Eve Davidian (ISEM Montpellier), nominated by Mélissa Barkat-Defradas, was rewarded for the quality of her work in the following article:
Davidian, E., & Höner, O. P. (2022). Kinship and similarity drive coordination of breeding-group choice in male spotted hyenas. Biology Letters, 18(12), 20220402. https://doi.org/10.1098/rsbl.2022.0402
It's 6:25 in the morning...
It's 6:25 in the morning. The sun is just rising over the rim of the Ngorongoro Crater, a UNESCO site in northern Tanzania. It's been almost 40 minutes since I left the research station for the stony road that descends into the sumptuous 250 km² caldera, and I finally reach the communal burrow of the Shamba clan. This is one of the 8 spotted hyena clans that inhabit the Crater, and which the Ngorongoro Hyena Project has been studying for 28 years. I approach slowly and switch off the car engine. It's crowded. The atmosphere is peaceful. Youngsters are playing with their tails out, teenagers with full bellies are lounging around, and a few females are nursing their young (hyena litters range from 1 to 3 cubs). I spot an individual standing apart from the group, shyly sniffing the scent of the females. This is Damu, officially "M-298", a young male who immigrated three months ago from the neighboring Munge clan. Incidentally, his brother Fujo "M-297" is also spending a lot of time with the Shamba at the moment. The two brothers are often found together...
Brothers or peers from the same clan often make similar choices and, within a few weeks or months of each other, settle in the same clan to reproduce. This phenomenon has never been recorded elsewhere, certainly due to the loss of information when individuals disperse outside the study group. By tracking the life history of each of the 3,000 hyenas that have visited the Crater since 1996, we have access to this key information: we know the pedigree of the individuals, and the origin and fate of the males after dispersal.
Migration "coordination": a passive or active process?
Passive or active process? The causes of this male "coordination" were less clear to us at the time. A passive process can occur when males with similar genetic or social backgrounds, and the same needs or abilities, think and act in the same way, and are thus more likely to "accidentally" choose the same clan to reproduce with. An active or adaptive process, shaped by the selective advantages of settling with a close relative or ally, would involve flexibility in male behavior according to the costs and benefits of such coordination.
We tested these two hypotheses by comparing 148 pairs of males. The main criterion for this "artificial" matching was membership of the same cohort. To be matched, two males had to be born within 2 months of each other. We then matched the males to create three categories based on a gradient of similarity in terms of maternal environment (mother's identity and social rank), socio-ecology (clan of origin) and genetics (parentage).
Brothers are males from the same litter, raised by the same mother and from the same clan, and are therefore closely related. Peers have grown up in the same clan, but have different mothers and may be more or less closely related. Outsiders have the fewest similarities. They come from different clans, have different mothers (but may have similar social ranks) and are generally distant relatives.
The results of our study suggest that both processes are probably at work. In short, in line with the passive process hypothesis, the probability of choosing the same clan follows the similarity gradient: it is highest between brothers (70% of pairs), followed by peers (36%), and strangers (7%). In line with the hypothesis of an adaptive and flexible process, coordination varies with population density for close relatives (but not for distant relatives). That is, coordination is all the more frequent between brothers and related peers the more clans contain many females (sexual partners) and males (competitors), and the benefits in terms of social alliance and competitiveness against other males outweigh the reproductive costs of competing with one's brother or a close relative for access to females.
Conclusion
Dispersal, like group or habitat choice, is often seen as a solitary process, where individual decisions are made independently of those of fellow creatures. After all, each individual is unique and seeks to reach his or her own optimum. Much research has naturally followed the heterogeneity of dispersal strategies and focused on the factors that might explain it. By showing that brothers and familiar individuals can coordinate and modulate their choices according to the level of competition, our study adds nuance to our understanding of the role dispersal plays in social dynamics and gene flow in wild populations.
