Tradeoffs associated with production of horns

One surprising result that has emerged from our studies of beetle horn development is that allocation of developmental resources to one structure (such as an exaggerated male weapon) influences the expression of other structures (Nijhout & Emlen 1998[PDF]).  What this means, in our case, is that horn growth stunts the growth of nearby structures.

We first recognized this as a tradeoff between the relative size of male horns, and the relative size of eyes: males with the largest horns also had the smallest eyes (Nijhout & Emlen 1998; Emlen 2000[PDF]; Emlen 2001[PDF]).  However, recent evidence points to a more complex (and exciting) situation: horn expression occurs at the “expense” of either eyes, antennae, or wings, depending on where the horns are located.   Animals with horns that extend from the base of the head have tradeoffs with eyes; animals with horns at the front of the head have tradeoffs with antennae, animals with horns on the pronotum (thorax) have tradeoffs with wings.

Thus, there are important functional costs associated with expression of exaggerated male weapons of sexual selection (reduced/ impaired vision, olfaction, or flight), and the nature of these costs differs depending on the type of horn produced (Emlen 2001). 

Our comparative studies suggest that beetle horns have radiated in form tremendously.  They presently occur at five different physical locations on the beetle, and each of these horn ‘types’ has been gained and lost repeatedly in the history of the genus.  We are now exploring whether some of this extraordinary diversification can be understood in the context of horn tradeoffs.  Perhaps the functional costs of horn expression, rather than, or in addition to, the reproductive benefits, provide the key to understanding horn diversity. These findings offer new possibilities for understanding how exaggerated morphological structures evolve, and reveal novel avenues for future studies of sexual selection (Emlen et al. 2005b[PDF]; Simmons & Emlen 2006 [PDF] ).

Yet at the same time it is clear that tradeoffs are not always present – we find almost as many examples where predicted trait-trait associations are not found (e.g. Allen & Emlen in prep.).  This points to a much richer evolutionary scenario, where the evolutionary significance of developmental resource-allocation tradeoffs varies, clearly biasing trajectories of morphological evolution in some situations, but not in others.