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Stable isotope analysis is useful for examining the feeding strategies of mammals. Isotopes in the annual deposition growth layers of dentine in teeth permit assessment of ontoge- netic dietary shifts in individuals, because this metabolically inert tissue is not resorbed after deposition. Profiles of stable isotope ratios of nitrogen (δ 15 N) and carbon (δ 13 C) from the teeth of northern elephant seals Mirounga angustirostris, aged between 1 and 11 yr, were described from stranded individuals in San Benito and Magdalena Islands, Mexico, between 2000 and 2009. δ 13 C and δ 15 N values differed with age in each sex indicating shifts in feeding habits throughout life, as well as in their physiological condition. Although steady δ 13 C values within each individual sug- gest fidelity to feeding grounds, differences in δ 13 C values between males and females were likely a reflection of sexual segregation on the feeding areas, which begins during the juvenile stage. Mean δ 15 N values of males (18.3‰) and females (18.2‰) would suggest that both groups feed at a similar trophic level; however a combination of differences in benthic versus pelagic and longi- tudinal foraging habitats would be masking real sex differences in trophic level, estimated for the species at 4.6. δ 15 N and δ 13 C values varied markedly between individuals of the same sex, sug- gesting the potential existence of diverse feeding strategies leading to a resource partitioning in this species. The alleged fetal growth layer has a δ 15 N value 1.8‰ higher than the average of the adult female stage, suggesting that the fetal layer is more likely the product of the intensive nurs- ing during the first month of life.

Human exploitation can lead to genetic bottlenecks associated with reduced genetic variability and lower fitness. The population of California sea lions (Zalophus californianus) in the Gulf of California, Mexico, was hunted during the 19th and 20th centuries, potentially leading to a genetic bottleneck; however, even exploitation that leads to low population sizes does not always cause genetic bottlenecks. Understanding the genetic consequences of past sea lion hunts is critical to the conservation of the Gulf of California sea lion population, which is currently declining and is genetically distinct from other populations. We used available data from 10 amplified polymorphic microsatellite loci in 355 individuals from six Mexican colonies. Microsatellite data were analyzed using diverse approaches (BOTTLENECK and M-ratio) to determine if a genetic bottleneck had occurred. Our results indicate that human exploitation did not cause a genetic bottleneck in the sea lion population of the Gulf of California. Simulation analyses revealed that a reduction in genetic variability would have been detected if fewer than 100 individuals had remained after exploitation. We conclude that past exploitation was not as severe as previously thought and did not cause a genetic bottleneck in the Gulf of California sea lion population. Nevertheless, historical hunts specifically targeted adult males and this sexbiased exploitation may have influenced the population dynamics and overall fitness. Key words: genetic variability, male-biased exploitation, Pinnipedia, Gulf of California. La explotacion humana puede causar cuellos de botella geneticos asociados con una variabilidad genetica reducida y menor aptitud. La poblacion del lobo marino de California (Zalophus californianus) en el Golfo de California, Mexico, fue objeto de una explotacion prolongada durante los siglos XIX y XX que pudo haber causado un cuello de botella genetico. Sin embargo, la explotacion que deriva tamanos poblacionales bajos no siempre causa cuellos de botella geneticos. El conocimiento de las consecuencias geneticas de la caza del lobo marino de California es critico para la conservacion de la poblacion del Golfo de California, que esta en declive y es geneticamente distinta de otras poblaciones. En este estudio usamos una base de datos de 10 marcadores polimorficos de microsatelite amplificados en 355 individuos pertenecientes a seis colonias mexicanas. Estos datos fueron analizados con diversos metodos (BOTTLENECK y razon M) para determinar si ocurrio un cuello de botella genetico. Nuestros resultados indican que la explotacion historica del lobo marino no causo un cuello de botella genetico en la poblacion del Golfo de California; sin embargo, las simulaciones sugieren que si la poblacion hubiese sido reducida a menos de 100 individuos si hubiesemos detectado una reduccion de la variabilidad genetica. Con base en estos resultados concluimos que la explotacion historica de esta poblacion no fue tan intensa como se pensaba previamente y, por tanto, no causo un cuello de botella genetico. No obstante, la explotacion se concentro en machos adultos y este sesgo podria haber afectado la dinamica poblacional de estas colonias y su aptitud general.

Understanding the spatial structure of a population is critical for effective assessment and management. However, direct observation of spatial dynamics is generally difficult, particularly for marine mammals. California sea lions (Zalophus californianus) are polygynous pinnipeds distributed along the Pacific coast of North America. The species’ range has been subdivided into three management stocks based on differences in mitochondrial DNA, but to date no studies have considered nuclear genetic variation, and thus we lack a comprehensive understanding of gene flow patterns among sea lion colonies. In light of recent population declines in the Gulf of California, Mexico, it is important to understand spatial structure to determine if declining sea lion colonies are genetically isolated from others. To define population subdivision and identify sex biases in gene flow, we analysed a 355-bp sequence of the mitochondrial DNA control region and 10 polymorphic microsatellite loci from 355 tissue samples collected from six colonies distributed along Mexican waters. Using a novel approach to estimate sex biases in gene flow, we found that male sea lions disperse on average 6.75 times more frequently than females. Analyses of population subdivision strongly suggest a pattern of isolation by distance among colonies and challenge current stock definitions. Based on these results, we propose an alternative classification that identifies three Mexican management units: Upper Gulf of California, Southern Baja Peninsula, and Upper Pacific Coast of Baja. This revised classification should be considered in future assessment and management of California sea lion populations in Mexican waters.

International audience; When 2 closely related species co-occur, each exhibiting sex-specific differences in size, resource partitioning is expected. We studied sex-specific foraging behaviour of 2 sympatric seabird species in the Gulf of California to disentangle the respective influence of species and sex, but also mass and size of individuals, on observed foraging behaviour. We used highly accurate data loggers to study movements, diving behaviour and activity of brown and blue-footed boobies rearing young chicks. Interspecific differences were limited; brown boobies had longer foraging trips and spent less time on the water than blue-footed boobies. The major differences observed were sex-specific; females of each species tended to have longer foraging trips, foraged farther from the colony, flew greater distances and had larger zones of area-restricted search. These sex-specific differences were more prominent in brown than in blue-footed boobies. Diet and stable isotope analyses showed that, during the study period, both species fed mainly on sardines, at similar trophic levels and in similar zones; outside the breeding season, the carbon and nitrogen signatures from feathers were also similar on average. In these sympatric species that feed on a superabundant prey, sex-specific differences appear to have a greater role than species-specific differences. We suggest that sex-specific differences may be mainly related to breeding involvement, as males are more involved in nest attendance and defence and females are greater provisioners. However, we show that several sex-specific differences in observed foraging behaviour were partly or totally explained by body size (flight speeds, foraging range, flapping frequency) or by body mass (depths attained during diving, duration of dives), which are parameters influenced by biomechanical constraints such as flight and diving.

Tooth wear in marine mammals has been attributed to age, feeding habits, behavior, and contaminants. Advanced tooth wear in some California sea lions, including some of very young age (0.09; ANOVA, p>0.15); however, the Ca concentration of the teeth was inversely proportional to the age of the animal (sexes combined, p=0.026) and particularly significant for the females (r 2=0.112, r=−0.335, p=0.039). Females could be more prone to decalcification because of their annual bone investment in their offspring. Animals of both sexes were susceptible to tooth wear as their age increased, but the higher frequency of animals between 4 and 7 yr suggests an impact on survival at early stages probably linked to deficient feeding and chronic malnutrition.