On July 25th Darwin described the distinct layers of freshwater and saltwater in the Río de la Plata estuary (recall that freshwater is less dense than saltwater):
“To day the water from its calmness & reddish muddy colour looked like that of a river: of course however, the Southern bank is far beyond the reach of vision.— The fresher discoloured water from its less specific gravity floats on the surface of the salt.— this was curiously shown by the wake of the vessel, where a line of blue might be seen mingling in little eddies with the adjoining fluid.— in this case instead of stirring up the mud, it was the reverse & stirred up the clear water.” (July 25)
He also makes a passing references to one of the largest South American sea lion breeding grounds on the east side of the continent – Isla de Lobos:
“A fine breeze has carried us to an anchor within six or seven miles of Monte Video.— At about noon we passed between Maldonado & the little island of Lobos covered with seals.” (July 25)
Location map of Isla de Lobos (from Wikipedia Commons)
Isla de Lobos is a small island located about 5 miles offshore of Maldonado near the northern shore of the Río de la Plata. Assuming the island is a continuation of the geology to the north, I would assume that it is made up of old (Precambrian) granite and/or gneiss – similar to the rocks around Rio. The small rocky island is now a nature reserve.
The nature reserve was, in large part, created to protect the population of “seals” that Darwin saw “covering” the island. The “seals” were almost certainly South American sea lions (Otaria flavescens), similar to those that inhabit the island today. So a short lesson on seals and sea lions seemed in order today.
Seals and sea lions belong to the subgroup (or superfamily depending on who you talk to) called Pinnipedia (the “fin foots”). Pinnipeds include the “true” seals (Family Phocidae – 20 species), the fur seals and sea lions (Family Otariidae – 16 species) and the walruses (Family Odobenidae – 1 species). Many of the species are regional – for example, there are 7 species of sea lions each found in a different part of the world (e.g., California sea lion, Japanese sea lion, etc.). The “fur seals” are more closely related to the sea lions, hence they are in the same family.
Male South American sea lion, with a harem of several females (from Wikipedia Commons)
There are several differences between “true” seals and sea lions. The most obvious of these is ears – sea lions and fur seals have ears, the “true” seals do not – hence the two families are also sometimes called the “eared seals” and the “earless seals”. Sea lions also tend to be able to better support themselves on their front flippers and have back flippers that are more “feet-like” than “true seals”. This is why they are also sometimes referred to as “walking seals”. In any case, this difference in body shape (see image below from Wikipedia Commons) is largely indicative of how well adapted they are to life in water. The “true seals” are “further along” in their adaptations to life in the water. Sea lions have still retained the ability to “walk” (to some degree at least).
Now to my favorite part – the fossil record. The pinnipeds are all part of the Carnivora order of mammals, which makes them closely related to dogs, cats, bears, raccoons, pandas, skunks, weasels, etc. Within this order, the pinnepeds are most closely related to the bears. Pretty cool, eh? Basically there was a split (speciation) among the early ancestors of bears and seals. Some of these ancestors spent enough time in the water that the species began to adapt to aqueous conditions – these became the pinnepeds. Others became more adapted to life on land and evolved into the bears (and other close relatives, such as raccoons). Even Darwin recognized this concept, writing later in his life in Origin of Species:
“A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly aquatic as to brace the open ocean ” (On the Origin of Species)
And to take one more step to tie everything together – the earliest known common ancestor to the pinnipeds was named in Darwin’s honor – Puijila darwini. This early ancestor, just discovered in Canada in 2007, looked more like a weasel than a bear or seal (see the reconstruction below from a 2009 article in Nature by Rybczynski, Dawson, and Tedford).
This fossil suggests that the pinnipeds split off from their land-based relations about 23 million years ago and that they originated in the Arctic – later spreading all around the world. What would Darwin have said if someone told him this as he sailed by the barking “seals” of Isla de Lobos in 1832? One can only imagine.
By the way, Darwin would have passed by the island just as the South American sea lions were beginning the mating season. So it is likely that what he saw were the males arriving to set up territory. South American sea lions exhibit sexual dimorphism – the males are about twice as large as the females. So he would have seen the “big boys” – up to almost 3 meters (~9 ft) long weighing up to around 350 kg (~750 lbs). I’m surprised he did not write more!
Sea lions on Isla de Lobos (from Wikipedia Commons):
Later in the day the Beagle passed by Maldonado (where they would later return). Darwin’s impression of the landscape was underwhelming:
“At some future time we shall lay in the harbor at Maldonado.— the country in the neighbourhood is more uneven than in the other parts of the coast, but from the sandy hillocks has a dreary uninteresting appearance.” (July 25)
By tomorrow they would be in Montevideo. (RJV)