Rarely observed circling behaviours of endangered basking sharks have now been explained as ‘shark speed dating’ courtship displays.

Marine biologists from the Marine Biological Association (MBA), the Irish Basking Shark Group and colleagues have led ground-breaking research which reveals the circles of basking sharks seen off western Ireland are engaged in annual reproductive behaviour, the first place in the world where this has been verified.

Circling formations have been documented on a few occasions over the past 40 years in the north-west Atlantic off Canada and the USA. Although basking sharks are often seen filter-feeding plankton in UK and Irish coastal waters in the summer, the circling formations were rarely seen, and until now, scientists could not explain the behaviour.

Photo: Irish Basking Shark Group

Scientists captured footage of 19 circling groups using underwater cameras and aerial drones off County Clare, Ireland, from 2016 to 2021. They found each group comprised between 6 and 23 sharks swimming slowly at the surface, with others below them deeper down, in a three dimensional ring structure the researchers termed a ‘torus’.

The team found that the sharks in circle formations were equal numbers of sexually mature male and females and were not filter-feeding. Some females had a paler body colour than males, a difference seen during courtship and mating behaviour in other shark species.

Gentle giant’s ‘love dance’.

The study also showed that despite courtship torus duration lasting several hours, and perhaps even several days, individual females and males associated with most other members within a few minutes. In that time, the sharks interacted through gentle fin-fin and fin-body touching, rolling to expose ventral surfaces to following sharks, and breaching behaviour perhaps as a signal of their readiness to mate.

Professor David Sims, Senior Research Fellow at the MBA and University of Southampton who was lead author of the study said: “How usually solitary basking sharks find a mate in the ocean’s expanse has been an enduring mystery. Incredibly we now find that a courtship torus not only forms but acts like a slow motion ‘speed-dating’ event for assessing lots of potential mates in one go.”

The research team hope their findings can inform identification of other basking shark courtship grounds in the UK and further afield in the Mediterranean Sea and Pacific Ocean to ensure appropriate conservation measures are put in place to safeguard this gentle giant’s ‘love dance’.