The Ability to Dissipate Heat Is Likely to Be a More Important Limitation on Lactation in Striped Hamsters with Greater Reproductive Efforts under Warmer Conditions

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The limitations on energy availability and outputs have been implied to have a profound effect on the evolution of many morphological and behavioral traits. It has been suggested that the reproductive performance of mammals is frequently constrained by intrinsic physiological factors, such as the capacity of the mammary glands to produce milk (the peripheral limitation [PL] hypothesis) or that of the body to dissipate heat (the heat dissipation limitation [HDL] hypothesis). Research on a variety of small mammals, however, has so far failed to provide unequivocal support for one hypothesis over the other. We tested the PL and HDL hypotheses in female striped hamsters (Cricetulus barabensis) with artificially manipulated litter sizes of two (three or four pups removed from natural litter size), five, eight (two or three pups added to natural litter size), and 12 (five to seven pups added to natural litter size) pups at ambient temperatures of 21° and 30°C. Energy intake and milk output of mothers, litter size, and litter mass were measured throughout lactation. Several markers indicating digestive enzyme activity and the gene expression of hypothalamic neuropeptides related to food intake were also measured. Food consumption and milk output increased with increasing litter size but reached a ceiling at 12 pups, causing 12-pup litters to have significantly lower litter mass and pup body mass than litters composed of fewer pups. Litter mass and maternal metabolic rate, milk output, maltase, sucrase, and aminopeptidase activity in the small intestine, and gene expression of hypothalamic orexigenic peptides were significantly lower at 30°C than at 21°C, and these differences were considerably more pronounced in 12-pup litters. These results suggest that PL and HDL can operate simultaneously but that the HDL hypothesis is probably more valid at warmer temperatures. Our results suggest that increased environmental temperatures in future climates may limit reproductive output through heat dissipation limits.