Sunday, 16 July 2017

Pinnipeds: Decline of the Monk Seals

Mediterranean monk seal
(museum specimen)
The great majority of seal species are found in cold, often freezing waters. Even harbour and elephant seals, which are frequently found in temperate, or even subtropical, climes, also inhabit colder latitudes for at least part of the year. There are just three modern species of true seal that are found solely in warm waters, and they all happen to be closely related. These are the monk seals.

It's not actually known why these animals are called "monk" seals. The oldest known reference to the term comes from Johann Hermann in 1779, when he wrote the first scientific description of the Mediterranean monk seal (Monachus monachus), and gave it its scientific name. The only reason he gave for doing so was that he'd heard that the animal was called that in France, and thought that maybe that was because, seen from behind, the head and shoulders looked a bit like a robed and hooded man. But he was guessing about that latter part, and there doesn't seem to be any independent corroboration that the animal really was called a "monk seal" in France (or, indeed, anywhere else) prior to his naming of it. Presumably, he'd got the name from somewhere, but, for all we know, he might have misremembered the details.

At any rate, as you might imagine, Mediterranean monk seals have been known to Europeans for a very long time indeed, and they appear, for example, in Greek mythology. Back in those days, they seem to have lived throughout the entirety of the Mediterranean and Black Seas, and out, past the Pillars of Hercules, into neighbouring regions of the Atlantic.  This is, however, very far from being the case today, and the remaining Mediterranean populations are restricted to the Aegean, the west coast of Greece, Cyprus, and the western and southern coasts of Turkey. A couple of isolated populations do survive in the Atlantic, though, around the Portuguese islands of Madeira and off the small peninsula of Cabo Blanco, which marks the northern border of Mauritania in Africa.

Mediterranean monk seals are big; with males up to 280 cm (9' 2") long and 300 kg (660 lbs) in weight, and the females only marginally smaller than that, they're slightly bigger than hooded seals, the largest seals of the North Atlantic proper. Adult females are dark grey, and the males black; both have paler underparts and a distinctive white patch on the belly. In the Aegean, they apparently feed almost entirely on cuttlefish and octopuses, although elsewhere they have been reported to also eat a wide range of bottom-dwelling fish. Studies have shown that they can dive to at least 123 metres (400 feet) in search of food, although this seems to be something of a rarity.

In the past, Mediterranean monk seals hauled out on sandy or pebbly beaches, often at the bottom of cliffs. Today, while this is still largely true along the Saharan coast at Cabo Blanco, most seals seem to have decided that even cliff faces are not sufficient protection from humans. Instead, they give birth in well-hidden caves, ideally ones that can only be reached from the sea. This may have affected their choice of mates, too, since at Cabo Blanco and Gyaros they form social colonies, where males mate with only a small number of females each, while elsewhere a given male typically guards and visits a number of isolated breeding caves.

Breeding takes place throughout the year, perhaps because, with no harsh winters to put up with, food remains plentiful all year round. Since they aren't born on ice, baby monk seals are black, not white, although they do have the same fluffy fur, which they keep for an unusually long time before moulting and entering the water. While that first coat of fur is shed at around two months, it's at least a further two months before they are fully weaned and able to make their own way in the world - longer than any other species of seal. Many, sadly, don't make it this far, with many ending their lives dashed against the rocks of their preferred habitat.

Given that risk, and given how few caves are suitable for resting in, let alone safe enough to raise pups, one might ask why they bother to use that habitat at all. But it would seem that, in the modern world, the alternatives are worse. Both the Romans and medieval Europeans extensively hunted the animals, reducing their population dramatically over the course of the centuries, and wiping them out through most of the Mediterranean, and all of the Black Sea. Even today, deliberate killing by humans to preserve fish stocks remains the number one cause of death among the seals, with accidental capture in fishing nets being responsible for most of the remainder. Loss of habitat, largely due to the growth of the tourist industry in that part of the world, is, unfortunately, not helping matters.

In fact, Mediterranean monk seals are likely the most endangered species of seal alive today. A 1978 study estimated that less than a thousand remained alive in the Mediterranean, and, since then, the number is thought to have declined to no more than 450, mostly in Greece. The Cabo Blanco population is about half this, having suffered a devastating decline in the 1990s, perhaps due to a disease outbreak. The remaining population, at Madeira, probably includes no more than 40 adults.

As one might expect, this has led to significant inbreeding, and the resulting loss of genetic diversity may make recovery more difficult. (It has been pointed out that the two main populations are at least genetically different from one another, so if one could somehow get them to interbreed, it might well help... but it's difficult to see how that might be done in practice).

Fortunately, it's not all doom and gloom. In 2015, the species was officially upgraded from "Critically Endangered" to merely "Endangered", since the population, small though it is, does at least seem to be increasing again. At Cabo Blanco, for instance, it seems that females fared better than males during the population crash - which isn't great for genetic diversity, but at least maximises the number of pups they can have. Even in Madeira, effective conservation measures have resulted in a recent rise in the population, and re-colonisation of the main island from human-free refuges elsewhere in the archipelago. And, in 2011, it was reported that Mediterranean monk seals had once again been sighted in Israel, for the first time since the 1950s.

