Field of Behavioral Ecology, The
The study of the ecological and evolutionary basis of animal behavior.
Behavioral ecology has its origins in field studies that examined associations among behavior, inter-individual interactions, and the environment (Martin and Bateson 2007). The primary focus is on the study of the ecological and evolutionary basis of animal behavior, specifically the functions of behaviors and their adaptive advantages. Adaptive behaviors allow an individual to increase or maximize its reproductive success, or fitness. Behavioral differences with an underlying genetic basis are subject to natural selection or the differential survival and reproduction of organisms that differ in heritable ways.
Under selection, behaviors best adapted to the local environment increase in frequency over generations relative to less well-adapted behaviors. The behavior that maximizes fitness will depend both on interactions with other individuals as well as the ecological context (environment). Since natural selection can only operate on genetic differences among individuals, for a behavior to evolve, there must be heritable variation in a population and some behaviors must result in greater reproductive success (increased fitness) than others (Krebs and Davies 1993).
However, behaviors do not always vary in ways that are adaptive. Traits may have evolved because they were beneficial in the past, but no longer confer a selective advantage. Traits may also evolve due to neutral processes, such as genetic drift or gene flow. Genetic drift refers to variation in the frequency of genotypes in a small population due to the disappearance of particular alleles, or gene variants, as individuals die or fail to reproduce. Gene flow refers to a substantial change in allele frequencies, typically due to migration.
Proximate Versus Ultimate Causation
Behavioral ecologists distinguish between proximate versus ultimate causation (Mayr 1961). Proximate causes describe how the behavior is performed and provide explanations for it based on immediate causes. Examples include understanding how a behavior develops, determining what stimuli elicit the behavior, and identifying the genetic and phenotypic factors that underlie behavior. Behavioral ecologists focus primarily on ultimate causes, which take an evolutionary approach to understand why a behavior is performed, proximate mechanisms occur, and organisms respond the way they do. Examples include understanding the adaptive value of a behavior, whether a behavior maximizes fitness, and how behaviors differ among species.
In 1973, the Nobel Prize in Physiology or Medicine was awarded to Nikolaas (Niko) Tinbergen, Karl von Frisch, and Konrad Lorenz for their research on individual and social behaviors. One of Tinbergen’s major contributions to ethology, the study of animal behavior, was to organize the types of questions biologists can ask about behavior into categories: (1) cause, (2) development, (3) function, and (4) evolutionary history (Tinbergen 1963). In one of Tinbergen’s studies of survival values, or how an animal’s behavior allows it to increase its own or its offspring’s survival, he determined that parental gulls remove eggshells from their nests after their offspring have hatched because crows use the white interior of a broken eggshell to find and prey upon unhatched eggs (Tinbergen et al. 1962). Karl von Frisch’s major contribution to ethology was in describing the honeybee’s (Apis mellifera) waggle dance (von Frisch 1967), which is used to relay directions to a patch of flowers to the colony. Konrad Lorenz investigated imprinting behavior, the process by which some birds bond instinctively with the first moving object they see shortly after hatching (Lorenz 1935).
J.H. Crook and David Lack developed a comparative approach in which they associated the behavior of birds with environmental variation. Crook (1964) focused on comparing species of weaver birds in evergreen forests and savannahs. He found that Birds living in forests foraged primarily on insects, were monogamous, and defended large territories, in contrast to the birds in the savannah, which were granivorous, nested in large colonies, and were typically polygynous. Since food supply in the forest is scattered and unpredictable, reproductive success is limited by the ability of parents to provide food for their offspring. Monogamy is therefore the preferred mating strategy because biparental care is essential for obtaining food. In contrast, in the savannah, food supplies are unstable, but locally plentiful. The polygynist mating strategy used here is likely linked to risk of predation, with the number of mates males acquire dependent upon the number of nest sites they are able to defend. David Lack associated differences in bill morphology and foraging behavior in Darwin’s finches in the Galapagos to adaptations to specific food resources (Lack 1947). Their comparative approach highlighted the importance of ecological factors in shaping behavior and the difficulty in distinguishing between correlations and causations in associations between behavior and the environment.
