Posted by: Rob Viens | September 1, 2013

Paleobotany and Sloth Food

The end of August found Darwin refreshing supplies and supervising the latest fossil discoveries.  His diary entries are brief as the month came to a close:

“We the next morning set out for Fort; but did not arrive there till 9 oclock at night.” (Aug 30)

“My guide or Vacciano not having come, I rode to Punta Alta, in order to superintend the excavation of the bones. — It is a quiet retired spot & the weather beautiful; the very quietness is almost sublime, even in the midst of mud banks & gulls, sand hillocks & solitary Vultures.” (Aug 31)

Sloth Week logo

As we come to the last couple of days of “Sloth Week”, I thought I’d share Darwin’s thoughts tonight regarding the ecosystem that the giant ground sloths and glyptodonts called home.

Darwin was a leader in the field of paleoecology – the study of how plants and animals of the past interacted with one another and formed a functioning ecosystem. In the case of South America, Darwin gave much thought to the vegetation of Pleistocene environment in which the sloths and glyptodonts lived. He wrote quite a bit about it in Voyage of the Beagle – clearly spending a lot of time pondering the possibilities.  Here are a few excerpts of Darwin’s thoughts on the subject.

On sloth feeding habits:

“The great size of the bones of the Megatheroid animals, including the Megatherium, Megalonyx, Scelidotherium, and Mylodon, is truly wonderful. The habits of life of these animals were a complete puzzle to naturalists, until Professor Owen† lately solved the problem with remarkable ingenuity. The teeth indicate, by their simple structure, that these Megatheroid animals lived on vegetable food, and probably on the leaves and small twigs of trees; their ponderous forms and great strong curved claws seem so little adapted for locomotion, that some eminent naturalists have actually believed, that, like the sloths, to which they are intimately related, they subsisted by climbing back downwards on trees, and feeding on the leaves. It was a bold, not to say preposterous, idea to conceive even antediluvian trees, with branches strong enough to bear animals as large as elephants. Professor Owen, with far more probability, believes that, instead of climbing on the trees, they pulled the branches down to them, and tore up the smaller ones by the roots, and so fed on the leaves. The colossal breadth and weight of their hinder quarters, which can hardly be imagined without having been seen, become, on this view, of obvious service, instead of being an incumbrance: their apparent clumsiness disappears. With their great tails and their huge heels firmly fixed like a tripod on the ground, they could freely exert the full force of their most powerful arms and great claws. Strongly rooted, indeed, must that tree have been, which could have resisted such force!”

Harlan’s Ground Sloth, Natural History Museum, San Diego
Harlan's Ground Sloth San Diego

Darwin goes on to raise the quintessential paleoecology question – in effect, what do the animal bones tell us about the entire ecosystem:

“What, it may naturally be asked, was the character of the vegetation at that period; was the country as wretchedly sterile as it now is?”

He then goes on to answer the question, making a strong argument that just because their were big browsers around, it did not mean that this region once contained lush vegetation.  He notes:

“Nevertheless, from the following considerations, I do not believe that the simple fact of many gigantic quadrupeds having lived on the plains round Bahia Blanca, is any sure guide that they formerly were clothed with a luxuriant vegetation: I have no doubt that the sterile country a little southward, near the Rio Negro, with its scattered thorny trees, would support many and large quadrupeds.”

Darwin’s main argument is based on the great herds of megabeasts that can be found in the grasslands of South Africa.  Here are a few of his thoughts on Africa:

“That large animals require a luxuriant vegetation, has been a general assumption which has passed from one work to another; but I do not hesitate to say that it is completely false, and that it has vitiated the reasoning of geologists on some points of great interest in the ancient history of the world. The prejudice has probably been derived from India, and the Indian islands, where troops of elephants, noble forests, and impenetrable jungles, are associated together in every one’s mind. If, however, we refer to any work of travels through the southern parts of Africa, we shall find allusions in almost every page either to the desert character of the country, or to the numbers of large animals inhabiting it. The same thing is rendered evident by the many engravings which have been published of various parts of the interior.”

“Dr. Andrew Smith, who, at the head of his adventurous party, has lately succeeded in passing the Tropic of Capricorn, informs me that, taking into consideration the whole of the southern part of Africa, there can be no doubt of its being a sterile country. … Now, if we look to the animals inhabiting these wide plains, we shall find their numbers extraordinarily great, and their bulk immense. …  He informs me, that in lat. 24°, in one day’s march with the bullock-waggons, he saw, without wandering to any great distance on either side, between one hundred and one hundred and fifty rhinoceroses, which belonged to three species: the same day he saw several herds of giraffes, amounting together to nearly a hundred; and that, although no elephant was observed, yet they are found in this district. At the distance of a little more than one hour’s march from their place of encampment on the previous night, his party actually killed at one spot eight hippopotamuses, and saw many more. In this same river there were likewise crocodiles. Of course it was a case quite extraordinary, to see so many great animals crowded together, but it evidently proves that they must exist in great numbers. Dr. Smith describes the country passed through that day, as “being thinly covered with grass, and bushes about four feet high, and still more thinly with mimosa-trees.” The waggons were not prevented travelling in a nearly straight line.”

Black Rhino (from National Geographic Society:

rhino

Based on South Africa, Darwin speculates on how numerous large animals can survive on such sparse vegetation:

“The larger quadrupeds no doubt roam over wide tracts in search of it; and their food chiefly consists of underwood, which probably contains much nutriment in a small bulk. Dr. Smith also informs me that the vegetation has a rapid growth; no sooner is a part consumed, than its place is supplied by a fresh stock. There can be no doubt, however, that our ideas respecting the apparent amount of food necessary for the support of large quadrupeds are much exaggerated”

He even makes a comparison with the large mammoth remains from the northern latitudes that lived on the tundra, where mere grasslands could support herds of “elephants”:

“These remarks, I may be permitted to add, directly bear on the case of the Siberian animals preserved in ice. The firm conviction of the necessity of a vegetation possessing a character of tropical luxuriance, to support such large animals, and the impossibility of reconciling this with the proximity of perpetual congelation, was one chief cause of the several theories of sudden revolutions of climate, and of overwhelming catastrophes, which were invented to account for their entombment. I am far from supposing that the climate has not changed since the period when those animals lived, which now lie buried in the ice. At present I only wish to show, that as far as quantity of food alone is concerned, the ancient rhinoceroses might have roamed over the steppes of central Siberia (the northern parts probably being under water) even in their present condition, as well as the living rhinoceroses and elephants over the Karros of Southern Africa.”

Woolly Mammoth – a northern contemporary of the ground sloth (from Wikipedia Commons):
Woolly mammoth

There are a lot more tools available to paleoecologists today, but in the end (from what I can tell), Darwin’s basic premise is correct. To read the full account of Darwin’s thoughts on the subject, see pages 81-89 in Voyage of the Beagle (the source of all of the above quotes).  (RJV)

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