Posted by: salamandercandy | July 30, 2007

Cultural Orcanization

I recently returned from the San Juan Islands, where my sister-in-law is studying how anthropogenic noise affects the calling behavior of killer whales (orcas). I got to spend many hours watching killer whales merely hundreds of yards away or less, as they surfaced to breathe, breeched, and slapped their pectoral and caudal fins against the sea. A microphone below the waves let us hear their squeaky vocalizations. The population is so well studied that every individual whale can be visually recognized by the local experts based on the dorsal fin and the white dorsal patch.orcas

The San Juans are one of the few places where you can be virtually guaranteed to see whales any day of the year, because of the unique behavior of these particular animals. While other “transient” killer whales are out roaming the open ocean hunting marine mammals, the “resident” killer whales in the San Juans stay near the islands and catch salmon. Hunting is a learned behavior in these highly social animals, so there are effectively two distinct killer whale cultures: the wolflike transients and the piscivorous residents. Even when transients visit the Straight of Juan de Fuca, the two whale societies have nothing to do with each other, and apparently rarely mate.

As an evolutionary biologist, I am intrigued by these cultural differences promoting genetic differentiation. It’s tempting to speculate that these cultural differences could lead to speciation, but in the case of the orcas this seems unlikely. Even in our own species, which is more culturally diverse than any other, cultural speciation has never occurred. Throughout history, when two different cultures encountered each other, at least some individuals have been willing to have sex with someone from the other group. Thus, no human group, no matter how culturally isolated, is a distinct species. For orcas, cultural speciation is even more unlikely, since young orcas are raised entirely by their mother and her family, and don’t even know their father. Thus, a young hybrid doesn’t have to suffer lower fitness by straddling two lifestyles; instead, it learns the hunting behaviors of its mother regardless of the population its father came from. As a result, an adult orca has nothing to lose by mating with an orca from another population. On the other hand, might residents avoid the whale-killing transients out of fear for their own safety, and thus shun them as mates? As a reproductive isolating mechanism, this is unheard of, but it’s still possible, I suppose. Molecular evidence shows that residents and transients have different mitochondrial lineages (as expected in a matriarchal species), but are only moderately different at neutral nuclear markers, consistent with occasional male-mediated gene flow between populations (Hoelzel et al. 2007). These groups are not yet different species, but I wonder what could happen far into the future…

If these whale survive far into the future, that is. The resident whales are threatened by several factors. Sitting at the top of the food chain, they accumulate massive amounts of toxic chemicals, espeicaly PCBs and PBDEs, from their food. The food itself, salmon, is declining due to overfishing. Finally, the tiny population size of the residents puts them at risk for inbreeding depression and stochastic extinction. Unfortunately, we don’t have a good legal mechanism for saving them. The specialized culture of the residents should be preserved, but because these whales are not a different species, and because the global numbers of killer whales are still high, they aren’t really the type of entity that the Endangered Species Act is supposed to target. Either we need a law that protects endangered non-human cultures, or else the local human community needs to recognize the ecotourism value of these whales and step up to protect them on their own.

Hoelzel AR, Hey J, Dahlheim ME, Nicholson C, Burkanov V, Black N. 2007. Evolution of Population Structure in a Highly Social Top Predator, the Killer Whale. Molecular Biology and Evolution 24(6):1407-1415


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