Prolonged copulation, sperm reserves and sperm competition in the Aquatic Warbler Acrocephalus paludicola

Abstract

The Aquatic WarblerAcrocephalus paludicolahas a quasi‐promiscuous mating system with high levels of multiple paternity and, thus, intense sperm competition. The duration of copulation in the Aquatic Warbler is unusually long. In hand‐reared, captive birds the duration of mounting was 23.7 ± 11.8 min (mean ± s.d.). On average, six cloacal contacts (inseminations) occurred during each copulation. Between inseminations the male remained in contact with the female, either on top of or directly behind her. Protracted copulation may be a form of contact mate guarding, a behaviour not previously recorded in birds. Copulation was most frequent during the evening and early morning. Male Aquatic Warblers also exhibit extreme morphological adaptations of their reproductive system compared with otherAcrocephalusspecies and other birds: their testes, cloacal protuberance and seminal glomera (which contain 198 × 106spermatozoa) are all relatively large. Frequent insemination during protracted copulation and contact mate guarding may be alternatives to paternity guarding found in other birds.