Difficulty: Intermediate | Prerequisites: Basic genetics and natural selection concepts
This section of Chapter 29 shifts from how muscles produce movement to why animals move the way they do. Behavioural ecology asks how evolution shapes behaviour: why a bird sings a particular song, why a squirrel buries nuts, why some animals are monogamous and others are not. You will need to understand the difference between innate and learned behaviours, and how natural selection acts on behavioural traits the same way it acts on physical ones. If you are comfortable with natural selection and fitness from earlier chapters, you are ready.
Animal behaviour is a product of both genetics (innate behaviour) and experience (learning). Behavioural ecology uses evolutionary logic to explain foraging strategies, mating systems, and social behaviours such as altruism. Hamilton's rule and kin selection explain why organisms sometimes sacrifice their own fitness for relatives.
Behaviour
An action carried out by muscles under the control of the nervous system in response to a stimulus. In simple terms, anything an animal does that involves a motor output.
Behavioural ecology
The study of the ecological and evolutionary basis for animal behaviour. It asks: given the pressures of natural selection, why does this animal behave this way?
Innate behaviour
Behaviour that is developmentally fixed and does not require learning or prior experience. Examples include fixed action patterns, courtship stimulus-response chains, and pheromone signalling. Think of it as behaviour that is "hardwired."
Fixed action pattern (FAP)
A sequence of unlearned acts directly linked to a simple stimulus. Once triggered, it runs to completion regardless of changes in the environment. A classic example is the egg-retrieval behaviour in greylag geese.
Sign stimulus
The specific external cue that triggers a fixed action pattern. In simple terms, the "button" that starts the pre-programmed behaviour.
Pheromones
Chemical substances released by one animal that affect the behaviour or physiology of another member of the same species. Used in mate attraction, territory marking, alarm signalling, and trail-following.
Learning
The modification of behaviour based on specific experiences. Unlike innate behaviour, learned behaviour can change within an individual's lifetime.
Imprinting
The establishment of a long-lasting behavioural response to a particular individual, typically a parent, during a sensitive period early in life. Konrad Lorenz famously demonstrated this with greylag geese following him as a "parent."
Sensitive period (critical period)
A limited time window during development when an organism is especially responsive to certain stimuli and able to acquire particular behaviours. If the window closes, the learning may never occur normally.
Spatial learning
The establishment of a memory that reflects the spatial structure of the environment. Think of it as building an internal map.
Cognitive map
A representation in the nervous system of the spatial relationships between objects in an animal's surroundings. It allows navigation without having to retrace a known route step by step.
Associative learning
The ability to associate one environmental feature with another. Classical conditioning (Pavlov's dogs) and operant conditioning (trial-and-error learning) are both forms of associative learning.
Cognition
The process of knowing that involves awareness, reasoning, recollection, and judgement. In animal behaviour, it refers to higher-order mental processes such as problem solving.
Social learning
Learning through observing others. It enables behaviours to spread through a population without each individual needing to learn from scratch.
Culture
A system of information transfer through social learning or teaching that influences the behaviour of individuals in a population. Not limited to humans: tool use in chimpanzees and song dialects in birds are cultural phenomena.
Foraging
Food-obtaining behaviour. Optimal foraging theory predicts that animals will behave in ways that maximise energy intake relative to the costs of searching, handling, and consuming food.
Monogamous
A mating system in which one male mates with one female, at least for a given breeding season. Often associated with biparental care of offspring.
Polygamous
A mating system in which an individual of one sex mates with several of the other. Includes polygyny (one male, multiple females) and polyandry (one female, multiple males).
Altruism
A behaviour that reduces an animal's own individual fitness but increases the fitness of other individuals in the population. It poses a puzzle for natural selection, which Hamilton's rule resolves.
Inclusive fitness
The total effect an individual has on the proliferation of its genes, both by producing its own offspring and by providing aid that enables close relatives to produce offspring.
Coefficient of relatedness (r)
A measure of the proportion of genes shared by common descent between two individuals. Siblings share r = 0.5; parent-offspring share r = 0.5; first cousins share r = 0.125.
Hamilton's rule
Natural selection favours altruism when rB > C, where r is the coefficient of relatedness between altruist and recipient, B is the benefit to the recipient, and C is the cost to the altruist.
Kin selection
Natural selection that favours altruistic behaviour by enhancing the reproductive success of relatives, even at a cost to the altruist's own direct reproduction.
