If you want to read about foraging decisions click here,
cooperative intergroup aggression click here
or the social lives of Colobus angolensis ruwenzorii click here


Individual’s fitness depends on their ability to optimize food intake, survive as long as possible, choose mates wisely, and maximize the number of offspring they produce. Thus, successful foraging decisions are key to an individual’s success. Optimal foraging theory predicts that foraging animals can maximize their fitness by maximizing net energy gained by selecting high-value foods, decreasing travel distance between food patches, or minimizing the time they spend handling food items.  

Therefore, when moving between food patches, foraging individuals should choose the route that minimizes travel distance. For example, if moving between five patches laid out in a pentagon, they should take the following path:

How does social context impact foraging decisions?

In social species, individuals are typically surrounded by their group mates, and so foraging strategies that optimize net energy gain need to be adapted to maximize food intake in the face of feeding competition. We used an experimental set-up in which we provided wild vervet monkeys with five food patches set out in a pentagon array (Arseneau-Robar et al., In Review, Frontiers in Ecology and Evolution). Four were baited with a low-value food reward (3 corn kernels), while the fifth was baited with a food reward the monkeys preferred (a half banana) that was placed in a box that required handling.

The preferred banana was placed such that it was never the nearest platform, and we tried to place it so it was on the opposite side of the array from the focal monkey. Then we examined the first 2 platforms that decision-makers chose, and from this, identified 3 main foraging strategies:

  1. Prioritizing their preferred-food reward by rushing for the platform with the banana at the start of the trial (purple arrow). The alternative choice was to start with the nearest platform of corn (blue arrow).
  2. Still prioritizing the preferred-food reward but taking the time to stop for 1 platform of corn on the way (green arrows).
  3. Taking the path that minimized travel distance by starting with the nearest platform of corn, then continuing on to the next platform of corn (solid red arrows). Individuals who did so typically continued on to retrieve the banana, as well as more corn platforms depending on whether a competitor arrived at the array or not (dotted red arrows).

Who uses which strategy and when?

As expected, when foraging alone, the monkeys took the most efficient path when moving through the array. However, when in competition they were more likely to rush for the preferred-food platform at the start of the trial (Arseneau-Robar et al., In Review, Frontiers in Ecology and Evolution). But what was really cool was that low-ranking monkeys, who experienced a lot of contest competition at the experiment, also calculated the risk of losing food to a dominant competitor. They did this by taking into account who was in their audience, if these potential competitors were dominant or subordinate to themselves, how far away potential competitors were (i.e. their travel time to arrive at the array), and their own handling skill (i.e., how fast they were able to retrieve the banana from the box).

When the risk was very high, because they had poor handling skills and/or a dominant competitor was close by, low-ranking monkeys rushed for the banana right away. When the risk was moderate, because dominant competitors were further away (e.g., 50-75m), they took the time to stop for one platform of corn en route to the preferred-food platform. When the risk was low, they took the route that minimized travel distance as they moved through the array (Arseneau-Robar et al., In Review, Frontiers in Ecology and Evolution).

Why are these findings exciting?

While there are numerous hypotheses for why humans are so intelligent, most of them fit into two categories: hypotheses that posit that intelligence evolves in response to ecological challenges (e.g., efficiently exploiting clumped food resources, dealing with seasonality, or extracting foods that would otherwise be unobtainable), and hypotheses that assume intelligence evolves in response to social challenges (e.g., the need to form and maintain social bonds, as well as to deceive, manipulate, and exploit others).Our findings suggest that when foraging takes place in dynamic social contexts, decision-makers must attend to and process a multitude of social and ecological stimuli, and then flexibly adapt their foraging strategies to not only optimize energy intake, but also mitigate the costs of competition. This flexible decision-making likely requires numerous social and ecological cognitive processes simultaneously. Thus, our findings suggest that an important avenue of future research is to investigate how social and ecological cognitive processes integrate together to determine the capacity for flexible decision-making behaviour.


Humans are unique in the animal kingdom in the extent to which we cooperate with others. We perform multiple cooperative acts each day, help strangers we are unlikely to meet again, engage in dangerous activities like warfare, and we can coordinate highly effective collective actions, even in very large groups. Our prolific cooperation relies on our ability to flexibly use a number of different mechanisms to promote cooperation and limit defection. Many cooperative investments are directed towards relatives and so provide inclusive fitness benefits. But we also reciprocally trade goods and services, form mutually beneficial social bonds, and help others in order to maintain a good reputation or to advertise our willingness/ability to be a good cooperative partner. We behave cooperatively to receive rewards or other incentives, or because if we do not, we might be punished, fined, or sanctioned.

