Adaptive value of behaviour
A behaviour is what an animal does: feeding, fleeing, mating, calling, caring. Behavioural ecology asks two questions about each one: how does it work (the nerves and hormones), and why does it exist (what does it do for survival and reproduction).
A behaviour has adaptive value if animals that do it leave more surviving young than those that do not. Part of the behaviour comes from genes and part from learning. Genes that build helpful behaviour become more common over many generations by natural selection.
Behaviour is always a trade-off: energy and risk (cost) against food, safety or young (benefit).
Direct and indirect fitness
Fitness here means how many copies of your genes reach the next generation. It has two parts.
- Direct fitness: your own young that survive.
- Indirect fitness: extra young that your relatives raise because of your help, counted according to how closely related they are.
Relatedness (r) is the chance that two animals share a gene by common descent. Parent–child 0.5, full siblings 0.5, grandchild 0.25, first cousin 0.125, an unrelated animal about 0.
Inclusive fitness = direct fitness + indirect fitness. This explains why helping relatives can pass on your genes.
Cost-benefit and optimal foraging
Cost-benefit thinking says an animal should do an action when benefit minus cost is positive, in units such as energy or number of young.
Optimal foraging asks how long to feed in one patch of food. The patch gives diminishing returns: the first berries are easy, later ones are scarce. Leaving too early wastes the trip; staying too long wastes time. The best choice is the stay time that gives the largest energy per minute including travel. In the 3D model the patch gives 20 × (1 − e−t/4) berries and a trip costs 4 minutes, so the best stay is about 4.6 minutes (1.6 berries a minute). If travel takes longer, the best stay becomes longer.
Real animals also weigh danger from predators and their own need for food.
Cooperation and altruism
Cooperation means animals work together and both gain, like lionesses hunting together. Altruism means one animal pays a cost (C) to give a benefit (B) to another. Helper birds, worker bees and alarm-calling squirrels are examples.
Kin selection and Hamilton’s rule: an altruistic gene can spread when r × B > C. Example: r = 0.5 (sibling), B = 4, C = 1: 0.5 × 4 = 2 > 1, so helping evolves. For a cousin (r = 0.125) with B = 4, C = 1: 0.5 < 1, so helping does not pay.
Help between non-relatives can evolve too, through reciprocity: you help me today, I help you tomorrow, and cheats are punished or avoided.
Animal communication signals
A signal is an act or structure that changes another animal’s behaviour and was shaped for that job. Signals use sound, sight, smell or touch.
- Alarm calls: warn relatives and neighbours of a predator; the caller may pay a small risk.
- Dances: a honeybee waggle dance tells the direction and distance of food.
- Displays: a peacock’s tail or a frog’s call tells a mate about its health. Costly displays are honest signals, because only a strong animal can afford them.
- Chemical signals: pheromones from ants mark a food trail.
A signal evolves when both sender and receiver gain on average.
Try it: the helper at the nest
- Open the last story step and choose the Cost–benefit view.
- Set relatedness to sibling (½), benefit 4, cost 1. What is the verdict?
- Change relatedness to cousin (⅛). Does helping still pay? Raise B until it does.
- Switch to Foraging and move the time slider to find the best stay.
Key formulas and definitions
- Inclusive fitness = direct fitness + indirect fitness
- Hamilton’s rule: helping evolves if r × B > C
- Relatedness r: parent/child 0.5, sibling 0.5, grandchild 0.25, cousin 0.125
- Net benefit = benefit − cost
- Foraging rate = food gained ÷ (time in patch + travel time)
Worked examples
1. A bird helps feed a sibling’s chicks. B = 4 chicks gained, C = 1 chick lost, r = 0.5. Does helping pay?
r × B = 0.5 × 4 = 2. This is more than C = 1, so helping pays.
2. Same helper but the chicks belong to a first cousin (r = 0.125), B = 4, C = 1. Does helping pay?
r × B = 0.125 × 4 = 0.5, which is less than C = 1. Helping does not pay.
3. A bird raises 2 chicks of its own and, by helping, 4 extra chicks of its full sister survive. Find its inclusive fitness (in chick-equivalents).
Direct = 2. Indirect = r × 4 = 0.5 × 4 = 2. Inclusive fitness = 2 + 2 = 4.
4. A forager gets 12.6 berries from a patch in 4 minutes and the trip to the patch takes 4 minutes. What is the rate?
Rate = 12.6 ÷ (4 + 4) = 12.6 ÷ 8 = about 1.6 berries per minute.
5. Staying 8 minutes gives 17.3 berries, travel is 4 minutes. Is staying 8 minutes better than 4.6 minutes (1.6 berries a minute)?
Rate = 17.3 ÷ (8 + 4) = 17.3 ÷ 12 = about 1.44 berries a minute. This is less than 1.6, so 8 minutes is too long.
6. A meerkat’s alarm call gives a benefit B = 6 to relatives with average r = 0.25 and costs C = 1. Is calling favoured?
r × B = 0.25 × 6 = 1.5 > 1, so calling is favoured.
Common mistakes
- Saying animals act "for the good of the species". Behaviours spread because they help the genes of the individual and its relatives.
- Thinking animals calculate. The costs and benefits are balanced by natural selection over generations, not by thinking.
- Forgetting r in Hamilton’s rule: it is r × B > C, not B > C.
- Believing the longest stay in a patch is best. After a point the rate of food per minute falls.