The Cost Of Cohesion: Social Foragers Sacrifice Diet Quality To Stay Together

Group-living has evolved many times across a very broad range of animal taxa – from swarms of bees to herds of elephants, and even human societies. Why is living in groups so common? Individuals may experience several benefits from being social rather than solitary, including improved abilities in two very important facets of life: to eat and avoid being eaten.

Foraging as a group could help make the process of finding food more efficient because individuals can learn the locations of high quality food from their peers. For this to work, individuals would need a way to transfer information about food quality between them. On the other hand, competition and interference between individuals could lead to reduced foraging efficiency in groups. In this case, individuals might be choosing to forage in groups to gain other benefits, such as reducing their individual risk of predation or reducing variability in their food intake, at the cost of foraging less efficiently.

To find out what drives cohesion, we tested the foraging response of a herd of fallow deer, a model social herbivore, to different distributions of food. Specifically, we asked: will deer sacrifice efficient foraging to stay together? If so, this would suggest that social foraging is not driven by advantages in foraging efficiency but rather by other mechanisms that could benefit herbivore fitness in the longer term.

We created foraging patches using bowls of pelleted deer fodder varying in quality. To manipulate food quality, we added tannins to the fodder in either low or high concentrations. Tannins are a group of compounds found in many plants that generally make consumption less appealing to herbivores because of their bitter and astringent taste, and because some of these compounds can reduce the digestibility of food. We changed the spatial arrangement of bowls to create two different patch shapes: rectangular blocks and single lines. We also manipulated the pattern of food quality within patches whilst maintaining equal numbers of high and low quality bowls in each patch.

With all the bowls constituting a patch being easily accessible, the herd should have maximized its consumption of high quality (low tannin) food relative to low quality (high tannin) food to forage efficiently. But the arrangement of some patches made it impossible to both maintain cohesion and for all individuals in the herd to eat the best food. So how were these competing demands traded-off at the group level?

While the deer clearly preferred the high quality food, they were not equally selective in different types of foraging patches. They selected a better diet – containing more high quality food relative to low quality food – when bowls were arranged in a block rather than in a line. This contrasts with the hypothesis that social foragers share and benefit from information about the location of better food. Although the distance to the closest bowl in both patch shapes was the same, the blocks are likely to have been better aligned with the shape of the herd, which tends to be rounded.

We also found evidence that deer relied on their own observations rather than cues from their peers. When deer foraged from bowls in lines, they were better at picking the high quality food when it occurred in every second bowl rather than in one half of the patch. Alternating quality probably made direct comparison between bowls easier for the individual.

Interestingly, consumption was elevated at the centres of patches. As a result, the herd consumed a poorer diet when the centre of the block consisted of low rather than high quality food. Even though the herd consistently ate more high than low quality food, the deer were not as good at selecting for high quality food when it was in peripheral compared to central bowls.

If increased foraging efficiency were a key driver for the evolution of group living, the distribution of resources should not affect diet selection – the best food should still be selected. Yet we found that the fallow deer herd made poorer diet choices to maintain cohesion at the centres of food patches. This suggests that other factors must make group foraging beneficial. For example, by sharing information about the location (and not necessarily the quality) of food, foraging in groups could reduce the variability in food intake for individuals in environments with fluctuating resource availability.

Foraging cohesively could also reduce the individual risk of predation by, for example, facilitating the detection of predators through group vigilance, or decreasing each individual’s chances of being eaten through dilution. Since group living often results in social hierarchies, the costs of foraging with others to reduce these risks are not likely to be shared equally amongst all members of a group. A critical next step is to understand how the costs in foraging efficiency are distributed within social groups and how this manifests in individual fitness.

These findings are described in the article entitled Cohesiveness reduces foraging efficiency in a social herbivore, recently published in the journal Animal Behaviour. This work was conducted by Pasi Rautio and Juha Tuomi from the University of Oulu, and Rebecca S. Stutz, Ulrika A. Bergvall, and Olof Leimar from Stockholm University.

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