A stereotyped, species-specific sequence of behaviours triggered by a specific stimulus
Once initiated, the sequence runs to completion even if the stimulus is removed
Example: a male stickleback fish attacks anything with a red underside (the sign stimulus) during breeding season, including non-fish objects painted red
FAPs are under strong genetic control and show little variation between individuals of the same species
The sign stimulus is the minimal cue that triggers the FAP
A releaser is a signal specifically evolved to trigger a FAP in another individual (e.g. the red belly of the stickleback)
Supernormal stimuli are exaggerated versions of the sign stimulus that can produce an even stronger response (e.g. a bird preferring to incubate an artificially oversized egg)
A signal is a stimulus transmitted from one animal to another
Communication occurs when the signal changes the behaviour of the receiver
Signals can be visual, auditory, chemical (pheromones), tactile, or electrical
Pheromones are especially important in insects for mate attraction, alarm signalling, and colony coordination
A regular, long-distance change in location, often seasonal
Involves innate navigational abilities (using the sun, stars, Earth's magnetic field) combined with learning
Studied through cross-fostering experiments and displacement experiments
Cross-fostering studies place the young of one species in the care of adults from another species. If behaviour follows the biological parents, it suggests a genetic basis.
Twin studies compare the behaviour of identical twins raised apart with those raised together. Greater similarity in separated twins points to genetic influence.
Occurs during a sensitive period early in life
The young animal forms a lasting attachment to a specific individual (usually the parent)
Not easily reversed once the sensitive period closes
Lorenz's geese imprinted on him because he was the first moving object they saw after hatching
Animals learn the layout of their environment and encode it as a cognitive map
Allows flexible navigation: the animal can take shortcuts and detours rather than relying on a single memorised route
Demonstrated in studies where animals find hidden food even when starting from novel positions
Classical conditioning: an animal learns to associate an arbitrary stimulus with a reward or punishment (Pavlov's bell → salivation)
Operant conditioning (trial and error): an animal learns to associate its own behaviour with a consequence (a rat pressing a lever → food)
Both forms allow animals to modify behaviour based on experience, which is adaptive in changing environments
Cognition encompasses awareness, reasoning, recollection, and judgement
Problem solving is the cognitive activity of devising a method to overcome an obstacle
Examples include chimpanzees stacking boxes to reach bananas and crows bending wire into hooks to extract food
Animals can acquire behaviours by watching others, bypassing the need for individual trial and error
When socially learned behaviours persist across generations and spread through populations, they constitute culture
Examples: Japanese macaques washing sweet potatoes, regional song dialects in bird populations, tool-use traditions in chimpanzees
Optimal foraging theory predicts that natural selection favours foraging strategies that maximise net energy gain per unit time
Animals weigh the energy content of food against the time and energy spent searching for and handling it
Explains why predators sometimes ignore small prey items that are abundant: the handling time per calorie is too high
Monogamy: one male pairs with one female. Common when biparental care is essential for offspring survival (e.g. many songbirds)
Polygyny: one male mates with multiple females. Common when males defend territories rich in resources or when females cluster
Polyandry: one female mates with multiple males. Rarer; seen in species where males provide most parental care (e.g. jacanas)
Mating systems correlate with ecological conditions, the degree of sexual dimorphism, and the relative investment of each sex in offspring
Altruistic behaviour reduces the altruist's own fitness while increasing the fitness of others
Under a naive reading of natural selection, altruism should be selected against, because the altruist leaves fewer offspring
Hamilton's rule resolves the puzzle: altruism is favoured when rB > C
r = coefficient of relatedness between altruist and beneficiary
B = reproductive benefit to the beneficiary
C = reproductive cost to the altruist
Kin selection is the mechanism through which Hamilton's rule operates: natural selection favours genes for altruism when the recipients are close relatives
Inclusive fitness accounts for an individual's total genetic contribution, including genes passed on through helped relatives
Example: worker bees (r = 0.75 to sisters in haplodiploid species) sacrifice their own reproduction to help the queen raise more sisters, because the indirect genetic payoff exceeds the cost
Parent to offspring: r = 0.5
Full siblings: r = 0.5
Half-siblings: r = 0.25
Grandparent to grandchild: r = 0.25
First cousins: r = 0.125
Haldane's quip captures the intuition: "I would lay down my life for two brothers or eight cousins."
Understanding imprinting and sensitive periods informs conservation breeding programmes: captive-reared birds can imprint on humans, which undermines their ability to mate and survive in the wild. Programmes now use puppet-rearing to prevent this.
Pheromone traps exploit insect communication to control agricultural pests without broad-spectrum pesticides.
Optimal foraging theory helps ecologists predict how habitat degradation will change animal diets and, in turn, how those dietary shifts ripple through food webs.
Kin selection theory explains the social structure of eusocial insects (bees, ants, termites), which has practical implications for understanding colony collapse and managing pollinator health.
Students often assume "innate" means "completely unaffected by environment." Innate behaviours still develop within an environmental context; "innate" means the basic pattern is genetically determined and does not require specific learning experiences.
