action potential A neural signal that transmits information via a self-regenerating change in membrane electrical charge that travels the length of a nerve cell, sometimes triggering further action potentials in adjacent nerve cells.

activational effect An immediate but temporary regulatory effect of a hormone. Also see organizational effect.

acute stressor Short-term event than can directly and rapidly affect an animal’s behavior by temporarily elevating glucocorticoid levels.

adaptation A characteristic that confers higher inclusive fitness to individuals than any existing alternative exhibited by other individuals in the population. An adaptation is a trait that has spread, is spreading, or is being maintained in a population as a result of natural selection.

adaptive value The contribution that a trait or gene makes to inclusive fitness.

allele A form of a gene. Different alleles typically code for distinctive variants of the same enzyme.

alternative hypotheses When only one of a series of competing hypotheses could explain a given behavior.

alternative mating strategy A type of behavioral polymorphism in which a mating behavior has a strong genetic component. Different forms of the behavior may be seen in different individuals (i.e., the behavior is polymorphic), but the form is fixed throughout a given individual’s life.

altricial Referring to young that reside in the nest for an extended period of time.

altruism Cooperative behavior that lowers the donor’s reproductive success while increasing the reproductive success of the recipient of the altruistic act.

amplitude Intensity of a waveform.

ancestral state reconstruction The recreation of traits of extinct species. Requires extrapolation from measured characteristics of extant species back in time to their common ancestors.

animal personality Individual behaviors that are repeatable through time.

anisogamy The fusion of two gametes that differ greatly in size.

aposematism Warning coloration, in which an organism’s highly visible or vivid coloration signals to potential predators that it is distasteful or dangerous. Also see Batesian mimicry; Müllerian mimicry.

armaments Elaborate morphological traits that have been selected for because they can act as weapons in intrasexual battles.

associated reproductive pattern The onset of reproductive behavior is tightly correlated with seasonal cues that trigger changes in circulating hormones and the gonads. Also see dissociated reproductive pattern.


badges of status Signals that reveal information about an individual’s size or dominance status. Although badge of status signals are typically not costly to produce, they may be costly to maintain due to social enforcement, a phenomenon referred to as a maintenance cost.

Bateman’s principle Because males achieve greater reproductive success they tend to have more mates than do females. As a result, males tend to have higher reproductive variance than females. 

Batesian mimicry When an edible species resembles a distasteful or dangerous one.

behavioral polymorphism When behavioral phenotypes become relatively more influenced by genetic factors than by the environment.

behavioral syndromes Behavioral consistency within individuals across contexts.

bottom-up forces When food and other resources influence behavioral decisions. Also see top-down forces.


carotenoid A type of dietary pigment that provides coloration to animal tissues and scavenges free radicals such as antioxidants that have been linked to disease and aging.

central dogma of molecular biology Information encoded in DNA is transcribed into RNA, which in turn is translated into a protein.

central pattern generator A group of cells in an organism’s nervous system that produces a particular pattern of signals necessary for a functional behavioral response.

chronic stressor Prolonged event that results in chronically elevated glucocorticoid levels, leading to negative physiological effects.

circadian clock An internal oscillator modulated by external cues such as sunlight or temperature that regulates physiological processes.

circadian rhythm A roughly 24-hour cycle of behavior that expresses itself independent of environmental changes.

circannual rhythm An annual cycle of behavior that expresses itself independent of environmental changes.

citizen science data Observations recorded by the public and collated by scientists to test hypotheses.

coefficient of relatedness (r) The probability that an allele present in one individual will be present in a close relative; the proportion of the total genotype of one individual present in the other as a result of shared ancestry.

coevolutionary arms race When two parties in conflict exert reciprocal selection pressure on each other, with an adaptive advance made by one often leading eventually to an adaptive counterresponse by the other.

cognition The mental process of acquiring knowledge and understanding through thought, experience, and the senses.

collective behavior The synchronized movements of individuals following a series of basic interaction rules.

command center A neural cluster or integrated set of clusters that has primary responsibility for the control of a particular behavioral activity.

communication The transfer of information from one individual (sender) to another (receiver) that affects current or future behavior and the fitness of one or both individuals.

