The transmission of behavioural traditions by learning from others — cultural learning — was once thought to be a uniquely human attribute. However, evidence increasingly indicates that this phenomenon is widespread among animals, shaping behaviours from foraging for food to mate choice to predator avoidance1. Claims for human uniqueness in our cultural skills have therefore been pinned on our species’ capacity for what is called cumulative culture: the ramping up of cultural sophistication as each generation builds on their ancestors’ cultural achievements2,3. Writing in Science, Jesmer et al.4 now challenge this view in a study of the development of migratory skill in wild populations of bighorn sheep (Ovis canadensis) and moose (Alces alces) populations that have been moved to unfamiliar locations. Their findings have implications for understanding the evolution of cumulative culture in both humans and other animals, and for conservation policies5,6.
In the wild, bighorn sheep (Fig. 1) and moose normally migrate in spring and move between distinct seasonal ranges. These movements follow a pattern known as green-wave surfing, whereby the animals’ migration tracks the availability of high-quality vegetation, which peaks at different times in different places depending on factors such as altitude. How animals evolved the capacity for this type of migratory behaviour remains unknown.
Jesmer and colleagues investigated the migration of bighorn sheep and moose that had been moved to unfamiliar areas in recent decades to repopulate regions in which these types of animal had been wiped out by disease or hunting. The authors compared the migration of such relocated populations with that of animals in long-established populations that had been migrating for many generations in a particular region. They noted that when individuals had been moved to an unfamiliar location, the animals usually ceased migrating, although migratory behaviour gradually re-emerged in subsequent generations.
The researchers fitted animals with a collar containing a Global Positioning System (GPS) device that enables accurate tracking of an animal’s position. They combined this information with the corresponding satellite imagery for the region that revealed where and when vegetation was at peak quality. To measure animals’ green-wave surfing skills, the authors counted the number of days between the peak forage quality at a location and the arrival of an animal there. When the authors analysed bighorn sheep from migratory populations that had been relocated at times ranging from 0 to 35 years ago, these animals surfed the green wave approximately half as effectively as animals from populations that had been established in a particular region for more than 200 years.
Jesmer and colleagues then combined these and other bighorn records with similar data for moose that had been relocated to a given region between 10 and 110 years ago. The combined results for these 267 bighorn and 189 moose were consistent with a model in which it took up to 30 years (between 4 and 5 generations) for migration to distinct seasonal ranges to re-emerge in these species. It took almost a century for a relocated population to reach a point at which half its number migrated in this way. Animals that do not migrate to distinct seasonal ranges might begin to undertake green-wave surfing over small distances.
The bighorn-sheep data span more than two centuries, and the authors found that migratory behaviour had spread to nearly all of the bighorn sheep individuals that had been established in a location for at least around 200 years. Most interestingly, green-wave-surfing knowledge steadily increased over the decades, indicating that migratory skill progressively rises to the highest levels over long time frames.
The authors suggest that their findings can be explained by a cumulative process of acquisition of migratory skill involving cycles of individual and cultural learning that span many decades and generations. Individuals might acquire some initial surfing knowledge by personal learning, which then becomes available to their young through social learning, and the next generation might build on this knowledge through further exploration. The refinement of skills in the next generation could be similarly enhanced, and so on. Repeated cycles of individual and social learning might thus generate a cumulative culture of progressively refined surfing expertise and an increase in the proportion of migrants in the population.
Unfortunately, no direct evidence of social learning in these animals has yet been documented in the wild to support this interesting idea. However, a previous analysis of the homing of domesticated pigeons7 provides data suggesting that cumulative effects of social learning can occur in animals. In this study, two birds were tracked using GPS monitoring as they flew homing flights together. One animal of the pair was then replaced by a pigeon that had not flown the route before, and this pair of birds flew a series of homing flights. After a series of successive replacements of one bird of the pair, the efficiency of the homing flights improved significantly from that observed at the outset. The birds in the later pairings were different from those that made the initial flights, so this improvement is consistent with a model of individual learning coupled with social transmission across these ‘cultural generations’.
The bighorn and moose findings might well reflect similar learning processes. Moreover, for these animals, cultural learning will probably involve the acquisition of a diverse range of expertise relating to different aspects of migration in addition to green-wave surfing skill, such as knowledge of the predation risks in what are known as ‘landscapes of fear’8, which is of particular consequence given that offspring migrate with their mothers. The findings of Jesmer et al. provide an advance for this area of research by investigating learning in the wild, across multiple generations and over many decades, illuminating our understanding of animal culture and the collective behaviour of a population over time.
Jesmer and colleagues interpret the long-term growth over time in the populations’ green-wave surfing skills to imply that, over successive generations, the individuals of a particular population develop more-refined migration skills than those in earlier generations. However, a possibility worth investigating is whether the improvements in a relocated population’s ability to track peak vegetation might be driven mainly by an increase in the proportion of animals that learn migratory skills from others, rather than because the migratory skills of individuals increase over successive generations. Nevertheless, what would develop under this scenario is also the progressive, collective enhancement in migration skills of the population as a whole, an example that is relevant to the topic of collective intelligence in animal groups9,10.
The United Nations Environment Programme has been considering how evidence for cultural learning in animals should inform conservation policies. This is of particular note for animal populations that migrate through, or are located in, areas that cross national borders. A panel of scientists has recently assembled key evidence and recommendations related to this in a report for policymakers6. The findings of Jesmer et al. underscore the importance of such considerations if wild-animal populations develop skills that enhance their survival over a time span of centuries. In the case of migratory skills, the blocking of traditional migratory routes by human-made barriers such as roads could lead to the loss of animals’ hard-won cultural knowledge.
Conservation efforts need to take into account the significance of such knowledge, the scope of which we are perhaps only starting to recognize1,10, and our understanding of which is extended by long-term perspectives such as those reported by Jesmer and colleagues. Cumulative culture of this kind might be more widespread in nature than was previously assumed, and not unique to humans. Accordingly, understanding the gulf between these and our own species’ cumulative cultures might require us to consider more-specific aspects of cultural transmission, including modes of learning such as intentional teaching, or cultural contents, such as adopting qualitatively improved materials for tools. As the latter example suggests, human culture could progress by incorporating qualitatively distinct innovations. It remains a controversial question whether this ability is also found in animals — can they go beyond just achieving gradual refinements in a skill, such as green-wave surfing, to add a transformative new approach to solve a particular problem?
Nature 562, 198-200 (2018)