Log Transcript

Early this afternoon, a series of splashes off the bow alerted us a to a school of dolphins. After listening to the patterns of clicks and whistles through the acoustic array, we knew that these were Risso's dolphins. The animals appeared to be milling in a tight group, between abrupt bursts of speed just beneath the surface. We assumed they were probably chasing a school of fish and decided to take a closer look.

Risso's are unmistakable dolphins - they are large and robust with a blunt, squarish head that lacks the characteristic beak of most species. A vertical crease running from the blowhole and down the front of the melon is visible from the front. A long mouth line that turns up toward the eye gives the dolphin a conspicuous 'smile'. These are a distinctive species and a favorite of the crew.

Young Risso's are uniformly brown to almost black. By adulthood they are silvery-grey except for the dorsal, flippers and flukes, which remain almost entirely dark throughout their lives. The white face, extensive scarring and crisscross scratches that are indicative of this species, gives them a distinctly battered appearance.

Risso's are a pelagic species usually encountered in the deep waters of the tropical and temperate oceans. However, they can be found in coastal waters where the edge of the continental shelf is relatively close to land, as it is in Sri Lanka. This is also the reason why the normally oceanic sperm whale can be found in high concentrations relatively close to shore.

Extensive white scatches give this species a distinct battered appearance. It seems possible that the heavy scarring is a direct result of aggressive group mating behavior.

Photo: Chris Johnson

As we moved closer to the group, we noticed them swimming rapidly, even franticly as they chased a single animal in tight circles, their blunt heads burrowing down and pushing forward white capped bow waves. The dolphins clashed violently as bodies were jostled and pushed beneath the surface, several piled on top of one another in a writhing mass of heads, flippers and tail flukes. The otherwise still sea was whipped into a turbulent, frenzied mass of white water. We watched in amazement as the dolphins leapt clear of the water, half breached - slapped their heads against the surface, tail slapped and fluked. We drifted closer in the hopes of better observing this unusual flurry of raucous activity and to make acoustic recordings at close range.

The unusually loud clicks and whistles detected on the acoustic array were heard from Odyssey's bow. Bob cut the engine and Peter deployed our wideband hydrophone system that can record ultrasound frequencies in an effort to capture the unique sounds of the Risso's - recordings of this species in the wild are rare. The dolphins hastily changed course and headed directly toward Odyssey at high speed, which we found surprising as boats do not normally hold a strong attraction for Risso's dolphins, which usually maintain a comfortable distance. However, these Risso's seemed to be so pre-occupied, they barely noticed our close proximity. The group consisted of roughly 20 animals, 8-10 of which were intent on the pursuit of a single dolphin, presumably a female that attempted to remain ahead of her pursuers.

They continued rolling and leaping around the lead animal as it swam back and forth beneath Odyssey's bowsprit in what appeared to be an effort to escape. Several animals spy-hopped (lifted their heads vertically above the surface), affording us a rare close up of their scratched rounded melons, large eyes and open mouths, others swam on their backs, their milky-white bellies and flippers exposed and facing the sky. We were certain that the purpose of this behavior was mating. This was perhaps our best opportunity of the voyage so far to observe rarely seen dolphin behavior at exceptionally close range.

For over forty-five minutes the Risso's surrounded Odyssey, swimming furiously only feet from the hull, the calm sea conditions giving the crew a perfect view and the opportunity to capture the entire sequence on film.. The scientists and crew has often commented on the severe scratching and scarring that is characteristic of this species while speculating about the cause.

A Risso's dolphin rolls on its side to take a look at the Odyssey.

Photo: Chris Johnson

Risso's dolphins primarily feed on squid. Most other odontocetes (toothed whales) that specialize in squid usually have teeth in the lower jaw. This is evident in sperm whales, and several beaked whale (Ziphiids) species, which are usually characterized by the absence of all but one or two pairs of mandibular teeth. It has long been recognized that male odontocetes of certain species use these lower teeth to rip and tear at each other's bodies in fights for access to receptive females.

Risso's have an average of only four pairs of teeth in the lower jaw and none in the upper. It has long been suggested that much of the scarring on the bodies of adult Risso's dolphins result from confrontations with squid and not from confrontations between other members of their own species. Such conjecture is, at first glance, supported by the normal behaviour of Risso's dolphins at sea, where apparently there is little close interactions between animals while travelling or foraging. However, it seems now more probable that the majority of scarring on the body is a direct result of the dolphins inflicting and receiving heavy scarring during these aggressive group mating rituals such as we witnessed today. This contention is also supported by the fact that much of the parallel scarring on the animals evidently is inflicted by teeth and not by squid beaks.

The modification of teeth as a secondary sexual characteristic in several male squid eating odontoctes results in heavy scarring that may serve to advertise an individuals strength, longevity and viability as a potential mate. Because of the pronounced sexual dimorphism in species such as sperm whales and beaked whales and the fact that it is usually only the males that have teeth, we can often determine their sex in the wild and therefore better understand their behavior. However, as dolphins of both sexes have teeth and are of a similar size - there is no evidence of size difference in Risso's dolphins, we are unable to determine at this point if adult male Risso's dolphins are more heavily scarred than females.

It is extremely exciting for the Odyssey scientists and crew to have the opportunity to witness this poorly understood behavior. We look forward to analysing photographs and video footage taken in an effort to promote our understanding of the body scarring, mating behavior and social structure of Risso's dolphins.

This is Genevieve Johnson speaking to you from the Odyssey in Sri Lanka.

A Risso's dolphin exposes its milky-white belly while being pursued by the group.

Photo: Chris Johnson

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