Differences in diet composition among conspecifics (dietary specialization) have been documented across a broad range of taxonomic groups and habitats, and such variation at the individual level is increasingly recognized as an important component of diversity in trophic interactions. Accurate identification of individual dietary specialization, however, requires longitudinal dietary records that are labor-intensive and cost-prohibitive to obtain for many species. Here we explore the use of stable isotopes (delta13C and delta15N) as a promising technique for detecting and quantifying patterns of individual dietary specialization. Southern sea otters (Enhydra lutris nereis) offer a unique opportunity for testing this approach because (1) they consume a wide variety of prey that span multiple trophic levels, habitats, and ecologically defined functional groups; and (2) individual diet specialization can be validated with existing observational data. We analyzed the isotopic composition of sea otter vibrissae (n = 31) in order to characterize inter- and intra-individual variation in sea otter diets at Monterey Bay, California, USA. At the population level, sea otters showed substantial variation in both delta13C and delta15N values, occupying nearly all of the "isotopic space" created by the diversity of isotopic signatures of potential prey taxa. Most of the variation in sea otter vibrissae was accounted for by differences between individuals, with much less contributed by within-individual variation. A majority of sea otters (approximately 80%) showed relatively little temporal variability in isotopic composition, suggesting that the proportional composition of most individuals' diets is relatively constant over time; a few individuals (approximately 20%) exhibited a high degree of intra-vibrissa isotopic variability, suggesting seasonal shifts in diet composition. These results and our interpretation of them were supported by long-term observational data on the diets of radio-tagged sea otters from the same population (n = 23). Our results demonstrate that stable isotopes can provide an efficient tool for measuring individual- and population-level dietary breadth and may be useful for studying populations where longitudinal data on individuals would otherwise be impossible to acquire. This will be critical for examining the causes and consequences of dietary variation within and among consumer populations, thereby improving our understanding of these important ecological and evolutionary processes at the community level.

Lactation represents the greatest postnatal energetic expenditure for mammalian mothers, and a mother's ability to sustain the costs of lactation is influenced by her physical condition. Mothers in good condition may produce infants who weigh more, grow faster, and are more likely to survive than the infants of mothers in poor condition. These effects may be partially mediated through the quantity and quality of milk that mothers produce during lactation. However, we know relatively little about the relationships between maternal condition, milk composition, milk yield, and infant outcomes. Here, we present the first systematic investigation of the magnitude, sources, and consequences of individual variation in milk for an Old World monkey. Rhesus macaques produce dilute milk typical of the primate order, but there was substantial variation among mothers in the composition and amount of milk they produced and thus in the milk energy available to infants. Relative milk yield value (MYV), the grams of milk obtained by mammary evacuation after 3.5-4 h of maternal-infant separation, increased with maternal parity and was positively associated with infant weight. Both milk gross energy (GE) and MYV increased during lactation as infants aged. There was, however, a trade-off; those mothers with greater increases in GE had smaller increases in MYV, and their infants grew more slowly. These results from a well-fed captive population demonstrate that differences between mothers can have important implications for milk synthesis and infant outcome.Keywords milk composition and yield; parity; infant growth; maternal effects; life history For mammalian females, lactation is the most costly form of parental investment. Mothers vary in their ability to meet the costs of lactation, and maternal effects on infant outcomes are common among mammals. As such, maternal influences on infant phenotype contribute to the variation on which natural selection acts (Mosseau and Fox, 1998;Räsänen and Kruuk, 2007). In nonhuman primates, infant growth and survival have been linked to variation in maternal weight, parity, age, and social rank as well as the timing of birth (in Chlorocebus aethiops; age and weight Fairbanks and McGuire, 1995; in Papio sp maternal age; Cheney et al., 2004; rank and parity;Altmann and Alberts, 2005; in Macaca mulatta maternal age and adiposity; Johnson and Kapsalis, 1995; birth-timing, Drickamer, 1974; Fairbanks, 1996 andMaestripieri, 2009). It is likely that these maternal effects are at least partially mediated through the quantity or quality of milk that mothers provide their infants during lactation, but currently little is known about interindividual differences in milk synthesis.To develop a full understanding of parent-offspring conflict and life-history theory in primates, it is important to identify the sources, magnitude, and consequences of variation in milk composition and output (Trivers, 1974;Bowman and Lee, 1995;Johnson and Kapsalis, 1995;Lee, 1996;Johnson, 2003). Although variation in...

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