Physiological and behavioral determinants of lactation efficiency in the northern elephant seal.

The efficiency with which mothers convert acquired energy and body reserves into milk production is a key determinant of both the rate and magnitude of parental investment in mammals. Despite this, relatively few studies have examined lactation efficiency and its underlying physiological and behavioral characteristics in free-ranging mammals. Pinnipeds represent an ideal system with which to address these questions because the physiology of lactation in pinnipeds is significantly impacted by constraints resulting from the temporal separation of foraging and parental investment. This makes it possible to look at the costs of reproduction without the confounding variable of food intake. It has been suggested that one strategy available in the case of energy limitations on allocation toward reproduction is compensatory shifts in behavior that reduce energy spent on activity and reduce maternal energy requirements. From a physiological standpoint the key to lactation efficiency is the rapid mobilization of stored body lipids and their uptake and use in milk synthesis by the mammary gland. Lipoprotein lipase (LPL) is the primary enzyme involved in directing triglycerides mobilized from tissue stores to tissues for utilization. For my M.A. thesis research, I will investigate these behavioral and physiological features in the northern elephant seal.

Background

Elephant seals spend the majority of their lives at sea but they are tied to the land for the birth and suckling of their young. When pregnant females arrive at the rookery they seek each other out for safety from the aggressive males and form harems. The fighting between the males results in a dominance hierarchy, with only the most dominate males having access to the females. Harems can range in size from a small group controlled by a single male to larger sizes where other males take up position in the periphery. During this period, elephant seals fast completely from food and water. Previous investigations of female reproductive energetics have revealed significant positive relationships between maternal age and lactation efficiency. It was suggested that this was because older females have more central harem positions and therefore expend less energy on agonistic encounters than younger females on the periphery (Crocker et al., in press). An alternative explanation for this increase in efficiency is that older females have higher levels of LPL that could increase the rate of lipid uptake by the mammary gland and increase the rate of milk synthesis relative to the females own maintenance metabolism. It has been suggested that Lipoprotein Lipase (LPL) activity plays an important role in high milk fat transfer from mother to pup in gray and hooded seals. The goal of this study is to determine which factors explain why older females are more efficient at milk synthesis than young females. She will test the following hypotheses: 1) Older females spend less time on agonistic interactions with other females and males and 2) Older females have higher LPL levels than younger females.

Significance of results

The results from this study will contribute to the understanding of the underlying physiological and behavioral characteristics of lactation by investigating what causes differences in lactation efficiency in the two age classes. A female can maximize the amount of energy available for milk production by minimizing the amount of energy expended on her own maintenance metabolism while on shore, thereby increasing the lactation efficiency. Two ways that a female can minimize her maintenance metabolism is by decreasing activity and through metabolic compensation.

The increased lactation efficiency in older females could be explained by behavioral modifications if it is found that older females spend less time than younger females involved in agonistic interactions with other females and males and/or if the older females are spending more time sleeping than younger females. In sleep the body temperature set point drops so the elephant seals do not expend as much energy on thermoregulation. In gray seals it has been found that lactation efficiency increased throughout lactation and that this increase in efficiency cannot be explained by behavioral modifications. It is suggested that the increase in efficiency is due to metabolic compensation. A possible explanation for this metabolic compensation in gray seals could be an increase in LPL activity levels, which did show an increase from the beginning to the end of lactation. If it is found in the proposed study that the older females have higher LPL activity levels than the younger females it would suggest that LPL activity levels contribute to the metabolic compensation and thereby increases lactation efficiency in the older females. If it is found that, in addition to having higher LPL levels, older females produce milk with a higher fat content this would further support the hypothesis that increased LPL activity levels increase lactation efficiency.