Chapter 53
Behavioral Ecology
Behavioral Ecology
• Introduction
• Responding to Environmental Variation
• The Evolution of Animal Societies
• Behavioral Ecology, Population Dynamics, and Community Structure
Introduction
• Ecology is the science that deals with all kinds of biological interactions.
• Individuals of all species interact in various ways with individuals of their own and other species and with their physical environment.
• The term environment includes both abiotic (physical and chemical) and biotic factors (all other organisms living in an area).
• Behavioral ecology is the study of how __________ make “decisions” that influence their survival and reproductive success.
Responding to Environmental Variation
• Throughout their life, all organisms must make many decisions and life choices in a changing environment.
• Some environmental changes, such as the approach of danger, require an immediate response; others allow for a more gradual response.
• Some plants detach their leaves in a wind storm and regrow leaves afterward.
• Lizards bask in the morning sun and then move to the shade when it gets too hot.
Responding to Environmental Variation
• Organisms may evolve life __________ that anticipate __________ environmental change.
• Insectivorous birds leave high latitudes in autumn for more favorable wintering grounds; grazing animals may migrate to follow the rains.
• Other animals hibernate until environmental conditions have improved.
Responding to Environmental Variation
• The environment in which an organism normally lives is called its habitat.
• For choosing a habitat, an animal seeks food, resting places, nest sites, and escape routes from predators.
• Numerous factors influence how animals choose environments, but in general, habitat selection cues are good predictors of general conditions suitable for survival and reproduction.
Responding to Environmental Variation
• Habitat selection by red abalone involves chemical cues.
• The abalone begins its life as a motile larva and swims in the open ocean until its yolk sac is gone (about seven days).
• Guided by chemical signals from coralline algae (the abalone’s food source), the abalone settles on the seafloor and __________ into an adult.
• Only the coralline algae produce the chemical, so the larvae always settle onto suitable habitat.
Responding to Environmental Variation
• Many animals use the presence and success of already settled individuals as an indication that the habitat may be good ground.
• When collared flycatchers arrive on their breeding grounds in the spring, they peer into the nests of other individuals.
• In studies in which broods were artificially enlarged by researchers, birds settled preferentially in those areas.
Responding to Environmental Variation
• An animal may leave an area temporarily or permanently if the population has grown too large to be supported there.
• When a colony of the ant Lepidothorax albipennis has grown too large for its nest site, recruiter ants look for a new site.
• A recruiter returns to the nest and releases a pheromone that attracts a second recruiter, and both visit the new site. Then a third recruiter is brought in, and so on.
• Once a threshold number of workers has been recruited to a site, the recruiters begin carrying eggs and larvae from the old to the new site.
Responding to Environmental Variation
• The most common way for an animal to improve its survival and reproductive success is to establish an exclusive territory.
• Advertising and defending the territory takes time and energy.
• To understand the evolution of these types of behavior, ecologists use a method called cost–benefit analysis, based on two assumptions:
An animal has a limited amount of time and energy to devote to any particular activity.
Animals generally do not perform behaviors whose __________ __________ are greater than the sum of their benefits.
Responding to Environmental Variation
• The cost of behavior has three components:
The __________ cost is the difference between the energy the animal would have expended had it rested and the energy expended in performing the behavior.
The __________ cost is the increased chance of being injured or killed as a result of performing the behavior.
The __________ cost is the sum of benefits the animal forfeits by not being able to perform other behaviors during the same time interval.
Responding to Environmental Variation
• An experiment conducted with Yarrow’s spiny lizards demonstrates all three costs.
• Male lizards with implanted testosterone patrolled their territories more, performed more displays, and expended more energy than control males did (energetic cost).
• They had less time to feed (opportunity cost), captured fewer insects, stored less energy, and died at a higher rate (risk cost).
• Normally, the lizards only defend their territories vigorously during the breeding season.
Responding to Environmental Variation
• Foraging theory is used to predict how animals will behave when searching for food.
