Abstract
Males can gather information on the risk and intensity of sperm competition from their social environment. Recent studies have implicated chemosensory cues, for instance cuticular hydrocarbons (CHCs) in insects, as a key source of this information. Here, using the broad-horned flour beetle (Gnatocerus cornutus), we investigated the importance of contact-derived rival male CHCs in informing male perception of sperm competition risk and intensity. We experimentally perfumed virgin females with male CHCs via direct intersexual contact and measured male pre- and post-copulatory investment in response to this manipulation. Using chemical analysis, we verified that this treatment engendered changes to perfumed female CHC profiles, but did not make perfumed females "smell" mated. Despite this, males responded to these chemical changes. Males increased courtship effort under low levels of perceived competition (from 1-3 rivals), but significantly decreased courtship effort as perceived competition rose (from 3-5 rivals). Furthermore, our measurement of ejaculate investment showed that males allocated significantly more sperm to perfumed females than to control females. Together, these results suggest that changes in female chemical profile elicited by contact with rival males do not provide males with information on female mating status, but rather inform males of the presence of rivals within the population and thus provide a means for males to indirectly assess the risk of sperm competition.
Highlights
Sperm competition occurs when sperm from 2 or more males compete within the female genital tract to fertilize a female’s ova (Parker 1970)
Our analyses showed that contact-derived male cuticular hydrocarbons (CHCs) retained on female cuticles had a significant effect on courtship effort (F3,115 = 3.096, P = 0.03)
Our findings indicate that male G. cornutus are able to detect the local risk and intensity of sperm competition from chemical cues transferred between males and females during contact, as well as through physical interactions with rival males as has been shown previously (Okada et al 2010)
Summary
Sperm competition occurs when sperm from 2 or more males compete within the female genital tract to fertilize a female’s ova (Parker 1970). A male’s ability to respond to changes in sperm competition risk and intensity is entirely dependent on his ability to gather information to assess this risk and intensity accurately (Parker et al 1997) Males can acquire such information from a variety of cues in their socio-sexual environment [e.g., visual—presence of rival males during mating Drosophila pseudoobscura (Price et al 2012), Mediterranean fruit flies Ceratitis capitata (Gage 1991); acoustic— male song in crickets Teleogryllus oceanicus (Gray and Simmons 2013); Tactile—Drosophila melanogaster (Bretman et al 2011)], and recent empirical evidence illustrates that males rely on these cues, often in combination (Bretman et al 2011; Thomas 2011). Unlike the aforementioned cues, chemical cues facilitate both the detection of competitors and the assessment of female mating status
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