Willingness to help depends on family size


By Eve Davidian and Oliver Höner

Why is it that in some species individuals are more willing to help their groupmates than in other species and why does the willingness to help change with age? In our new study led by colleagues from the University of Exeter and just out in the journal Nature Ecology and Evolution, we shed light on the possible roots of differences in willingness to help others across animal societies and between individuals of different age and sex. Using data from seven mammal species – including spotted hyenas – we show that the number of kin an individual has in a group can change over its lifetime. These ‘kinship dynamics’ often differ between males and females and profoundly influence the incentive of individuals to help their groupmates.

When you live in a group of close kin, it might be in your best interest to help your groupmates because helping individuals who share genes with you such as your offspring and siblings is a bit like helping yourself. In contrast, when you live among distantly-related or unrelated individuals – say, second-degree cousins or total strangers – your best strategy may be to be selfish or even harmful to the others. But what happens when the number of kin in a social group fluctuates over time, for example because some family members disperse or die? To answer this question and get an idea of what is going on across mammals, we teamed up with scientists working on other awesome animals: the killer whale, yellow baboon, banded mongoose, chimpanzee, rhesus macaque, and European badger.

The seven species were not picked at random. Our Exonian colleagues looked for the “crème de la crème” of wildlife datasets. This means species for which scientists compiled a detailed genetic pedigree (aka ‘family tree’) of a population and demographic information over the lifetime of many individuals of the population. To give you an idea, the hyena dataset used for this study contained data on the life history – i.e., the birth, reproduction and death – of more than 2000 spotted hyenas over nine generations. This required 26 years of continuous monitoring (see how we do) of our eight study clans in the Ngorongoro Crater. And it was crucial for the study that the species differed from one another in two key features of their social system:

  1. Dispersal: Are some individuals (males, females, both, neither) more likely to leave their birth group when adult?
  2. Mating: Do individuals reproduce with members of their own group (‘local’ mating) or mainly seek mates in other groups (‘extra-group’ mating)?

Dispersal and mating patterns shape kinship dynamics. We found that the number of kin an individual has in a group changes over its lifetime but also that the direction of the change – that is, whether it increases or decreases – is determined by the species dispersal and mating patterns. As a result, kinship dynamics vary from species to species but patterns also often differ between males and females of the same species.

Kinship dynamics may explain why female killer whales go through menopause. In killer whales, neither sex disperses from their birth group (called a ‘pod’). Sons and daughters thereby both remain with their mother and adult females live among a growing number of offspring and grand-offspring as they age. This pattern was suggested to explain why old female killer whales – and possibly also women in humans – experience menopause as a ‘cooperative’ strategy to avoid the costs of competing with their daughters and grand-offspring (see study here). For male killer whales, patterns are reversed because of the “extra-group” mating system, which implies that males sire offspring with females that live in other pods. As a result, the offspring that are recruited in their pod are not closely related to the native males.

In spotted hyenas, males should become more willing to help others as they age. We found that the number of close kin increases for male hyenas but decreased for females. This pattern was largely shaped by the fact that dispersal is strongly male biased in spotted hyenas: most males leave their natal group after reaching sexual maturity and establish themselves as immigrants in other clans whereas the daughters remain in their birth clan all their life. Females therefore live among fewer close kin as they get older, because their mother and older sisters and aunts eventually die. In contrast, males have no or few kin upon immigration into their new clan but their numbers increase over time, as males sire more and more daughters (that remain in the clan). The distinct kinship dynamics in male and female hyenas predicts that males should be more willing to help other group members as they age while females should become more selfish. More detailed investigation of hyena behaviour is needed to assess if this in fact is the case. So stay tuned!

Original publication

Ellis S, Johnstone RA, Cant MA, Franks DE, Weiss MN, Alberts SC, Balcomb KS, Benton CH, Brent LJN, Crockford C, Davidian E, Delahay RJ, Ellifrit DK, Höner OP, Meniri M, McDonald RA, Nichols HJ, Thompson FJ, Vigilant L, Wittig RM, Croft DP (2022) Patterns and consequences of age-linked change in local relatedness in animal societies. Nature Ecology & Evolution.

Further information

Davidian E, Courtiol A, Wachter B, Hofer H, Höner OP (2016) Why do some males choose to breed at home when most other males disperse? Science Advances 2 e1501236.

Höner OP, Wachter B, East ML, Streich WJ, Wilhelm K, Burke T, Hofer H (2007) Female mate-choice drives the evolution of male-biased dispersal in a social mammal. Nature 448:798-801.