• A scientist first specifies the objective of the behavior and then attempts to determine the behavioral choices that would best achieve that objective.
• This approach is known as optimality modeling.
• The underlying assumption is that natural selection has molded the behavior of animals so that they solve problems by making the best choices available to them.
Responding to Environmental Variation
• In the case of food selection, for example, optimality modeling would support the energy maximizing hypothesis:
If the most valuable prey type is abundant, a predator gains the most energy per unit of time spent foraging by taking only that prey type.
As the abundance of the most valuable prey type decreases, the predator adds less valuable prey to its diet.
Responding to Environmental Variation
• The energy maximizing hypothesis has been tested using bluegill sunfish.
• Prey types were three sizes of Daphnia, the water flea. They were ranked according to their energy quantity and the energy required for the fish to capture and ingest them.
• If all three sizes of Daphnia were present in low numbers, the fish ate every one encountered.
• If large Daphnia were abundant, the fish ignored the smaller ones.
Responding to Environmental Variation
• Animals select certain foods for reasons other than energy content. Many species of mammals and birds, for instance, get minerals by eating mineral-rich soil.
• One hypothesis to explain human’s taste for spices is that spices have antimicrobial properties against food-borne bacteria.
Responding to Environmental Variation
• The most basic mating decision is the choice of a partner of the correct species.
• Decisions are made based on the __________ of a potential mate, the resources it controls, and nest sites.
• The reproductive behaviors of males and females are often very different, in part because of the costs of producing sperm and eggs.
• Sperm are cheap to produce (energetically). Therefore, males of most species can increase their reproductive success by mating with __________ females.
Responding to Environmental Variation
• Eggs are energetically more expensive to produce.
• To improve their reproductive success, females need to assess the quality of potential mates, including health, genetic quality, potential for parental care, and quality of resources they control.
• Males use a variety of tactics to induce females to copulate.
• Females favor signals at which males can’t “cheat”, and have favored the selection of reliable signals.
Responding to Environmental Variation
• Experiments with bluethroats have shown that females respond to the UV light reflected by the bright blue throat patch of males.
• They prefer normal males to those to whom sunscreen has been applied because intense plumage is an indicator of a male’s health.
The Evolution of Animal Societies
• Social behavior evolves when individuals who cooperate with others of the same species have, on average, higher rates of survival and reproductive success than those achieved by solitary individuals.
• Today’s animal social systems are the result of long periods of evolution, but behavior leaves few traces in the fossil record.
• Biologists infer possible routes of the evolution of social systems by studying current patterns of social organization.
The Evolution of Animal Societies
• Three important concepts in understanding animal social systems:
Social systems are best explained by how they benefit the individuals who join together, not according to how they benefit the species as a whole.
They are dynamic, as individuals constantly communicate and adjust relationships.
The costs and benefits to specific individuals differ according to their age, sex, physiological condition, and status.
The Evolution of Animal Societies
• Group living may improve hunting success or expand the range of prey that can be captured.
• Hunting in groups, our ancestors were able to kill larger animals than they could have if they had hunted alone.
• Small birds forage in flocks; flocking has been shown to provide protection against predation.
The Evolution of Animal Societies
• Social behavior has costs as well as benefits.
• Individuals in a group may compete for food, interfere with one another’s foraging, injure one another’s offspring, inhibit one another’s reproduction, or transmit diseases to their associates.
The Evolution of Animal Societies
• The most widespread social system is the family, an association of one or more adults and their __________ offspring.
• If parental care or the breeding season lasts a long time, older offspring may be available to help parents care for younger siblings.
•
• Parental care is altruistic—it involves tremendous costs for parents and helpers. How has it been possible, therefore, for altruism to evolve?
The Evolution of Animal Societies
• Altruistic behaviors are most easily understood in terms of close genetic relatedness.
• An individual contributes to its own individual fitness by producing offspring and may also help relatives in ways that increase their fitness.
