Since the sex ratio on the spawning grounds ranges from 4-11 males/1 female, there is a high level of competition for females. Males on the spawning grounds are sexually dimorphic by size. Large males (mean 370 mm mantle length, ML) pair with and guard females (mean 250 mm ML) temporarily (pre- and postcopulation). Large lone males search for lone females or challenge consorts with agonistic displays. Large males engage in agonistic contests that pass through increasingly complex stages of display, culminating in a dramatic passing cloud display and sometimes physical contact and biting (see video of courtship behavior). Contests ended with one male swimming away. Small lone males guard females if there is no large male in the vicinity, or search for lone females with whom they mate without guarding. Small males (150–250 mm ML) may also use opportunities for extrapair copulations (EPCs). They use "open stealth" (overt sneak mating), "hidden stealth" (concealed sneak mating, e.g. under a rock) and female mimicry to achieve sneaker EPCs (see video of males mimicking females). Female mimicry leads to increased acceptance of mating with the female, and to immediate fertilization, as demonstrated by DNA fingerprinting.

References

Hall, K. C. and R. T. Hanlon. 2002. Principal features of the mating system of a large spawning aggregation of the giant Australian cuttlefish Sepia apama (Mollusca : Cephalopoda). Mar Biol 140: 533-545

Hanlon, R. T., M. J. Naud, P. W. Shaw, and J. N. Havenhand. 2005. Behavioural ecology - Transient sexual mimicry leads to fertilization. Nature 433: 212-212

Naud, M. J., R. T. Hanlon, K. C. Hall, P. W. Shaw, and J. N. Havenhand. 2004. Behavioural and genetic assessment of reproductive success in a spawning aggregation of the Australian giant cuttlefish, Sepia apama. Anim Behav 67: 1043-1050

Norman, M. R. 2000. Cephalopods, a world guide : Pacific Ocean, Indian Ocean, Red Sea, Atlantic Ocean, Caribbean, Arctic, Antarctic, ConchBooks, Hackenheim, Germany 318p