Monk seals differ from other seals in that the pups have black rather than white fur, and the adults have unmarked brown or grey dorsal colouration. Their common name is based on the supposed similarity between the uniformly coloured coat of the seals and the cassocks worn by monks.
Adult Caribbean monk seals reached a length of 200-240 cm. Their ecology is believed to have been similar to that of other monk seals, but very little specific information was recorded about the species before it became extinct. Other monk seal species give birth to a single calf every 1-2 years on a sandy beach; the peak birth period of M. tropicalis appears to have been in November-December, with a relatively long pupping season. Recently published field notes from 1900 indicate that the species may have occurred in large groups (up to 100) when abundant, probably ate fish and crustaceans, and was preyed upon by sharks.
The species was hunted extensively as an easily exploited source of oil in the tropical Atlantic, with large-scale European exploitation beginning during the second voyage of Columbus in 1494, when his sailors killed ‘eight sea wolves which were sleeping on the sands’ on a small island near Hispaniola. This persecution continued relentlessly. Remnant populations of monk seals were also subject to additional persecution by fishermen for allegedly depleting fish catches, and extensive collecting of specimens for museums and zoos further contributed to its demise. This is illustrated by E. W. Nelson's account of a trip to the Arrecifés Triangulos in June 1900, which describes the overharvesting of one of the last known monk seal populations by both fishermen and museum staff:
'We found these seals much less numerous than they were reported to be by men at Campeche who have visited the Triangles to kill them for oil during the past few years. The man from whom we hired our schooner has made two sealing expeditions to the Triangles and under his directions hundreds of the seals have been killed with clubs. The blubber was removed and tried out [i.e. melted] on the spot and taken back to the mainland in 5 gal. oil tins and sold to the R. R. Co. for $3 per tin for lubricating purposes. In this way the great majority of the existing seals of this species have been destroyed within the last ten years. At the time of our visit we landed on two of the 3 islets making up the group called the Triangles and the total number of seals observed during our week's stay did not exceed 75 of which we obtained a good series [i.e. of museum specimens] about one half the number. Should the sealers again visit the islands it is possible that all of the survivors will be killed.' Indeed, in January 1911, fishermen returned to the Arrecifés Triangulos and killed about 200 more seals.
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