Stranded sperm whales in 2016 belonged to two different groups

© Sonja von Brethorst

In January and February 2016, 30 young male sperm whales stranded on the coasts of Germany, the Netherlands, the United Kingdom, Denmark and France, of whom 24 could be autopsied. Scientists of the Institute for Terrestrial and Aquatic Wildlife Research (ITAW) of the University of Veterinary Medicine Hannover (TiHo) investigated these stranded animals together with their colleagues from the affected neighbouring countries for pollutants. They found that the animals belonged to two different social groups. The results of the study were published by the scientists in the scientific journal "Scientific Reports".

It is known that different mixtures and concentrations of chemical pollutants can accumulate in the tissues of individual marine mammals depending on their geographical habitat and food, resulting in different contamination profiles. On the basis of these contamination profiles, the researchers drew conclusions about the social structures in which the young male sperm whale (Physeter macrocephalus) lived. To do so, they examined the tissue samples from the 24 animals stranded in the North Sea during the autopsies and analysed organic compounds and trace element concentrations in muscles, liver, kidneys and fat.

Dr. Joseph Schnitzler was able to show with other scientists of the ITAW that the sperm whales, which stranded in January on Texel in the Netherlands, on Heligoland and in front of Büsum, came from areas that were more heavily polluted with organic contaminants. "Probably these animals came from southern regions and belonged to the same group," explains Schnitzler. "The fact that we can also detect higher concentrations of arsenic in the samples of these animals supports our assumption. Arsenic is found above all in geothermal active regions such as the Azores and volcanic hotspots, such as the Canary Islands and Cape Verde. "Among the group of eight bulls stranded off Dithmarschen end January 2016 and two other animals in early February however, researchers found lower concentrations of organic contaminants and arsenic but higher concentrations of zinc and barium. Similar to nutrients, dissolved zinc only occurs in very low concentrations in surface waters in the oceans, but concentrations below 1000 meters are high. Adding to this the fact that barium is an indicator of Arctic water masses, observations show that these animals originated from the lower North Atlantic feeding areas around the Norwegian shelf edge. Genetic analyses also indicate an origin from the Canary Islands and the northern part of the Atlantic. They also made it possible to uncover relationships. "The combination of toxicological and genetic data suggests that two groups were found among the stranded sperm whales: one group sojourned around the Canary Island and one around the northern part of the Atlantic," explains Schnitzler.

Sperm whales show the strongest social behaviour within the group of large whales. Except during the mating season, adult male and female sperm whales live separately in the oceans. The group structures, group sizes and home regions of the female groups have already been studied relatively well. Adult females live with young of both sexes in stable groups of groups. These groups occur mainly in subtropical waters of low latitudes. The young males leave these groups at the age of ten and then gradually move to higher latitudes with colder surface waters. Studies show that these young sperm whales are grouped into male bachelor groups, although little is known about their group structure. Apart from these bachelor groups, male sperm whales are usually later spotted singly or occasionally in groups of two with adult males. Only in their late twenties did the sexually mature males eventually return to lower latitudes in the areas where the females live to mate.