Whale calves are more vulnerable to gull attacks

Off the coast of Península Valdés in Argentina, southern right whales must endure frequent attacks by kelp gulls. The birds land on the whales — often zeroing in on mothers and calves — and peck the skin and blubber. Now researchers have found that calves’ injuries have risen more rapidly than those of adult whales over the past four decades, perhaps partly because the young whales are less adept at avoiding the gulls.

Female southern right whales and their young spend about three months at Península Valdés during calving years. Since the 1970s, they have increasingly been targeted by gulls, which may harass a mother-calf pair for more than an hour at a time. In response, the whales are forced to expend energy on swimming away from the birds instead of resting and nursing.

The study authors analyzed aerial photos of 2,680 mothers and calves taken from 1974 to 2011 and searched for the lesions left by gulls. The researchers counted the number of lesions on each whale’s back and noted the size of the wounds. The team also studied photographs of 192 dead calves that had been stranded from 2003 to 2011.

Figure 3

Photographs showing different sizes of lesions caused by gull attacks on two living whales (left and center) and one dead whale (right). Credit: John Atkinson and SRWHMP; Marón CF et al. (2015) PLoS ONE 10(10): e0139291.

In the 1970s, only 2 percent of the living mother-calf pairs had lesions; by the 2000s, nearly all of the pairs did. But the injuries grew worse in the calves. Toward the end of the study period, the mothers showed an average of four lesions each, covering less than 1 percent of the animal’s back. But calves had an average of nine lesions covering 2.5 percent of the back. In one extreme case, nearly one-fifth of the calf’s back was wounded by gulls.

The mother whales can avoid injuries by arching their backs, leaving less skin exposed to the birds. Since calves are smaller, it may be harder for them to adopt this position. Calves also need to spend more time at the surface breathing, giving the gulls more opportunities to attack.

In the analysis of dead calves, the researchers didn’t find a link between the extent of the gull-inflicted injuries and the number of calf deaths each year. So while the birds may be partly responsible for calf mortality, they’re not the only factor at play, the researchers conclude. Roberta Kwok | 22 October 2015

Source: Maron, C.F. et al. 2015. Increased wounding of southern right whale (Eubalaena australis) calves by kelp gulls (Larus dominicanus) at Península Valdés, Argentina. PLOS ONE doi: 10.1371/journal.pone.0139291.

Image © Stubblefield Photography | Shutterstock