comparative approach An approach to behavioral biology that involves using comparisons among species that have evolved independently to study relationships among traits or historical and physical constraints on trait evolution.

conditional strategy with alternative mating tactics The genetically based capacity of an individual to use different mating tactics under different environmental conditions; the inherited behavioral capacity to be flexible in response to certain cues or situations. More simply called conditional mating tactics.

conspecific (intraspecific) brood parasitism When an animal exploits the parental care of individuals of their own species.

convenience polyandry A form of polyandry in which a female will struggle with a male but acquiesces to his mating attempts in order to save time and energy.

convergent evolution The independent acquisition over time through natural selection of similar characteristics in two or more unrelated species. Also see divergent evolution. 

cooperative breeding A social system in which more than two individuals care for young.

cryptic female choice The ability of a female that receives sperm from more than one male to choose which sperm get to fertilize her eggs.

cue An unintentional transfer of information between a sender and a receiver.


Darwinian puzzle A trait that is maintained in a population even though it appears to reduce the fitness of individuals that possess it. Traits of this sort attract the attention of evolutionary biologists.

deceitful signaling A form of communication in which a sender uses a specially evolved signal to manipulate the behavior of a receiver such that the sender receives a fitness benefit but the receiver pays a fitness cost. Sometimes referred to as manipulation.

density-dependent habitat selection When settlement decisions are influenced by the intensity of intraspecific competition as reflected by the density of conspecifics in a location.

descent with modification The foundational idea of evolution, that individuals pass varied genetic traits to their offspring, and that differences in reproductive success among individuals in a population cause a population or species to evolve over time.

developmental homeostasis The capacity of developmental mechanisms within individuals to produce adaptive traits, despite potentially disruptive effects of mutant genes and suboptimal environmental conditions.

developmental plasticity The ability to respond (often with changes in neural connections) to environmental cues through the adjustment of genotypic expression during early development.

dilution effect When associating in groups makes it less likely that any one individual will be depredated.

diplodiploidy Having two sets of the chromosomes, and therefore two copies of genes (one from a mother and from a father). Synonymous with diploidy. Also see haploidy.

diploidy Having two sets of the chromosomes, and therefore two copies of genes (one from a mother and from a father). Synonymous with diplodiploidy. Also see haploidy.

direct benefits Material benefits provided by a male that can increase a female’s fitness. Examples include parental care, access to resources, safety from predators, and reduced harassment by other males.

direct fitness A measure of the reproductive (genetic) success of an individual based on the number of its offspring that live to reproduce. Also see inclusive fitness, indirect fitness.

dispersal The permanent movement from the birthplace to somewhere else.

dissociated reproductive pattern The onset of reproductive behavior is apparently not triggered by a sharp change in circulating hormones. Also see associated reproductive pattern.

divergent evolution The evolution by natural selection of differences between closely related species that live in different environments and are therefore subject to different selection pressures. Also see convergent evolution. 

DNA methylation Chemical modifications (typically the addition of methyl groups) to DNA molecules that influence gene expression.

dominance hierarchy Social ranking within a group, in which some individuals give way to others, often conceding useful resources without a fight.


eavesdropping The detection of signals from a legitimate signaler by an illegitimate receiver, to the detriment of the signaler and the benefit of the receiver.

economic defensibility The trade-off in costs versus benefits for maintaining a territory.

entrain To reset a biological clock so that an organism’s activities are scheduled in keeping with local conditions.

epigenetic modification Alterations to the genome that do not change the DNA sequence. Examples include DNA methylation. 

ethology The study of the proximate mechanisms and adaptive value of animal behavior is a discipline founded by Niko Tingerben and Konrad Lorenz.

eusociality Describes species with overlapping generations, cooperative care of young, and reproductive division of labor where many individuals in a group are permanently sterile.

evolutionarily stable strategy (ESS) That set of rules of behavior that when adopted by a certain proportion of the population cannot be replaced by any alternative strategy. 

evolutionary constraint A limitation or restriction on adaptive evolution.

evolutionary game theory An evolutionary approach to the study of adaptive value in which the payoffs to individuals associated with one behavioral tactic are dependent on what the other members in the group are doing.

evolutionary genetics A field of study that examines how DNA leads to evolutionary change by comparing sequence variation among species or individuals that exhibit different traits.

