The behavioral ecology of threshold evolution in a polyphenic beetle

Abstract

Facultative expression of alternative male morphologies is thought to allow individual males to select the phenotype with the highest fitness gain given their competitive status relative to other males with which they compete for females. Choice of, or switching between, morphs commonly relies on developmental threshold responses. Evolutionary changes in developmental threshold responses are thought to provide an important avenue for phenotypic diversification and the evolution of morphological and behavioral novelties. However, the extent to which alternative male phenotypes and their underlying threshold responses actually evolve in natural populations is unclear. Likewise, the ecological factors that shape the evolution of threshold responses in natural populations are unexplored for most organisms, as are the consequences of such modifications for patterns of morphological diversity. I examined the ecological basis of rapid threshold evolution in exotic populations of the horn-polyphenic dung beetle Onthophagus taurus. Male O. taurus vary continuously in body size as a function of larval feeding conditions. Only males that exceed a critical threshold body size develop a pair of long horns on their heads, whereas males below this threshold remain hornless. Populations in two exotic ranges of this species, the eastern United States and western Australia, have diverged in the mean threshold body size, which has resulted in the evolution of highly divergent and novel horn length-body size allometries in these populations. Populations in a third and previously unstudied exotic range of O. taurus in eastern Australia exhibit threshold body sizes roughly intermediate between the eastern U.S. and western Australian populations. I tested three hypothesis to explain how differences in ecological and demographic factors can drive allometric divergences between populations, using data derived from comparative, standardized sampling of a large number of populations in each exotic range. Results suggest that differences in the intensity of both intra-and interspecific competition have contributed to the evolution of divergent thresholds in these populations. My results do not support the hypothesis that shifts in threshold body sizes to larger body sizes are a consequence of increases in the mean body size of competing males. I discuss my results in the context of Onthophagus mating systems and the evolutionary implications of threshold evolution.

Seasonal constraints on the mandible allometry of Lucanus cervus (Coleoptera: Lucanidae)

“…Moczek et al (2002) suggest that the optimal body size at which the switch between two morpho-types occurs may vary as a function of external conditions. Environmental factors that are known to influence allometric functions in beetles are food quality and competition (Emlen, 1997;Moczek, 2002Moczek, , 2003. In Onthophagus, adult body size and male horn morphology are determined by the quality and quantity of the food that the parents supply their larvae (Moczek, 1998).…”

“…The threshold mechanism and the phenotypic alternatives it regulates are thought to be independent modules (Emlen et al, 2005): the threshold body size can therefore be subject to selection through the fitness of the alternative phenotypes (Tomkins & Hazel, 2007) and evolve rapidly. Despite a well developed theoretical background there are only a few studies that empirically assess intra-specific variation in the threshold that regulates male dimorphic traits (Moczek, 2002(Moczek, , 2003Moczek et al, 2002;Tomkins et al, 2004;Hongo, 2007). There are even fewer studies on the plastic responses of thresholds during the course of a season (but see Emlen, 1996Emlen, , 1997 and the ecological factors that influence polyphenism in natural populations are largely unexplored.…”

Integrating micro- and macroevolution of development through the study of horned beetles

“…More importantly, these studies also showed that evolutionary changes in several of its components provided important avenues for evolutionary changes in patterns of horn expression, ultimately driven by population-wide changes in ecological conditions (summarized in Figure 5; Moczek, 2003;Moczek and Nijhout, 2003). However, to fully appreciate how the endocrine regulation of beetle horns links ecological to evolutionary change, we first need to briefly review the behavioral and ecological context in which beetle horns and horned beetles function.…”

“…Any ecological factor that somehow alters the composition of males within a population, or the nature of interactions between males, would be likely to also affect the exact location of the optimal body size threshold. Populations subject to divergent ecologies might then diverge in the exact scaling relationship between body size and horn length (Moczek, 2003).…”

“…Contrasting a large number of local populations in each of the two exotic ranges introduced above, as well as a third exotic range in eastern Australia, Moczek (2003) tested several hypotheses regarding the ecological causes of threshold divergences (Figure 5e-h). For example, threshold divergences might be brought about by population-wide changes in the distribution of male body sizes, causing the same absolute male size to be relatively large in one population, but relatively small in another.…”