Intra-specific morphological variations can result from adaptation to local environments and, ultimately, lead to distinct patterns of geographical variation. Therefore, morphology is thought to represent aspects of the relationship between an organism and its environment. Within this framework, there should be greater similarity in morphological characters among populations that share similar environments than among populations that do not. In the tropics, Pachymerus beetles (Family Bruchidae) have evolved as specialist predators of Attalea palm seeds. Beetle larvae have a complex head and mouth morphology to allow efficient seed penetration and emergence. This relationship, however, can be altered in populations where an alternate seed predator is present, such as tropical squirrels of the genus Sciurus. Therefore, we can expect differences in the head and mouth morphology of bruchid beetles between populations where squirrels are abundant and those where they are rare. Here, we conducted larvae rearing experiments during two years to examine the relationship between beetle larvae head and mouth morphology, and the occurrence of tropical squirrels across four localities in the Bolivian lowlands: two with high, and two with low densities of squirrels. Fifty fruits from 20 trees per locality were collected and exposed to beetle infestation. After five months larvae were reared and, measurements of head (greatest capsule length, width and height), and mouth (total length and distances among condyles of both left and right mandibles) were taken for each individual.  

Results/Conclusions

Head structure did not differ between populations with high and low squirrel density. In contrast, mandibles were shorter in populations with high squirrel density than in those with low density, even after controlling for differences in size among individuals. Moreover, mandibles were more symmetrical in these populations. The pattern of asymmetry in populations with low squirrel density was not consistent between the two populations, being the left mandible longer in one, and the right in the other. These results show that there is an association between the values of morphological traits and environmental variables such as the presence of a competing species. Our results suggest that reciprocal selection among species across space may not always result in the best phenotypic match, but the one that is the best compromise.