evolutionary history The sequence of changes that have occurred over time as an ancestral trait becomes modified and takes on a new form (and sometimes, a new function). Also see phylogeny.

experimental approach An approach to behavioral biology that involves manipulating features of the animal or its environment to more directly establish a causal relationship among traits.

explosive breeding assemblage The temporary formation of a large group of mating individuals.

extra-pair copulation A mating by a male or female with an individual other than his or her partner in a socially monogamous species.


facultative siblicide Behavior that occasionally results in an individual killing a sibling or siblings.

fitness A measure of the genes contributed to the next generation by an individual. Often stated in terms of the number of surviving offspring produced by the individual, technically called direct fitness. Also see inclusive fitness; indirect fitness. 

fitness benefit The positive effect of a trait on an individual’s reproductive (and genetic) success.

fitness cost The negative effect of a trait on an individual’s reproductive (and genetic) success.

fitness payoff The fitness gain or loss from a social interaction or other behavior.

fixed action pattern (FAP) An innate, highly stereotyped response that is triggered by a well-defined, simple stimulus. Once the pattern is activated, the response is performed in its entirety.

forward genetics The identification of the genes responsible for a given phenotype.

free-running cycle The cycle of activity of an individual that is expressed in a constant environment.

frequency The rate at which amplitude of a stimulus, such as sound, increases and decreases.

frequency-dependent selection A form of natural selection in which those individuals that happen to belong to the less common of two types in the population are the ones that are more fit because of their lower frequency in the population. Can be negative (the fitness of a phenotype decreases as it becomes more common) or positive (the fitness of a phenotype increases as it becomes more common) in nature.

functional genomics A field of study that examines the relationship between genotype and phenotype by comparing gene or protein expression in individuals or species that exhibit different traits.

fundamental asymmetry of sex Males produce small sperm (and many of them) and females produce large eggs (and relatively few of them), based on the premise that sperm are energetically “cheaper” to produce than eggs.


gene A segment of DNA, typically one that encodes information about the sequence of amino acids that make up a protein.

genetic monogamy A form of monogamy in which a male and female form a pair-bond and mate only with each other.

genotype The genetic constitution of an individual. Can refer either to the specific alleles of one gene possessed by the individual or to the individual’s complete set of genes.

group augmentation Individuals survive or reproduce better by living in larger groups.

group selection The process that occurs when groups differ in their collective attributes and these differences affect the survival chances of the group.


habitat saturation Occurs when territories or breeding sites are limiting to a population because most of the best sites are already occupied by other members of that species.

Hamilton’s rule The argument made by William D. Hamilton that altruism can spread through a population when rB > C (where r is the coefficient of relatedness between the altruist and the individual helped, B is the fitness benefit received by the helped individual, and C is the cost of altruism in terms of the direct fitness lost by the altruist due to its actions).

haplodiploidy A sex-determination system in which males develop from an unfertilized egg and are haploid (have one set of chromosomes), whereas females develop from a fertilized egg and are diploid (have two sets of chromosomes).

haploidy Having one set of the chromosomes, and therefore one copy of each gene. Also see diploidy and diplodiploidy.

histone modification A type of epigenetic modification to histone proteins that affects gene expression by altering chromatin structure or recruiting histone modifiers. [3] 

honest signaling A form of communication in which both the sender and the receiver obtain a fitness benefit.

honest signal Signal that indicates the quality of the sender, facilitates decision making by a receiver, and cannot be easily produced by the sender.

hypothalamic–pituitary–adrenal (HPA) axis A cascade in which corticotropin-releasing hormone (CRH) secreted from the hypothalamus stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland, which in turn leads to the production of glucocorticoids in the adrenal cortex.

hypothalamic–pituitary–gonadal (HPG) axis A cascade of reproductive hormones in which gonadotropin-releasing hormone (GnRH) produced in the hypothalamus triggers the release of luteinizing hormone from the pituitary, and eventually the production of testosterone (in males) and estrogen (in females) in the gonads.

hypothesis A possible, even speculative, explanation that is used as a starting point for further examination via testing predictions taken from the hypothesis.


ideal free distribution theory A theoretical framework that enables behavioral biologists to predict what animals should do when choosing between alternative habitats of different quality in the face of competition for space, food, or other critical resources.

