The History of Müllerian Mimicry Part 2: How far we have come

If I have seen further it is by

standing on the shoulders of giants.

~ Isaac Newton ~

Since Friz Müller first attempted to explain the phenomenon of two poisonous or unpalatable species mimic each other there has been a considerable amount of research into the topic. As scientists have explored the relationships that mimics have with each other and their predators, they have been able to build on Müller’s ideas. Unfortunately, Müller has also been found to be wrong in some cases (which is understandable considering he made these claims 135 years ago!). Never the less, his contributions to the topic have not gone unnoticed and this phenomenon is still commonly referred to as Müllerian mimicry.

Last week, I posted a general explanation of Müller’s model he put forward. In his model he assumed that animals need to learn to avoid unpalatable species and that, in learning to avoid them, they eat a fixed amount of them before being turned off them all together. Let’s see how far research has come since then.

Do predators really need to learn to avoid prey? 

Müller’s first proposition was that animals need to learn to avoid prey that tastes bad. This is usually the case but not always. In studies concerning some very poisonous prey items (eg. Snakes), predators seem to innately avoid these species (Cladwell & Rubinoff, 1983).  Apart from these few cases, the majority of predators researched have demonstrated that they need to learn by their mistakes before they will avoid an unpalatable prey species. Examples include young birds learning to avoid noxious insects and butterflies in studies by Mostler in 1935 and Chai in 1996 (Cited in Sherratt, 2008) as well as inexperienced lizards attacking unpalatable butterflies before learning to avoid them (Boyden, 1976).

Do predators take a fixed number of prey regardless (independent) of the abundance? 

Müller’s second assumption was that there was a fixed number (n) of individuals a predator would take from a population before it had learnt to avoid it. In Müller’s theory he supposed that the number of individuals taken from two species not mimicking each other would be the same regardless of the size of the population. For example, if there were 100 individuals of one species and 900 individuals from another species that looked different from each other, Müller predicted that both species would have an equal absolute number taken from each population and resulting in the rarer population having a far greater proportion of the total population eaten relative to the more abundant population. Unfortunately for Müller, there has been no research to date showing that a “fixed n” exists (Sherratt, 2008).

Contrary to Müller’s hypothesis, an experimental study by Greenwood et. al. (1989) found that when birds were left to forage for unpalatable prey of two different appearances and in different abundances that the absolute number taken from the two different appearances were different. Greenwood et. al. (1989) randomly placed pieces of pastry flavoured with quinine hemi-sulphate (proven to be unpalatable to the birds) on grid in a ratio of yellow to red of 1:9 and repeated with a ratio of 9:1. Of all the trials conducted, the more common form had a larger absolute number of pieces of pastry eaten by the birds compared to the rarer form. This disproves Müller and confirms that there is not a ‘fixed n’. Never the less, the proportion of rarer forms eaten was larger than the proportion of abundant forms taken and therefore proves it would still be beneficial if these two forms were a mimic of each other. In a way, Müller was right, this development only means that there is no ‘fixed n’ and that the amount of benefit gained by mimicry is less than what Müller originally anticipated (Greenwood, et al., 1989).

Building on Müller’s Ideas: Research into other factors influencing mimicry

As well as the influences of abundance on predation on unpalatable food, there have been many studies that have found or proposed other factors could influence the number of unpalatable prey that will be eaten. Greenwood et. al. (1989) suggested that hungrier birds may be more likely to eat unpalatable if they assessed the cost of eating a bad tasting individual to outweigh the benefit of acquiring food (for instance, I don’t like beetroot and usually avoid it. But if I hadn’t have eaten for a few days, I’m sure I would jump at the chance if I had no other choices). Other possible influences on predation of mimetic species include predators’ ability to distinguish between mimetic species, differences in defences of species that mimic each other (eg having different toxins that make them them unpalatable) which may cause one predator to view both species as distasteful but another predator to view only one of the species as distasteful, and the intensity of unpalatability of the two mimetic species to name a few.

References

Fritz Müller in Brazil, n.d., photograph,  viewed 6 April 2014,
<http://en.wikipedia.org/wiki/File:Fritz-muller-1821-1897.jpg>

Boyden, T. C., 1976. Butterfly palatability and mimicry - experiments with Amevia lizards. Evolution, 30(73-81), pp. 1992-1998.

Cladwell, G. S. & Rubinoff, R. W., 1983. Avoidance of venomous sea snakes by naive herrons and egrets. The Auk, 100(1), pp. 185-198.

Greenwood, J. D., Cotton, P. A. & Wilson, M. D., 1989. Frequency-dependent selection on aposematic prey - some experiments. Biological Journal of the Linnean Society, 36(1-2), pp. 213-226.

Müller, F., 1879. Ituna and Thyridia; a remarkable case of mimicry in butterflies. Transactions of the Entomological Society of London, pp. xx-xxix.

Sherratt, T. N., 2008. The Evolution of Müllerian Mimicry. Naturwissenschaften, 95(8), pp. 681-695.