Living in groups can be beneficial and costly at the same time. Animals have to make decisions about whether they want to live in a group or not and they weigh the benefits against the costs. Two benefits of group living are decreased predation risk and increased foraging efficiency. In other words, living in groups provides protection from predators because it is easier to catch an individual that is alone compared to one that is part of a group and being part of a group means that there are more eyes watching for predators. Living in groups also makes it easier to find food as there are more eyes searching for food at the same time.
In addition to these benefits, there are costs of living in groups, such as increased predation risk and increased competition for food. In other words, predators may spot a group more easily than a lone individual because a group is larger and more visible from a distance. When the group finds food, there are more individuals around to share in the resources and so competition is higher in groups.
Two main hypotheses have been proposed as possible mechanisms driving grouping behavior: predation risk and activity budgets. Individuals in groups also assort phenotypically, such as by species, body size, age, and sex, to further increase the benefits and decrease the costs of group living. Explanations for phenotypical assortment can be found within the main hypotheses regarding grouping in general.
Scientist 1 explains assortment in the context of the predation risk hypothesis, whereas Scientist 2 explains it in the context of the activity budget hypothesis.
Odd-looking individuals are usually different in color, size or behavior. The oddity effect hypothesis states that odd‐looking individuals in a group are preferred and therefore, are targeted more often by predators. Odd‐looking individuals are preferred because they stand out visually against the rest of the group, which makes it easier to target them for pursuit. What follows from this hypothesis is that if all individuals in the group look alike, the chance of any particular individual being targeted is reduced.
Therefore, when an animal is looking to join a group, a group where all individuals look alike should be preferred over a group that contains odd‐looking individuals. In nature, group‐living animals are often found in groups composed of similar‐sized individuals. Predation risk (and thus the oddity effect) is one of the main reasons why animals group according to size.
An activity budget is the time an individual invests in different activities such as looking for food, moving around, sticking together with the group and keeping an eye out for food and potential predators. The activity budget hypothesis suggests that grouping by similarity in size may be driven by the cost of behavioral synchrony.
Behavioral synchrony is individuals keeping in sync with each other’s movements. Group living animals must synchronize their behavior or else the group will split apart causing individuals to get separated from the group and therefore, risk easily being preyed upon.
Groups in which all individuals are the same size are more synchronous in behavior than groups that contain odd‐sized individuals because similar‐sized individuals tend to have similar activity budgets, whereas individuals that differ in size have different activity budgets—they have different energy requirements and movement rates. The behavioral synchrony hypothesis states that different‐sized individuals have to adjust when and how fast to move in order to keep in sync with the group, which can incur the cost of decreased foraging efficiency. Not being able to search and find food effectively will cause an individual to lose weight over time. They will spend more time searching and less time eating, which causes weight loss. The cost of weight loss, and thus lower fitness, may drive individuals to group by similarity in size; therefore, activity budget (and thus behavioral synchrony) is one of the main reasons why animals group by size.