Post‐mortem studies of Northern elephant seal pups

Abstract

Thirty‐four pups of the Northern elephant seal, Mirounga angustirostris, collected from Mexican and Californian coastal islands were studied to determine causes of death, growth of certain organs, and development of the supernumerary bones of the occiput. Dead pups from Añ Nuevo Island off central California from 1965 through 1969 appear to have died due to crushing by adults, probably the large breeding bulls. Recent deaths through the 1972 breeding season indicate that some deaths are attributable to bites inflicted by agitated cows. Organ weights were taken on several pups and compared with weights recorded for pups of the Southern elephant seal. Some organs appear to grow proportional to body weight, and other organs show disproportionate growth. The supernumerary bones of the occiput called “extraoccipitalia” were found to be embryonically preformed in cartilage. Ossification centres were shown to be developing in late term foetuses. These extraoccipitalia evidently allow rapid growth of the neurocranium during the first few months postpartum and later fuse with adjacent cranial bones at about one year of age. The rising elephant seal population in relation to the types of mortality is discussed in relation to available breeding grounds.

Postmortem examination and causes of death of northern elephant seal (Mirounga angustirostris) pups of the San Benito Islands, Baja California, Mexico

“…Causes of pup death observed at SB largely agree with the ones found previously (Morejohn and Briggs , Le Boeuf and Briggs ), and TSS was the main cause as it was at AN (Le Boeuf et al . , Spraker et al .…”

“…Differences in pup mortality, female aggression, and TSS rate between NES and SES were often attributed to differences in population and harem density (Le Boeuf et al . , Morejohn and Briggs , McCann , Baldi et al . , Galimberti and Boitani ), and in particular to higher female crowding of the AN harems.…”

“…). Most studies of preweaning mortality in NES have been carried out at Año Nuevo (AN; Morejohn and Briggs , Le Boeuf and Briggs , Spraker et al . ) in California, which is a fast increasing colony where the main causes of preweaning mortality seem to be of social origin, including crushing by males and wounds caused by female aggression (Morejohn and Briggs , Le Boeuf and Briggs ).…”

“…The starting point of TSS is mother‐pup separation, followed by definitive abandonment, such that the pup is not able to suckle and is not protected by the mother, being vulnerable to aggression by alien females and by male harassment. Pup necropsies conducted at AN by Morejohn and Briggs () found internal bleeding and fractures related to crushing, and wounds likely produced by female canine teeth, and suggested the intensity of aggression and pup mortality to be related to harem density. More recently, Spraker et al .…”

“…Determination of causes of mortality in wild populations is an important but difficult task, because death is often produced by multiple causes that are difficult to assess in the field (Spraker et al 2014). Most studies of preweaning mortality in NES have been carried out at Año Nuevo (AN; Morejohn and Briggs 1973, Le Boeuf and Briggs 1977, Spraker et al 2014 in California, which is a fast increasing colony where the main causes of preweaning mortality seem to be of social origin, including crushing by males and wounds caused by female aggression (Morejohn andBriggs 1973, Le Boeuf andBriggs 1977). Le Boeuf and Briggs (1977) introduced the "trauma starvation syndrome" (TSS), to indicate a set of factors that jointly determine pup mortality, including starvation, wounds produced by alien female aggression, crushing by males (adult harem holders in particular), and general trauma due to male harassment carried out mostly by young males.…”