John Maynard Smith coined the term evolutionary stable strategies to describe, using mathematical tools and game theory, how natural selection maintains a balance between opposing characteristics within a species. Game theory analyzes situations in which the best option for one individual depends on the decisions of another (von Neumann 1928). Smith used game theory to demonstrate that natural selection may favor both aggressive and nonaggressive behaviors in a context-dependent manner using a comparison of hawk versus dove strategies. In this example, hawks will always battle over resources, whereas doves will never behave aggressively. A population entirely comprised of doves would be unstable because it would be destroyed by the introduction of a hawk. However, in a hawks-only population, an introduced dove would have a long-term advantage because the hawks’ consistently aggressive behavior would lead to frequent injury among hawks, while the dove would avoid harm. Smith demonstrated that a certain ratio of hawks to doves forms an evolutionary stable strategy and that balancing selection is able to maintain both characteristics within a population (Maynard Smith 1976).
Behavioral ecologists study the effects of behaviors on foraging, territoriality, mating success, parental care, communication, and likelihood of predation, and how different behavioral strategies contribute to fitness.
Foraging Behavior. Natural selection acts on behaviors that increase feeding efficiency. Foraging includes not only eating, but also searching for, identifying, and capturing food, and involves a trade-off between a food’s energy content and the cost of obtaining it. Foragers need to make decisions about what types of food to consume as well as where and how long to search for it. Large food items may contain more energy, but are often more difficult to capture and less readily available than smaller items. Efficient foraging may mean that individuals consistently or periodically maximize energy intake or minimize variation in intake.
Optimal foraging theory states that natural selection will favor individuals whose foraging behavior is as energetically efficient as possible. This theory proposes that animals tend to consume prey that maximizes their net energy intake per unit of foraging time. Shore crabs (Carcinus maenas), for example, preferentially feed on intermediate-sized mussels, which provide the greatest energetic gains. While larger mussels provide more energy, they also require considerably more energy to open (Elner and Hughes 1978).
One assumption of optimal foraging theory is that natural selection will only favor behavior that maximizes energetic gains if increased energy is associated with increased reproductive success. For example, direct relationships have been found between net energy intake and reproductive success in captive zebra finches (Taeniopygia guttata; Lemon and Barth 1992).
Optimal foraging theory also assumes that foraging behavior is subject to natural selection. In the fruit fly (Drosophila melanogaster), variation in a gene called forager (for) underlies the foraging behavior of larvae. Larvae with the for R (“Rover”) allele travel nearly twice the distance while feeding as larvae with the for S (“Sitter”) allele. Experimental studies confirmed that this locus is under selection by demonstrating that individuals maintained at low numbers foraged a shorter distance than those maintained at high numbers. In the low-density group, the for S allele increased in frequency, while the for R allele increased in the high-density group. At low population densities, short foraging trips yield sufficient food, while in high density groups, individuals may have to travel long distances as nearby resources become depleted (Sokolowski et al. 1997).
Optimal foraging theory also has limitations. Animals have needs besides energy acquisition that can conflict with foraging, such as watching for predators, searching for mates, and defending territories or nest sites (Milinski and Heller 1978). Another limitation is that individual differences in foraging success are often due to development, rather than natural selection (Wunderle 1991). Ontological differences in habitat or patch use may be due to dominant adults displacing juveniles, the inability of juveniles to adequately choose patches, diet differences, and/or changing nutritional needs throughout development.
Territoriality. Territoriality refers to the behavior that an individual displays to maintain exclusive rights to an area that contains a limited resource, such as food, nesting sites, or mates. Individuals must make decisions about whether to be territorial and what size territory to defend. Territorial displays and aggressive behaviors are costly because they require energy, can harm the individual, and may alert a predator to an individual’s location.
Despite these potential costs, there are benefits to territoriality, including increased access to resources or protection from predators. Since territorial behavior requires time and energy, a territory must provide net benefits greater than those available to an individual that is using the space non-territorially (Brown 1974). For hummingbirds and songbirds, the benefits of territory defense depend upon the amount of nectar in flowers and the birds’ ability to efficiently harvest it. If flowers are scarce or nectar levels low, the bird may not get enough energy to offset the cost of territoriality. Alternatively, if flowers are abundant, then the bird may be able to meet its energy requirements without having to defend a flower patch. Therefore, territoriality is only advantageous at intermediate levels of flower availability and high levels of nectar production (Gill and Wolf 1975).
Territory size may be correlated with food availability, body size, and number of competitors. Territory sizes tend to be smaller when food is more plentiful or nutritious and are often positively correlated with body weight (Schoener 1968). More competitors are attracted to resource-rich areas, making them more costly to defend.
