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Weighty Extinction

Theory proposes that excessive rainfall altered the vegetation and eliminated large mammals in South America, but preserved them in Africa

Eduardo CesarTake a good look at the two pachyderms on this page, a tapir and an elephant. In spite of their visible difference in size, both are superlative animals in their continents. With a maximum weight of 300 kilograms and length of 2 meters, the tapir is the largest terrestrial mammal in South America. In its natural habitat, its measurements are unmatched by any other. Even at that, its physical make up is timid next to that exhibited by its neighbor on the page. Up to twenty times heavier than its South American colleague, and with at least triple its size, the elephant is the most colossal non-marine animal in Africa – in truth in the world.

On the savanna, the long-necked giraffe is taller, the ferocious lion holds the title of king of the animals, but truly majestic is the elephant. Why is the largest terrestrial mammal of South America so much smaller than its African counterpart? Why is it that here, as on the majority of the planet, the so-called megafauna died out completely, in a manner as yet not well explained, at some moment of recent history, while in Africa some of the lineages, such as those generating the current elephants, giraffes, rhinoceros, found ways of preserving themselves throughout time? Right, but here comes the real question: if fifteen thousand years ago South America had a fauna for diverse mammals and similar in size to those of Africa, in the end why did our megafauna die out and theirs did not?

According to a new theory, drawn up by the researcher Mario de Vivo, from the Zoology Museum of the University of São Paulo (USP), with the collaboration of the biologist and doctorate student Ana Paula Carmignotto, a significant climatic change could have been the key element to explaining the disappearance of the South American megafauna as the reasonable preservation in Africa: the above normal quantity of rain that fell upon the two continents in the middle of the Holocene, the geological era (warmer) that began close to eleven thousand years ago at the end of the Ice Age, and which has extended on up until today.

It rained so excessively that the ancient areas of the savanna-cerrado ( wooded savanna, typical of Brazil) the excellent habitat for medium and large mammals, generally situated intropical of moderate to low humidity turned themselves extremely dense and closed, with lots of trees, and practically became extensions of their neighboring tropical rainforests.

In Africa, the majority of the mammals of large size, generally herbivores that lived in bands, managed to migrate to new zones of open vegetation, with few trees and some pasture. As a consequence of the climatic change, this type of vegetal formation appeared in areas that are today desserts, situated in the northern and southern extremities of the continent.

Here the largest animals, concentrated in the central-north portion of South America, did not find a nearby environment compatible with their style of life. There was no of savanna for them. “Most authors say that the maintaining of examples of the megafauna in Africa was due to a reason that had nothing to do with its disappearance in South America”, says De Vivo. “I disagree with this vision and believe that the two processes were the consequence of the same cause, the excess of rainfall that changed the vegetation in both continents.”

Climate and vegetation
To plan out his theory on the megafauna, which will shortly be published in a paper article in the Journal of Biogeography, the professor and his student from USP carried out detailed multidisciplinary research into the mammals, both extinct and living, of South America and Africa. They also put together data about how the climate and vegetation were, or might have been, in these two continents over the last twenty thousand years.

The major part of the work was done under a thematic project, funded by FAPESP and coordinated by De Vivo, who studies evolution and the conservation of the present current mammals in the east of Brazil. The zoologist is the first to admit that his model is not perfect, nor even capable of responding to all of the questions concerning the megafauna. Even though, he believes that his theory, in spite of its limitations, stands on its feet. “The explanation makes sense when one looks at the past and present of the mammals in Africa and South America.”, he says.

The logic of the point of view defended by the pair from USP is based on a relatively simple sequence, but ingenious, of deductions and conclusions starting from the analysis of a series of pieces of data and articles about the South American and African megafaunas.

De Vivo saw that the largest terrestrial mammals of both continents those extinct in South America and those of large size still present in Africa, such as elephants, rhinoceros and giraffes need large open spaces, with pasture and without a lot of trees, to maintain their lifestyle. “In Africa there still are species of elephants and buffalo that live in the forest, actually areas of clearings in the middle of the closed forest”, the zoologist ponders. “But these animals live in small bands and are a lot smaller than the typical elephants and buffalo of the savanna.” Therefore, if today the large mammals inhabit prairies with sparse trees, then some thousands of years ago this must also have been the natural environment of the megafauna.

Up until this point there is nothing very new. All the fossil records lead to this type of reasoning. The next step was to create a climate-vegetation model that would be reasonably trustworthy and that would indicate where the savannas may have been, or something close to this, in Africa and South America, between the end of the geological era called Pleistocene more or less between twenty and thirteen thousand years ago, at the peak of the last ice age and the middle of the Holocene era, close to five thousand years ago.

