Complex adaptive traits between mating behaviour and post-copulatory sperm behaviour in squids

Emergence of male dimorphism within a species is the evolutionary process of disruptive selection. In squids, two types of male mating behaviour, known as alternative reproductive tactics (ARTs), are causally associated with adult body size. Males inseminate promiscuously with the same females; large “consort” males internally, and small “sneaker” males externally. Previously we found that in Heterololigo bleekeri, sneaker (but not consort) spermatozoa are able to swarm by sensing self-emitted CO_2. This suggests that a swarming trait might have arisen in sneakers as a “sperm cooperation” strategy among sibling sperm in order to compete with consort males, or as a consequence of adaptation to external fertilization. To address these possibilities, we examined six species where three patterns of insemination are present, namely, only internal, only external, or both ARTs. In three species that employ both ARTs (H. bleekeri, Loligo reynaudii and Uroteuthis edulis), sneaker spermatozoa always exhibited self-swarming capacity. In Idiosepius paradoxus and Todarodes pacificus, which use only external insemination, spermatozoa formed a swarm. However, in Euprymna morsei, which use only internal insemination, sperm were unable to swarm. These results suggest that the self-swarming trait is likely to be linked to the mode of insemination rather than the alternative strategy used by sneaker males. Thus we propose a new hypothesis in which cooperative sperm behaviour has evolved not only through kin selection against sperm competition risks, but also through adaptation to the insemination/fertilization environment.