Studying the relationship between parental and mating effort helps us to understand the evolution of parental care and, consequently, has been the subject of many theoretical and empirical investigations. Using burying beetles as a model, we found no correlation between the intensity of a sexual signal (sex pheromone quantity) and the amount of care provided by males. However, males that were given the opportunity to breed and care for young went on to produce a higher amount of their sexual signal and attracted three times more females in the field than control males that were not given the opportunity to breed. The likely explanation for our finding is that specific aspects of care in burying beetles, that is the defense and preservation of a nutrient rich breeding resource, a small vertebrate cadaver, is not only beneficial for the offspring but also for the adults themselves. Obtaining a good carrion meal possibly enables males to store resources that they can subsequently allocate toward sexual signaling. Collectively, our results highlight that conditions can exist where male participation in brood care has a positive effect on its sexual attractiveness. This in turn might have facilitated the evolution of male assistance in parental care.

Life history theory predicts a trade-off between male sexual trait expression and immunocompetence. Using burying beetles, Nicrophorus vespilloides, as a model, we investigated the relationship between male immune function, sex pheromone emission, and attractiveness under field conditions. In the first experiment, we tested whether there is a positive correlation between immune capacity, sex pheromone characteristics (quantity, relative composition, and time invested in pheromone emission), and male attractiveness. As a measurement of immune capacity, we used an individual's encapsulation ability against a novel antigen. In the second experiment, we specifically examined whether a trade-off between chemical trait expression and immune function existed. To this end, we challenged the immune system and measured the subsequent investment in sex pheromone emission and the attractiveness of the male under field conditions. We found that a male's immunocompetence was neither related to the emission of the male's sex pheromone nor to its attractiveness in the field. Furthermore, none of the immune-challenge treatments affected the subsequent investment in pheromone emission or number of females attracted. However, we showed that the same males that emitted a high quantity of their sex pheromone in the laboratory were able to attract more females in the field. Our data suggest that the chemical signal is not a reliable predictor of a male's immunocompetence but rather is a general important fitness-related trait, with a higher emission of the sex pheromone measured in the laboratory directly affecting the attractiveness of a male under field conditions.