12.2 Applications and skills

12.2.6 Testing hypotheses about behaviour (HL)

  • Natural selection acts on behaviours because animals interact with each other and with their environments, using different behaviours that result in differential survival and reproduction.
  • The theory of natural selection gives rise to many hypotheses about the advantages of specific behaviours.
  • These hypotheses should be testable and falsifiable – otherwise they risk becoming simple ‘just so’ stories.
  • A ‘just so’ story is an ad hoc fallacy – an explanation that is added in order to make a something appear to fit the established theory of natural selection.
  • Which of the following hypotheses is testable, and which is a ‘just so’ story?
    • Human noses evolved wider bridges as an adaptation to wearing eyeglasses.
    • Human noses evolved wider bridges as an adaptation to cold air temperatures.

Application: Migratory behaviour in blackcaps has a genetic basis

  • Hypothesis: The observed differences in migratory behaviour of blackcaps, Sylvia atricapilla, are genetically determined.
  • Background: Migration patterns of blackcaps have been monitored for many years. Some groups migrate from Germany to Britain over winter (travelling west) and some groups migrate from Germany to Spain (travelling southwest). The proportion of blackcaps migrating to Britain has increased recently.


Figure 12.2.6a – Migration
The genetics of migratory behaviour has been investigated experimentally.

  • Method: Remove eggs from nests of parents with different migration patterns, hand-rear the hatchlings without contact from other birds. Record where the isolated birds migrate when they reach adulthood.
  • Results: Birds whose parents had migrated to Britain tended to fly west, and birds whose parents migrated to Spain tended to fly southwest.
  • Conclusion: Orientation of migration behaviour in blackcaps is genetically determined.
  • Theory: All migrating birds respond to environmental cues that result in morphological and physiological changes which prepare them for migration. Many of the structural and at least some of the behavioural differences are genetic.

Application: Development of reciprocal altruism in vampire bats

  • Hypothesis: Natural selection can lead to reciprocal altruism. Food sharing in vampire bats Desmodus rotundus is an example.
  • Background: Vampire bats feed at night on blood of livestock. When bats return to the roost, they often regurgitate food to share with members of the group who have not had a blood meal. Vampire bats cannot survive more than two days without a blood meal. Sharing increases survival for the receiver but it is unclear if the sharer benefits. Any behaviour that is maintained by natural selection should be advantageous to the individual performing the behaviour.
  • Method: Monitor incidence of sharing by donors in a roost over a two-year period based on the following factors: genetic relatedness to recipients and incidence of past sharing with recipients.
  • Results: Past experience of sharing (Figure 12.2.6b graph b) is a greater predictor of sharing than relatedness (graph c). Reciprocation increases as a function of donation (graph d).

Figure 12.1.26b – (from Carter and Wilkinson, 2013)

Figure 12.1.26b – (from Carter and Wilkinson, 2013)
Relationships between food donated and predictor variables. z-score for log food donated was predicted by z-scores of (a) log food received (R2 = 0.27, p < 0.0002), (b) allogrooming received (R2 = 0.14, p < 0.0002) and (c) relatedness (R2 = 0.04, p < 0.0012). A bubble plot (d) shows multivariate relationships by scaling bubble size to relatedness and bubble darkness to allogrooming received.

  • Conclusion: Food sharing is probably an example of reciprocal altruism since the data shows that a bat that shares more often is likely to be shared with.
  • Theory: Reciprocal altruism is a behaviour strategy through which the individual benefit of performing the behaviour is greater than its cost. Bats appear to share food in the expectation that sharing will be reciprocated when they are in need. Food sharing is a learned behaviour, because bats make choices about who to share with.
  • Individual survival is increased by this strategy, which can be thought of as ‘tit for tat’.

Application: Optimal prey choice increases survival in foraging shore crabs

  • Hypothesis: Survival is increased if foraging animals use a behavioural strategy that maximises energy intake while minimising energy output.
  • Background: The velvet crab, Necora puber, forages for mussels on the coast of England. A crab will hold a mussel in its carapaces for two or three seconds before deciding to crack it open or release it.
  • Method: Observe crabs as they choose prey from a constant supply of mussels of varying sizes. Calculate the time required to access the food energy and the amount of energy obtained from each mussel.
  • Results:

Figure 12.2.6c – Optimal prey choice

Figure 12.2.6c – Optimal prey choice
Prey choice optimises profitability and increases survival in Necora puber. (Image: T. ap Reinhallt, 1986)

  • Conclusion: When there is a constant availability of prey of different sizes, crabs will choose the prey of intermediate size most often.
  • Explanation: Larger mussels are more difficult to crack, but smaller mussels have less energetic value. The most profitable prey, in terms of energy intake per unit time, are those of intermediate size. Survival is increased by optimum prey choice because crabs spend less time foraging for their nutritional needs.

Exam tip

Make sure you can explain different behaviours in terms of how they increase the chances of survival and reproduction. 

12.2.6dFigure 12.2.6d – European blackcap
Migratory behaviour has a genetic basis in the European blackcap, Sylvia atricapilla.

12.2.6eFigure 12.2.6e – Vampire bat
The vampire bat Desmodus rotundus ensures her own survival by sharing her food with her roost-mates through reciprocal altruism.

12.2.6fFigure 12.2.6f – Velvet crab
Velvet crab survival is increased by optimal prey choice. Necora puber (alternative name Leocarcinus puber)


An ad hoc ‘just so’ story is named after the Just So Stories by Rudyard Kipling. The stories are fun, literary explanations of animal origins and behaviour (e.g. ‘How the leopard got its spots’). How are scientific explanations different from historical or literary explanations?

12.2.6gFigure 12.2.6g – Just So Stories by Rudyard Kipling

Further reading

The Just So Stories by Rudyard Kipling is part of the public domain:

Recent experimental evidence for reciprocation in bats:
Gerald G. Carter and G.S. Wilkinson (2012) ‘Food sharing in bats: reciprocal help predicts donations more than relatedness or harassment’, Proceedings of the Royal Society B 280:201222573.

Prey sizes and foraging behaviour:
T. ap Rheinallt (1986) ‘Size selection by the crab Liocarcinus puber feeding on mussels and shore crabs: the importance of mechanical factors’. Mar. Ecol. Prog. Ser. 29:45-53.