Geometry of Maternal-Offspring Contact in Two Rodents

Abstract

We determined the importance ofphysical contact amongpups, and between pups and their dams and the substrate on which they rest, for the thermal relations of pups and dams in two species of rodents differing substantially in developmental mode: the altricial wood rat Neotoma floridana and theprecocial cotton rat Sigmodon hispidus. We used two different methods to quantify contact. One, which makes use ofacrylic casts, enables quantification ofall areas of contact. The other, using ink blotting, measures only areas of direct contact in which skin surfaces are within close contact but achieves measurements nearly equal to those ofacrylic casting Contact amongpups is small, and heat exchange among thepups is relatively unimportant while thepups are being brooded by the dam. This contrasts withprevious work establishing the importance ofhuddling by juvenile endotherms duringperiods ofparental inattention. Nonetheless, contactper se within the mother-pups unit is important under all conditions. Contact of the pups with the substrate did not differ between species and averaged about 25% of totalpup surface area. Contact ofpups with their dam depends strongly on both age and litter size, but the effects of these two factors differ between species. Heat transfer between the dam and her pups is more strongly affected by contact geometry than by interspecific differences in fur morphology. Litter size can havepotentially important effects on the energetic demand of theparent. Heat loss from the dam increases with litter size, but does not necessarily increase with age of the pups because of changes in the fur insulation of thepups. Development of the fur in the growingpups can significantly inhibit heat flow from the dam. Compression of the fur because of the weight of the dam is the most importantfactor in reducing insulation between the dam and herpups, although changes in thermal conductivity with compression and contact between ventral, rather than dorsal surfaces, are also important. Early development of the fur, althoughpermitting improved thermoregulation duringperiods ofparental inattentiveness, can reduce heat gain from the broodingparent. Thus, the ontogeny of insulation must be tied to the development of thermogenic capabilities.