Vullioud C*, Davidian E*, Wachter B, Rousset F, Courtiol A*, Höner OP* (2019) Social support drives female dominance in the spotted hyaena. Nature Ecology & Evolution 3:71-76. *contributed equally

To eat or to mate that’s the question


By Eve Davidian

To reproduce is the ultimate goal of all living creatures. But to achieve this, individuals first have to survive long enough, and so must eat. The more, the better? Not so sure. A good diet may make you strong and appealing to potential mates but an excess of food can turn out to be counterproductive.

Jage, a male from the Ngoitokitok clan wanted it all, to feast and to mate, but he learnt the hard way that one sometimes has to choose. One day, after months spent courting females of his clan, he was lucky enough to find favor with Uvumiliva, a young and high-ranking female (to say, a nice piece). But there was a catch; Jage had feasted earlier in the morning and his belly was so bloated that he could not seal the deal.

It should be pointed out that mating is a very tricky task for male spotted hyenas because females’ private parts are masculinized into a pseudo-penis. This means that reaching seventh heaven requires full cooperation from the female – she has to stand still, put her head down and retract her pseudo-penis. But even when the female does cooperate, the male still has to show great acrobatic skills. It does take males some time and practice to get it right, and those with little experience usually are clumsy and sorely try the female’s patience.

On that hot day, Jage tackled the task. But no matter how much he wiggled and waggled, his huge, flabby belly kept barring the way. Jage was exhausted by all this physical activity. Just picture yourself running a marathon while aiming at a bullseye with an arrow, all this after having devoured a huge platter of meat! Jage was panting and drooling, and had to take regular breaks to catch his breath. Meanwhile, Uvumiliva remained amazingly calm and supportive – she actually got her name, meaning ‘patient’, after that day.

Jage really was not at his best that day. In a moment of weakness, he even tried to blame it all on Nyemeleo, a rival male born in the Shamba clan that had just joined the Ngoitokitok clan. Of course Nyemeleo was not responsible for Jage’s misery; apart from standing beside the couple and peeping at them, which let’s admit was slightly sleazy, Nyemeleo was not really interfering. Maybe Nyemeleo simply wanted to show off his athletic body and perfectly-sized belly, hoping that Uvumiliva would change her mind and pick him instead of the chubby guy. But quite clearly Nyemeleo’s presence did not help Jage to keep his focus on the target. Hyena sweethearts do like to have their privacy and usually do their ‘business’ in remote areas of their clan territory. After spending hours peeping (yes, but for Science!) at the trio, we had to leave them. We hoped for the best but our expectations were low.

Four months later – the duration of gestation in spotted hyenas – Uvumiliva did not give birth to cubs, suggesting both that Jage’s attempts failed and that Uvumiliva did not give Nyemeleo a chance. Let’s hope that Jage at least learnt a lesson from this unfortunate turn of events and that he now thinks twice before having an umpteenth slice of meat.

Moral of the story: Better eat light than miss the boat.

Here is the video showing Jage’s setbacks:

It’s tough to be a male in love


By Eve Davidian

Spotted hyena males can be very persistent when they have a crush on a female. They may follow the female’s every movement: lie down a few meters away from her when she decides to rest, observe her when she changes her position or lifts her head, and get up and follow her when she walks away – in human society, such a behavior would be considered as stalking. In spotted hyenas, males use this rather exhausting strategy to foster and maintain a friendly relationship with the female – and more if they hit it off. Staying nearby a female also allows the male to displace rivals if they come too close to their loved one or to defend the female if she is being aggressively harassed by another male – harassment by males occurs regularly but is a far less successful strategy than a charming courtship.

There was a period when Kondo, one of the males in the Ngoitokitok clan, was very much into the female Aiba and was following her wherever she went. Aiba seemed ok with this rather old guy sticking around. They actually made quite a cute couple sleeping close to each other. One day, as we were approaching them by car, Aiba got up and walked away from her resting place – and from Kondo. Kondo was then deep asleep and did not react to her departure. When he woke up 15 minutes later, yawning and stretching his legs, he immediately checked the place where his girlfriend was resting. As soon as he realised that Aiba was gone he jumped up and scrutinised the surroundings to see where she was. But Aiba was nowhere to be found. Kondo looked very nervous indeed; running around and sniffing the ground for cues, pricking his ears up, running again in great excitement. It was obvious that he was desperate to find Aiba again.

We took pity on Kondo and tried to show him the direction Aiba had taken but Kondo was not paying attention. He kept trying to pick up her scent and eventually ran in the wrong direction. Poor Kondo! For more than two weeks, he had bet everything on Aiba and would have to start from scratch with another female if he did not find Aiba quickly. But no worry, when we saw Kondo again three days later he was reunited with his beloved. It seemed that after all Aiba did not resent Kondo for his lack of attention.

Here is what a male looks like when desperately looking for his lost female: