When Should Herbivores Scrounge For Food From Other Group Members?

At some point in our lives, we have all had at least one friend that constantly mooched off us. You know the one — they steal chips off your plate, borrow money and don’t pay you back, and take the last piece of pizza that they did not pay for. Surprisingly, many animals that live in groups face similar challenges from other group members. Obviously, they don’t lend each other money, but “scroungers” definitely take food.

Within groups, scrounging individuals spend their time watching other group members and waiting for them to find food patches. Once they do, a scrounger rushes over and joins the food patch. As you can imagine, this increases competition between group members and reduces the amount of food that the patch finder can ultimately eat. This is because, by eating, the scrounger reduces the amount left for another individual, or the scrounger chases the finder away from the patch and gets all the food for themselves.

When individuals find food, they can cut their losses by increasing the rate at which they eat, thus ingesting a large amount of food before a scrounger joins them (Shrader et al. 2006). The amount of food that the patch finder eats is called the “finder’s share.” The longer this individual feeds on its own, the less food is available for a scrounger. As such, it would make sense for scroungers to pay attention to how long other group members feed in patches and thus keep track of which patches have the most food and are thus worth scrounging from. Yet, do scroungers actually do this?

To answer this question, researchers at the University of KwaZulu-Natal set up a series of experiments using domestic goats (a well known group-living herbivore). They first trained the goats to associate different plastic crates (artificial patches) that varied in color and size with specific amounts of food. For example, a black rectangular crate contained 300 g of food, a white rectangular crate contained 100 g, and a small round white crate contained 40 g. Goats are actually quite smart, so they quickly learned to associate the different crates with the amount of food found in them.

Once the goats had learned this, they then ran an experiment where a scrounger could watch two goats feeding from similar patches (e.g. large black crate vs. large black crate). The first goat was allowed to feed immediately, while the second had to wait 10, 30, 60, or 120 seconds before it could feed from its crate. What this did, was allow the first goat to eat more from its patch (i.e. obtain a larger finder’s share). Once the second goat started to eat, the researchers released the scrounger and recorded which patch it went to.

What they found, was when the artificial patches contained a small amount of food (40 g and 100 g), scroungers avoided the first goat’s patch if it had fed for more than 30 seconds and preferred joining the patch of the goat who had just started eating. In other words, after 30 seconds, the scroungers figured that there was not enough food in the patch to make it worth their while. However, if the patch contained 300 g of food, scroungers would join the first goat at its patch even if it had fed for 120 seconds. This was because, despite the removal of a large finder’s share (54 g), scroungers felt that there was enough food left in the patch to make it worth scrounging from.

Ultimately, these findings show that scroungers combine information about food availability with how long other group members have fed from food patches as a way to weigh up the costs and benefits of scrounging from specific patches. There are several animals that live in groups, and in many of these groups, there are scroungers. As a result, despite focussing their experiments with goats, the researchers suggest that many species use the removal of finder’s share as a cue to determine which patches to join. I wonder if our mooching friends are as calculating?

This study is described in greater detail in the article Does the removal of finder’s share influence the scrounging decisions of herbivores?, which was published in the journal Animal Behaviour. At present, Ryan Kok is a Junior Environmental Scientist at Eco-Pulse, South Africa, Keenan Stears is a postdoctoral fellow in the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara, USA, and Adrian M Shrader is an Associate Professor of Mammalian Behavioural Ecology in the Mammal Research Institute at the University of Pretoria, South Africa.

References:

1. Kok, R. B., Stears, K., & Shrader, A. M. (2017). Does the removal of finder’s share influence the scrounging decisions of herbivores? Animal Behaviour, 133, 229-235.
2. Shrader, A. M., Kerley, G. I. H., Kotler, B. P., & Brown, J. S. (2007). Social information, social feeding and competition in group living goats. Behavioral Ecology, 18(1), 103-107.