How Killer Whale Culture Impacts Their Evolution
Culture can be defined as information or behavior in a group that is obtained from conspecifics through social learning. Human culture has been studied extensively, but biologists are starting to examine cultures in other animals, and killer whales are one of them.
Killer whales can be found in every ocean on the planet. Apart from us, they are the most widespread mammal on earth. They live in matriarchal societies and generally have strong maternal philopatry (basically, they stick close to mom for most of their life). While currently only one species of killer whale is recognized, there are at minimum 10 different kinds of killer whales around the world, called ecotypes. Each of these ecotypes differs in morphology, diet, vocalizations, genetics, and behavior. They are fairly xenophobic, meaning the ecotypes do not associate with one another and don’t intermix.
Each ecotype also has its own unique culture, mainly in the form of diet and hunting techniques. For example, large type B killer whales––also called pack ice killer whales––specialize in hunting seals through a hunting method called wave washing:
This is a learned behavior and has to be taught to calves in order for them to master it.
Another example comes from resident and transient killer whales. Resident killer whales feed solely on fish, mainly salmon, while transient killer whales prefer marine mammals such as seals, sea lions, and other cetaceans. This, again, is a cultural preference.
Photo: Jodi Frediani
Photo: Center for Whale Research
Dialects are another important factor in killer whale culture. Each population and pod has its own unique vocalizations. Killer whales are vocal learners and learn these dialects from family members, thus making this culture. It is thought that killer whales prevent inbreeding by mating with individuals whose dialects are different from their own. However, they also seem to avoid mating with whales whose dialects are too different, thus preventing mating between ecotypes.
Cross-ecotype dispersal has also never been observed. This is probably because it would be far too difficult for a killer whale of one ecotype to migrate to another. A killer whale that’s been raised to eat salmon and hunt salmon would have a hard time learning how to hunt marine mammals and consume marine mammals.
How might these cultures be impacting how killer whale ecotypes are evolving?
This is still a fairly new field of research, so not much is known quite yet. However, we do have some ideas of how these cultures might be affecting killer whales on a genetic level. Let’s take a look at residents and transients again.
Studies on killer whale genomes have shown that there is some sort of evolutionary pressure occurring on some genes related to diet in residents and transients. Biologists have found that genes involved in the early development of the digestive tract differ in residents and transients, as well as genes that are involved in the hydrolysis of long chain fatty acid esters. Additionally, biologists have found differences amongst the two ecotypes in the genes involved in the methionine cycle. Methionine is an essential amino acid that the body cannot produce on its own, and it must be obtained from the diet via protein intake. They have hypothesized that transient killer whales are consuming large amounts of mammal protein in a sporadic manner, while residents are eating smaller amounts of fish protein in more consistent intervals, and thus, these differences could be exerting different selection pressures (namely, cultural preferences for food) on these genes. It’s also been noted that transient killer whales have more robust jaws and differently shaped skulls.
Another gene that is different between killer whale ecotypes is the FAM83H gene. This gene is involved with keratin cytoskeletons in epithelial cells. The gene is different in the ecotypes found in Antarctica compared to those found in the North Pacific. Antarctic killer whales, particularly type B and type C, usually have a layer of yellowish algae called diatoms found on the surface of their skin:
Photo: NOAA Fisheries Service
In order for killer whales to shed their outer layer of dead skin and grow new ones, they need to have ample blood supply to those cells, Antarctic killer whales, however, live in an extremely cold environment and likely direct blood flow to more vital parts of their body to keep warm, so skin regeneration is likely slow. This allows for the build-up of diatoms to occur. It’s been discovered that type B killer whales make long migrations to the warm waters off of Uruguay and Brazil, likely to aid in the sloughing of old, diatom-infested skin. When they return to Antarctica, those previously yellow spots are now a shiny white color again. Culture doesn’t have a clear role in this gene, but it is certainly worth investigating further as culture does have geographical implications at times.
There are also other morphological differences between killer whales that could be caused by culture, though scientists have not yet identified candidate genes. Here is a part of a table from Hal Whitehead’s paper “Genome-culture coevolution in whales and dolphins”:
Culture and its role in evolution is an exciting and new field of research. Knowing that culture can have an impact on organisms at a genetic level adds a whole level of complexity to evolutionary biology, but it can explain some of these fascinating and profound differences amongst killer whale ecotypes.
Header image source: Amadeo Bachar