Ever since the slugs evolved out of their ancestors' shells, their slime, or mucus, has become their major defense against the elements and the enemies. When a substance is so vital a component of survival, it shouldn't come as a surprise if evolution has also turned it into a medium of communication. This follows from the results of a series of simple experiments carried out by Jordaens et al.1 that demonstrate that the slug Deroceras laeve responds to the mucus of other D. laeve (conspecifics) and other Deroceras species (heterospecifics).
When D. laeve was given the choice of being either in an area containing mucus of conspecifics or in an area without mucus, the time they spent in areas with mucus was significantly higher than the time spent in areas without mucus (A in the figure below).
The vertical axis shows mean time spent (and standard deviation) in each experimental area either with mucus of conspecifics (white bars) or without mucus (black bars in A & B) or with mucus of heterospecifics (black bars in C).
But when the same experiment was repeated using mucus from stressed conspecifics2, the slugs went bananas: "Nineteen out of the 30 individuals almost immediately left the plastic container, despite the copperfoil. Eight other individuals were situated on the area without mucus and three individuals were situated on the area with mucus from stressed individuals." In another experiment, the slugs spent significantly longer time in areas without mucus than in areas with the mucus of one stressed individual (B in the figure). So, obviously there is some secretion in the mucus of stressed slugs that the other slugs don't like.
In yet another experiment, D. laeve showed no preference for areas that had either the the mucus of other D. laeve or that of D. panormitanum. And the time spent in the area with heterospecific mucus was not significantly different from the time spent in the area with conspecific mucus (C in the figure).
Slugs do seem to pay attention to the slime they encounter during their travels. And sometimes, doing so may save their lives.
1. Kurt Jordaens, Hilde Gielen, Natalie Van Houtte, Gary Bernon, and Thierry Backeljau (2003). The response of the terrestrial slug Deroceras laeve to the mucus and air-borne odours of con- and heterospecifics (Pulmonata: Agriolimacidae). J. Mollus. Stud. 69:285-288.
2. How do you stress a slug? You make it think that it is about to get eaten: "Mucus from stressed individuals was obtained by simulating the attack of beetles which grasp slugs from behind after following the pedal mucous trail. This was done by touching the posterior part of three slugs with pincers, without breaking the skin, a few times over 1 min. Such a stimulation changes the amount and composition of the mucus."