Category: blog

Willingness to help depends on family size

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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

Emotions and cultural importance help us understand the “beef” between pastoralists and large carnivores

A dive into the fascinating world of human-carnivore relationships

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By Arjun Dheer

There isn’t a “one size fits all” solution to the problems that can arise in areas where humans live near wild animals. But one thing is consistently important: making sure that the people who live alongside the animals are on board with whatever management decisions are put into place to promote coexistence. This is especially true when it comes to potentially dangerous animals like large carnivores – for example, lions, tigers, and bears (oh my). Humans have long had a love-hate relationship with large carnivores: they’re aesthetically pleasing, awe-inspiring, and powerful, but also potentially damaging to human livelihoods due to their habit of killing livestock. They might be easy to admire, but they’re definitely not easy to live with!

Maasai woman milking a cow

How can we understand what makes people tick when it comes to managing large carnivores? The traditional focus has been on livestock depredation: the propensity of large carnivores to kill animals such as cattle, sheep, goats, and donkeys. Predictably, most studies have found that the more livestock killing that goes on, the more people want the large carnivores either killed or moved away. More recently, scientists have investigated the importance of psychological factors like the emotions people feel towards large carnivores and of the importance carnivores have in their culture. Positive emotions and cultural importance were found to predict the acceptance of conservation-oriented management strategies; and negative emotions, more invasive strategies. Past research looked at these factors separately, but did not compare them at the same time to see which is the best predictor, which would provide wildlife managers with information on what to target when looking to reduce human-carnivore conflict.

So what did we do? We looked at all of them simultaneously in order to draw comparisons. We chose to do this for multiple large carnivore species – spotted hyenas, lions, and leopards. And we zeroed in on three primary management strategies that have been used at our field site (Ngorongoro Conservation Area, or NCA) and are applicable worldwide wherever humans and large carnivores live in the same area: no action (letting the carnivores be), relocation (moving them away), and lethal control (killing them). We conducted interviews with 100 members of the Maasai community across the NCA to gather the data. Respondents rated the emotions of joy, disgust, and fear and the cultural importance they felt towards the large carnivores. They were also asked to report how many cattle, goats, sheep, and donkeys they lost to the carnivores over the past three years. And we asked them to what degree they’d accept the management strategies for each species.

What was the outcome? Interestingly, emotions and cultural importance were stronger predictors for the acceptance of management strategies than livestock depredation. Between the emotions, joy was the strongest predictor; it was positive for no action and negative for relocation and lethal control. Cultural importance showed a trend similar to joy. Overall, respondents accepted no action towards the carnivores and rejected relocation and lethal control. Between the species, lions were viewed the most positively and hyenas the least.

Implications of the study:

All this goes to show how important emotional and cultural aspects are in human-wildlife relationships. That’s not to say that livestock depredation isn’t important, but that it might be overemphasized. Multi-pronged approaches that combine psychological and cultural factors with the close involvement of local communities can help pave the way for continued human-carnivore coexistence. Maybe scientists have been barking up the wrong tree with so much focus on livestock depredation!

Original Publication:

Dheer A, Davidian E, Jacobs MH, Ndorosa J, Straka TM, Höner OP (2021) Emotions and cultural importance predict the acceptance of large carnivore management strategies by Maasai pastoralists. Frontiers in Conservation Science 23, 1–13.

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Why do the top dogs get the pretty ladies?

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A story of sex, stress and hyena poops

By Eve Davidian

In most animal societies, resources are not shared equally among members of a group. Those at the top of the social hierarchy eat the tastiest food, get the comfiest sleeping spots, and can hang out – and more if they hit it off – with the most attractive and fecund mates. In the animal world, where lifetime achievement is largely determined by the number of offspring one leaves behind, it is rather straightforward why individuals should work hard to reach the top and try to remain there for as long as they can.

What we still don’t quite understand is how social rank influences reproductive success. Do high-ranking males sire more offspring and offspring of higher quality because they are stronger and more attractive? Or is it because they are less “stressed” by competition with other males and can invest more in courting females?

