Tag: Ngorongoro

Friends over muscles: How female hyenas came to dominate males

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By Oliver Höner and Eve Davidian

In most animal societies, members of one sex dominate those of the other. Is this, as widely believed, an inevitable consequence of a disparity in strength and ferocity between males and females? Not necessarily. Our new study shows that in spotted hyenas, females dominate males because they can rely on greater social support, not because they are stronger or more competitive in any other individual attribute. The main reason for females having, on average, more social support than males is that males are more likely to disperse and that dispersal disrupts social bonds. This study was conducted in collaboration with the Institut des Sciences de l’Evolution de Montpellier (ISEM-CNRS, France). Our results were published in the journal Nature Ecology & Evolution.

Spotted hyena females are often portrayed as archetypes of powerful and ferocious females. They are on average heavier than the males, have highly masculinised outer genitalia (a ‘pseudo-penis’ and a ‘pseudo-scrotum’), and usually occupy the highest position in the society. But according to the new study, it is not their manliness that allows them to dominate males.

When two hyenas squabble, the one that can rely on greater social support wins, irrespective of sex, body mass or aggressiveness. Differences in social support between two individuals correctly predicted who will be the dominant in almost all encounters and in all contexts – between natives and immigrants, members of the same and different clans, residents and intruders, and individuals of the same and opposite sex.

Female dominance thus emerges from females being more likely to receive greater social support than males. What is so fascinating is that it all works without any direct involvement of other hyenas. In the end, it’s all about assertiveness and how confident a hyena is of receiving support if needed.

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For this study, our team analysed the outcome of 4133 agonistic interactions between 748 hyenas from eight different clans monitored for 21 years in the Ngorongoro Crater in Tanzania. To estimate potential social support, we developed an algorithm that predicted for each clan member, which of two interacting hyenas it would support; this algorithm was derived from behavioural observations of social support and relatedness estimates based on one of the most comprehensive pedigrees of a free-ranging mammal. To tease apart the effects of social support and intrinsic attributes such as body mass, one needs to evaluate each effect while controlling for the presence of the other. When this is done, the effects of sex and body mass appear negligible.

Clear dominance relationship

In many social contexts, female and male hyenas can rely on equal social support and are equally likely to win. But there is one exception: when natives interact with immigrant clan members. Hyena society is highly nepotistic and social support is primarily given to kin. Native clan members live among their relatives and have a competitive advantage over immigrants because immigrants lose their social bonds when they disperse from home. In this context, females have the upper hand because immigrants are usually males.

Female dominance in spotted hyenas is therefore driven by the sex bias in dispersal and the demographic structure of the clan: when the clan contains a high proportion of immigrant males, female dominance is nearly absolute. But when the clan contains many native males, males win almost as often as females and the sexes are co-dominant.

Implications of the study:

Identifying the determinants of dominance relationships between the sexes is fundamental to understanding the evolution of reproductive strategies, gender roles, and sexual conflicts. Our findings show that social dominance of one sex over the other – a trait that characterises gender roles – does not need to be a direct consequence of sex or physical strength, but can be shaped by the social environment.

By demonstrating the key role of social support in mediating the establishment of dominance – and sex-biased dominance – the study improves our understanding of the social impact of nepotism, political alliances, as well as emigration and immigration patterns in animal and human societies.

Original publication

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

Media outlets

Hyena girl power explained. CNRS-news, 19th November 2018.

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.

To eat or to mate that’s the question

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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
Uvumiliva

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

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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.

Aiba
Aiba
Kondo
Kondo

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:

Mama’s boys are not losers in spotted hyenas!

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By Eve Davidian and Oliver Höner

Males that stay at home are not second-class males but can breed as successfully as their more adventurous competitors that leave home, our new study on spotted hyenas shows. The results were published in the open access journal Science Advances.

In most mammals, there are two kinds of males: those that stay at home and those that disperse from home to breed elsewhere. In the scientific literature, stay-home males were generally considered as ‘losers’ that failed to join another group and that father few offspring. Our new study now demonstrates, for the first time in a group-living mammal, that stay-home (philopatric) males and dispersers are not inherently different and have similar reproductive success. Our study further shows that their choice to stay at home or leave home is the outcome of a process during which all males answer the same question: which group will provide me with the best fitness prospects? Whether a spotted hyena male stays at home or hits the road simply depends on whether his birth clan or another clan contains the highest number of young females when he wants to start breeding.