imprinting A form of learning in which individuals exposed to certain key stimuli early in life form an association with an object or individual and may later attempt to mate with similar objects.

inclusive fitness A total measure of an individual’s contribution of genes to the next generation generated by both direct fitness (derived from reproduction) and indirect fitness (which depends on social interactions with relatives).

indirect (genetic) benefit Genetic benefit that does not benefit a female directly but does increase the fitness of her offspring.

indirect fitness A measure of the genetic success of an altruistic individual based on the number of relatives (or genetically similar individuals) that the altruist helps reproduce that would not otherwise have survived to do so. Also see direct fitness, inclusive fitness.

indirect reciprocity A form of reciprocity in which a helpful action is repaid at a later date by individuals other than the recipient of assistance.

infanticide The intentional killing of offspring, usually by a parent.

innate releasing mechanism A conceptual neural mechanism thought to control an innate response to a sign stimulus.

instinct A behavioral pattern that reliably develops in most individuals, promoting a functional response to a releaser stimulus the first time the behavior is performed.

Institutional Animal Care and Use Committee (IACUC) An institutional ethics committee whose permission is required for research studies on nonhuman vertebrates.

Institutional Review Board (IRB) An institutional ethics committee whose permission is required for research studies on humans.

interactive theory of development The development of behavioral traits requires both genetic information and environmental inputs.

interneuron A nerve cell- that relays signals either from sensory receptor neurons (e.g., touch receptors, odor receptors, light receptors) to the central nervous system (a sensory interneuron) or from the central nervous system to neurons commanding muscle cells (a motor interneuron).

intersexual selection Sexual selection usually involving male courtship behavior or appearance that influences a female’s choice of mate.

interspecific brood parasitism When an animal exploits the parental care of individuals of another species.

intrasexual selection Sexual selection in which members of the same sex compete for access to mates.


kin recognition The ability to distinguish between close genetic kin and non-kin.

kin selection A type of natural selection that favors the reproductive success of an organism’s relatives, even at a cost to the organism’s own fitness.

kleptoparasitism Stealing food from an individual that has caught or collected it.


landscape of fear The spatially explicit elicitation of fear in prey when cues in the environment (such as odors, alarm calls) lead to the perceived risk of predation.

lek A traditional display site that females visit to select a mate from among the males displaying at their small, resource-free territories.

levels of analysis The proximate (developmental and physiological) and ultimate (historical and adaptive evolutionary) causes of a behavior.

load-lightening Helpers reduce the workload of parents in offspring care.


maintenance cost A cost associated with the socially enforced maintenance of a signal.

major histocompatibility complex (MHC) A group of cell surface proteins critical for the immune system to recognize foreign molecules. Allelic diversity in the MHC leads to a more robust immune system and thus may have a selective advantage.

marginal value theorem A type of optimality model that predicts that an animal should leave a foraging patch when its rate of food intake in that patch drops below the average rate for the habitat, and that this marginal capture rate should be equalized over all patches within a habitat.

maternal effect Where an individual’s phenotype is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother.

melanin A type of color-producing pigment that animals produce in tissues such as skin and hair. The two main forms of melanin are eumelanin, which produces brown and black coloration, and pheomelanin, which produces red and yellow coloration.

migration The regular movement back and forth between two relatively distant locations by animals that use resources concentrated in these different sites.

migratory connectivity The movement of individuals between summer and winter populations, including the stopover sites between the breeding and wintering grounds. 

monogamy A mating system in which one male mates with just one female, and one female mates with just one male in a breeding season. Also see genetic monogamy; social monogamy.

monogynous Among social insects, refers to a colony that has only a single reproductive queen.

