Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address when studying animal behaviors: what are the proximate causes, ontogeny, survival value, and phylogeny of a behavior?
If an organism has a trait that provides a selective advantage (i.e., has adaptive significance) in its environment, then natural selection favors it. Adaptive significance refers to the expression of a trait that affects fitness, measured by an individual's reproductive success. Adaptive traits are those that produce more copies of the individual's genes in future generations. Maladaptive traits are those that leave fewer. For example, if a bird that can call more loudly attracts more mates, then a loud call is an adaptive trait for that species because a louder bird mates more frequently than less loud birds—thus sending more loud-calling genes into future generations. Conversely, loud calling birds may attract the attention of predators more often, decreasing their presence in the gene pool.
Individuals are always in competition with others for limited resources, including food, territories, and mates. Conflict occurs between predators and prey, between rivals for mates, between siblings, mates, and even between parents and offspring.
Competing for resources The value of a social behavior depends in part on the social behavior of an animal's neighbors. For example, the more likely a rival male is to back down from a threat, the more value a male gets out of making the threat. The more likely, however, that a rival will attack if threatened, the less useful it is to threaten other males. When a population exhibits a number of interacting social behaviors such as this, it can evolve a stable pattern of behaviors known as an evolutionarily stable strategy (or ESS). This term, derived from economic game theory, became prominent after John Maynard Smith (1982) recognized the possible application of the concept of a Nash equilibrium to model the evolution of behavioral strategies.
Evolutionarily stable strategy In short, evolutionary game theory asserts that only strategies that, when common in the population, cannot be "invaded" by any alternative (mutant) strategy are evolutionarily stable strategies, and thus maintained in the population. In other words, at equilibrium every player should play the best strategic response to each other. When the game is two-player and symmetric, each player should play the strategy that provides the best response.
Therefore, the ESS is considered the evolutionary end point subsequent to the interactions. As the fitness conveyed by a strategy is influenced by what other individuals are doing (the relative frequency of each strategy in the population), behavior can be governed not only by optimality but the frequencies of strategies adopted by others and are therefore frequency dependent (frequency dependence).
Behavioral evolution is therefore influenced by both the physical environment and interactions between other individuals.
Evolutionary models also indicate that learning is favored when environmental conditions change at intermediate levels of predictability, and recent syntheses have emphasized the need for consistent definitions of ecological change to better link theoretical predictions with empirical systems.
An example of how changes in geography can make a strategy susceptible to alternative strategies is the parasitization of the African honey bee, A. m. scutellata.
Resource defense The term economic defendability was first introduced by Jerram Brown in 1964. Economic defensibility holds that defending a resource entails costs, such as energy expenditure or risk of injury, as well as benefits, such as priority access to the resource. Territorial behavior arises when benefits are greater than the costs.
Studies of the golden-winged sunbird have validated the concept of economic defendability. Comparing the energetic costs a sunbird expends in a day to the extra nectar gained by defending a territory, researchers showed that birds only became territorial when they were making a net energetic profit. When resources are at low density, the gains from excluding others may not be sufficient to pay for the cost of territorial defense. In contrast, when resource availability is high, there may be so many intruders that the defender would have no time to make use of the resources made available by defense.
Sometimes the economics of resource competition favor shared defense. An example is the feeding territories of the white wagtail. The white wagtails feed on insects washed up by the river onto the bank, which acts as a renewing food supply. If any intruders harvested their territory, then the prey would quickly become depleted, but sometimes territory owners tolerate a second bird, known as a satellite. The two sharers would then move out of phase with one another, resulting in decreased feeding rate but also increased defense, illustrating advantages of group living.
Ideal free distribution
One of the major models used to predict the distribution of competing individuals amongst resource patches is the ideal free distribution model. Within this model, resource patches can vary in quality, and there is no limit to the number of individuals that can occupy and extract resources from a particular patch. Competition within a particular patch means that the benefit each individual receives from exploiting a patch decreases logarithmically with the increasing number of competitors sharing that resource patch. The model predicts that individuals will initially flock to higher-quality patches until the costs of crowding bring the benefits of exploiting them into line with the benefits of being the only individual on a lesser-quality resource patch. After this point is reached, individuals will alternate between exploiting higher- and lower-quality patches so that the average benefit for all individuals in both patches is the same. This model is ideal in that individuals have complete information about the quality of a resource patch and the number of individuals currently exploiting it, and free in that individuals are freely able to choose which resource patch to exploit.
