‘Broken jaw’ mystery: Study to boost conservation effort for humpback dolphin

Marine biologist Shanan Atkins, who heads the Richards Bay Endangered Humpback Dolphin Project, has shared information on a study on how the rostra (jaws) of the endangered Indian Ocean humpback dolphin can become damaged and broken.

Habitat degradation, pollution, predators, ship propellers, shark nets, climate change, disease and many other issues make theirs a life that is constantly threatened.

Humpback dolphins found with injured jaws

At the same time, some of these dolphins appear to have an extreme ability to cope with the disability of a disfigured or seriously injured rostrum.

Working with colleagues of the SouSA Consortium – a group studying Indian Ocean humpback dolphins (Sousa plumbea) along the South African coastline – they have over a period of years photographed and documented such injuries.

“The characteristic surfacing pattern of the humpback dolphins gives plenty of opportunity to observe their rostra,” Atkins was quoted as saying in the Zululand Observer.

“Typically, the rostrum first breaks the surface, followed by the head, a quick breath then the arching of the back – revealing the dorsal fin – and it disappears under the water, leaving only two widening circular ripples as the only sign it had been there.”

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Researchers collaborate in coastal study

Researchers were perplexed when they noticed abnormalities on the snout of a dolphin, and on investigation found that colleagues in other study areas along the coast had documented similar sightings.

They decided to work together to describe the phenomenon, figure out its extent and examine individual survival information.

“Ten researchers from seven institutions collaborated – collecting and sharing dolphin photographs taken in seven areas: False Bay, Gansbaai, Struisbaai, St Sebastian Bay, Mossel Bay, Plettenberg Bay and Richards Bay,” said Atkins.

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Abnormalities in jaws pose ‘serious implications’ to dolphins

“They found that 11% of the individuals surveyed had evidence of abnormal rostra.

“In total, 19 of the 31 individuals had clearly broken jaw bones, nine had aberrant shapes (for example twisted or maligned jaws), two had minor injuries and one had some fishing line causing a problem.

“Some of these abnormalities would likely have had serious implications for the dolphins, which need their jaws/rostra intact to catch the fish they eat,” Atkins explained.

“In addition, the jaws are important for echolocation – for sound transmission and echo reception. Therefore, damaged rostra potentially make it much harder for a dolphin to perceive the environment, and to find and catch fish.

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Surprise findings and the case of White Tip

“We were surprised to find that many of the dolphins documented in the study were surviving despite these sometimes severe abnormalities,” Atkins said.

“Four individuals were observed with their injuries over a ten-year period and another over seven years, with most detail coming from Richards Bay.

“From the start of our project in 1998 we saw White Tip regularly, with her calf Junior. By April 1999, Junior was independent and White Tip had a new baby, Spike.

“In October 1999, we noticed there was something wrong with White Tip’s rostrum, but it wasn’t until February 2000 that we could confirm photographically that her upper jaw was badly broken.

“We continued to see her and Spike frequently until July 2001 when they disappeared for nearly a year. Imagine how pleased we were to see her in June 2002 – with a new calf!

“Sadly, we didn’t see them again until November 2002 when they were both fatally entangled in the shark nets.

“This female not only survived her disability but even reproduced – successfully raised her calf and gave birth to another three, years after the injury,” said Atkins.

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Possible causes of broken jaw phenomenon in humpback dolphins

The cause of these abnormalities in humpback dolphins is unclear, but potential reasons included the long, thin shape of the jaws (skull morphology), hunting strategies, defence mechanisms, aggression from predators and other dolphins, and exposure to contaminants.

“Humpback dolphins may be more susceptible to injuries owing to their longer rostra, and their hunting behaviour could result in collisions or getting stuck: they have been observed poised vertically, head downwards above reefs, investigating crevices and crannies with their long rostra, snapping with a sideways motion of the head at any rock-dwelling fish that emerged,” Atkins explained.

“They could be defending themselves, for example against sharks or other dolphins. It is possible that a particularly high chemical pollution load could also affect their bone density and make them more prone to abnormalities.

“These abnormalities could be indicators of poor underlying health of the population.

Effort to develop conservation strategies

“Researchers are now collaborating with the International Union for Conservation of Nature’s (IUCN) Conservation Planning Specialist Group to conduct a threat analysis and engage stakeholders to develop conservation strategies for the endangered Indian Ocean humpback dolphin at this the southern edge of its range,” Atkins concluded.

Edited by Cornelia le Roux.

This article originally appeared in Zululand Observer and was republished with permission. Read the original article here.