• By helping its relatives, an individual can increase the representation of some of its own genes in the population. This process is known as kin selection.
• Together, individual fitness and kin selection determine the inclusive fitness of an individual.
The Evolution of Animal Societies
• Occasional __________ acts may eventually evolve into altruistic behavior if the benefits of increasing reproductive success of relatives outweigh the costs in terms of the individual’s own reproductive success.
• Breeding pairs of white-fronted bee-eaters are assisted by nonbreeding adults who help them incubate their eggs and feed nestlings. Helpers choose nests with young that are most closely related to them.
• This behavior likely evolved through kin selection. Individual birds do not gain anything other than inclusive fitness, and nests with helpers produce more fledglings than nests without helpers.
The Evolution of Animal Societies
• Species such as ants, bees, and wasps, whose social groups include sterile individuals, are said to be eusocial.
• In these species, worker females defend the group against predators or bring food to the colony, but they do not reproduce; only a few females, known as queens, reproduce.
• Some ant species have soldiers with large defensive weapons.
The Evolution of Animal Societies
• Genetic factors may facilitate the evolution of eusociality.
• Among the Hymenoptera, a diploid egg hatches into a female and a haploid egg hatches into a male.
• Therefore, if a female mates with only one male, her daughters share all of their father’s genes but only half of their mother’s.
• Because workers are more genetically similar to their sisters than they would be to their own offspring, they increase their own fitness by caring for their sisters rather than by reproducing themselves.
The Evolution of Animal Societies
• Eusociality may be favored if establishment of a new colony is difficult and dangerous. Nearly all eusocial animals construct elaborate nests or burrow systems within which their offspring are reared.
• High __________ rates may account for the eusociality of naked mole rats, who live underground in colonies of 70–80 individuals and restrict breeding to a single queen and several kings.
• Inbreeding could help explain the evolution of eusociality among many hymenopteran species in which queens mate with many males, and among termites and naked mole-rats, in which both sexes are diploid.
Behavioral Ecology,
Population Dynamics, and Community Structure
• The ways in which organisms make decisions about habitats, food, and associates may have important implications for the structure and function of ecological systems.
• First, animals with complex social organizations often achieve high abundances.
• Second, the decisions animals make about the above three matters may influence the range of habitats and foods used by a species.
Behavioral Ecology,
Population Dynamics, and Community Structure
• For ants and termites, living in colonies allows them to harvest vital resources from other organisms.
• Most of the biomass of arthropods in the canopies of tropical rainforests are social ants.
• Termites,
which live in dense colonies, are the primary consumers of plant tissues in the
savannas of
• Both actively cultivate fungi that break down difficult-to-digest plant tissues, including wood.
• Some ants tend phloem-sucking aphids and other insects that provide the ants with the carbohydrates they need.
Behavioral Ecology,
Population Dynamics, and Community Structure
• Social
living also enables organisms to use temporally and spatially patchy foods. An
example is the wildebeest, which travels in large herds and is the most
abundant wild mammal in
• Social organization allows humans to live in high densities and to specialize in different activities.
Behavioral Ecology,
Population Dynamics, and Community Structure
• Despite the “rule of thumb” accuracy of the optimality modeling approach, interspecific interactions may prevent animals from living in those environments in which they would do best.
• Individuals of a behaviorally dominant species may be able to exclude individuals of a subordinate species from its preferred foraging areas.
Behavioral Ecology,
Population Dynamics, and Community Structure
• Hummingbirds extract nectar from flowers and defend flower patches.
• In an
experiment in
• Larger male blue-throated hummingbirds kept smaller male black-chinned hummingbirds away from the rich feeders.
• Nevertheless, the smaller hummingbirds achieved about the same amount of energy from the dilute feeders because they were able to feed longer at them.
Animation 53.1 The Costs of Defending a Territory
Animation 53.2 Foraging Behavior
Video 53.1 Cooperative behavior in weaver ants
Video 53.2 The social system of leaf cutter ants
Video 53.3 The social system of honeybees