Abstract
Complex signals are commonly used during intraspecific contests over resources to assess an opponent's fighting ability and/or aggressive state. Stomatopod crustaceans may use complex signals when competing aggressively for refuges. Before physical attacks, stomatopods assess their opponents using chemical cues and perform threat displays showing a coloured patch, the meral spot. In some species, this spot reflects UV. However, despite their complex visual system with up to 20 photoreceptor classes, we do not know if stomatopods use chromatic or achromatic signals in contests. In a field study, we found that Neogonodactylus oerstedii meral spot luminance varies with sex, habitat and, more weakly, body length. Next, we conducted an experimental manipulation which demonstrated that both chemical cues and chromatic signals are used during contests. In the absence of chemical cues, stomatopods approached an occupied refuge more quickly and performed offensive behaviours at a lower rate. When UV reflectance was absent, stomatopods performed offensive behaviours more frequently and contest duration trended towards shorter fights. These results provide new evidence that UV reflectance and/or visible spectrum luminance is used to amplify threat displays. Our results are the first to demonstrate that chemical and chromatic cues comprise a multimodal signal in stomatopod contests.
Highlights
In many animal interactions, complex signals are used to improve communication ability
Female spots were twice as bright in the visible spectrum compared with males, and stomatopods from rubble habitats had meral spots twice as bright in the visible spectrum compared with stomatopods from seagrass habitats
Complex signals have been demonstrated in several crustacean taxa including hermit crabs [36], Aegla decapods [37], fiddler crabs [38], blue crabs [6], crayfish [39], snapping shrimp [9] and copepods [40]
Summary
Complex signals are used to improve communication ability. If 2 such signals use more than one modality (e.g. visual, acoustic and chemical), they are multimodal signals, otherwise they are known as unimodal signals In these interactions, each signal may transmit different information. This could be beneficial if the receiver responds more appropriately to two signals than to one [3], or if particular signals are more effective under different environmental conditions [4] Due to these benefits, complex signals are common in many animal interactions, including mate choice [5,6,7], intraspecific competition [8,9,10] and even predator/prey interactions [11,12]
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