Each year, more than a thousand marine mammals die during mass strandings, grisly events in which large numbers of whales or dolphins become beached on the shore together. But scientists still don’t know exactly why these strandings occur. Climatic events, unfamiliar underwater topography, and noise from seismic surveys and naval exercises have all been suggested to play a role. In another theory based on family ties, one or a few whales, driven by disease or starvation, veer off in the wrong direction and draw well-meaning family members into shallow, dangerous waters as they try to help.

However, a paper in this week’s Journal of Heredity suggests that the role of relatives in mass strandings may not be quite so straightforward.

The researchers collected skin samples and information about the spatial distribution of 490 long-finned pilot whales stranded in 12 events across New Zealand and Tasmania. By studying the whales’ mitochondrial DNA (a type of genetic information passed on from mother to offspring), the researchers could determine how closely related the stranded animals were.

To their surprise, the researchers found that many of the whales stranded together were completely unrelated. Furthermore, those that were related weren’t even close to each other on the beach, as might be expected of relatives trying to support each other. And perhaps the most surprising finding was the fact that the mothers of many of the beached juveniles—up to 57 percent of them in one instance—weren’t involved in the stranding. The data suggest a very different picture than what most scientists previously envisioned.

The study’s authors suggest some strandings may be the result of a disruption of family ties. Strandings may be triggered by dramatic events such as the formation of “super pods,” a phenomenon where several groups of whales come together to feed or mate. The competition and aggression that results from such a large number of whales gathering together may separate families, causing confusion and increasing the likelihood of a stranding event.

Journal of Heredity, 2013. DOI: 10.1093/jhered/est007 (About DOIs).