Testosterone amplifies the negative valence of an agonistic gestural display by exploiting receiver perceptual bias

Many animals communicate by performing elaborate displays that are incredibly extravagant and wildly bizarre. So, how do these displays evolve? One idea is that innate sensory biases arbitrarily favour the emergence of certain display traits over others, leading to the design of an unusual display. Here, we study how physiological factors associated with signal production influence this process, a topic that has received almost no attention. We focus on a tropical frog, whose males compete for access to females by performing an elaborate waving display. Our results show that sex hormones like testosterone regulate specific display gestures that exploit a highly conserved perceptual system, evolved originally to detect ‘dangerous' stimuli in the environment. Accordingly, testosterone makes certain gestures likely to appear more perilous to rivals during combat. This suggests that hormone action can interact with effects of sensory bias to create an evolutionary optimum that guides how display exaggeration unfolds.

Claw regeneration, waving display and burrow characteristics of Austruca perplexa (H. Milne Edwards, 1852) (Brachyura, Ocypodidae) from southern Thailand

We studied the effects of claw regeneration on male waving rate and burrow characteristics (i.e., important mate choice criteria) by examining the waving rates and burrow characteristics (diameter, total and horizontal lengths, depth, volume, maximum width, entry and burrow angles, and presence and location of chambers) of large-sized original-clawed males (OCMs) and regenerated-clawed males (RCMs) of Austruca perplexa (H. Milne Edwards, 1852). Female burrows were also examined. The results showed that female burrows were smaller than male burrows, with no chamber and, thus, female burrows are not used for breeding; however, 80% of RCM burrows, and 65% of OCM burrows, had chambers. Other characteristics were not different between RCM and OCM burrows, except for maximum width, which was larger in RCM burrows. The waving rates of OCMs and RCMs were not different. Our results indicate that claw regeneration do not have detrimental effects on male waving rate and burrow characteristics.

Receiver female body size and distances affect the claw-waving rate of signaller males in fiddler crabs (Brachyura, Ocypodidae)

We tested for the first time how Austruca bengali Crane, 1975 signaller males adjusted their waving rates based on receiver female body sizes and their distances. We video recorded the waving display of 46 males (9–12 mm carapace width) for 30 s, and counted their waving rate. Receiver females were categorised as small (8–10 mm carapace width) and large (>10 mm). Distances between males and females were categorised as short (⩽12 cm) and long (>12 cm) distances. Our results indicate that males are able to measure distances and female sizes, and adjust their waving display by actively reducing waving rate (1) towards small females, as usually small females have lower fecundity compared to large ones and (2) towards females at very close distance because at this point, the females would make their mating decision, and thus males start to lead/hit the females towards their burrow rather than waving vigorously.

Multimodal communication in courting fiddler crabs reveals male performance capacities

Courting males often perform different behavioural displays that demonstrate aspects of their quality. Male fiddler crabs, Uca sp., are well known for their repetitive claw-waving display during courtship. However, in some species, males produce an additional signal by rapidly stridulating their claw, creating a ‘drumming’ vibrational signal through the substrate as a female approaches, and even continue to drum once inside their burrow. Here, we show that the switch from waving to drumming might provide additional information to the female about the quality of a male, and the properties of his burrow (multiple message hypothesis). Across males there was, however, a strong positive relationship between aspects of their waving and drumming displays, suggesting that drumming adheres to some predictions of the redundant signal hypothesis for multimodal signalling. In field experiments, we show that recent courtship is associated with a significant reduction in male sprint speed, which is commensurate with an oxygen debt. Even so, males that wave and drum more vigorously than their counterparts have a higher sprint speed. Drumming appears to be an energetically costly multimodal display of quality that females should attend to when making their mate choice decisions.