View of the western side of the Ngorongoro CraterFor this study, we monitored the entire population of spotted hyenas inhabiting the Ngorongoro Crater in Tanzania. We combined two decades of demographic data from the eight clans with data on the behaviour, survival, and reproductive success of more than 250 males to investigate the causes and fitness consequences of male clan choice.

In natural populations, group sizes and the number of young females fluctuate due to chance and environmental effects. Thus, when there are more than two groups, the home group is less likely to contain the highest number of young females than a non-home group. As expected, most hyena males in the Ngorongoro Crater dispersed to another clan. Yet, we found that twice as many males stayed at home than we had expected based on the distribution of young females only, suggesting that staying at home comes with advantages.

The main advantage is provided by their mothers. In the matriarchal system of spotted hyenas, females can influence the competition among males. Mothers provide social support to their stay-home sons and ensure they acquire a high social rank among breeding males. This gives the mama’s boys privileged access to both food and females, allowing them to invest a lot of time consorting females. And this pays off. Genetic paternity analyses showed that stay-home males father their first cubs at a younger age than dispersers and father almost exclusively cubs with high-ranking females — females of high reproductive value because they are most successful at rearing offspring. This is the first empirical evidence in a group-living mammal, that stay-home males can be equally, if not more, successful as dispersers.

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Dispersal is a key driver of ecological and evolutionary processes, yet scientists still do not fully understand why individuals of the same sex of a species differ in their propensity to disperse. By showing that dispersal patterns can be shaped by the distribution of breeding partners across groups, our study expands our understanding of the processes leading to the coexistence of philopatry and dispersal within a sex.

Original publication

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.

Further information

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.

Höner OP, Wachter B, Hofer H, Wilhelm K, Thierer D, Trillmich F, Burke T, East ML (2010) The fitness of dispersing spotted hyaena sons is influenced by maternal social status. Nature Communications 1: 60.

Triplets survived!

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By Oliver Höner

We just got confirmation that a new triplet litter survived to adulthood in the Crater! This is very exciting news because spotted hyenas only very rarely give birth to triplets and raising all three cubs of a triplet litter to adulthood (2 years of age) is extremely rare! Spotted hyena females only have two functional teats, forcing mothers of triplet litters to show great organisational skills to allocate sufficient suckling time for each cub.

This is only the third ever reported case of surviving triplets in spotted hyenas! The mother is Mkakati, the alpha female of the Forest clan. The other two litters were also reared in the Ngorongoro Crater, both by Kiwanda, the former alpha female of the Ngoitokitok clan. We had previously performed genetic analyses to verify that Mkakati was indeed the mother of the three cubs. Confirming maternity with genetic data is important because hyena mothers can, in rare cases, adopt cubs form another female.

Mwanga with triplet

We now also finalised the paternity analysis for all three cubs. In contrast to the two previous triplet litters that were sired by a single male, the three cubs of Mkakati were sired by two fathers. This is in line with our observations from twin litters where 84% of litters are sired by a single male but 16% are sired by two different males. These results are very interesting for our study of female mate choice and of male reproductive success.

Wild dogs back in the Crater!

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By Oliver Höner

After more than 30 years of absence, African wild dogs have been sighted again recently on the floor of the Ngorongoro Crater! A group of four strong and healthy adult dogs roamed though the territory of the Forest clan on the western side of the Crater floor.

African wild dogThese are very exciting news because African wild dogs (Lycaon pictus) went almost extinct in the Serengeti ecosystem in the 1990s. They  are highly social and live in a complex social and breeding system that is quite different from that of all other social carnivores, including free-ranging domestic dogs.

Unfortunately, some of the characteristics of their social system makes it difficult for African wild dogs to repopulate areas with no or very low densities of wild dogs and highly fragmented areas. Wild dogs have been, and still are, persecuted by humans when they are seen as threat to domestic lifestock. Furthermore, there is strong evidence to suggest that invasive handling and vaccination have adverse effects that may even lead to the death of entire packs. And once packs are reduced to small sizes, and suitable habitats are fragmented and altered by humans, populations rarely recover. As a result, African wild dogs have gone extinct in many areas in Africa and are considered ‘endangered’ by the International Union for Conservation of Nature (IUCN) (see the website of the IUCN Red List of Threatened Species).

We hope that these iconic, beautiful and athletic carnivores feel comfortable in the Crater and stay for a long time to come!

African wild dogs resting

Further Information

On the social system of African wild dogs and the adverse consequences of invasive handling and vaccination: Websites of the Painted dog Research Trust and African Wild Dog Watch.

Scientific publications on African wild dogs and the Achille’s heel of sociality, Allee effects in cooperative breeders, and potential ecological traps.