May 30, 2016
Humans and killer whales have so much in common that the latter are now serving as a model for our species by showing how ecology, culture and evolution reveal themselves in genomes.
Both killer whales, also called orcas, and humans are two of the world’s most cosmopolitan animals, given that both include subgroups that migrated and quickly diversified from an ancestral population likely due to climate change. The new study, published in Nature Communications, finds that such life histories can leave their mark in genes.
Our species is not closely related to orcas — toothed whales that are the largest species in the dolphin family — making the similarities all the more surprising.
“Both humans and killer whales have big brains and are able to master complex behaviors, and perhaps, most importantly, innovate new behaviors that help them adapt to different environments and to hunt different prey often using quite specialized hunting behavior,” lead author Andrew Foote told Discovery News.
Orcas exist within at least five different cultures, also called ecotypes. The various groups live in different areas, look a bit different and have different foraging strategies. One group, for example, mostly hunts seals, while others mostly subsist on penguins or fish. Social learning within the cohesive groups appears to facilitate the specializations.
Foote, an evolutionary ecologist who is a visiting scholar at the University of Bern, and his team studied and compared the genomes of these different orca cultures, and found that their predecessors came from an ancestral population that began to migrate starting at about 250,000 years ago.
The great journeys are told in the DNA, as the migrant relatives exhibit less genetic diversity than the ancestral population. Rare genetic mutations in the migrants also help to trace travel paths, since these mutations were passed on to successive generations. The same phenomena are seen in our species.
“In humans, we see this signal of changes in the frequency of mutations, and loss of genetic diversity associated with the migration out of Africa,” Foote said. “We also see this in our genomic data for each killer whale ecotype.”
The researchers point out that humans and orcas have each colonized a portion of the globe in a relatively short period of time, from an evolutionary standpoint. Both sets of travelers were exposed to differences in climate and available food sources.
“It is the extent of this geographic scale and consequently the range of climatic, habitat and dietary variables that make the comparison between killer whales and humans an attractive one, and in which humans differ from other great apes (such as chimps and gorillas),” Foote said.
For both orcas and humans, there is a “chicken and egg” question: Which came first, cultural change or genetic change?
The scientists suspect that behavioral flexibility in both animal groups leads to cultures inherited through social learning over many generations. As time goes on, genetic changes that may contribute to the adaptations can also rise in frequency and gradually become widespread throughout the population as it expands. Cultural and genetic change therefore go hand in hand, but the behavioral flexibility that leads to the former probably happens first.
Orca experts contacted by Discovery News said that the new paper represents a milestone in marine mammal research.
Hal Whitehead, a biologist at Dalhousie University, said, “I had hoped for some time that someone would begin to look at gene-culture coevolution in killer in killer whales using genomics, but the extent of this research goes beyond well my hope. The results are fascinating. We now see how in killer whales, as in humans, culture is not only an important factor in the lives of the whales, but also drives genetic evolution.”
Michael Krützen of the University of Zurich’s Anthropological Institute & Museum said, “So far, we have not been able to investigate the link between adaptive evolution and the demographic history of a marine mammal species in such great detail. The genomics analyses are state of the art and allowed novel insights into the processes shaping the genome of a marine mammal.”
Krützen added that “there is no reason to assume gene-culture co-evolution is not also happening in other highly cultural species, such as for instance great apes and some marine mammals.” Future studies might therefore apply the same approach to other intelligent, social animals, such as bottlenose dolphins, which also exhibit gene-culture coevolution.