Hawaiian monk seal
The closest living relative of the Mediterranean species is the similarly endangered Hawaiian monk seal (Neomonachus schauinslandi). This lives solely in the tropical waters of Hawaii and Midway, primarily around the long chain of atolls that stretch away for hundreds of miles to the northwest of the main group. Despite having been known to Europeans for at least a hundred years by that point, they weren't named as a distinct species until 1905. Originally placed in the genus Monachus, along with their Mediterranean kin, in 2014 they were shown to have enough genetic differences to be split off into a new genus, Neomonachus.

Hawaiian monk seals are noticeably smaller than the Mediterranean sort, and are paler in colour, without the white stomach patch, but otherwise look rather similar. Uniquely, at least among living species of seal, they feed among coral reefs, diving to over 300 metres (1,000 feet) in search of prey that consists largely of reef fish such as marine eels, with a substantial side order of squid and octopus.

While the presence of some sort of dry land is essential for seals, and there really isn't anywhere else nearby that they could go, it is clear that the heat ot the tropical sun is something of a problem for Hawaiian seals when they're out of the water. They don't pant, and there's no clear evidence that their sweat glands actually work, so instead, their tactic seems to be to lie upside down - with their paler undersides facing skywards - and move as little as they possibly can. Even then, they will rarely move far from the water's edge during daylight, unless it's particularly overcast or rainy. There is also some evidence that cooler weather brings with it a greater supply of food, so that particularly hot years are damaging to their population.

Perhaps because of this, unlike the Mediterranean seals, Hawaiian monk seals do have a distinct breeding season, with the young being born between March and June. They are weaned much more quickly, too, with their mothers abandoning them at around the six week mark. At this time, testosterone levels rise dramatically in the males, who patrol the beaches in order to fight off rivals and pursue any females attempting to enter the water to feed. The sex itself, which, as is typical for seals, occurs underwater, is often violent, with females sustaining severe injuries, especially when several males try to mate with them at once. In fact, such injuries may be a more common cause of death for female seals than shark attacks, and can seriously weaken those that survive.

This, of course, doesn't sound like the sort of behaviour that's particularly adaptive. And, indeed, it may well not be. While aggressive sexuality has presumably been part and parcel of the lives of Hawaiian monk seals for thousands, if not millions, of years, serious injuries were not such a common feature until recently. That has been because of a decline in the number of females compared with males, making the latter more desperate, and more violent when they do find a suitable female, willing or otherwise.

Females can be aggressive towards one another, too, while nursing their pups. In fact, they don't appear to be able to recognise their own offspring, perhaps because they all sound pretty much the same, and it is common, especially where population densities are high, for a female to end up fostering a pup that isn't theirs.

Large scale hunting of Hawaiian monk seals is a more recent phenomenon than for the Mediterranean kind, dating back only to the nineteenth century. Nonetheless, it has been significant, reducing the population to no more than 1,500 or so by the mid-twentieth century. Hunting has long since ceased, to be replaced by net entanglement, loss of habitat, and pollution as the main threats to the species' survival. Their genetic diversity is even lower than that of Mediterranean monk seals, perhaps because it was never very high to start with. Despite efforts to save them, and minor recoveries in some specific localities, overall, their population is continuing to decline, and there may be only around 650 or so adults alive today.

Caribbean monk seal
Could they really disappear forever? In my opinion, it's likely that conservation has come in sufficient time to save them for the foreseeable future. But the same cannot be said for the remaining tropical species, the Caribbean monk seal (Neomonachus tropicalis). This was discovered in 1494, by none other than Christopher Columbus (or, at least, his crew). At that time, they were widespread, breeding and resting on beaches throughout the Caribbean islands and along coasts from Yucatan to Guyana. One is even recorded as having made it as far north as Georgia, although that would be well outside their regular range.

Europeans began hunting them in large numbers as soon as there were enough Europeans around to make that an option. Aside from the fact that this was, by all accounts, relatively easy to do, there were many reasons for doing so. Skins and blubber top the list, however, with the latter being rendered into oil to lubricate plantation machinery. Just as with the Mediterranean and Hawaiian monk seals, the population of the Caribbean species went into a sharp decline.

By the dawn of the twentieth century, they were already rare, and the numbers only continued to drop from then on. Finally, they became a protected species in 1945, but, to the extent that anyone took any notice of those laws, they only applied in Jamaica anyway. And, besides, by that point, it was likely already far, far too late.

A small group of Caribbean monk seals were spotted at Seranilla Bank in 1952. Which is about as remote a part of the Caribbean as one could wish for. But, despite numerous further attempts to find them over the years, not one has ever been seen since.  They have become the only species of seal to go extinct in modern times, and one can but hope that that will remain the case.

Despite living in the Northern Hemisphere, monk seals are in fact, more closely related to the seals found south of the equator. They are not, however, the only species to have made this crossing from south to north, and next time we'll be looking at the solitary other example...

[Photos by G.dallorto and the US Fish and Wildlife Service, illustration by George Brown Goode, all from Wikimedia Commons.]

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