Body lipid reserves and the reproductive cycle: towards a better understanding

The aim of this paper was to develop a framework describing the relationship between body lipid reserves and the reproductive cycle. This is derived from consideration of the underlying cycle of reproductive priorities and strategies of dealing with environmental constraints. A way to quantify reproductive priority is proposed, based on the rate of maternal energy investment in the offspring. The majority of environmental constraints alter body energy mobilisation, which in turn affects reproductive factors such as the duration of post-calving anoestrus and subsequent conception rate. The accumulation of body lipid reserves in pregnancy is an important strategy for safeguarding reproductive investment. Consequently, body lipid reserves vary through the reproductive cycle in a predictable manner. These issues are discussed and then developed into a conceptual framework. Simple functions describing the negative effects of increasing lipid mobilisation rate and decreasing body fatness on the likelihood of onset of oestrus cycling postpartum are presented. The approach taken appears to be of value, not least because it raises a number of issues that merit further investigation. The resulting framework provides a basis for evaluating the pressures on reproduction and a way to predict the impact of future management and selection strategies. (C) 2003 Elsevier B.V. All rights reserved.