How collective sensing, ontogeny, and individual condition influence migration in a gregarious African fruit bat

When thinking about migration in the animal world, most people probably think at first about the migration of birds. But what about the migration of other animal species? Edward Hurme and his colleagues Martin Wikelski and Dina Dechmann are studying the migration of straw-coloured fruit bats within their project How collective sensing, ontogeny, and individual condition influence migration in a gregarious African fruit bat. They track the green wave across Africa with ICARUS tags. “This is the first project to conduct long-term GPS tracking of migratory bats,” says Edward Hurme. “While migration has been inferred in several species, it is yet to be clearly demonstrated through tracking.” The postdoctoral researcher and his colleagues also collect samples to look at age, diet and viral load in order to understand how these factors influence migration.

The aim of the study is not only to explore the migration route, but also how individuals and groups respond to natural changes in the environment on a large scale. “We already discovered that bat migration is correlated with the green wave, i.e. increase in greening typically seen during spring or the beginning of the wet season,” says Hurme. “Additionally, this pattern was stronger in larger colonies, suggesting that collective sensing may improve migration timing. We can now develop new hypotheses from these results that we can test on data from individually tagged bats.”

Field study in four African countries

To achieve this, they tag roughly 30 individuals per country. They plan to implement the field study in Rwanda, Zambia, Cameroon, and Ghana. “Additionally, we will monitor colonies across Africa through a monitoring network composed of volunteers conducting monthly counts as well as use camera arrays combined with imaging software to provide detailed counts for colonies that are too large to estimate manually,” comments Hurme. The CASCB members are therefore working together with local researchers, conservation groups and community leaders to conduct monthly counts of bat colonies. In addition, they have developed a computer vision method to conduct more accurate counts in colonies with over one million individuals.

Edward Hurme feels certain that they will advance the study of collective behaviour: “Focusing on the migration of a large fruit bat in Africa will add to our understanding of migration because there is little information on migration in bats, frugivores, or species that move solely throughout the tropics.” What particularly impresses him is that this species forms colonies of thousands to millions, providing opportunities for individuals to benefit from social information. “While we have yet to fully tease apart their collective behaviour, this species seems to be a good candidate for collective migration.”