Investigating the effect of larval diet quality on adult body size and male horn morphology in the beetle Onthophagus taurus

Background/Question/Methods

Phenotype is the product of the genotype and the environment in which those traits are expressed. Threshold traits are quantitative traits that are expressed in a limited number of phenotypes, but are based on numerous factors that contribute to that particular trait. The environment is one such contributor. One aspect of the environment that is particularly important for a threshold trait is the resource quality of food. The aim of this experiment is to test the extent to which two resources, commonly used by the sexually dimorphic burrowing dung beetle Onthophagus taurus, differ in their effects on adult body size and male horn development. This experiment is based on those done by Moczek (1998) and Hunt and Simmons (2004). Pairs of beetles (96 total pairs, 48 per treatment) were placed in breeding tubes with 150g thawed dung in its natural form. After 8 days of breeding, the resulting brood balls were collected and weighed. With the use of digital imaging and ImageJ, the horn length and thorax width were measured for each emerging offspring. 

Results/Conclusions

A Combination of statistical analyses (Mann-Whitney-Wilcoxon, Regression, and Chi Square) tested the following hypotheses: there is a difference in the number and mass of brood balls that produce live offspring; there is a difference in the thorax size and horn length of resulting male offspring based on resource quality. A Chi Square test revealed that cow dung produced significantly more emerging offspring than horse dung. Brood balls of cow dung were statistically smaller than those of horse dung and larger brood balls (mass) produced male offspring (Mann-Whitney-Wilcoxon). However, a regression analysis showed no significant correlation between brood ball mass and thorax width and horn length of offspring. The results of this work imply that the form in which the resource is available may affect parental energy investment in offspring, which may in turn affect the sex of the offspring.