Reproductive Behavior. Reproductive success is determined by an individual’s lifespan, mating success, and number of offspring produced per mating. Reproductive behavior is influenced by natural selection and involves searching for a nesting site, finding a mate, and rearing offspring. Reproductive strategies are sets of behaviors that have evolved to maximize reproductive success.
Many species exhibit mate choice, or an evaluation and selection of a potential mate or mates. Since each reproductive event is relatively inexpensive for males, they are able to maximize their fitness by mating with as many females as possible. In contrast, reproductive events are much costlier for females, as they are limited by the number of eggs that they can produce. Females, therefore, have an incentive to be selective in choosing the mate that can provide the best benefit to future offspring (Darwin 1871). Mate choice by both males and females may be found in species with parental care in which both parents contribute equally to the cost of rearing offspring. In some instances, males invest more in reproduction than females. For example, male Mormon crickets (Anabrus simplex) transfer a spermatophore to a female during mating that is nearly one-third of the male’s body weight (Gwynne 1981).
Competition for mates is termed sexual selection and operates through variance in reproductive success. Selection usually acts more strongly on males because they experience greater variance in success than females (Trivers 1972). Sexual selection leads to the evolution of two types of male secondary characteristics. Intrasexual (male-male) selection influences traits involved in fighting other males, while intersexual (male-female) selection influences traits important in attracting mates (Darwin 1871). For example, aggressive defense of females can lead to increased male strength and size. In many territorial species, males are considerably larger than females because the largest males have greater mating success. These physical differences between the sexes are called sexual dimorphisms.
Another example of sexual dimorphism includes the horns, antlers, and tusks of males, which may serve as weapons against predators or other males and/or indicators of strength, fighting ability, vigor, and quality. For example, male caribou (Rangifer tarandus) with smaller antlers tend to withdraw from sparring matches with caribou that have larger antlers (Barrette and Vandal 1990). In forked fungus beetles (Bolitotherus cornutus), males with the longest horns have increased access to females (Brown and Bartalon 1985).
Individuals may select mates based on a variety of criteria. In some species, mates with high fecundity or fertility are preferentially chosen. For example, male Mormon crickets prefer larger females because they carry more eggs than smaller females (Gwynne 1981). Mates may also be chosen based on ability to provide nuptial gifts, typically in the form of food (Vahed 1998). In the hangingfly (Hylobittacus apicalis), females tend to only accept males that provide insect prey above a certain size. Because mating occurs while the female is eating, presentation of a larger insect allows for longer copulations and increased likelihood of fertilization. The males who are able to capture larger prey may also be of higher quality (Thornhill 1976).
Mates may also be chosen based on parenting ability. Female threespine stickleback fish (Gasterosteus aculeatus) are more likely to lay eggs in nests that already contain eggs (Ridley and Rechten 1981). Having more eggs in the nest reduces the chance that any particular egg will be preyed upon and males who already have eggs in their nests expend more energy defending them than males who are engaging in courtship. In fifteen-spined stickleback (Spinachia spinachia), females are more likely to choose males that shake their bodies frequently during courtship because this behavior is associated with increased fanning of fertilized eggs, which increases rates of successful hatc-hing (Ostlund and Ahnesjo 1998).
Sexually selected traits should be correlated with aspects of fitness such as growth rate, predator avoidance, disease and parasite resistance, and competitive ability (Kodric-Brown and Brown 1984). For example, in zebra finches, carotenoids are associated with increased immune responses (McGraw and Ardia 2003). Females have been found to prefer experimentally carotenoid-enhanced males over controls. Female canaries (Serinus canaria domestica) select mates based on their singing ability, which is an honest signal of health (Suthers 1990).
Preference for a specific trait may develop and be maintained if it is correlated with male quality or made males easier to find. If the phenotype has a genetic basis, this advantage will be passed on to future sons and the genes that cause females to prefer the trait will also be favored. This positive feedback is termed runaway selection. A classic example of this is in widowbirds (Euplectes progne; Andersson 1982), in which females select males with long tails. Mating success decreases for males with experimentally shortened tails and increases for those with elongated tails.
However, conspicuous male traits may represent a handicap if they reduce survival (Zahavi 1975). Strong, healthy individuals will produce larger ornaments because they can take on greater handicaps at a lower cost. Handicap size therefore serves as a reliable indicator of quality.
