When evolution is also driven by culture: killer whales

Killer whales (Orcinus corca) are highly social marine mammals, belonging to the Dolphinidae family (Delphinidae). These cetaceans are found in all oceans and seas of the world, from the frigid Arctic and Antarctic waters to tropical seas, and have occasionally been sighted near the mouths of some rivers. Killer whales exhibit both sedentary and migratory behaviour: they prefer cold waters, but some populations migrate towards the equator in summer. Unbeatable predators, they can reach speeds of up to 55 km/h and their bite is considered one of the most powerful in nature, far superior to that of the white shark, for example. Finally, the size of killer whales is generally around 6-8 metres long for males and 5-7 metres for females, with a weight of around 4-6 tonnes.

Highly intelligent animals with a complex social structure, killer whales live in groups called pods, generally no larger than 25 individuals. The pod structure is matrilineal and consists of an adult female and her young, both males and females, her daughters' offspring, sterile older females and an adult male. All pods have their own language or 'dialect', handed down from generation to generation but which may be influenced by close proximity between two different groups.

Killer whales are formidable killers and have no predators in the wild. For this reason, they are considered superpredators at the top of the food chain. Their diet depends on the individual populations and is very varied: some killer whales feed exclusively on fish, others hunt birds and penguins, while others hunt marine mammals such as seals, dolphins, sea lions and even whales. In some locations, the different killer whale populations divide into specialised ecotypes, i.e. groups belonging to the same species but occupying different ecological niches and presenting diversified hunting strategies.

For example, there are two distinct ecotypes of killer whales in the North Pacific Ocean: the transient ecotype, which specialises in hunting marine mammals, and the resident ecotype, which feeds mainly on fish. While hunting, transient killer whales become very quiet to surprise their prey and their attack requires a high level of coordination, with each individual playing a specific role within the group. Again, in 1988, a new type of killer whale population living in the open sea was discovered and dubbed offshore: much more numerous and comprising around 60 individuals, it feeds and hunts with complex strategies mainly fish, sharks and large marine mammals.

If we define culture as 'information acquired through teaching, imitation or other forms of learning and capable of influencing individual behaviour', we can attempt to state that the hunting and behavioural practices of different ecotypes, which need to be passed on, learned and improved from generation to generation, are a form of animal culture. Indeed, the hunting strategies of killer whales require the acquisition of specific skills through learning and result in marked cultural differences between ecotypes, differences that are maintained through cultural transmission. The importance of teaching and transmission of behavioural/cultural practices (but also of the nurturing of young) is so important in killer whales that it explains evolutionarily - according to the so-called "grandmother hypothesis" - another phenomenon that is very rare in nature and that we have in common with these animals: menopause.

Well, according to a study published in Nature communications and titled "Genome-culture co-evolution promotes rapid divergence of killer whale ecotypes", cultural practices may have influenced, and may still influence, the evolution of these extraordinary animals. Genetic analyses conducted by the researchers revealed that the separation between the two main ecotypes (transient and resident) occurred around 68,000 and 35,000 years ago, respectively. Furthermore, it emerged that the males rarely changed ecotype and mated mainly with females with the same feeding habits, thus gradually contributing to a form of genetic isolation between the different ecotypes.

Furthermore, according to the study, the diet of different killer whale groups affects certain adaptations at the molecular level. In particular, it has been observed that a diet rich in methionine, an amino acid abundantly found in mammals, has a major impact on the selection of metabolic pathways and markedly modifies the DNA of transient killer whales, leading in the long term to a possible phenomenon of speciation between different ecotypes. Although the relationship between genes and culture is an issue that has yet to be clarified within evolutionary studies, this study is very important and could represent an example of co-evolution between genes and culture. It opens up the possibility of studying behavioural and cultural phenomena not only as the result of genetic evolution, but also as possible co-factors - albeit limited - of species origin and diversification.

Despite being incredible predators of extraordinary intelligence, the survival of orcas is under constant threat. A recent study has, for instance, highlighted the inexorable decline of the last 75 resident killer whales along the Pacific coast of North America, attributing it mainly to the intensive fishing of king salmon, their main food. Not only that, despite the official 1982 hunting ban by the IWC, humans and their illegal activities continue to threaten these magnificent animals. Killer whales are considered a nuisance to fishing vessels, and are often intentionally attacked, as they are wrongly blamed for the dwindling fish stocks in the seas. Furthermore, factors such as the degradation of marine ecosystems, pollution and climate change are sources of dramatic decline for the populations of this species, which exhibits more human-like behaviour than others.