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Oh Mother, where art thou? Mass strandings of pilot whales

By Marc Oremus and C. Scott Baker

Biologists since Aristotle have puzzled over the reasons for mass strandings of whales and dolphins, in which large groups of up to several hundred individuals drive themselves up onto a beach. To date, efforts to understand mass strandings have largely focused on the role of presumably causal environmental factors, such as climatic events, bathymetric features or geomagnetic topography. But while these studies provide valuable information on the spatial and temporal variation of strandings, they give little insight into the social mechanisms that compels the whales to follow their counterparts to an almost certain death (at least without human intervention).

So how can we go further in our understanding of these enigmatic events? We believe that deciphering the relationship of the whales and their behavior at the time immediately before and during the stranding is the key. Indeed, social behavior appears to play a critical role in these events as suggested by several accounts of group cohesion during mass strandings; the most striking example of this being the intentional restranding of whales after being refloated during rescue efforts. Yet, previous attempts to describe mass stranding have largely failed to consider these social and behavioral aspects.

That’s where our own interests in mass stranding began. Our idea was to define a testable framework in which we could investigate how social relationships play into the dynamics of the strandings. Our starting point was a long-standing hypothesis regarding the reason for strandings, in which “care-giving behaviors” are mediated by family relationships. In this scenario, the stranding of one or a few whales, because of sickness or some kind of disorientation, triggers a chain reaction in which other healthy individuals are drawn into the shallows in an effort to support their family members. We draw two predictions from this scenario: first, that the whales in a stranding event should all be related to each other through a single ancestral female or matriarch. Second, that close relatives, especially mothers and calves, should be found in close proximity to each other when they end up on the beach during a stranding event.

Long-finned pilot whale spyhopping in Cape Breton Island, Nova Scotia, Canada.
To test these assumptions, we were fortunate to have access to a unique collection of genetic samples from long-finned pilot whales that stranded in New Zealand and Tasmania, two renowned hotspots for mass strandings. Long-finned pilot whales were the perfect candidates for this study as they are the most common species to strand “en masse” worldwide. Compelling evidence also indicates that in this species, neither males nor females disperse from the group into which they were born (a community structure also found in killer whales, but otherwise thought to be rare in mammals), suggesting the critical importance of kinship bonds in their social life.

Over 400 genetic samples were used to describe the kinship of individual long-finned pilot whales involved in 12 different mass strandings. Our investigation revealed that stranded groups are not necessarily members of a single extended family, which challenges the notion that mass strandings are driven primarily by kinship-based behavior. To test the second prediction, we needed to assess whether the individuals found near each other when strandings occur were kin related. To do so, we were lucky enough to have the position of whales stranded on the shore for two large mass strandings. To our surprise, no correlation was found between spatial distribution and kinship. Even mothers and nursing calves were not necessarily together when the whales drove themselves up onto the shore, and in many cases no identifiable mother of stranded unweaned calves was found among the beached whales. We called them the “missing mothers”. We believe that several scenarios could account for this lack of spatial cohesion, including the disruption of social bonds among kin before the actual strandings. It is even possible that the separation of related whales might actually be a contributing factor in the strandings.

Whatever the exact reason for this pattern, the results of this study have important implications for rescue efforts aimed at “refloating” stranded whales. Often, stranded calves are refloated with the nearest mature females, under the assumption that this is the mother. Well-intentioned rescuers hope that refloating a mother and calf together will prevent re-stranding. Unfortunately, the nearest female might not be the mother of the calf. Our results caution against making rescue decisions based only on these assumptions as the refloating of a juvenile and unrelated female could increase the tendency to restrand after rescue, as those individuals seek their still-stranded kin.

Many questions remain unanswered. For instance, where are the “missing mothers”? To answer this, we should make sure that genetic samples are collected from all whales involved in future mass strandings, including from those individuals who do eventually make it back to sea. We also need to know more about the genetic relationship among groups in the open ocean, for comparison to the composition of stranded groups. It seems likely that some form of social disruption takes place prior to strandings, but it remains unknown whether this is simple a consequence of the stranding or is actually a causal force, perhaps due to competitive or even aggressive interactions between multiple social groups.

Marc Oremus is a marine mammal biologist that earned his PhD at the University of Auckland, working on the population and social structure of several species of dolphins. Most of his work is based on a combination of molecular and demographic approaches; he is a member of the South Pacific Whale Research Consortium and IUCN Cetacean Specialist Group. Marc Oremus and C. Scott Baker are co-authors of the paper ‘Genetic Evidence of Multiple Matrilines and Spatial Disruption of Kinship Bonds in Mass Strandings of Long-finned Pilot Whales, Globicephala melas’, which appears in the Journal of Heredity.

The Journal of Heredity covers organismal genetics: conservation genetics of endangered species, population structure and phylogeography, molecular evolution and speciation, molecular genetics of disease resistance in plants and animals, genetic biodiversity and relevant computer programs. The journal is published by Oxford University Press on behalf of the American Genetic Association.

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Image credit: Long-finned pilot whale spyhopping in Cape Breton Island, Nova Scotia, Canada. Photo by Barney Moss [CC-BY-2.0], via Wikimedia Commons

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