Copulatory Sequences and Sexual Struggles in Millipedes

Abstract

Behavioral sequences were followed in the millipede genus Centrobolus (Diplopoda: Spirobolida: Pachybolidae). Precopulatory struggles included a combination of female coiling, the antipredator response, and male coercion and uncoiling. During copulation three male behaviors were always performed—antennating, glandular secretion and sexual collaring—all three were candidates for cryptic female choice. In species with highly prolonged copulation (e.g., Centrobolus inscriptus, 170.0 ± 49.3 min), the male coiled around the female and produced a large “sexual collar,” defined upon timely hydraulic movements in the region of genital contact. In species with shorter copulation (e.g., C. ruber, 39.8 ± 13.2 min), the pair copulated in parallel and lacked the sexual collar. Variance in copulation duration produced significant trends, e.g., female size correlated with the prolonging of copulation duration, and there were indications of a conflict of sexual interests for controlling when copulation would be terminated. Stand-by males that were unsuccessful at take-over accompanied copulatory triplets and clusters of mating pairs. Polygynandrous mating systems of these millipedes incorporate diametric pressures on males to maximize copulation numbers and duration, and females to perform endurance rivalry based upon sexual size differences. This study provides a useful comparison for insects where copulation is also prolonged.