Mates may also be chosen based on access to resources. In green frogs (Rana clamitans), females choose males with territories in dense vegetation that is favorable for laying eggs (Wells 1977). In choosing a mate with high status, mating success may be increased because copulation is likely to occur without interruption by other males. In cockroaches (Nauphoeta cinerea), for example, dominant males mate more often than subordinates (Breed et al. 1980). Progeny may benefit from their father’s status by receiving increased parental care and protection and/or inheriting traits that will enable them to achieve high status as well.
Species use various mating systems to maximize their fitness. In polygynous species, one male mates with multiple females, which increases the variance in male reproductive rates, resulting in heightened selection for increased size and strength (Alexander et al. 1979). Polygyny may be favored if males hold territories that have enough resources for multiple females or in precocious species, in which offspring require little parental care. Polygyny is more common than polyandry, in which females mate with multiple males. One example of polyandry is in spotted sandpipers (Actitis macularius), in which males incubate the eggs and care for young, while females mate with multiple males. Monogamy may be favored in altiricial species, such as birds, that require two parents to care for young.
Some species of fish (including salmonids and sticklebacks) have two genetically distinct groups of males. One is large and defends territories to acquire mates, while the other, called jacks or sneaker males, is small and remains at the edge of large male territories rather than defending their own. When the territorial male is fertilizing eggs, the smaller jack will dart in and release his own sperm, thus fertilizing part of the clutch.
Parental Care. There is a trade-off between rate of population increase and parental care. R-selected species maximize r, which is the rate of population increase, and minimize parental care, while K-selected species maximize survival and reproduction only when population size is at or near K, which is the carrying capacity, and exhibit greater parental care. Parents typically care for young that face harsh environments, increased predation, and/or intense competition. Females are typically the primary care-giving sex because the costs of producing eggs exceed that of sperm (Trivers 1972). In mammals, females are also responsible for gestation and lactation, which are energetically expensive.
At some point, it becomes more profitable for parents to stop caring for their current offspring and instead devote energy to producing additional offspring, which often results in inter- and intrabrood conflicts (Trivers 1974). Since parental investment is fixed, nestlings may try to manipulate parents and outcompete siblings to gain a greater investment. For example, in American robin (Turdus migratorius) nestlings, begging is positively associated with likelihood of being fed and nestlings respond to increased begging of a sibling by increasing the intensity of their own begging (Smith and Montgomerie 1991).
Social Behavior. There are both costs and benefits to living in groups. Living in groups may help individuals forage more efficiently than they would if they were on their own. Foraging groups may increase the rate at which patches of food are found because individuals learn what or where to eat by observing others. In Great tits (Parus major), foraging success is greater in flocks than when birds forage alone (Krebs et al. 1972). Foraging groups can also reduce variation in feeding rate (Baker et al. 1981) if individuals share information about the location of food patches (Krebs 1974). For example, naïve red-billed Quelea (Quelea quelea; DeGroot 1980) are able to find resources more often when in the presence of trained birds. Individuals in groups may be able to capture larger prey and have increased success when competing against other species for access to food resources.
Group living may also confer increased protection from predators. Individuals in groups may shelter themselves by placing other group members between themselves and a predator and increased group sizes will decrease the likelihood of any one individual being preyed upon. Group members may be able to detect predators more efficiently than solitary individuals. When individuals pay attention to other group members’ behavior, the entire group is alerted if one individual detects a threat. Certain individuals or species (in interspecific groups) in a group may serve as lookouts. For example, in the Amazon, white-winged shrike-tanagers (Lanio versicolor) act as sentinels in canopy flocks, while bluish-slate antshrikes (Thamnomanes schistogynus) serve as sentinels in understory flocks (Munn 1986). These species are typically the first to alarm when predators approach, alerting other species of imminent threat. Group members may also cooperate to drive predators away. Nesting bluegill sunfish (Lepomis macrochirus) work together to keep bullhead catfish from preying on their eggs (Gross and McMillan 1981).
There can also be drawbacks to living in a group. Group members may have to expend energy to maintain high status positions (Taborsky and Grantnerm 1998), experience reduced fitness (Hotker 2000), compete with each other for access to food (Scheel and Packer 1991), and be more vulnerable to infection (Alexander 1974). If groups get too large, individuals may become more vulnerable to predators. For example, wolves (Canis lupis) can capture caribou in large herds more easily than those in small herds, implying that individuals in large groups may not be able to detect approaching predators (Crisler 1956).
