Impossible to confuse a male and a female in this species of pinniped (sea elephants, seals, walruses, sea lions…). By the imposing mass of the first, which easily exceeds one and a half tons, three times that of the female. Not to mention its nasal appendage, a kind of thick trunk that earned it its name of marine pachyderm.
This sexual dimorphism results in a radically different mode of feeding, and a team of Californian scientists, led by Professor Sarah Kienle, of Texas’s Baylor University, thinks they know why.
The foraging strategy of elephant seals represents a trade-off between its reward and the risk of mortality, according to the study published in Royal Society Open Science.
It all starts on land, in a bay, where colonies of North Pacific elephant seals form harems,
varying from a few females up to 40 or 50, led by an alpha male, with a few other beta males at the margin, explains Pre Kienle to AFP.
one of the most competitive breeding systems on the planet, note the authors of the study, who analyzed data from a Californian colony of 39 males and 178 females over ten years.
Only a small percentage of males, the larger and more dominant ones, mate with females, according to Prof. Kienle.
The shooting window is short, from one to three months per year, when the colony forms from December for reproduction. The rest of the year is devoted to a long fishing expedition at sea, interrupted by a short stay for the moult.
On land, the males engage in a series of confrontations, vocal and postural, and if necessary in sumo wrestling accompanied by biting, which determines their hierarchy.
The animal’s weight is decisive in gaining the dominant role, then maintaining it during the breeding season, during which it cannot feed and where it can hope to mate until it is over fifty. of times.
There is enormous pressure on the males to get bigger, as fast as possible, in order to compete for those rare chances to reproduce., explains Prof. Kienle.
Females have a different constraint. Giving birth once every year or two, their priority is to last, to procreate as much as possible.
These two sex-specific traits result in starkly different foraging strategies.
The females venture up to a thousand kilometers from the Pacific continental shelf, favoring the ocean floor, down to -500 or -600 m, to feed on fish and squid.
They leave the feeding area of the males, located on the edge of the continental shelf, and at much lower depths, on average -230 m. An area with
a high concentration of prey, according to the study, based on fish, cephalopods and small sharks.
An adult male of about one ton will thus gain half his weight during the campaign. A gain up to six times greater than that of females, but not without consequences on its life expectancy.
Males are six times more likely to die while foraging at sea than females, according to observations.
Even if the main cause of mortality remains to be examined, Pr Kienle makes
the hypothesis that predation explains the higher mortality rate of males, as they forage in areas prized by their predators : the white shark and the killer whale.
This reduces the population of males capable of reproducing accordingly. Without changing much in the end, since a very small number of them perpetuate the species, anyway.