ESR 5:55-64 (2008)  -  doi:10.3354/esr00122

Variation in the relationship between offspring size and survival provides insight into causes of mortality in Hawaiian monk seals

Jason D. Baker*

Pacific Islands Fisheries Science Center, National Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, Hawaii 96822-2396, USA

ABSTRACT: The reasons why wildlife populations decline in abundance are often unknown; thus, competing hypotheses are offered as causal factors, and research focuses on attributing relative support to them. The widely documented relationship between offspring size and survival during the period following parental investment has been typically attributed to larger offspring having greater energy reserves, though size may also confer other survival advantages. Here, it is hypothesized that variation in the relationship between offspring size and survival in a population should provide insight into causes of mortality; for example, size-dependent and size-independent mortality will affect the shape of the size–survival curve differentially. This hypothesis is examined using weaning girth measurements and subsequent survival outcomes for Hawaiian monk seal Monachus schauinslandi pups, a species which is currently declining in abundance as a result of low juvenile survival. The probability of surviving the first year is strongly influenced by weaning condition (girth), as well as subpopulation and year. Comparing size–survival curves during years with high, medium, and low first-year survival suggests that an intensification of size-dependent mortality, probably related to food limitation, is commonly associated with poor survival. A notable exception at one colony, French Frigate Shoals, indicates that size-independent mortality, probably a result of a known high incidence of shark predation, is associated with high juvenile mortality. Analyzing variability in the relationship between body size and survival can be a useful tool for evaluating sources of mortality and, ultimately, aid in diagnosing population declines.


KEY WORDS: Body size · Hawaiian monk seal · Monachus schauinslandi · Offspring size · Survival


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Cite this article as: Baker JD (2008) Variation in the relationship between offspring size and survival provides insight into causes of mortality in Hawaiian monk seals. Endang Species Res 5:55-64

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