Lines selected for different durations of tonic immobility have different leg lengths in the red flour beetle Tribolium castaneum

Tonic immobility is an adaptive anti-predator behaviour observed in many species. This anti-predator behaviour is often correlated with a species' movement motivation, so a relationship between the duration of tonic immobility and morphological traits supporting movement would be expected. Using the red flour beetle, Tribolium castaneum, we carried out two-way artificial selection for the duration of tonic immobility over more than 43 generations, establishing populations with longer (L) and shorter (S) tonic immobility durations compared to those of a non-selected control (C) population. Here, we investigated differences in walking motivation and leg length between the selected populations. Walking motivation was significantly higher in beetles from the S population than that in those from the L population. Moreover, S-population beetles of both sexes had significantly longer legs than those from L and C populations. The present results suggest the evolution of longer legs in response to selection pressure for a shorter duration of tonic immobility in T. castaneum.

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