How and why did we evolve this incredibly propensity to cooperate? Are all these mechanisms for promoting cooperation unique to humans or did some of them exist deeper in our evolutionary history? What were the selective pressures that drove the evolution of these mechanisms for cooperation?

Most of the work on intergroup competition has focused on the resources that individuals fight to defend. Yet, in many species, multiple group members actively participate in intergroup conflicts, and need to cooperate together to successfully win the fight. So in looking at participation during intergroup conflicts, I focused on understanding when and why individuals cooperated together effectively. I first examined the causes of interindividual variation in participation (i.e. the different resources that different individuals fight to defend), then identified the contexts in which male and female group members would disagree on whether to fight versus flee from a neighbouring group. And most importantly, I investigated the mechanisms that each sex used to manipulate the participation of their opposite-sexed group-members.

Females and males fight in intergroup conflicts for different reasons

High-ranking females, who stand to gain the most by defending monopolizable resources, and females without infants, who are less averse to the risks associated with fighting, are most active in intergroup conflicts. Females are primarily interested in defending access to food resources, but also defend important areas of their home range. (Arseneau-Robar et al. 2017 Animal Behaviour)

Males who are likely to have sired infants in the group are very protective, responding reactively when the other group is aggressive such that offspring might be at risk. Males often support females who are trying to instigate a fight, but they primarily do so during the mating season when doing so is associated with higher mating success. (Arseneau et al. 2015 Animal Behaviour, Arseneau-Robar et al. 2016 Scientific Reports)

Manipulating the participation of group members

Conflicts of interest often arise between likely sires, who are averse to the risks intergroup conflicts pose to infants, and females, who need access to high-quality food resources to successfully produce and raise offspring.

When conflicts of interest arise, females use both punishment and rewards to obtain higher levels of male support. Females direct aggression towards males who are not participating; punished males are more likely to participate aggressively afterwards, and these increased levels of participation are higher than would be expected given each male’s baseline levels of participation in intergroup conflicts. (Arseneau-Robar et al. 2016 Proceedings of the Royal Society B)

Conversely, females preferentially groom males who have supported them in the intergroup conflict and males who receive this reward maintain their high levels of participation. The observed levels of participation after being rewarded are higher than each male’s baseline levels. (Arseneau-Robar et al. 2016 Proceedings of the Royal Society B)

Males who are likely sires also have strategies for achieving more ideal outcomes when conflicts of interest arise. They use punishment/coercion to prevent encounters from escalating into intergroup fights that could put offspring at risk. Likely sires direct aggression towards group members who (try to) instigate a fight, and those who are punished/coerced are less likely to keep on fighting than would be expected given their baseline participation levels. Likely sires are most likely to use this strategy when they themselves are wounded, and so might feel unable to protect offspring should the need arise. (Arseneau-Robar et al. 2018 Proceedings of the Royal Society B)

Why use social incentives?

Female fitness is largely dependent on having access to the resources required to produce and raise offspring to independence. This creates selective pressure for female strategies that increase the odds of winning an intergroup conflict. Conversely, males who pay very little cost to reproduce can best maximize their fitness by ensuring that intergroup encounters do not escalate, which could put their offspring at risk. Because the number of active participants, relative to the number in the opposing group, determines whether the group wins or loses, using punishment and rewards to recruit males likely improves the odds females win access to fitness-limiting resources. (Arseneau-Robar et al. 2016 Proceedings of the Royal Society B)

Conversely, male punishment/coercion was highly effective in preventing intergroup conflicts from erupting when used when the two groups were near to one another but no fighting had yet occurred. When males used punishment/coercion in the middle of an ongoing intergroup conflict, this strategy successfully ended the fight in ~50% of cases and intergroup conflicts tended to end sooner than expected. (Arseneau-Robar et al. 2018 Proceedings of the Royal Society B)

Why are these findings exciting?

Warfare is one of the riskiest cooperative activities that humans engage in. Research on modern day forager groups, who are our best window into our own recent evolutionary history, shows that social incentives are important in promoting participation in primitive warfare. Warriors often fight to avoid being punished or ostracized by their group members, to gain social prestige, or build/maintain a good reputation. My findings provide the first quantitative evidence that other species can also use social incentives to manipulate the participation of group members during intergroup fights, and highlights that they do so when conflicts of interest arise between group members. As more researchers focus on the cooperative nature of intergroup aggression, and look for the role that social incentives can play in driving participation, we will improve our understanding of how and why social incentives may have evolved in our own species.


