human child and some chickens have displayed the ability to perform arithmetic. So, the "dumb animals" argument fails on at least two fronts: first, by establishing intelligence as an arbitrary and irrelevant criterion for moral consideration, and second, by failing to recognize (or simply ignoring the fact) that human beings do not have a monopoly on intelligence.
I will give the last word on this subject to the late British philosopher Jeremy Bentham, who exposed the callowness of the “dumb animals” argument exquisitely in his 1789 treatise, Introduction to the Principles of Morals and Legislation:
The French have already discovered that the blackness of the skin is no reason a human being should be abandoned without redress to the caprice of a tormentor. It may one day come to be recognised that the number of the legs, the villosity of the skin, or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same fate. What else is it that should trace the insuperable line? Is it the faculty of reason or perhaps the faculty of discourse? But a full-grown horse or dog, is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day or a week or even a month, old. But suppose the case were otherwise, what would it avail? The question is not, Can they reason? nor, Can they talk? but, Can they suffer?
Correction: Bentham gets the penultimate word. For the sake of consistency, the final word goes to me: Your argument is invalid.