Pinniped have evolved a range of reproductive and foraging patterns to deal with the constraint of terrestrial parturition and marine feeding. To understand the link between these constraints it is necessary to understand both reproductive periods on land and foraging at sea. I examined the foraging behavior of a phocid (northern elephant seal, Mirounga angustirostris) and otariid (California sea lion, Zalophus californianus) species to provide insight into the divergent life histories of these families. To understand foraging behavior it is necessary to know when and where animals find prey. Using stomach temperature telemetry, I conducted rigorous feeding experiments with both species to determine whether measures of stomach temperature can be used to identify and quantify prey consumption. Feedings were identified with high accuracy and the equations to estimate mass consumed were not significantly different between species.

Until recently, studies on the foraging behavior of marine predators have relied on indirect measures of feeding, such as changes in diving behavior or movement patterns. When combined with measures of at sea behavior, stomach temperature telemetry can provide information about when and where animals uccessfully capture prey. I deployed stomach temperature telemeters along with time-depth recorders and satellite transmitters to examine the foraging behavior of northern elephant seals. This resulted in the first measures of feeding behavior and foraging success in elephant seals, as well as provided a validation of indirect indices of foraging behavior.

Finally, I examined the foraging behavior of free-ranging California sea lions and documented how their behavior changed annually in response to environmental variation that occurred over three years. Although stomach temperature telemeters were deployed on sea lions, I was only able to maintain the telemeters in the animals for long enough to measure at sea feeding in one female. Therefore, foraging behavior was examined based on indirect measures of feeding, such as diving behavior and at sea distribution. I found females alter movement patterns in response to even modest environmental variation. Such detailed information on the foraging behavior of these species will make it possible to examine the link between reproductive pattern and at sea foraging behavior.