The dark dolphins – when climate change surfaces in the ocean

It was a fresh spring morning and our boats were already anticipating the arrival of a calm school of Short-beaked common dolphins (Delphinus delphis) that were soon spotted gently gliding towards us. These magnificently coloured cetaceans can instantly be identified at sea through the characteristic yellow hourglass pattern decorating their flanks. I suddenly spotted an individual, however, that looked a little different. They were darker in colour and lacked the hourglass pattern so we wondered whether it was another species, maybe even a hybrid? The animal however had the elegant physique of a common dolphin and was perfectly integrated into the pod. Unsure of my judgement, I did the natural thing us whale-watchers do when we are in doubt of our assertions; send the photos to scientists. The scientists confirmed a theory I had when I spotted the animal; it had a pigmentation anomaly known as melanism. Here the dark pigment melanin, that is responsible for protecting the skin from the suns harmful UV rays, is overproduced as a result of a mutation and causes the animal to look a lot darker.

After having a confirmed sighting of this anomaly, I kept a sharp lookout for another melanistic individual and soon began seeing plenty of these animals; some even with melanistic calves. Melanistic calves bring this anomaly to a whole new level; they prove that it is dominant in the hereditary process, a phenomenon known as adaptive melanism. This suggests that the animals must gain some sort of advantages and these are usually linked to foraging or mating. Another theory is that they may be linked to climate change and the expanding hole in our ozone layer. As the waters of our oceans are gradually heating up it makes sense for surface active cetaceans like common dolphins to be more resistant to the harmful UV rays of the scorching sun.

Pigmentation anomalies and hybrids are expected observations in the wake of climate change. Sudden changes in climate are known to cause changes in distribution patterns and provoke range shifts in several species, causing the overlapping of ecological niches. Of course such effects usually remain more diluted in a vast habitat lacking in geographic barriers like the ocean but hybrids aren’t a rare sight with cetaceans anymore. Extreme weather conditions can also cause disorientation in large groups of cetaceans prompting them to approach sheltered bays where they may be surprised by low tides, resulting in mass strandings. Temperature shifts also change current dynamics affecting seasonal prey abundance in designated feeding areas for cetaceans, especially those of baleen whales.

So while the dark common dolphins we saw on that fine day may have experienced a mutation that will make them more resilient in the face of climate change, anticipated changes may also provoke negative mutations that can trigger a detrimental state of the animals and make them more susceptible to infectious and chronic diseases.

In a nutshell there is nothing positive about climate change, all we can do is admire how nature still manages to surprise us beautifully in the face of this serious issue.

By Paula Thake