This blog post is provided by Monica L. Bond, Derek E. Lee, Arpat Ozgul, Damien R. Farine, and Barbara König and tells the #StoryBehindThePaper for the paper ‘Leaving by staying: Social dispersal in giraffes’, which was recently published in the Journal of Animal Ecology.
Like humans, animals seek independence around the time that they become sexually mature. They often do so by moving away from family, known as natal dispersal, which we studied in a population of giraffes in Tanzania. Dispersal helps animals to find new mating partners that are not closely related, which decreases the chance of inbreeding. Dispersal is driven by both spatial and social processes. While its initiation depends mostly on local social (inbreeding avoidance) and environmental factors (resource competition), a disperser’s fate is determined by regional social (mating opportunity) and environmental and spatial factors (availability of and distance to settlement sites). Therefore, dispersal is a social as well as a spatial process.
Some mammals have a highly dynamic grouping behaviour known as fission-fusion, where groups merge and split throughout each day. This type of social system is seen in a diversity of taxa including chimpanzees, dolphins and whales, elephants, several species of bats, and our study species, the giraffe. Giraffes reside in a fission-fusion social system, but the process of natal dispersal—who disperses and when, and how these dispersing giraffes realise dispersal—remains largely unknown. Given that male and female giraffes become sexually mature at about four to five years of age, documenting natal dispersal patterns requires longitudinal studies of individual giraffes. Our team has been studying Masai giraffes in northern Tanzania since 2012. We identify individuals by their unique and unchanging spot patterns and monitor them over time using rigorous survey techniques. We investigate patterns of births and deaths and movements to understand how giraffe populations are structured and sustained.
In a previous study, we found that adult female giraffes in our study population form a number of distinct social communities, each consisting of around 60 to 90 females. Within each community, females form and dissolve groups, meaning that the turnover in group membership is high and group size fluctuates almost continuously. However, the pool of females from which groups are formed is highly non-random—from our observations taken over many years we consistently find the same set of females forming groups together. This suggests that females maintain enduring social bonds with others, forming a stable community. What is quite striking is that these individuals also share the same physical space with members of different communities, but despite the opportunities to encounter and form groups together, we rarely observed females from different communities together. This means these female communities are spatially overlapping, but socially discrete and stable over time.
The complex social structure of adult female giraffes got us thinking… could dispersers effectively move through ‘genetic space’ without having to move far through physical space? That is, do they join other communities that overlap in the same area as the community they were born into? This type of dispersal, remaining close to home but joining a different social community, requires new approaches to the study of dispersal. Specifically, it requires focused data collection not only on spatial movements, but also on the social associations that individuals are forming over time.
The first step we had to take was to assess who disperses in the first place. We were lucky in our study population to be able to observe the locations and social associations of 67 male and 70 female giraffes, which we first documented as tiny calves, over a seven-year period. Our study area is huge (2200 km2) and unfenced, and our systematic surveys over this whole area enabled us to quantify whether individuals stayed within the same community into which they were born, or switched communities, as well as whether they stayed close to home or moved long distances.
In a new paper just published in the Journal of Animal Ecology, we show that while only about one quarter of young females dispersed, nearly three-quarters of males did. This confirms that in giraffes, like in the majority of other mammals, males are the dispersing sex. The results of our analysis testing whether any giraffes effected dispersal without having to make large spatial movements were particularly interesting. We found that the majority of the males that dispersed —three out of four young males—moved long distances away from where they were first seen as little calves. Of the dispersing males, more than 4 of 5 switched communities, yet around one quarter switched communities while remaining relatively close to where they grew up. Among the one quarter of females for which we suspected dispersal took place, half stayed in the same community. Regardless of sex, we found that all giraffes that dispersed did so at about four years of age which is the time of sexual maturity.
Giraffes are an African icon and one of the Earth’s last remaining megaherbivores (land mammals weighing more than 1000 kg), but their numbers are declining in the wild. Given the critical importance of genetic diversity for maintaining healthy populations, the more we understand the process of natal dispersal, the better we can help conserve wildlife. Our findings provide some fundamental information about dispersal in giraffes, confirming that most dispersers are males, that male giraffes leave the area that they grew up in around the time of sexual maturity, and that they do so mostly by moving to new areas, although some stay closer to home. By contrast, females transitioning into adulthood are much more likely to remain within the community into which they were born. Because of this natural behaviour of giraffes, conservation management plans would benefit by preserving large ecosystems in order to maximize genetic diversity and maintain the complex social system in a species that is “Vulnerable” to extinction according to the IUCN Red List.
Bio: Monica Bond is a post-doctoral research associate in the Department of Evolutionary Biology and Environmental Studies as the University of Zurich. Derek Lee is an associate research professor at Pennsylvania State University. Arpat Ozgul is an associate professor of population ecology at the Department of Evolutionary Biology and Environmental Studies at the University of Zurich. Damien Farine is an Eccellenza Professor at the Department of Evolutionary Biology and Environmental Studies at the University of Zurich. Barbara König is professor emerita in Animal Behaviour at the University of Zurich.