Japanese Spear Squid Hatch Dates Shape Mating Tactics

If you favour science, horoscopes and their claims about a preordained future may hold little appeal, even when those forecasts are linked to the positions of stars and planets on the night of your birth.

New research from Japan suggests that some squid may nevertheless be fated to become either furtive or aggressive mating partners according to when they hatch.

University of Tokyo marine biologist Shota Hosono and his colleagues examined Japanese spear squid (Heterololigo bleekeri). They discovered that the reproductive approach taken by males is shaped by the month in which they emerge.

Japanese spear squid mating tactics

Squid hatching early in the breeding period, from early April to mid-July, were relatively larger by the time they mated. They therefore became consorts: individuals that drive away competing males to fertilise a female, then remain with her as she lays her eggs.

By contrast, squid that hatched later, between early June and mid-August, were generally smaller and adopted the role of ‘sneakers’. These males discreetly place sperm on the outside of a female near the site where she deposits her eggs, hoping to fertilise them.

Individuals hatching in early July were equally likely to become sneakers or fighters.

This pattern is known as the relative age effect. It is clearly illustrated by children born earlier in the year, who are more likely than others in their age group to become professional athletes because they have had longer to develop before selection takes place.

Before this squid research, however, the ‘birthdate hypothesis’ - as biologists also call it - had been examined only in several teleost fish species and, beyond humans, in no other taxonomic group across the wider animal kingdom.

“Typically, large males adopt a tactic of competing with rivals for mating, while small males adopt a tactic of stealing fertilization opportunities from the large males,” Hosono and colleagues explain in their published paper, following their study of 201 male squid and 68 mature females.

This marks the first evidence for the ‘birthdate hypothesis’ in aquatic invertebrates, indicating that male H. bleekeri may be set on particular mating behaviours from birth.

Hatching dates and squid growth

Even when males born early in the season did not grow as large as expected, they did not abandon the birthday pattern by becoming sneakers. Instead, they delayed mating and kept growing until they were large enough to act as consorts.

“Our results showed that the hatching date determines the whole life trajectory in this species,” says Yoko Iwata, a marine ecologist at the University of Tokyo.

Hosono, Iwata and their colleagues also argue that these mating strategies are not merely passed down through the paternal line.

“Female squid store sperm from both consort and sneaker males throughout the breeding season and use a mixture of sperm from both sources to fertilize their eggs during a spawning event,” the researchers explain.

Genetic variation arising from alternative reproductive tactics can affect animal mating behaviour, including in birds and damselflies, although recorded cases are thought to be uncommon.

Yet the findings from Hosono and colleagues indicate that the male mating strategies of H. bleekeri are probably shaped by a potent combination of physical, biological and environmental conditions present at birth. In beetles, for instance, food availability strongly influences physical characteristics connected with reproductive success later in life.

Humans show a comparable seasonal pattern: the time of year in which you are born can leave a mark on your DNA, influencing, for example, the likelihood of developing allergies.

“The difference in hatch date means that the squid experience different environmental conditions in early life, which may influence the growth trajectory,” Iwata says.

Squid are highly sensitive to environmental conditions, especially water temperature. Iwata says an extreme occurrence, such as a marine heatwave during the hatching season, could alter the squid’s adult body size and, in turn, its later mating tactic.

“This would also impact the amount that could be commercially caught enormously,” Iwata adds.

The study nevertheless found little difference in early-life growth rates between consorts and sneakers, despite their developing in different seasons. Further research is therefore required to clarify how environmental conditions might influence squid mating behaviours.

The study was published in Proceedings of the Royal Society B Biological Sciences.