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All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.




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Capital Expenditure, Lactation Energetics, and the Importance of Foraging to Weddell Seals and their Pups

Lactation is the process by which energy and nutrients are transferred, in the form of milk, from a female mammal to offspring. Lactation is a costly physiological process for the milk provider, who must convert energy and nutrients she has ingested, in the form of food, into milk. Lactation is quite variable among species in terms of its duration, the nutrient and energy content of milk produced over the entire course of lactation and at its different stages, and the efficiency with which lactating individuals convert the food they ingest into milk.

As dependent young grow, they require ever-increasing amounts of energy and nutrients (more milk), placing even greater demands on the provider, which must produce not only greater quantities of milk but, in many cases, milk with higher energy and nutrient content. Many female mammals forage during lactation and thereby take in energy and nutrients that replace at least some of what is lost from milk production. But even lactating females who forage convert some of their own body stores during lactation and typically lose at least some of their own body mass in the process.

At one end of the lactation weight-loss continuum are mammals where mothers lose very little of their own body mass during infant rearing. Dominant female meerkats are an excellent example of this. After dominant female meerkats give birth, they leave pups in the care of foster-mothers who nurse the young almost exclusively. Some of these foster mothers are females that have lost their own young, but some are non-reproducing females who begin to lactate spontaneously. As a consequence, the actual mother has to invest very little, if anything at all, in lactation and uses her acquired and stored energy to produce more babies. This is an unusual lactation strategy found only in cooperatively breeding species.

At the other extreme are mammals in which mothers forage very little, if at all, during lactation. These species use energy and nutrients they have stored in their own body tissues (in the form of fat or protein) to make all the milk their young will need over the course of lactation. Such species are referred to as "capital breeders" because they rely almost exclusively on stored reserves—as in body "capital"—to support reproduction. We say "almost exclusively" because it is possible that some capital breeders actually do forage during lactation and we simply have not been able to detect or verify it. Phocid seals, which include the Weddell seal, are the best studied capital breeders.

Species that forage during lactation are referred to as "income breeders." In contrast to capital breeders, which rely on what they have stored up ("capital"), income breeders depend to one extent or another on what they take in ("income") during reproduction. There is a continuum between income and extreme capital breeders: at one end are otariid seals, such as the California sea lion, which remain ashore in a fasting state for about a week after giving birth and, after losing about four percent of their body protein and 12 percent of body energy to their pups, resume foraging trips to sea. These species employ the capital strategy at the beginning of lactation and then switch to the income strategy. At the other end of the continuum are large phocid seals, such as the elephant seal, which appear to fast throughout the entire duration of lactation—these are the extreme capital breeders. Large body size and limited availability of food during lactation may be important factors favoring the evolution of extreme capital breeding whereas the opposite appears to be true for income breeders.

Somewhere in between are species, like the Weddell seal, where there are larger individuals, which may fast for most or all of lactation, and smaller females that do not have many stored reserves and may actually forage at some point during lactation to obtain the energy and nutrients needed to sustain themselves and their young. Diving studies and the patterns of maternal weight loss during lactation seem to verify that this is indeed the case. Thus, Weddell seals may be unusual in that there may be a continuum from income to extreme capital breeding within a single population.

At this stage, we really don't know whether some, all, or no lactating Weddell seals must forage for successful reproduction. If it turns out that some or all have to forage, it would mean that breeding colonies would be limited to places where there is adequate prey and suitable access to that prey. These would-be limitations are very sensitive to environmental conditions that affect critical factors such as prey population size, sea ice cover and water currents, among others. If, on the other hand, Weddell seals need not forage during lactation, populations would be much less affected by changing environmental conditions that affect food resources and their physical environment. These are two very different scenarios and we want to know which one (or perhaps it's both) apply to Weddell seals.

To get to the bottom of all this, we will study a number of things in a sample of Weddell seals in the McMurdo population. These are:

  • How female "capital stores" (such as body energy, fat, and protein) change over the course of lactation;
  • The extent of maternal energy expenditure via heat production;
  • Maternal milk production and the transfer of energy and nutrients to pups;
  • The timing of onset of feeding in mothers and pups;
  • How important food intake is to the overall energy budget of mothers during lactation;
  • The duration of lactation and how the weaning process unfolds.