The role of behaviour in evolution

Polar bear

Polar bears evolved from coastal populations of brown bears that began to hunt for marine prey. This behavioural shift created a selection pressure that favoured feet adapted for swimming, leading to a morphological change in the population © John. Licensed under CC-BY-2.0, via Wikimedia Commons.

Principal Investigator

Prof Adrian Lister

Project summary

  • Focus: Studying 'fossilised behaviour' to investigate the role of behavioural changes in morphological evolution

Species behaviour has the potential to lead morphological evolution, by placing the organism under novel selection pressures. Many adaptations of living species could have originated in this way, although there are few documented examples.

Our research focuses on the fossil record, a potentially rich but under-exploited source of information on the role of behaviour in evolution.

The behaviour of fossilised species is traditionally deduced from their morphology. This prevents researchers from observing behavioural changes that may occur prior to morphological evolution. Examining behavioural proxies independent of adaptive morphology may help us to address this problem.

Preserving behaviour

Examples of 'fossilised behaviour' include dietary information (wear traces on teeth and stable isotopes) and trace fossils indicating locomotor mode (footprints). The signature of a behavioural lead would be an observed shift in behaviour from one horizon (or species) to another, followed later by a morphological change relating to function.

Fossil case studies that suggest a behavioural role in evolution include:

  • feeding shifts in finely resolved sequences of vertebrates, ranging from freshwater fish to terrestrial ungulates
  • locomotion changes crucial to major evolutionary transitions in the origin of tetrapods, birds and humans 

These examples suggest that behaviour is central to the process known as exaptation, in which structures acquire new functions.

Using fossil sequences to clarify behavioural roles is challenging. Problems include insufficient stratigraphic resolution and uncertainty over the adaptive function of observed traits. Researchers should consider these limitations in order to select promising research and formulate testable hypotheses about evolutionary modes.

Read more about behavioural leads in evolution in Dr Adrian Lister's paper in the Biological Journal of the Linnean Society.

Publications

Lister AM (2014) Behavioural leads in evolution: evidence from the fossil recordBiological Journal of the Linnean Society, 112(2): 315–331.

Case study: the role of behaviour in elephant evolution

Carbon isotope analysis suggests that early elephant species switched to a grass-dominated diet around eight million years ago, despite the continued availability of their favoured woodland vegetation. This behavioural change led to a morphological adaptation in the species.

The elephant family arose in Africa around ten million years ago from a broader group of mammals known as the Proboscidea. Around that time a major shift in the global climate led to the spread of large areas of grassland.

We can identify dietary changes in early elephant species by analysing carbon isotope ratios in their fossilised teeth. In the tropics, most grasses have a significantly higher proportion of the rare isotope carbon-13 than trees and shrubs.

A change in feeding behaviour

Isotope analysis suggests that many proboscidean species in east Africa, including the earliest elephants, switched to a grass-dominated diet around eight million years ago, even though woodland habitats were still available.

Grass is a more abrasive food for herbivorous mammals than the leaves of trees or shrubs. It is tougher to chew, and grass-eating animals tend to pick up more grit from the ground. These factors mean that grass-eating animals' teeth usually wear down more quickly than the teeth of those that feed on leaves. 

Delayed morphological adaptation in teeth

Analysis of the size and shape of fossilised elephant teeth suggests that adaptations for grass-eating did not begin to evolve until around four million years ago, long after the switch to a grassy diet. 

These adaptations include:

  • an increase in the number of enamel ridges, improving resistance to abrasion
  • a heightening of the tooth crown so that it lasted longer before wearing out

A delay in morphological adaptation suggests that evolution was led by behavioural change, and that the dental adaptation (which also required significant changes in skull shape) took time to catch up. The dental changes then progressed continuously for at least three million years before reaching the final form seen in living African and Asian elephants today.

An alternative possibility is that dental changes were not triggered by the transition to grass-eating, but rather by another, as yet unknown factor.

Read more about the role of behaviour in adaptive morphological evolution of African proboscideans in Dr Adrian Lister's paper in Nature

Molar tooth of an elephant

The molar tooth of an African elephant, Loxodonta africana

African elephant

Taxonomic status of living elephants

Publications

Lister AM (2013) The role of behaviour in adaptive morphological evolution of African proboscideans. Nature, 500: 331–334.

Origins, evolution and futures

We study the Earth's origins and environment, and the evolution of life.

Fossil vertebrate research

Investigating the role of vertebrate evolution in shaping the history of life on Earth.

Fossil mammal collection

The collections contains around 250,000 specimens from around the world.