Dr Julie Teichroeb started a new research site near Lake Nabugabo in Uganda. This site is home to a subspecies of colobus that very little was known about, Colobus angolensis ruwenzorii. We have started putting together the puzzle pieces to understand the basic social organization and social structure of this subspecies, as well as how these are impacted by their ecology.

Lake Nabugabo field site in Uganda

Aren’t colobines folivores?

Colobines are a group of monkeys that have evolved complex, multi-chambered stomachs (much like a cow or other ruminant) to help them digest a diet that largely consists of leaves. Because leaves, and mature leaves in particular, are low in energy and high in toxic compounds, most colobines need to spend a lot of time resting so that they can digest their low-quality forage and conserve energy.

In examining the diet of the Nabugabo population, we found that their diet consists of 31% fruit, 65% young leaves, and only 3% mature leaves (Arseneau-Robar et al. 2021 Folia Primatologica). They were highly selective for fruits and ate them at any time of year that they were available (see below). These are levels of fruigivory that are quite unique among the African colobines. When we examined specific foods that were preferred vs avoided, we found that this population has access to preferred food resources all year round. Thus, it appears that the colobus at Lake Nabugabo have access to abundant, high-quality food resources that allow them to maintain a high-quality diet of fruits and young leaves.

(b) Food availability index scores for the different plant parts consumed. (c) The monthly diet of Colobus angolensis ruwenzorii near Lake Nabugabo, Uganda.

When we looked at their activity budgets, they spent 40% of their time resting, 25% moving, 28% feeding and 7% of their time socializing (Arseneau-Robar et al. 2021 Folia Primatologica). They maintained a fairly consistent activity budget throughout the year (see below). Most other colobines populations spend at least 50% of their time resting, so the low levels of forced resting-time in this population suggest that their high-quality diet allows them to employ an energy maximization strategy, similar to most frugivorous primate species.

Monthly activity budget for Colobus angolensis ruwenzorii near Lake Nabugabo

Other work by Dr Teichroeb and Sam Stead has shown that this population lives in a large, multi-level society (Stead and Teichroeb 2019 PLOS One) in which core units associated preferentially into clans, and clans into bands. Core units in this population spend a lot of time in close association with one or more other core units, and so are often found in large aggregations. And we are currently investigating the extent to which their high-quality diet, and particularly the availability of fruit, enables these large aggregations to form (Adams et al. 2021 Ecology and Evolution).

Why don’t other African colobines form such large groups?

There are other subspecies/populations of Colobus angolensis that have been studies and not all of them form large groups. While having access to a high-quality diet likely enables large aggregations to form, observations of very small groups in this species suggests that diet is not the only important factor regulating group size. Perhaps their social structure (i.e. who forms close relationships with who) also plays a role?

We investigated social relationships among different age-sex classes in the Colobus angolensis ruwenzorii living at Lake Nabugabo (Arseneau-Robar et al. 2017 Primates). After examining association and grooming patterns, we found that adult males tended to have the strongest relationships with adult females, but also spent a considerable amount of time grooming with other males. These high levels of tolerance amongst adult males are atypical in other species of colobus, and may be key to producing the high levels of inter-unit tolerance that are necessary for large aggregations to form. These high levels of male-male tolerance may arise because dispersal in this population seems to be female biased (Stead and Teichroeb 2019 PLOS One). While males typically disperse out of their natal core unit, they do not tend to leave their band; conversely, females tend to disperse outside of their band. These dispersal patterns likely cause high levels of male relatedness within bands, promoting higher levels of male-male, and therefore inter-unit tolerance.

Why are these findings exciting?

Through much of our evolutionary history, humans are also thought to have formed multi-level societies. At the lowest level is the family unit, which is typically a monogamous male-female pair and their offspring, although some families are were polygynous. Families lived together in a camp, and these families were highly interdependent on one another as they shared food and helped each other raise offspring. Although dispersal patterns were flexible, female exogamy and marriage outside the band was common. Male patrilocality facilitates male-male bonding, and strong male bonds within camps is thought to have been critical to humans evolving our incredible propensity to hunt cooperatively and engage in cooperative intergroup violence (i.e. warfare). Because Colobus angolensis ruwenzorii also live in multi-level societies, show female exogamy and high levels of male tolerance, this species may be an excellent model to understand the pressures underlying the evolution of some key traits present in humans.