(ORDO NEWS) — Zoologists have found that the number of Mediterranean monk seals, which are considered one of the rarest pinnipeds, declined sharply in the era of antiquity and the early Middle Ages.
Apparently, the ancient Romans, and perhaps some other peoples, hunted seals so extensively for their fat, skin, and meat that they undermined most populations.
For comparison, other species of pinnipeds experienced a similar decline in numbers only in the 19th-20th centuries, during the period of commercial fishing.
Mediterranean monk seals (Monachus monachus) are among the rarest pinnipeds in the world. Once this species was widespread in the Mediterranean and Black Seas, as well as on the Atlantic coast of southern Europe and North Africa.
However, due to hunting, competition with fishermen, pollution of the seas, and human development of the coasts, monk seals have disappeared from most of the places where their rookeries were once located.
Only three populations have survived to this day: in the Eastern Mediterranean (187-240 adults), on the Cabo Blanco peninsula in Mauritania and Western Sahara (about 350 individuals) and on the island of Madeira (about 20 individuals).
A team of zoologists led by Jordi Salmona of Paul Sabatier University decided to learn more about how the monk seals ended up in such a plight.
To do this, the researchers analyzed 383 samples of tissues and droppings of this species: 314 modern (they were collected in 1989-2020) and 69 historical (collected in 1833-1975). The authors chose the 1970s as the boundary between modern and historical specimens, when monk seals disappeared from the western and central Mediterranean.
DNA microsatellite analysis has shown that Mediterranean monk seals can be divided into four populations: an Eastern Mediterranean population, an extinct Western Mediterranean population, and two Atlantic populations.
They form an east-west wedge, the existence of which is explained by the fact that in most cases monk seals settle within 500 kilometers of their native colony (although sometimes they can travel 1500 kilometers).
Individuals from the western Mediterranean Sea occupied an intermediate position between the Atlantic and Eastern Mediterranean counterparts.
The now extinct Black Sea population, represented in the sample by one specimen, also belonged to the Mediterranean group. And within the Atlantic group, the researchers found a clear division between populations from Cabo Blanco and Madeira.
The genetic diversity of nuclear DNA in all populations of Mediterranean monk seals – both modern and extinct – was the lowest among all pinnipeds (heterozygosity was 0.04-0.36 percent). The minimum level of all diversity indicators and one of the maximum levels of inbreeding were noted in individuals from Madeira.
Their genetic diversity is even lower than that of the Saimaa seals (Pusa hispida saimensis), which have lived in isolation in Finnish Lake Saimaa since the end of the Ice Age.
The extinct population from the western Mediterranean Sea had the greatest diversity of alleles, while the seals from Cabo Blanco and the eastern Mediterranean Sea had the most unique alleles. Mitochondrial DNA diversity was also found to be low in all populations, especially in the Madeira population.
Comparing historical samples with modern ones, the authors found more diverse alleles in the former and more alleles unique to individual populations, indicating a decrease in genetic diversity over time in 1840-2020.
However, judging from heterozygosity rates and patterns of increasing inbreeding rates, this decline was not drastic. For comparison, in many other species of pinnipeds in the 19th-20th centuries, as a result of a decline in numbers due to active fishing, the genetic diversity decreased sharply and quickly.
The results obtained demonstrate that the genetic diversity of monk seals was at low levels as early as the middle of the century before last, which may be related to the hunting of this species in earlier eras.
In monk seals from the two largest modern populations, from Cabo Blanco and from the Eastern Mediterranean, the authors found no evidence of loss of genetic diversity between 1990 and 2020.
The authors paid special attention to the populations from Cabo Blanco, since in 1997 more than two-thirds of the individuals died here for various reasons. Surprisingly, this sharp drop in numbers did not affect most indicators of genetic diversity (with the exception of the number of alleles unique to the population).
At the final stage, the researchers reconstructed the demographic history of the Mediterranean monk seals. It turned out that all the populations for which genetic data were available experienced one or two sharp, at least one order of magnitude, declines in numbers.
They occurred in the period from 800 BC to 600 AD. that is, in antiquity or in the early Middle Ages. At the same time, the maximum of the last glaciation, due to which the level of the Mediterranean Sea dropped 120-130 meters below the current level, did not affect the number of monk seals.
The decline in all populations of monk seals in ancient and medieval times coincides with the period when people actively colonized the coasts and islands of the Mediterranean Sea.
Ancient Greek and Roman sources repeatedly report hunting for these pinnipeds for meat, fat and skin, as well as in conflicts with fishermen (there is even a mention of hunting monk seals in Homer’s Odyssey).
Apparently, the number of monk seals has declined sharply precisely because of the active fishing, which was primarily carried out by the inhabitants of the Roman Empire.
The authors emphasize that people hunted these pinnipeds before (and continued to do so until recently), but in the ancient era, the fishery reached its peak.
In historical sources, one can also find evidence that monk seals were common, very trusting and formed extensive colonies on open coasts and headlands.
In comparison, modern members of this species are secretive and breed in caves and on remote islets, according to the researchers. before the start of fishing in ancient times, the number of monk seals was 100,000-356,000 individuals.
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