High Condition Male Rhinoceros Beetles Transfer More Nutrients to Females During Mating, Contributing to Female Preference for Body Condition Rather than Body or Weapon Size

Males in many insect species transfer nutrients along with sperm in an enclosed package known as a spermatophore. Females utilize these spermatophores to increase their fecundity. However, not all males offer the same size spermatophore. This can drive female preference toward males who are able to provide the largest spermatophores. The largest males typically produce the largest spermatophore sizes, but this is not always the case. Adult insects with rigid exoskeletons may suffer drastic reductions in body condition that are not visible on the outside. In these taxa, females may select males in excellent physiological condition rather than simply choosing males with the largest body size. Male rhinoceros beetles use their iconic “pitchfork” horns in battles with rival males over feeding territories. Once a male beetle has won a territory he will court females using stridulatory song and trembling dances. For years females were assumed to be passive in the courtship process because males who are able to win territories usually have the largest body size and the longest horns. However, it is now clear that females routinely reject males, including large males with long horns. We suspect that hours of repeated battles can exhaust males, depleting their energy reserves and thus their body condition. If poor condition males transfer smaller spermatophores, then females may be using courtship to select mating partners based on their body condition. Here I tested a critical prediction of this hypothesis. I restricted food intake for a subset of males and compared the spermatophore sizes of 25 nutrient-stressed and 25 well-fed males. I observed and filmed male courtships and then froze females immediately after mating for dissection. Spermatophore sizes significantly smaller in starved males, confirming that male short-term body condition does affect the amount of nutrients a male transfers to a female during mating. Female insects continue to amaze us with the sophistication of their mate choices, and this study reminds us that it isn’t always all about body or weapon size