Many students think altruism in biology means the same as altruism in everyday language (selfless kindness). In biology, altruism is defined strictly in terms of fitness costs and benefits, with no reference to intention or awareness.
Students sometimes believe Hamilton's rule says animals "calculate" relatedness before helping. Animals do not do arithmetic. Kin selection works because, over evolutionary time, genes promoting help toward relatives leave more copies of themselves.
A common error is treating "culture" as uniquely human. In biology, culture refers to any socially transmitted behaviour that persists across generations, and it has been documented in several non-human species.
⚠️ Be able to define and distinguish innate behaviour, fixed action patterns, and sign stimuli with examples. These are standard short-answer targets.
⚠️ Know the types of learning (imprinting, associative, spatial, social) and be able to identify which type is illustrated in a given scenario.
⚠️ Hamilton's rule (rB > C) is almost certain to appear. Be prepared to apply it: given r, B, and C values, determine whether altruism is favoured.
⚠️ Understand the difference between direct fitness (own offspring) and inclusive fitness (own offspring plus the effect on relatives' reproduction). This distinction is the conceptual foundation of kin selection.
⚠️ Be ready to explain why eusocial insects are a classic case of kin selection, and how haplodiploidy affects the coefficient of relatedness among sisters.
True or False: A fixed action pattern can be stopped midway if the sign stimulus is removed.
False. Once triggered, a FAP runs to completion.
Fill in the blank: The coefficient of relatedness between full siblings is ______.
0.5
True or False: Imprinting can occur at any point in an animal's life.
False. Imprinting occurs during a sensitive period early in development.
Fill in the blank: According to Hamilton's rule, altruism is favoured when rB > ______.
C (the cost to the altruist)
True or False: Culture in biology is exclusive to humans.
False. Non-human animals such as chimpanzees and certain bird species also exhibit culture.
Q: A newly hatched duckling follows a moving basketball instead of its mother. What type of learning does this illustrate, and why did it happen?
A: This is imprinting. The duckling was exposed to the basketball during its sensitive period and formed a lasting attachment to it as a "parent figure." Imprinting is triggered by exposure to a moving object during a narrow developmental window, regardless of whether that object is the biological parent.
Q: A ground squirrel gives an alarm call when it spots a predator, drawing attention to itself. Using Hamilton's rule, explain when this behaviour would be favoured by natural selection.
A: The alarm call is favoured when rB > C. If the squirrel's nearby relatives (high r) gain a substantial survival benefit (high B) from the warning, and the cost to the caller (C) is relatively low, then the indirect genetic benefit outweighs the personal risk. Kin selection favours the behaviour because the caller's genes, shared with those relatives, are more likely to be passed on.
Q: Distinguish between classical conditioning and operant conditioning. Give one example of each.
A: In classical conditioning, an animal learns to associate a neutral stimulus with an unconditioned stimulus. Example: a dog salivates at the sound of a bell after the bell has been repeatedly paired with food. In operant conditioning, an animal learns to associate its own voluntary behaviour with a consequence. Example: a rat learns to press a lever to receive a food pellet.
Q: A researcher observes that chimpanzees in one population use sticks to extract termites, while a neighbouring population does not. What concept does this illustrate?
A: This illustrates culture, specifically social learning that produces a behavioural tradition in one population but not another. The behaviour is transmitted through observation rather than genetic inheritance, and its geographic patchiness rules out a simple genetic explanation.
Q: Why is haplodiploidy relevant to the evolution of eusociality in Hymenoptera (bees, ants, wasps)?
A: In haplodiploid species, females are diploid and males are haploid. This means full sisters share r = 0.75 (they receive all the same genes from their haploid father and, on average, half the same genes from their diploid mother). Because sisters are more related to each other than they would be to their own offspring (r = 0.5), Hamilton's rule is more easily satisfied, making it more likely that worker females will forgo their own reproduction to help the queen raise more sisters.
Behavioural ecology ties directly to evolution and natural selection (the framework explaining why behaviours exist), population genetics (allele frequencies in the context of kin selection), and ecology (foraging theory, predator-prey dynamics, habitat use). The neurobiological basis of behaviour connects back to the nervous system and muscle physiology covered in the first half of this chapter.
Behaviour, behavioral ecology, innate behavior, fixed action pattern, FAP, sign stimulus, releaser, supernormal stimulus, pheromone, migration, cross-fostering, twin study, nature vs nurture, learning, imprinting, sensitive period, critical period, spatial learning, cognitive map, associative learning, classical conditioning, operant conditioning, Pavlov, cognition, problem solving, social learning, culture, foraging, optimal foraging theory, monogamy, polygamy, polygyny, polyandry, mating system, sexual selection, altruism, inclusive fitness, coefficient of relatedness, Hamilton's rule, kin selection, eusociality, haplodiploidy, reciprocal altruism