Geographic variation in some reproductive characteristics of diurnal squirrels. Bulletin of the AMNH ; v. 122, article 1

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New York : [American Museum of Natural History]
"The discovery is reported of a third line of evidence which separates the large tribe of Oriental tree squirrels Callosciurini from the Holarctic members of the tribe Sciurini and also associates the widely disjunct Bornean tassel-eared squirrel Reithrosciurus with the Holarctic Sciurini. The third line of evidence is the number of pairs of functional mammae, which are four in the Holarctic Sciurini but only two or three in the Callosciurini. Data in the new line of evidence are presented for samples of 34 of the 37 genera of the subfamily Sciurinae of diurnal squirrels. Their significance to the interrelationships of the eight tribes and their constituent subtribes and genera and occasionally lower categories is evaluated. In general the new line of evidence is equivocal or supports the latest classification of the subfamily Sciurinae. It also suggests some minor refinements. For example, within the large Oriental squirrel subgenus Callosciurus a previously unsuspected division of the species is discovered that separates grossly the Malaysian from the Indo-Chinese species. The species of the Indo-Chinese Subregion have two pairs of mammae; those of the Malaysian Subregion, generally three. In addition to being of systematic value in its geographic variation, the number of pairs of functional mammae is shown to vary with latitude. The number of pairs generally decreases from boreal and temperate climates to tropical. Data from the literature on the size of the brood are presented which reveal that it is correlated with a reduction in mammae from temperate to tropical climates. In the tree squirrels as a whole and in the ground squirrels as a whole, there seems to be a strong trend of larger towards smaller broods from the north temperate climates to tropical climates. This trend corresponds with the decrease in the size of the clutch in birds from the north temperate to tropical climates, but in squirrels the trend seems also to be correlated with a meristic morphological difference--the number of pairs of mammae. No clear answer is at hand for an important question concerning evolution: Do the squirrels of the temperate climate with more mammae and larger broods actually produce more young per year? A definite north-to-south increase of broods from one to two broods per year is found in Nearctic ground squirrels. Data from Funambulus pennanti in India and three species of Callosciurus in Malaya hint that the females of tropical species may rear three or four broods in a single year, instead of one or two as do their relatives in more northern latitudes"--P. 29.
32 p. ; 27 cm.
Includes bibliographical references (p. 29-32).