Influence of learned behaviours on predation

Dr Angus Jackson


Background
The reef crab Ozius deplanatus holding onto the gastropod Littorina unifasciata

Many factors can influence what, when and how much a predator feeds. Size, abundance, developmental stage and physiological condition of predators and prey, plus environmental conditions, have frequently been shown to influence predation. Often there are strong patterns in consumption where a predator eats a particular type of prey more than others. The usual, untested explanation is that the predator is expressing a preference for that type of prey. Preferences, however, involves an active behavioural choice and to demonstrate this requires complex and well designed experiments and appropriate analyses. The influence of prior experience or learned behaviours has seldom been considered when examining or investigating predation. Learned behaviours, how a predator remembers (or forgets) how to deal with a particular prey type, have the potential to alter selection of prey and, hence, the consequences for the assemblage of prey.

Aims

An intertidal reef crab (Ozius deplanatus) was used to investigate the expression of preference and role of learning in various aspects of predation. Preference was tested in aquarium experiments that compared the numbers of prey consumed when a choice is available to numbers consumed when there is no choice. Other experiments were used to explore how different types of prey were attacked and consumed and how the effectiveness or efficiency of predation changed with experience.



Importance
Numbers of prey of different size consumed by crabs when choosing from a range of sizes

Expression of a preference in prey choice is often assumed rather than tested. Other mechanisms can also explain observed patterns in prey consumption. Decisions based on erroneous assumptions of preference are often inappropriate or ineffective. The role of prior experience and learned behaviours is also seldom assessed when considering the influence of predators on their prey. Learned behaviour has the potential to exert strong influences on predatory behaviour and, thus, it is important to appreciate how and to what extent learning by predators can affect the populations of predators and their prey, particularly in comparison with other determinants of predatory behaviour. Being able to measure behavioural preference and to understand how learning influences their behaviour is important for the implementation of conservation or management of predators and their prey. Where predators are major and important components of an assemblage, understanding how they interact with their prey is vital to the understanding of the system as a whole.

Results

Expression of preference

I applied recently developed techniques for the analysis of choice of prey to my data. This showed that, although there appeared to be a clear tendency for crabs to consume particular sizes of prey, this pattern could not be attributed to preference. Following correction of the data to account for sample sizes, there was no statistical difference in the numbers of prey consumed when no choice was available compared to when choice was present. This result indicated that behavioural preference, which is often assumed rather than demonstrated, really does need careful experiments and analysis before safe conclusions can be made.

Changes in handling time

Two feeding experiments using Nerita atramentosa as prey failed to show any evidence of an ability of this crab species to learn. This may have been because learning occurred before the crabs were brought into the laboratory. To explore this possibility, an experiment was done using prey unlikely to be encountered by naturally foraging crabs (mussels, Mytilus galloprovincialis). This gave similar results. Some possible explanations for this lack of learning (no decrease in handling time) are that the crabs used in this experiment could not learn under aquarium conditions or that the variability in handling time caused by inherent differences in prey of the same size (e.g. shell thickness) swamped out any patterns in handling time occurring through learning. Alternatively, if crabs had previously fed on N. atramentosa they might have already learned (and not forgotten) the best way to deal with it and consequently could not show any improvements to handling time. There was, however, room for improve the handling of M. galloprovincialis that had never been encountered before.

The foraging behaviour of M. marginalba was studied by Giordana Cocco. Experiments on learning and handling time, similar to those for O. deplanatus, were done in the laboratory and experiments on diet were done in the field