The sciurid tribe Marmotini has a distinctive, highly specialized reproductive profile characterized by annually produced litters of many offspring, each of small mass and fast growth rate. However, not all genera of marmotines follow the reproductive profile of the tribe. In fact, included in the tribe are squirrels with the highest and lowest energetic investment into reproduction of the entire family. Because of its large litter size, Ammospermophilus has the largest energetic investment into reproduction of nearly all squirrels. Also, Ammospermophilus is not limited to 1 litter per year. At the opposite extreme, Marmota has the smallest energetic investment into reproduction of all squirrels. Of the other 4 marmotine genera, reproduction in Cynomys is similar to that of Marmota, whereas that of Tamias is similar to that of Ammospermophilus; reproduction in Sciurotamias and Spermophilus may reflect their phylogenetic positions. Litter size in the basal Sciurotamias (2.5) is the lowest for all marmotines and is closest to that of other squirrel taxa. Consistent with the probable paraphyly of the genus, spermophiline data are the most variable and many reproductive characters sort along the probable clades within Spermophilus. For spermophilines, a litter size of 4 is probably basal and the larger litter sizes of 6–8 in some clades are derived. Two other derived features are the low offspring biomass in Marmota and a short time between conception and weaning in a spermophiline clade of predominantly New World species. Overall, reproductive investment within marmotines has followed 2 patterns: the larger marmotines (Cynomys and Marmota) have a reduced annual energetic investment compared with a higher reproductive investment in the smaller marmotines (Ammospermophilus and Tamias). Thus, body size is a key aspect in the diversification of reproductive patterns in the Marmotini.
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