The optimal group size will vary among habitats and species and the group size that is optimal for one individual (a dominant one or an adult) may not be the same for another individual (a subordinate or a juvenile). Individuals should choose group sizes and habitats that maximize fitness (Fretwell 1972). The optimal group size is determined by comparing the fitness of the lowest-ranking group member to the fitness that the individual would have if they were solitary and in a poorer habitat.
Dispersal. Species may exhibit two types of dispersal. Natal dispersal refers to movement from the birth site to location of reproduction, and breeding dispersal is the movement of adults between breeding attempts. Dispersal is energetically costly and may expose the individual to predators. However, it can be beneficial for a variety of reasons. Innate dispersal refers to a genetic predisposition to disperse and is common in animals such as the spruce grouse (Falcipennis canadensis), in which the young disperse without receiving cues from their parents (Keppie and Towers 1992). Dispersal also has a genetic basis in fire ant (Solenopsis) queens, in which individuals carrying one genotype pursue new nesting sites, while individuals with the alternative genotype are more likely to return to their natal nesting place (DeHeer et al. 1999). Proximate causes for dispersal include responses to environmental factors, such as the absence of or competition for suitable resources. Ultimate causes of dispersal include inbreeding avoidance and competition for mates or resources.
In birds and mammals, dispersal is sex-biased. Dispersal may be costly to female mammals because their reproductive success is limited by nutritional constraints, while males are limited by the number of females with whom they can successfully mate. Female mammals may benefit more from philopatry, remaining in or returning to their birthplace to breed, because they would be more familiar with resources, whereas males may benefit from increased dispersal because they would have greater access to mates. Female Belding’s ground squirrels (Urocitellus beldingi) remain in close proximity to their birthplace because they need a territory in which to rear their young and their mothers are able to help them defend their territories (Holekamp 1984). Similarly, female lions (Panthera leo) remain near their birthplaces so they can benefit from access to familiar hunting grounds and safe places to rear offspring (Pusey and Packer 1987), while dispersing male lions are able to mate with nonrelatives (Hanby and Bygott 1987). In birds, males may be more successful at establishing territories in their natal area, whereas females may benefit from the opportunity to disperse to choose different mates.
Cooperation. Cooperative behavior evolves to provide a benefit to another individual and increases the fitness of both the giver and the recipient. Rodents, mammalian carnivores, and birds with young that require a high degree of parental care may engage in cooperative breeding, in which non-breeding adults contribute physically, but not genetically, to rearing young. Helpers may increase offspring survival by providing additional protection from predators and more food. Helpers’ fitness may increase if individuals are able to learn parenting behaviors or inherit a territory.
Cooperation may also be inter-specific. Aphids (Aphidoidea) secrete a liquid that ants (Formicidae) like to eat. In exchange, the ants protect aphids against predators and will sometimes raise their eggs and larvae inside their colony (Dawkins 1977). Some fishes, such as the goby (Gobiidae), will feed on ectoparasites of other fish.
Communication. Communication describes an action of one organism that changes the behavior of another organism. Communication can increase fitness if a stimulus or signal provides an advantage to either the signaler or its group.
Information may be communicated using visual, chemical, tactile, and/or auditory cues. The type of communication used by an individual is dependent upon their life history characteristics and environment. Both nocturnal and diurnal species use auditory cues to communicate. However, nocturnal animals are more likely to use olfactory signals that can be detected in the dark, while diurnal animals rely more heavily upon visual cues.
Individuals communicate to attract mates, scare off predators, and locate food. For example, male Mallorcan midwife toad (Alytes muletensis) calls stimulate the reproductive physiology of females (Lea et al. 2001). To alert gravid females that they are ready to reproduce, male threespine stickleback fish engage in elaborate courtship rituals that entice females to deposit eggs in their nests. Honeybees will engage in waggle dances to direct colony members to the location of food (von Frisch 1967). Signals also help individuals conserve their energy. For example, roaring in male lions and erection of spines in stickleback are less energetically expensive behaviors than attacking invaders.
Behavioral ecology has a rich history of study dating back to Darwin’s observations in the 1800s. Researchers focus on the purposes of behaviors and their adaptive advantages. Adaptive behaviors with a heritable basis increase individuals’ reproductive success and are under positive natural selection. The field focuses on ultimate causes of animal behavior, such as whether they maximize fitness and how behaviors vary among species. Foraging efficiency, territoriality, mating strategies, parental care, and communication are all types of behaviors that influence survival and reproductive success across species.
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