Male squid (Loligo bleekeri) employ different reproductive strategies depending on their body size. Research published in BioMed Central's open access journal BMC Evolutionary Biology shows that the divergent mating behavior of male squid has resulted in the evolution of different sperm sizes.
Large male squid compete for females by courting them with flashy skin color-change displays. Once a female has chosen her partner they mate in an above and below position so that the male can place his sperm inside the female's oviduct. He remains with the female until she spawns, ensuring that his sperm fertilize her eggs and that no other males have a chance to mate with her. At the moment a female lays her eggs, small 'sneaker' males rush in and mate with her, head to head. These small males place packages of sperm by the female's mouth in the hope that their sperm have a chance of fertilizing the eggs as they leave the female's body.
When researchers from London and Japan looked at the sperm produced by small sneaker males and large consort males they discovered that the sperm produced by the sneaker males was larger than that of the consorts. Dr Yoko Iwata from University of Tokyo said, "Sperm size is likely to be an adaptation to fertilization environment, either inside the female or externally, rather than competition between sperm, because the fertility and motility of sneaker and consort sperm were the same."
Overall, the larger males' strategy resulted in higher paternity rates -- but for smaller males, who cannot win a female by fair means, being sneaky gives them a chance of passing on their genes.
- Yoko Iwata, Paul Shaw, Eiji Fujiwara, Kogiku Shiba, Yasutaka Kakiuchi and Noritaka Hirohashi. Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours. BMC Evolutionary Biology, 2011
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