Study: Mass Whale Strandings not a Family Affair
Whales that beach themselves in large groups are not all members of the same family, a study has found, undermining long-held assumptions about the cause of mass strandings.
An international team of scientists studied 12 mass strandings in Australia and New Zealand to examine the theory that healthy whales beach themselves while trying to help sick or disorientated family members who have run aground.
After examining the DNA of 490 whales they concluded the so-called "caregiving model" did not stand up, as most of the whales were not related.
Further, it found that many stranded calves were discovered with no mother nearby.
The study was led by Marc Oremus of Auckland University in New Zealand, where mass whale strandings are common.
"If kinship-based social dynamics were playing a critical role in these pilot whale strandings, first, we would expect to find that the individuals in a stranding event are, in fact, all related to each other," he said.
"We would (also) expect that close relatives, especially mothers and calves, would be found in close proximity to each other."
In the study, the position of each stranded whale was mapped to determine if individuals found near each other were related.
No correlation was found between location and kinship, even with nursing calves and their mothers, who were often widely separated when the group drove itself onto shore.
Co-author Scott Baker from Oregon State University in the United States said the study, published Friday in the Journal of Heredity, suggested the break-up of family groups in deep water might be a reason for mass strandings.
"In the past people tried to explain mass strandings of pilot whales due to environmental causes or something that's social -- like pursuit of prey into shallow unfamiliar territory," he told Radio New Zealand.
"They might get disoriented, misled about depth and slope. That's been one of the primary hypotheses.
"The other is the individuals are supporting each other in some way, one may be ill and the others are supporting them and following them into the beach as a last resort."
With the second reason now in doubt, Baker was asked if the first was the only logical alternative, but said researchers now had another view.
"We actually pose a new hypothesis that there are social forces at work but these may not be specifically kinship driven care behaviour as assumed in the past," he said.
"We think there might be competition between groups or some sort of disruption prior to stranding."
The study suggested that disruptions prior to stranding could be due to feeding or mating competition, and even aggression among pods.
It speculated that distress calls from whales in trouble could create confusion among others in the vicinity, resulting in the separation of kin before they end up stranding themselves.
The researchers said the study had implications for attempts to refloat whales after they have stranded.
"Often, stranded calves are refloated with the nearest mature females, under the assumption that this is the mother," said Baker.
"Unfortunately, the nearest female might not be the mother of the calf -- our results caution against making rescue decisions based only on this assumption."