Müllerian mimicry When two or more distasteful or dangerous species resemble one another.

multilevel selection Selection that acts at both the group and the individual levels, originating with genes and progressing through cells, then organisms, and finally groups of organisms. According to the advocates of this form of selection, at higher levels, populations, multispecies communities, and even whole ecosystems can be subject to selection.

multimodal signaling The use of multiple traits to signal to the same or different individuals.

mutual benefit A type of social behavior when both interacting individuals receive a fitness benefit.


natural selection The process that occurs when individuals differ in their hereditary traits and the differences are correlated with differences in reproductive success. Natural selection can produce evolutionary change. Also see group selection; kin selection; sexual selection.

neural circuit A group of interconnected neurons that are able to regulate their own activity using a feedback loop.

neuron A nerve cell.

neurotransmitter A chemical signal that diffuses from one nerve cell to another across a synapse.

non-mutually exclusive hypotheses When multiple hypotheses could apply to a given behavior.

nuclei Dense clusters of neurons within central nervous systems.

nuptial gift A food item transferred by a male to a female just prior to or during copulation.


obligate siblicide Behavior that always results in an individual killing a sibling or siblings.

observational approach An approach to behavioral biology that involves watching animals behaving in nature or in the lab.

operant conditioning A kind of learning based on trial and error, in which an action, or operant, becomes more frequently performed if it is rewarded.

operational sex ratio The ratio of sexually active males to sexually receptive females in a population.

optimal foraging theory A model that predicts how an animal should behave when searching for food.

optimality theory An evolutionary theory based on the assumption that the attributes of organisms are optimal; that is, the attributes present in the organism are better than other alternatives in terms of the ratio of fitness benefits to costs. The theory is used to generate hypotheses about the possible adaptive value of traits in terms of the net fitness gained by individuals that exhibit these attributes.

organizational effect A relatively permanent effect of a hormone during development that causes changes to physiology and behavior. Also see activational effect.

ornament An elaborate morphological trait that has apparently been selected for because it attracts mates.

oscillogram A graph of amplitude of an acoustical stimulus as a function of time.


parent–offspring conflict Evolutionary conflict arising from differences in optimal parental investment in an offspring from the standpoint of the parent versus that of the offspring.

parental investment Costly parental activities that increase the likelihood of survival for some existing offspring but that may reduce the parent’s chances of producing offspring in the future.

parsimony The principle that the simplest explanation that fits the evidence is likely to be the correct one.

parthenogenesis Development from an unfertilized egg.

phenotype Any measurable aspect of an individual’s body or behavior that arises from an interaction of the individual’s genes with its environment.

photoperiod The number of hours of light in a 24-hour period.

phylogeny An evolutionary genealogy of the relationships among species or clusters of species; a representation of the evolutionary history of taxa.

pigment A type of molecule that differentially absorbs and emits wavelengths of visible light.

plural breeding A form of cooperative breeding system in which social groups contain more than one breeder of at least one sex.

polyandry A mating system in which a female has several partners in a breeding season. Also see convenience polyandry.

polygamy The human practice of having more than one wife or husband simultaneously.

polygynandry A mating system in which both males and females have several partners with whom they form a pair-bonds.

polygynous Among social insects, refers to a colony that has multiple reproductive queens.

polygyny A mating system in which a male fertilizes the eggs of several partners in a breeding season.

polyphenism The occurrence within a species of two or more alternative phenotypes whose differences are induced by key differences in the environments experienced by individual members of the species.

precocial Referring to young that are mobile soon after hatching.

predator swamping The movement of migratory individuals together in high densities to confuse predators or reduce the predation risk to the migratory individuals. Analagous to the dilution effect, where associating in groups makes it less likely that any one individual will be depredated.

prediction An expectation that should follow if a hypothesis is true.

preexisting bias An existing bias in an animal’s sensory system that detects some features of an organism’s world better than others.

preexisting trait An existing behavioral, physiological, or morphological characteristic that already provides an informative cue to receivers. If the sender benefits from the receiver’s response, the cue can be modified into a signal via a process called ritualization.

prisoner’s dilemma A game theory construct in which the fitness payoffs to individuals are set such that mutual cooperation between the players generates a lower return than defection (which occurs when one individual accepts assistance from the other but does not return the favor).

production cost A cost associated with the production of an energetically expensive signal.

promiscuity A mating system in which both males and females have several partners without forming a pair-bonds.

proximate cause An immediate, underlying reason for why a behavior is the way it is based on the operation of internal mechanisms possessed by an individual.


reciprocal altruism A helpful action that is repaid at a later date by the recipient of the assistance. Also known as reciprocity. Also see indirect reciprocity.

