John Locke (1632-1704), the English philosopher, is regarded as one of the most influential thinkers of the Enlightenment. Thomas Jefferson, for example, placed him on the highest rung of genius: “Bacon, Locke and Newton... I consider them as the three greatest men that have ever lived, without any exception” (letter to Richard Price Paris, Jan. 8, 1789). Through Jefferson, Locke's ideas shaped the Declaration of Independence.
What are we to think then when an individual with such a forbidding intellect claims to have seen a cat-rat hybrid? In An Essay Concerning Human Understanding (III, Ch. 6, 23), Locke gives the following brief, but eyewitness, report:
If in making this claim Locke was correct, the sire in the cross would almost certainly have been a black rat (Rattus rattus), since at that time the brown rat (Rattus norvegicus) had not as yet reached Britain.
In the next century, the French naturalist Jacques-François Dicquemare (1733-1789) refers to what is probably the same case (Dicquemare 1778, p. 213):
Is such a thing possible? The easy answer is “No.” But an honest one is that we simply know very little about the factors governing the ability of such disparate animals to hybridize. There is enough evidence that cats hybridize with rabbits that many people are convinced, which is a rather similar cross to the one under consideration here.
So who knows? Perhaps a rat gamete and a cat gamete can on rare occasions combine and sort things out sufficiently to produce a viable organism. But any definite conclusions on this point will have to await firmer evidence. In the meantime, it can only be said that the evidence supporting this cross is meager indeed (as indicated by the reality thermometer at right).
And then there's the question of whether a rat is physically capable of covering a cat. Certainly, it's true that rats and cats raised together get along well, and can even be snuggly friends, as can be seen in various YouTube videos.
Indeed, as surprising as it may seem, it's known that tame cats will on occasion nurse baby rats (as shown in the video below), a significant fact in the present context; most mammals (and birds) imprint on the type of animal that raises them, that is, when they reach sexual maturity, they will choose to mate with that kind of animal even if that kind is not their own (read a discussion of imprinting elsewhere on this website).
Accounts of such behavior seem to be nothing unusual in the older literature. In closing, I'll quote one such case here (from Bingley 1820, pp. 247-248):
The following is a list of reported cat crosses. Some of these crosses are much better documented than others (as indicated by the reliability arrow). Indeed, some might seem completely impossible. But all have been reported at least once. The links below are to separate articles. Additional crosses, not listed here, are covered on the cat hybrids page.
By the same author: Handbook of Avian Hybrids of the World, Oxford University Press (2006).
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology