Abstract
In animal contests, communicating aggressive motivation is most often mediated by visual or acoustic signals, while chemical signals are not expected to serve such a function since they are less able to be modulated by the sender during the changing behavioral context. We describe a rare example of ephemeral olfactory signals in terrestrial animals, signals that are emitted via protrusive scent glands in male cave crickets Troglophilus neglectus (Orthoptera, Rhaphidophoridae) to reflect the state of the signaler’s aggression. We correlate the intensity of behaviorally expressed aggression of the individuals in dyadic contests with the frequency and extent of their gland tissue protrusion, the latter serving as an indication of the amount of released odor. We detected large amounts of odor release during brief gland protrusions, and the absence of its release during gland retraction. Males protruded the glands during and after encountering a rival, with the degree of protrusion increasing with the intensity of the signalers’ aggression. During the encounters, the degree of gland protrusion increased most strongly with the occurrence of the elevated body posture, directly preceding the attack. This degree was significantly higher in encounter winners than in losers displaying such posture, suggesting the highly important role of the released odor for contest resolution. After the encounters, glands were protruded almost exclusively by winners, apparently announcing victory. We tested for the function of the olfactory signals also directly, by preventing gland tissue protrusion in symmetric and asymmetric treatments of the contestants. Treating only the dominant individuals decreased the percentage of encounters they won by over 60%, while treating both contestants elicited a significant increase in the frequency and duration of fights. During contests, the olfactory signals of T. neglectus apparently function as a highly effective threat, which prevents maximal contest escalation and decreases the conflict-related costs.
Highlights
In animal contests over resources, the opponents most often exchange information via signals and displays to determine the superior individual prior to engagement in a physical fight [1, 2, 3]
We have demonstrated that 5-Methyl-2-phenyl-2-hexenal (5,2,2-MPH), previously identified from gland secretion in T. neglectus [47], represents the major volatile compound in the emission from protruded glands and therewith the pheromone involved in agonistic behavior of the species
This substance has been found in different plants and plant-derived food [47, 48, 49], so its production in T. neglectus is most likely based on nutrition
Summary
In animal contests over resources, the opponents most often exchange information via signals and displays to determine the superior individual prior to engagement in a physical fight [1, 2, 3]. Dynamic conventional signals that can vary over short periods, such as visual or acoustic, are especially suitable for conveying information about short-term changes in the motivational state or intent of the signaler [10, 13]. Such signals may generally function as challenges emitted from a distance, as general aggressive motivation signals that reflect the need for a resource once the opponent has been detected, or as offensive threats that signal a high probability of subsequent attack [1]. Chemical signals are generally not supposed to signalize aggressive motivation and, the intention to attack, since they are difficult to modulate and provide poor directionality [1, 20]
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