An experiment by Manfred Malinski in 1979 demonstrated that feeding behavior in three-spined sticklebacks follows an ideal free distribution. Six fish were placed in a tank, and food items were dropped into opposite ends of the tank at different rates. The rate of food deposition at one end was set to twice that of the other end, and the fish distributed themselves with four individuals at the faster-depositing end and two individuals at the slower-depositing end. In this way, the average feeding rate was the same for all of the fish in the tank.
Mating strategies and tactics As with any competition for resources, species across the animal kingdom may also engage in competitions for mating. If one considers mates or potential mates as a resource, these sexual partners can be randomly distributed amongst resource pools within a given environment. Following the ideal free distribution model, suitors distribute themselves amongst the potential mates in an effort to maximize their chances or the number of potential matings. For all competitors, males of a species, in most cases, there are variations in both the strategies and tactics used to obtain matings. Strategies generally refer to the genetically determined behaviors that can be described as conditional. Tactics refer to the subset of behaviors within a given genetic strategy. Thus it is not difficult for a great many variations in mating strategies to exist in a given environment or species.
An experiment conducted by Anthony Arak, where playback of synthetic calls from male natterjack toads was used to manipulate the behavior of the males in a chorus, the difference between strategies and tactics is clear. While small and immature, male natterjack toads adopted a satellite tactic to parasitize larger males. Though large males on average still retained greater reproductive success, smaller males were able to intercept matings. When the large males of the chorus were removed, smaller males adopted a calling behavior, no longer competing against the loud calls of larger males. When smaller males got larger, and their calls more competitive, then they started calling and competing directly for mates.
Sexual selection
Mate choice by resources In many sexually reproducing species, such as mammals, birds, and amphibians, females are able to bear offspring for a certain time period, during which the males are free to mate with other available females, and therefore can father many more offspring to pass on their genes. The fundamental difference between male and female reproduction mechanisms determines the different strategies each sex employs to maximize their reproductive success. For males, reproductive success is limited by access to females, while for females, it is limited by access to resources. In this sense, females can be much choosier than males because they have to bet on the resources provided by the males to ensure reproductive success.
Resources usually include nest sites, food and protection. In some cases, the males provide all of them (e.g., sedge warblers). The females dwell in their chosen males' territories for access to these resources. The males gain ownership to the territories through male–male competition that often involves physical aggression. Only the largest and strongest males manage to defend the best quality nest sites. Females choose males by inspecting the quality of different territories or by assessing male traits that indicate resource quality. One example of this is with the grayling butterfly (Hipparchia semele), where males engage in complex flight patterns to decide who defends a particular territory. The female grayling butterfly chooses a male based on the most optimal location for oviposition. Sometimes, males leave after mating. The only resource that a male provides is a nuptial gift, such as protection or food, as seen in Drosophila subobscura. The female can evaluate the quality of the protection or food provided by the male so as to decide whether to mate or not or how long she is willing to copulate.
Mate choice by genes When males' only contribution to offspring is their sperm, females are particularly choosy. With this high level of female choice, sexual ornaments are seen in males, where the ornaments reflect the male's social status. Two hypotheses have been proposed to conceptualize the genetic benefits from female mate choice.
First, the good genes hypothesis suggests that female choice is for higher genetic quality and that this preference is favored because it increases fitness of the offspring. This includes Zahavi's handicap hypothesis and Hamilton and Zuk's host and parasite arms race. Zahavi's handicap hypothesis was proposed within the context of looking at elaborate male sexual displays. He suggested that females favor ornamented traits because they are handicaps and indicators of the male's genetic quality. Since these ornamented traits are hazards, the male's survival must indicate his high genetic quality in other areas. In this way, the degree to which a male expresses his sexual display indicates to the female his genetic quality. Zuk and Hamilton proposed a hypothesis after observing disease as a powerful selective pressure on a rabbit population. They suggested that sexual displays were indicators of resistance to disease on a genetic level.