Abstract
In his breeding experiments with sheep at Beinn Breagh, Alexander Graham Bell 2 found that as the number of nipples of the ewes increased there was a tendency towards a more frequent production of twins and triplets. Regarding this point he says (loc. cit., p. 383): “The indications are that our six-nippled stock will ultimately turn out to be twin bearers, as a rule, when they become fully mature.”
In reporting a case of unusually high, and probably inherited, fertility in the cow I have 3 noted that the individual exhibiting this high fertility bore two supernumerary mammá.
There is an obvious teleological aspect to this matter. In a general way it is clear that as the number of young born in a litter increases there must be a compensating increase in the number of mammá, unless there is some peculiarity of feeding habits in the young which would nullify the advantage, not to say necessity, of having enough nipples to “go around.” It would seem, a priori, that natural selection should have operated to bring about a high correlation, both intraand interracial between these two variables, size of litter and number of mammá of the dam. The purpose of this paper is to present the results of a study of interracial correlation between number of mammá and size of litter.
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