releaser A sign stimulus given by an individual as a social signal to another individual.

reproductive conflict Conflict among members within a social group over which individuals get to breed, as well as conflict over the resources that individuals need in order to breed. 

reproductive skew The unequal partitioning of reproductive success within a population or social group.

reproductive success The number of surviving offspring produced by an individual; direct fitness.

reproductive value A measure of the probability that a given offspring will reach the age of reproduction, or the potential of an individual to leave surviving descendants in the future.

resource-holding potential The inherent capacity of an individual to defeat others when competing for useful resources.

resource selection A method for characterizing the distribution of a species using the known spatial distribution of its resources.

reverse genetics The determination of which phenotypes will arise as a result of particular genetic sequences. Often achieved through experimental manipulation of the genetic code.


scientific method Observation, measurement, and experimentation to test hypotheses by seeing if the predictions produced by those hypotheses are correct. If not, hypotheses can be refined to undergo repeated testing.

selection pressure An agent of differential reproduction or survival that causes a population to change genetically.

selfish herd A group of individuals whose members use others as living shields against predators.

selfishness When the donor benefits from a social interaction but the recipient does not. Sometimes called deceit or manipulation.

sensory drive The fine-tuning of signals to work effectively in a particular environment.

sensory exploitation The evolution of signals that activate established sensory systems of signal receivers in ways that elicit responses favorable to the signal sender.

sex role reversal A change in the typical behavior patterns of males and females, as when, for example, females compete for access to males or when males choose selectively among potential mates.

sex-biased dispersal When individuals of one sex disperse farther than those of the other.

sexual arms race Conflict between males and females over mating as males evolve traits used in manipulating or forcing females to copute, and females evolve traits that help circumvent this (often violent) process.

sexual conflict Conflict between males and females over mate choice.

sexual dimorphism Difference in appearance between males and females of the same species. 

sexual selection A form of natural selection that acts on traits used to compete for mates with others of the same sex, or to attract members of the opposite sex in order to mate with them.

siblicidal behavior Behavior that results in an individual killing a sibling or siblings. Also see obligate siblicide; facultative siblicide.

sibling conflict Evolutionary conflict resulting from divergent interests among current and/or future siblings.

sign stimulus The effective component of an action or object that triggers a fixed action pattern in an animal.

signal A specially evolved message that contains information.

singular breeding A form of cooperative breeding system in which social groups contain only one breeder of each sex.

social monogamy A form of monogamy in which a male and female form a pair-bond but one or both sexes also may mate outside of the pair-bond.

spectrogram A visual representation of sound frequencies over time. In a spectrogram, intensity is shown using color or grayscale. Sometimes referred to as a sonogram.

sperm competition Competition among the sperm of different males that determines whose gametes will fertilize a female’s eggs.

spermatophore Often a type of nuptial gift given by a male to a female that contains both nutrients and sperm.

spite When neither the donor nor the recipient benefits from a social interaction, but instead both pay a cost.

stimulus Generally a sensory signal capable of triggering a complex behavior.

stimulus filtering The capacity of nerve cells and neural networks to ignore stimuli that could potentially elicit a response from them.

structural colors Colors produced by light interacting physically with the nanometer-scale arrangement of tissues and air.

supergene A region of DNA containing many linked genes that can influence the development of a behavioral phenotype.

syllable A distinct element of a vocalization, including birdsong.

synapse The point of near-contact between one nerve cell and another.


territory An area in which individuals exhibit a readiness to defend against intruders.

Tinbergen’s four questions A scheme developed by Nikolaas Tinbergen to address the proximate and ultimate causes of a behavior. Within the proximate level of analysis fall questions relating to development and to internal mechanism, and within the ultimate level of analysis fall questions relating to evolutionary history and adaptive function.

top-down forces When predation risk influences behavioral decisions. Also see bottom-up forces.

trait A character or feature of an organism. Also see phenotype.

transcriptome All of the mRNA being expressed in a given tissue at given time point.


ultimate cause An evolutionary, historical reason for why a behavior is the way it is.


waveform Graphical representation of the change in pressure over time that composes a sound. Also see amplitude; frequency.

worker policing Among eusocial insects, when a worker female eats or removes eggs that have been laid by other workers rather than those laid by a queen.