Gorda
Male hyena cuddling a female
Schnüfli

To answer these questions, we did fieldwork in the African savannah. Lots of fieldwork. Over 20 years of searching, identifying, assessing paternities, and monitoring the behaviour of thousands of free-ranging spotted hyenas in the Ngorongoro Crater in Northern Tanzania. We also collected over 400 steaming hyena scats to measure the concentration of cortisol, an estimate of the physiological costs – or so-called “stress” – borne by a hyena. [To know what it takes to be a hyena poop hunter click on the poopers below.]

In our study published in the journal Functional Ecology, we demonstrate that interactions with other males are more stressful for low-ranking males than for their high-ranking rivals and that this restricts the time and energy they can invest in courting the most fecund – and most contested – females. We also found that males have to juggle romance and more mundane duties like getting acquainted with new clan mates and maintaining old friendships and strategic alliances.

But low-ranking males shy away from these stressful activities and spend more time away from other clan members, munching on bones or chilling in stinky puddles. High-ranking males, on the other hand, need less time on their own to detox and de-stress, and can invest a lot more in fostering friendly relationships with females. And that’s something hyena females are very much into!

Another very neat result was that philopatric – “stay-home, mama’s boys” – males prioritise reproduction over the fostering of social ties, and focus their reproductive efforts on high-quality females – a likely consequence of their native status and higher social rank compared to immigrants. These results very nicely match our previous work on male reproductive skew and the fitness consequences of male dispersal strategies where we found that philopatric males reproduce earlier than immigrants and sire offspring almost exclusively with top ranking females.

In contrast to many species where males use their physical strength, long horns and sharp teeth to deter rivals – or even to sexually coerce females – male spotted hyenas do not engage in violent fights to get to the top of the hierarchy and to sire offspring [males couldn’t coerce females even if they wanted to because female genitals are masculinized into a rape-proof device]. Why then are low-ranking males more stressed out than high-ranking males?

Male hyenas may not be ruthless brutes but they are no peaceful hippies either. Dominance relationships in hyena society are primarily determined by how many social allies a hyena can count on when in conflict with others. Low-ranking males usually are newcomers and lack strong alliances. They are also more vulnerable to being used as scapegoats by others and this is likely to be a major source of stress. Scapegoating is frequent among hyenas (and other animals) and likely serves as a means to release frustration and cope with stress. In hyenas, it often takes the form of a chain of dominance whereby successive males redirect aggression onto another, lower-ranking male. And when the lowest-ranking male of such a chain has no scapegoat nearby, rushing tail up at an innocent jackal, a rock, or even our research vehicle seems to do the job.

But don’t feel too sorry for low-ranking males. Their time will come. The social rank of male spotted hyenas is determined by a queuing convention. Most males eventually climb the social ladder and get to enjoy the perks of being the top dog.

Our results reveal that even in a society where dominance relationships are formalised by strict social conventions, interactions among males can incur physiological costs that are high enough to trigger behavioural adjustments with (costly) reproductive implications. Our study reshapes our understanding of how social rank correlates with ‘stress’ and how male-male competition impacts physiology, sociality and fitness. It also provides new perspectives on the potential physiological mechanism underlying the emergence of alternative reproductive and dispersal strategies.

Original publication:

Davidian E, Wachter B, Heckmann I, Dehnhard M, Hofer H, Höner OP (2020) The interplay between social rank, physiological constraints and investment in courtship in male spotted hyenas. Functional Ecology

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.

Davidian E & Höner OP (2021) A king among queens. Frontiers EcoPics. Frontiers in Ecology and the Environment 19(10): 573.

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.

Davidian E*, Benhaiem S*, Courtiol A, Hofer H, Höner OP, Dehnhard M (2015) Determining hormone metabolite concentrations when enzyme immunoassay accuracy varies over time. Methods in Ecology and Evolution 6(5):576-583. *contributed equally