Gathering convincing behavioural
evidence of this phenomenon has proven to be a tricky task. Past experiments used systems in which passerine birds trained to attack both artificial prey and dead butterflies were monitored to see where they struck and how they were influenced by the eyespots. More recently however, Merilaita began using three-spined sticklebacks and was able to garner some fairly convincing results. “We were able to show that the behaviour of the fish was strongly influenced by the eyespots,” he says, “which has not been shown a lot in aquatic systems.” One explanation for the ubiquity of these marks developmental
is that they are, perspective, from a relatively easy to produce. “For instance, it is easier
in which humans were asked to search for monomorphic or polymorphic prey on a screen, and saw a clear effect showing that they were more effective at searching for monomorphic prey.” Camouflaged prey are difficult to detect,
and so predators react to the presence of two or more variants of a prey species by focusing
on the more common type,
allocating all of their attention to the one they are most likely to find and thus increasing their search efficacy. This is known as search image formation, and it has been theorised that this can lead to a stable state of fluctuation between prey morphs due to negative frequency- dependent selection on the more common type, after which the predator’s target will switch to the newly most prevalent type.
“We are very interested in how predators’ visual perception, decision making and behaviour are able to influence the patterns and colours of prey in an evolutionary sense”
for living things to produce circles of pigment than squares, and so they may not necessarily exist due to their resemblance to eyes,” explains Merilaita. “We have been investigating this idea with fish, comparing how they respond to different shapes, and it seems that the eye-like shape is an important protective function of eyespots that can influence the behavioural response of the attacker.”
Prey polymorphism Variation within species of animals and other organisms has long been a source of some confusion for scholars of evolution. One would expect natural selection to filter out variation within a population towards a single optimum form, yet in reality we see that species often have several variants. Studying the mechanisms by which this polymorphism is maintained makes up another part of Merilaita’s varied work.
“An example of one of these
mechanisms is that it may be harder for predators to search for a number of different morphs rather than just one, as they have to allocate their attention to more than one visual appearance.” “We were able to show the benefit of variation for prey with an artificial setup
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However, Merilaita is unsure as to the strength of this effect in reality. “There is some evidence for this in small scale experiments, but I am sceptical of the implications because there is a big difference between an experimental setup and a real life setting in which exists numerous predators, habitats and individual experiences. My simulations have shown that this mechanism is not as likely to maintain polymorphism as has been thought.” Merilaita’s work looks to see how colours
and patterns can be used to manipulate the visual perception and decision making of predators i.e. their immediate behavioural response. However, it also works the other way. “We are very interested in how predators’ visual perception, decision making and behaviour are able to influence the patterns and colours of prey in an evolutionary sense; in other words, how these traits can impose natural selection upon the prey species,” explains Merilaita. “Prey colouration and patterns have an immediate effect on the behaviour of predators, whereas predator perception works much more slowly, influencing the shape and appearance of prey species over evolutionary timescales.”
★ 69
Sami Merilaita Sami Merilaita is a behavioural and evolutionary ecologist who has mainly studied the use of animal body colours and marks against predation. He received a PhD in animal ecology in 1999 at Uppsala University and has worked at several universities, including University of California in Santa Barbara and Stockholm University.
AT A GLANCE Project Information
Project Title: Protective coloration
Project Objective: The project studies the function of protective prey coloration and marks, i.e., how they can be used to manipulate the perception and behavioural responses of predators. It also studies the optimisation of prey coloration and how natural selection imposed by predation influences prey appearance.
Project Funding: Academy of Finland
MAIN CONTACT
Contact: Tel: +358 (0)2 215 3355 Email:
sami.merilaita@
abo.fi Web:
http://web.abo.fi/fak/mnf/biol/ eco/beeg/
sami.html
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