Doping for sex: Bad for mitochondrial performances? Case of testosterone supplemented Hyla arborea during the courtship period.

Sexual selection has been widely explored from numerous perspectives, including behavior, ecology, and to a lesser extent, energetics. Hormones, and specifically androgens such as testosterone, are known to trigger sexual behaviors. Their effects are therefore of interest during the breeding period. Our work investigates the effect of testosterone on the relationship between cellular bioenergetics and contractile properties of two skeletal muscles involved in sexual selection in tree frogs. Calling and locomotor abilities are considered evidence of good condition in Hyla males, and thus server as proxies for male quality and attractiveness. Therefore, how these behaviors are powered efficiently remains of both physiological and behavioral interest. Most previous research, however, has focused primarily on biomechanics, contractile properties or mitochondrial enzyme activities. Some have tried to establish a relationship between those parameters but to our knowledge, there is no study examining muscle fiber bioenergetics in Hyla arborea. Using chronic testosterone supplementation and through an integrative study combining fiber bioenergetics and contractile properties, we compared sexually dimorphic trunk muscles directly linked to chronic sound production to a hindlimb muscle (i.e. gastrocnemius) that is particularly adapted for explosive movement. As expected, trunk muscle bioenergetics were more affected by testosterone than gastrocnemius muscle. Our study also underlines contrasted energetic capacities between muscles, in line with contractile properties of these two different muscle phenotypes. The discrepancy of both substrate utilization and contractile properties is consistent with the specific role of each muscle and our results are elucidating another integrative example of a muscle force-endurance trade-off.

Immune challenges and visual signalling in tree frogs.

In animals, mate-choice is often based on sexual signals that carry information and help the receiver make the best choice to improve the receiver's fitness. Orange visual sexual signals have been hypothesised to carry immune information because they are often due to carotenoid pigments which are also involved in immunity response. Although many studies have focused on the direct relationships between coloration and immunocompetence, few studies have simultaneously studied immunocompetent response and coloration variation after an immune challenge. We tested this hypothesis on starved and ad libitum-fed males of the European tree frog Hyla arborea. Our results show that male coloration is not a reliable indicator of its immune response capacity in this species. However, after an immune challenge induced by a PHA (Phaseolus vulgaris phytohaemagglutinin) injection, starved males presented a significant coloration loss and this alteration was related to the immune response intensity. Taken together, these results suggest that the brighter (lighter) coloration may be used as a cue by female to exclude males with a recent immune challenge, due to diseases or parasites for example.

Does testosterone affect foraging behavior in male frogs?

During the breeding season, males often produce costly and extravagant displays or physical ornaments to attract females. Numerous studies have established that testosterone could directly influence the expression of certain sexual signals. However, few of these studies have focused on the indirect role that testosterone could play in modulating prey detection and visual performance to improve the foraging ability of males and hence their acquisition of nutritional resource. In the present study, we experimentally modified the testosterone levels of European tree frog males (Hyla arborea), staying in the natural range previously measured in the field, and we investigated the effect of testosterone on the foraging ability of individuals. Foraging capacities were measured on males placed in an arena with a virtual cricket moving on a computer screen. Our results demonstrated a significant effect of testosterone on the hunting behavior of H. arborea. We observed that testosterone reduced the orientation latency to virtual prey for supplemented males compared to controls. In addition, testosterone significantly increased the attack promptness of male frogs. Finally, our experiment did not demonstrate any impact of testosterone on male attack success.