INEFFICIENCY OF LACTATION IN PRIMIPAROUS RATS - THE COSTS OF FIRST REPRODUCTION

To investigate the energetic costs of lactation in a female mammal in relation to previous reproductive history, we compared the performance of adult female Long-Evans rats that had preciously bred (multiparous ) with young females that had not previously given birth (primiparous) . All litters were standardized to 10 +/- 1 young. We compared materna l production (growth of pups), body mass, and energy intake (food cons umption) of mothers, as well as their energy expenditure (resting oxyg en consumption). The mass of litters at birth and the growth of pups d uring lactation did not differ according to reproductive history of th e mothers. The body mass of primiparous mothers was less than that of multiparous mothers, and primiparous mothers showed an increase in mas s during early lactation. To accomplish the essentially identical prod uction of offspring under these circumstances, the primiparous mothers consumed and expended more energy than the multiparous mothers. This remarkable performance of first-time mothers results in an overall eff iciency of energy allocation to reproduction amounting to only 25%, co mpared with 38% in multiparous mothers. The energetic inefficiency of primiparous female lactation results largely from the excessive expend itures associated with physiological and behavioral performances of fi rst-time reproduction, together with a small component of additional e xpenditure due to further growth by the primiparous mothers. We sugges t that this inefficiency probably contributes to the observed low repr oductive success of novice breeders; furthermore, active restraint of fecundity may be an evolutionary response to the constraints of the en ergetic inefficiency of primiparous breeding by female mammals.