Courtship and mating of the tailed frog (Ascaphus truei)

Frogs (Amphibia: Anura) have contributed greatly to our understanding of the functions and evolution of sexual behaviour patterns, but few studies have been conducted on 'primitive' taxa. Here we describe laboratory observations that characterize aspects of the courtship and mating of the tailed frog Ascaphus truei, one of the most basal of living frogs. Fertilization is internal (a rare and probably apomorphic state for anurans) and sperm transfer is via an erectile intrommitent organ, or 'tail.' This structure seems to be an autapomorphy. Inguinal amplexus accomplishes mating (probably a plesiomorphic state), but the combination of amplexus and copulation, previously termed copulexus, is unique (autapomorphic). Mate location is not mediated acoustically (sexual vocalizations probably are apomorphic), but may involve visual or chemical cues. In a competitive context, heavier males do not seem to enjoy an advantage in being the first to initiate mating, and rivals rarely displace males already engaged in copulexus by wrestling. The courtship and mating of the tailed frog consists of a mosaic of traits that are ancestral, derived and unique within the clade Anura. Efficient location and then (with internal fertilization) stimulation of females during copulexus may influence male reproductive success more than wrestling.