The only parameter found by De Vivo were the rates of humidity and rainfall of the two continents, one of the most important factors, beside that of temperature, in characterizing climate and by extension, vegetation for a region over a period of time. With the pre-historic indicators concerning the quantity of rain that fell upon the two blocks of earth separated by the South Atlantic, the researcher constructed two schematically and radically opposite scenarios about how the climatic variations could have promoted radical changes in the types of vegetation. This is the major contribution of the study.

The first scenario is situated around the so-called Last Glacial Maximum, between twenty thousand and thirteen thousand years ago, at the end of the Pleistocene era. Within the Ice Age, when a good part of the globe was covered with glaciers, this is the moment at which, taking the contemporary rates of rainfall as a base, the least amount of humidity was registered in Africa and South America. This was the peak of the dry spell (and of the cold).

The extremely dry environment guaranteed the existence of vast areas of open savanna, with sparse trees and lots of grass and of mosaics of open forest with enclaves of savanna in the major part of the two continents (see maps above). If in some regions the drought was extremely strong, in others it rained considerably in order to maintain lots of vegetation, albeit open vegetation.

Therefore, there was no lack of food and space for maintaining the lifestyle of the megafauna both in Africa and South America. “It’s not possible to be precise as to what was the exact level of humidity during the last Glacial Maximum,” De Vivo comments. “But it must have rained less than 1,500 millimeters in many areas.”

Today, areas with this rainfall rates do not support extremely dense tropical forests and the same thing must have been true in the past. Some authors believe that an even more intense cold and drought in this glacial phase could have been responsible for the death of the megafauna in North America. For the researcher from USP, this may well have been true there in the north but not here in the south. In truth, he thinks exactly the contrary.

“In this phase the life conditions of the large mammals in South America and Africa must have been excellent, since there must have been lots of areas of savanna for these animals”, De Vivo suggests. One cannot forget the fact that, due to their eminently geographical position between the tropics, the two austral continents were less affected by glaciation than, for example, Europe and North America, situated in temperate zones.

The second scenario is located during the Holocene Climatic Optimum, between eight and three thousand years ago. At this moment everything changes in relation to the previously described phase: the climate is humid as never before, perhaps some 30% more than today and the vegetation in South America and Africa suffers radical mutations. According to Professor De Vivo, it is now that the noose around the megafuana closed once and for all, and especially here. The excess of humidity transforms South America, almost from top to bottom, into a continent with vegetation formations so dense and closed that the maintenance of the largest lineages of terrestrial mammals became impractical.

Expelled from their original environment by the advance of the closed forest, the megafauna had to find new savanna lands to guarantee its survival “But in South America, contrary to what occurred in Africa, there are no remaining areas of cerrado-savanna close to the locations where the large mammals live”, the biologist Ana Paula Carmignotto says.

“In this case, the only region with these characteristics was Patagonia, in the south of Argentina and Chile, but this area was very cold and of difficult access.” And the majority of the mammals must not have managed the migration and were lost along the way. Through lack of physical space in which to move and of grasses to eat, fabulous animals died out in the South American lands. We said goodbye to gigantic sloths, glyptodonts (that remind one of giant armadillos), mastodons and saber-toothed tigers. Only animals of medium to small size survived, which would explain the fact as to why the modest tapir is currently the largest mammal on the continent.

Saved by the Sahara
On the other side of the South Atlantic, there was a similar process going on, but the consequences were a lot less tragic. In Central Africa, the abundant rains of the middle Holocene also radically changed the savannas and the open forests into much more closed jungles, improper for the life of the species that made up the megafauna. But, in compensation, the extra humidity of that period made cozier the two then arid and semi-arid areas of the continent, the deserts of the Sahara in the north and the Kalahari in the south.

In practice, always according to the model proposed by De Vivo, during this period of excess rains the extremities of Africa served as a refuge for the mammals of greater size that had been expelled from the central portion of the continent by the advance of the forest over the old savanna. “A series of stone-age paintings of up to eight thousand years in age, show that the Sahara (as an area of savanna) then houses populations of giraffes”, De Vivo comments.

Many years later, when the humidity quit being so excessive and the climate took on aspects similar to those of today, the deserts that had been savanna returned to being deserts and the savannas that had transformed themselves into forest returned to the condition of savannas. Therefore, the lineage survivors of the megafauna and the other mammals of medium size, which had found their oasis in the deserts during the Optimum Holocene Climate, could return to their classic environment, the savannas of Central Africa. According to the Vivo/Carmignotto model, it is for this reason that today there are elephants, rhinoceros, giraffes and hippopotamus in Africa and not in South America.

This is not the first time that a specialist has attributed the disappearance of the South American megafauna to weather changes and not to other reasons such as the arrival of man or of new diseases in the continent. But this is not to say that the ideas of the USP researchers are exactly the same as others who have studied the question. In truth, at least two points in the new theory are distinct from the other hypotheses that pointed to climate as the major villain in this story. Difference number one: the moment at which the final blow was given to the large mammals of South America, the last breath of life of these animals occurred between eight and three thousand years ago, in the middle of the Holocene period after the last great glaciation.

