Evolution of parental care inPhodopus: Conflict between adaptations for survival and adaptations for rapid reproduction

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Abstract
SYNOPSIS. Two species of dwarf hamsters are used to develop a model of the proximate and ultimate selective forces leading to the evolution of parental care strategies. One species, Phodopus campbelli, has been studied extensively as a socially monogamous, biparental species; the other, P. sungorus, rears its young withoutmale assistance. The cold, arid habitat occupied by both species has selected for shape, insulation, and physiology that provide tolerance of extremely cold ambient temperatures and enhance survival when water availability is limited. At the same time, the small body size and highly seasonal environment have selected for rapid maturation and the most compressed reproductive cycle known in a mammal. Rapid reproduction increases water demands for milk production and further stresses water balance because maternal hyperthermia demands increased evaporative heat loss. Thus, in Phodopus habitat, rapid reproduction requires heat tolerance and water availability, which conflict with adaptations for survival. In the habitat of P. sungorus, predictable rains allow a breeding season and females rear litters alone. In P. campbelli, water availability is insufficient for solitary reproduction. Instead, male presence alleviates thermoregulatory, and thus water balance, stresses on the female. The result is improved pup survival and growth. Therefore, an ‘ultimate’ reason for biparental care in P. campbelli is its harsh environment, and a ‘proximate’ reason is a need to reduce maternal water demand. Results confirm that independent physiological constraints, in addition to constraints on energy or time investment, can be essential selective pressures in the evolution of mammalian social organization and behavior.

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