Why do some animals live alone while others form groups?

Animals adopt different social strategies based on resource availability and survival benefits. Syrian hamsters are territorial and solitary because they cannot tolerate sharing resources, whereas Russian hamsters live in small groups and form lasting bonds. These behavioral differences reflect evolutionary adaptations to different environmental pressures and mating opportunities.

What is an evolutionarily stable strategy in animal behavior?

An evolutionarily stable strategy (ESS) is a behavioral strategy that provides greater payoff than alternative strategies, causing it to dominate a population over time. In sea lions, dominant males defending harems achieve higher reproductive success than non-dominant males using alternative mating tactics. Once an ESS arises through mutation or migration, natural selection favors its spread through the population.

How does game theory explain animal conflict and cooperation?

Game theory uses mathematical models to analyze how different behavioral strategies affect individual fitness by assigning benefits and costs to each interaction. The hawk-dove game demonstrates this by comparing aggressive hawks, which always fight for resources, against peaceful doves, which never fight. By calculating net gains across different interaction types, biologists can predict which strategies persist in populations.

What are polymorphic mating systems and how do they work?

Polymorphic mating systems occur when one sex, typically males, develops multiple distinct phenotypes and mating strategies within the same population. Sea lion males exhibit this by either defending harems on beaches or remaining in water to intercept females. These alternative strategies coexist because each provides fitness benefits under different circumstances, such as age or physical condition.

How do cooperative populations prevent cheaters from invading?

Cooperative populations have evolved mechanisms to prevent invasion by non-cooperators, including the ability to switch strategies when necessary or identify and punish cheaters through expulsion. These defenses are critical because in a population of 100% cooperators, uncooperative individuals can out-compete residents by gaining benefits without incurring costs. Cheater detection and punishment maintain cooperation's evolutionary stability.

What is the net gain calculation in the hawk-dove game?

Net gain equals the benefit obtained from a resource minus the cost incurred during interaction. When two doves meet, each receives half the benefit with no aggression costs. When a hawk meets a dove, the hawk gains all benefits while the dove gains nothing. When two hawks fight, they split the benefit but both incur fighting costs, reducing their net gain.

How does reciprocal altruism evolve in animal populations?

Reciprocal altruism evolves when individuals can identify each other and expect future interactions, making apparent self-sacrifice adaptive over time. Vampire bats regurgitate food to hungry individuals, expecting the favor returned later. Similarly, birds sound alarm calls when spotting predators, making themselves vulnerable but benefiting when others reciprocate. Game theory shows these behaviors maximize long-term fitness despite immediate costs.

Group Behavior

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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.