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In a previous post I wrote about individual snails having a turning preference (i.e. they are not equally likely to turn right or left), in more general terms this means that Lymnaea stagnalis is lateralised at the individual level.  Lots of different animals are lateralised at the individual level; turtles show a flipper preference giant water bugs show a turning bias, and octopus have a favoured arm.  However, many of the behaviours are exhibited in roughly equal numbers of left and right lateralised individuals meaning that there is no population level lateralisation.  Population level lateralisation is best defined as the average lateralisation value of a population.  In the case of my snails I used the average right turn proportion for each population I tested.

I tested several different populations (Table below), the key ones are the wild individuals, which are all dextral and assumed to be 0% inbred, and the very inbred laboratory reared generations.  The wild snails are from two separate populations which are geographically distinct, meaning they are not likely to be smaller pockets of one large population.  Also included are populations formed from their offspring which were part of the heritability analysis I conducted (explained in the future).  The laboratory populations have all been reared under aquarium conditions; the lab 1 snails are dextral and sinistral offspring from a reciprocal cross (also explained in the future) and the lab 2 to lab 6 populations are several generations of the same lineage of snails which are very inbred.

What the above table shows is the results of a one-sample t-test which was used to test whether the mean population r turn proportion was significantly different from a value of 0.5 (representing an equal number of left and right turns).  Rows where the p value is less than 0.05 are populations which have a mean r turn proportion that is significantly different from 0.5.

All but two of the populations have a right turn proportion that is significantly different from 0.5 meaning that pond snails also show lateralised behaviour at the population level.  This is biologically interesting for a few of reasons; first there must be some cue, either environmental or genetic, which results in the majority of the population being biased in the same direction.  Furthermore, both wild populations are significantly left biased which is interesting as the majority of inbred populations are right biased.  It may be possible that inbreeding affects the expression of lateralised behaviour, which I will cover in another post…

Photo by paperpariah via Flickr.

Related posts:

1. A Tale of Snails…

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