Abstract:
In the laboratory, ovulation is suppressed when a mammal is in negative energybalance whether that state is caused by inadequate food intake, excessivelocomotor activity or heavy thermoregulatory costs. In this paper, knowledgegenerated in the laboratory about the link between ovulation and energybalance is examined in relation to the kinds of energetic challenges mammalsactually face in natural habitats. When viewed in that context, severalconclusions can be drawn. First, females ovulate whenever extant energeticconditions permit unless the process is blocked by non-metabolic stress,social cues or a predictive seasonal cue such as photoperiod. In the lattercase, most mammals show at least a seasonal tendency in their reproduction andthe majority do not use a predictive cue; they reproduce opportunistically inrelation to seasonal variation in the energetic characteristics of theirenvironment. Second, the widely held assumption that a female’s fatreserves must exceed a critical level in order that she may ovulate finds nosupport in the literature dealing with natural populations. Third, thesurprisingly rapid responsiveness of the gonadotrophin releasing hormone(GnRH) pulse generator to energetic manipulation probably reflects the studyof animals that are in a pure survival mode. Fourth, the complexity of theenergetic challenges mammals face in the wild suggests that there are probablymultiple metabolic and neural pathways coupling ovulation to energy balanceand that these pathways are probably characterized by considerable overlap andredundancy. Thus, fifth, to develop a more realistic overview of thesepathways there is a need for experimental designs that present mammals withthe kinds of complex challenges they actually face in the wild habitats inwhich they evolved.