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.
Organisms in populations interact with one another in complex ways, where individuals compete for resources such as food, shelter, and mates. These interactions are costly since individuals invest energy to acquire the resource, therefore there are a variety of strategies that organisms adopt to gain an advantage over their competitors. This is observed clearly with polymorphic mating systems, in which one sex, predominantly males, display multiple mating strategies. For example, dominant male sea lions defend harems of females up on the beach, whereas non-dominant males try to increase their chances of mating by staying in or near the water, where they mate with females that have left the beach temporarily 1. Other types of strategies may determine how often an individual will fight for a resource, or how willing they are to cooperate with others.
In order to understand how different behavioral strategies evolve, ecologists turn to the game theory, which is a mathematical modeling approach that investigates the outcomes of multi-individual interactions, in which the payoff for any one individual depends its own strategy as well as the strategies of the others 2. In this approach, the relative cost of the interaction and the benefits obtained from the resource determines the net gain or, in some cases, loss incurred by the organism. Different strategies can be assigned costs and benefits based on those faced by an organism. For example, fighting for control of a resource may provide a large benefit, but it also comes with costs that reduce an organism’s net gain. On the other hand, a strategy to avoid conflict will reap fewer benefits but incurs no costs.
The organisms with the best interaction strategy maximize their net gain, which in turn will contribute to their fitness. Therefore, over the course of evolutionary time, one strategy may arise that outcompetes all others in a population. This is called an evolutionary stable strategy (ESS) 2. Populations evolve to adopt this strategy once it arises through mutation or is introduced through migration. Therefore, these strategies are mostly genetic or adopted at a young age, and changes in the strategies used by a population over time are determined by the action of natural selection. This concept is often illustrated using the Hawk-Dove game, which compares the success of two strategies for obtaining resources 3. In this example, “hawks” are aggressive, and always fight for resources. “Doves,” on the other hand, are passive, and never fight for resources. In an interaction between two doves, resources are split evenly. When a hawk and a dove interact, the hawk always wins and obtains all of the resources. However, when two hawks interact, they split resources evenly and suffer the cost of their conflict as well 3. Assessing these interactions over consecutive interactions allows modeling of how competing strategies in an evolving population fare against each other, thus emergence of ESS in an experimental setting.
As seen with the doves in the Hawk-Dove example, organisms not only compete against each other, but also display cooperative behaviors. The risk of being a population of 100% doves is the arrival of a cheater, or an individual that does not cooperate 4. Cheaters can invade and outcompete the residents, therefore many cooperating populations have strategies in place to prevent invasion, including the ability to switch strategy as necessary or to identify and cheaters, and in some cases, to communicate this information to others in their group to decrease the chances of cheater success 4.
The existence of altruism, or a reduction in an organism’s immediate fitness in order to benefit another’s, in wild populations has been held up as a counterpoint to the theory of natural selection, however game theory shows how altruism can evolve under certain conditions. Assuming organisms can identify one another or reasonably expect to interact again in the future, acts of apparent altruism may actually be beneficial over time, as an organism can count on the favor being returned. This is seen when flocks of birds or herds of mammals feed collectively – one individual may sound an alarm call when they spot a predator, making them more vulnerable to attack 5. The net benefit, however, of others frequently doing the same makes this act adaptive. Similarly, a vampire bat may regurgitate its food to feed hungry individuals. When it is unable to find food in future, it may benefit from the same behavior of another vampire bat 6.
In addition to the interactions within a single species, the evolution of social interactions can take place between species as well. Besides predator-prey interactions, different species can compete for the same resources and develop strategies to gain advantage over others. However, individuals of a species may also interact cooperatively with members of another species. Interspecies interaction that require cooperation include mutualisms, or situations in which two organisms provide mutual benefits to one another. Many plants form mutualisms with nitrogen-fixing bacteria in the soil, whereby the plants provide complex sugars to bacteria in exchange for nitrogen 7. Should bacteria fail to provide nitrogen, then plants can reduce the amount of sugars available.
Since cooperative interactions depend on either being able to identify or retaliate against cheaters, it is possible for invaders to take advantage of existing mutualisms in new environments. Invasive species have the potential to be closely related enough to local species that they are able to form mutualisms with other local species, but distantly related enough that existing recognition or defense mechanisms are ineffective. In Hawaii, an invasive dinoflagellate that forms a mutualism with coral takes more resources than local dinoflagellates. This has a negative impact on coral instead of the expected beneficial one 8. Therefore, studying intra- and interspecies interactions not only allows understanding the development of behavioral strategies in evolving populations but is also essential to assess the behavioral phenotypes of invasive organisms to develop effective strategies against them.
