Abstract

BackgroundUnder strong sexual selection, certain species evolve distinct intrasexual, alternative reproductive tactics (ARTs). In many cases, ARTs can be viewed as environmentally-cued threshold traits, such that ARTs coexist if their relative fitness alternates over the environmental cue gradient. Surprisingly, the chemical ecology of ARTs has been underexplored in this context. To our knowledge, no prior study has directly quantified pheromone production for ARTs in a male-polymorphic species. Here, we used the bulb mite—in which males are either armed fighters that kill conspecifics, or unarmed scramblers (which have occasionally been observed to induce mating behavior in other males)—as a model system to gain insight into the role of pheromones in the evolutionary maintenance of ARTs. Given that scramblers forgo investment into weaponry, we tested whether scramblers produce higher quantities of the putative female sex-pheromone α-acaridial than fighters, which would improve the fitness of the scrambler phenotype through female mimicry by allowing avoidance of aggression from competitors. To this end, we sampled mites from a rich and a poor nutritional environment and quantified their production of α-acaridial through gas chromatography analysis.ResultsWe found a positive relationship between pheromone production and body size, but males exhibited a steeper slope in pheromone production with increasing size than females. Females exhibited a higher average pheromone production than males. We found no significant difference in slope of pheromone production over body size between fighters and scramblers. However, scramblers reached larger body sizes and higher pheromone production than fighters, providing some evidence for a potential female mimic strategy adopted by large scramblers. Pheromone production was significantly higher in mites from the rich nutritional environment than the poor environment.ConclusionFurther elucidation of pheromone functionality in bulb mites, and additional inter- and intrasexual comparisons of pheromone profiles are needed to determine if the observed intersexual and intrasexual differences in pheromone production are adaptive, if they are a by-product of allometric scaling, or diet-mediated pheromone production under weak selection. We argue chemical ecology offers a novel perspective for research on ARTs and other complex life-history traits.

Highlights

  • Under strong sexual selection, certain species evolve distinct intrasexual, alternative reproductive tactics (ARTs)

  • Our results show that (1) α-acaridial production was positively correlated with body size, and this relationship was steeper in males than females, (2) α-acaridial production was influenced by the nutritional environment, (3) on average, females produced more α-acaridial than males, and (4) there was no significant difference in slope of α-acaridial production over body size between the male ARTs

  • We found a positive relationship between pheromone (α-acaridial) production and body size in bulb mites, but importantly, males demonstrated a steeper slope in pheromone production with increasing size than females

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Summary

Introduction

Certain species evolve distinct intrasexual, alternative reproductive tactics (ARTs). Morphological and physiological adaptations related to their ability to attract and compete for mates [1] Many of these adaptations have evolved under strong sexual selection, resulting in Zeeman et al BMC Ecology and Evolution (2022) 22:5 distinct, intrasexual Alternative Reproductive Tactics, or ARTs [2]. ART expressions are considered threshold traits, wherein one tactic is expressed when an environmental or (environmentally-driven) physiological cue reaches a certain, genetically determined threshold during ontogeny, and the alternative tactic is expressed if this threshold is not reached [e.g., 2, 13, 14] Such environment-dependent ARTs can only coexist if the fitness functions of the ARTs cross over the gradient of the environmental cue [15]. Such sneaking tactics have been observed across a wide range of taxa [e.g., 4, 7, 20], and would be successful if small(er) males can conceal themselves from rival, fighter males, for example by mimicking a female [21]

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