For other authors, the extinction occurred a little before, more than eleven thousand years ago, still in the Pleistocene era, the geological era that preceded the Holocene era and popularly known as the Ice Age. Difference number two: the climatic change that made the megafauna life impossible here in South America was the excess of rains in the middle of the Holocene, the era in which we live today and not its lack at the end of the Pleistocene era, as other researchers have advocated. “Many researchers believe that it was the driest and coldest period (the Pleistocene era) that killed off the megafauna in North America, but we believe that in South America exactly the opposite occurred”, Ana Paula says.

The hypotheses formulated to explain the extinction of the megafauna in the major part of the globe grouped themselves into three major categories, that form a word game in English: overkill (men over hunted the animals to extinction), overill (the culprit was the emergence of new and lethal illnesses) and overchill (the intense dry cold at the end of the glaciation era froze the animals to death).

In Brazil, it is difficult to find people that defend the first two theories. “I have already seen 150,000 relics (bones and artifacts) of the Brazilian Pleistocene era and I only found indications of intentional marks caused by man on one of them”, the paleontologist Castor Cartelle, from the Federal University of Minas Gerais (UFMG) and the Catholic Pontifical University of Minas Gerais (PUC-MG), and one of the best specialists on the megafauna in the country. “Honestly, this story of overkill is idiotic.

Also, I don’t know of a single case in history of a zoonosis that had eliminated a species (of megafauna) from an entire continent. “The retired paleontologist from the Federal University of Acre (Ufac), Alceu Ranzi believes, just like professor Cartelle, that the key element for the disappearance of the South American megafauna was some kind of climatic alteration, during the Pleistocene era or in the transition of that era to that of the Holocene.

“The entrance of man into the Americas (around eleven thousand years ago) was more or less contemporary to the extinction of the large mammals and some researchers state that one thing necessarily led to another”, Ranzi says. “The megafauna could have been the target of the huntsman, but it would not have been this that led to its extinction.

There are no cemeteries of these animals full of human arrows.” Years ago professor Ranzi found the remains of animals of the camel family (guanacos, alpacas, llamas) of eighteen thousand years of age in the Amazon, evidence that they must have lived there, a little before the end of the Pleistocene era, in a type of environment much closer to that of the savannas than that of the current tropical forest. More or less as De Vivo says in his climate-vegetation model for South America and Africa.

Mammals probably emerged close to 220 million years ago in the geological era denominated Upper Triassic, more or less at the same moment of pre-history in which the dinosaurs appeared. Their first examples were very small animals, of a few centimeters, similar to modern rats or wild squirrels. Apparently they ate insects and had nocturnal habits.

Their evolution was slow and during approximately 150 million years they lived under the shadow of the large reptiles. Only after the mysterious disappearance of the dinosaurs, some 65 millionyears ago, at the end of the Cretaceous era, did they move on to assume various forms and sizes. With time, the largest of them transformed themselves into creatures with as many advantages as those of the colossal reptiles that had preceded them, such as giant sloths and camels, mammoths, mastodons and glyptodonts, sometimes some meters in height and weighing tons.

Scientific literature shows that, although they always had contact with particular species, appropriate to their continent, South America and Africa had faunas of terrestrial mammals with similar degrees of diversity until the relatively recent past. Throughout all of the Tertiary period (between 65 million and 1.8 million years ago) and a good part of the Quaternary period (between 1.8 million years ago until today), there were even, according to some authors, more forms of non-flying and non-swimming mammals here than there. “South America had twenty orders of mammals (terrestrial) and Africa only thirteen”, De Vivo explains.

In the language of the taxonomists, an order is a category of classification of organisms that is made up one or various families, similar or intimately related to human beings. For example, within the order of primates various families of mammals figure, such as the Hominidae (large monkeys and humans), the Callitrichidae (marmosets and capuchin monkeys) and the Lemuridae (lemurs), among others. Today, Africa exhibits eleven orders of terrestrial mammals, one less than that of South America.

For some reason, or possibly various, eight orders disappeared from the left bank of the South Atlantic, above all those of the large and medium size animals that had lived in open vegetation, and only two on the right hand margin. If the weight of the animals was to be adopted as an indicator of their size, it was not by accident that the category of terrestrial mammals with less than 5 kilos is the only one in which there are more species in South America than in Africa (622 as against 587). In all of the others, the continent of giraffes and elephants shows more species of warm-blooded animals than Brazil and its Hispanic neighbors. “Basically we have remained with the animals of the forest, small; and they with those of the savanna, the largest”, De Vivo sums up.

The Project
Systematic, Evolution and Conservation of the Mammals of Eastern Brazil (98/05075-7); Modality: Thematic Project; Coordinator: Professor Mario de Vivo Zoology Museum/USP; Investment: RS$ 789,083.78

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