Investigation of a common physiological mechanism underlying progesterone-induced and maternal nesting in mice, Mus musculus.

These experiments addressed the possibility that genetic differences in plasma progesterone levels underlie genetic differences observed in maternal nesting (nest building by pregnant mice). Because this hypothesis requires that different circulating levels of progesterone result in a corresponding elevation of nesting, groups of outbred females were treated with varying doses of progesterone, and their nesting was measured. The relation between progesterone dose and nesting was not linear; rather, there appeared to be a threshold dose above which all groups responded similarly. Next, the progesterone levels of four inbred strains and lines selectively bred for differences in nonpregnant (thermoregulatory) nesting were measured directly by radioimmunoassay. The rank order of the inbred strains' and selected lines' progesterone levels did not correspond to the rank order of their nesting. Thus, genetically based differences in maternal nesting are not due to differences in circulating progesterone levels. Other results are presented which suggest a thermo-regulatory mechanism underlying progesterone-mediated maternal nesting which may involve genetic differences in thermal set point.