Group Behavior

Have you ever wondered why some animals are solitary, while others are social? Let's consider these different hamsters, for example. Syrian Hamsters are territorial, therefore unwilling to share resources. Since they can't tolerate each other well, they will fight others entering their territory, and can injure each other, potentially fatally. On the other hand, Russian Hamsters typically live in small groups, sharing resources, and forming long-lasting bonds, especially with their mating partners.

Speaking of mating, you may have noticed that some animals exhibit something called Polymorphic Mating Systems, meaning that one sex, generally males, develop different phenotypes and mating strategies. In sea lions, for example, males are much bigger and have more powerful jaws and necks than the females. In terms of the different mating strategies, dominant males gather harems of females up on the beach and fight any challengers trying to take their mates. However, some non-dominant males stay in the water around these groups, as a strategy to mate with females who have left the beach temporarily. These distinct behavior strategies can affect an organism's fitness differently, and so one strategy may dominate others in a population over the course of evolution.

Let's look at this more closely. This preferred strategy, here called strategy one, is known as an Evolutionarily Stable Strategy, or ESS, because the payoff is greater than any alternative strategy. Males that employ the less-beneficial strategy two only do so if their risks of fighting to gain a group of females is extremely high and likely to be unsuccessful, perhaps because they are extremely young or very old. To understand how Evolutionarily Stable Strategies like this arise, biologists turn to game theory, which is the study of cooperative and conflict behaviors between individuals, using mathematical models. First, biologists assign benefits and costs to different strategies. Benefits might be gaining control of a resource, like food or mates. Costs might be whatever risks are incurred by trying to take possession of the benefit, like the potential negative cost of losing a fight. So, sometimes, strategies like sharing benefits with no cost, i.e. risk of injury in this example, can be a good alternative.

We can model the net gain of an individual after an interaction, using the hawk-dove game, in which hawks are always willing to fight for resources, and doves are always peaceful. In an interaction between two doves, each individual will receive equal benefit without any aggression costs. Using this equation, we can calculate the net gain for each individual, which is the benefit minus the cost. That's half B, in this case. In an interaction between a dove and a hawk, the hawk will receive all the benefit, but neither bird will incur immediate costs, because the doves don't engage in conflict. If two hawks interact, they will fight and split the benefit, but also incur some costs, which end up reducing their net gain.

So how do populations strike a balance? In a predominantly sharing group, uncooperative cheaters can out-compete other residents, like this guy, sleeping on his watch. Because of this, many cooperative populations have developed ways to prevent invasion, such as the ability to switch strategies, or identify and punish cheaters with actions like expulsion from the group.

In this lab, you will perform the hawk-dove game, and demonstrate the persistence of two different strategies in a population, and the circumstances that may affect their use.