Brains Over Brawn

Image Credit: Kevin Pluck, CC BY 2.0, Image Cropped

Brain expansion in early hominins predicts carnivore extinctions in East Africa (2020) Faurby et al, Ecology Letters, https://doi.org/10.1111/ele.13451

The Crux

We’ve covered humans and their harmful effects many times here on Ecology for the Masses (see my recent breakdown from last month). Despite all of the colorful examples of our current effects on the wildlife of our planet, a significant amount of research has implicated Homo sapiens as the driver of the extinction of some of the megafauna of the prehistoric world, events that happens millions of years ago. Another possibility is that we as organisms (hominins, not Homo sapiens specifically) have been impacting other species for a very, very long time.

Today, East Africa is home to the most diverse group of large carnivores on the planet (though it is still less diverse than what was once seen in North America and Eurasia). Millions of years ago East Africa had an even more diverse assemblage of large carnivores, including bears, dogs, giant otters, and saber-toothed cats. The change in climate since that time may have caused the decline in large carnivore diversity, but another explanation is the rise of early hominins (our ancestors). Using fossil data, the authors of today’s paper wanted to figure out if it was indeed early hominins that drove many large carnivores extinct.

What They Did

The authors used a dataset of 88 different species, 9 still found today and 79 that are extinct. These species were then split into two groups: large and small carnivores. Large are those weighing 21kg (~44 lbs.) or more, and small are all of those weighing less than that. For their climate data the authors used mean annual temperature, precipitation, and forest cover. Hominin changes over time were modeled using changes in the size of the hominin brain, as larger brains are associated with more advanced behavior and technology.

To test if the factors above affected the extinction of large carnivores in Africa, the authors first tested for changes in extinction rates in both the large and small carnivore groups, after which they would compare any changes to relevant environmental factors (temperature, brain size, etc.). Basically, if one group lost one species per million years, but that then increased to three species per million years over a certain amount of time then you would expect that some factor is causing those increasing extinction rates.

To ensure that any patterns found would reflect changes due to one of the chosen environmental factors and not changes due to global declines in forest cover, the authors compared declines of carnivores and changes in forest cover in Africa to that of North America.

Did You Know: Kleptoparasitism

It wouldn’t be one of my breakdowns without mentioning parastism somewhere in here, but this is a very special kind. Instead of living on or in another organism, kleptoparasites simply steal resources from others. This was likely the way that early hominins began taking resources from large carnivores, either by scaring or (somehow) fighting the large carnivores for their prey items.

As far-fetched as this may sound, there are groups in Africa that still do this! They walk up to lions that are chowing down on their fresh kill and simply take it from them (DO NOT TRY THIS). Apparently the lions are so confused or frightened by these bold actions that they let it happen and/or run away. I can’t speak to the efficacy of this practice, but the fact that people still do this in some parts of the world strengthen the argument of the authors that kleptoparasitism contributed to the rise of hominins (and subsequent fall of large carnivores).

Charismatic megafauna like this saber-toothed cat would have been vulnerable to the increasing intelligence and advancing tool use of early hominins, losing out on prey and eventually going extinct as a result. (Image Credit: Charles R. Knight, Wikimedia Commons, Public Domain)

What They Found

For the small carnivores included in the study, the authors did not find any evidence for increasing extinction rates. The large carnivore extinction rates, however, increased over time, and the most likely causes were reduced forest cover and increasing hominin brain sizes. Mean annual temperature and precipitation did not affect extinction rates.

There was no evidence for a similar decline in large carnivores in North America, with the amount of large carnivores remaining mostly constant or slightly increasing over time. There was no relationship between declines of forest cover and large carnivore diversity in North America.

Problems

Working with fossil data is tricky, and it can be hard to tease apart certain factors. In this particular study hominin brain size and forest cover were highly correlated, meaning that if one changed, the other changed a similar amount. The authors made efforts to compare these changes in the fossil record to present-day carnivore diversity, forest cover, and weather data, but correlations like these make it difficult to tease apart the sole cause of a given phenomenon.

So What?

Humans have been implicated in the loss of animal diversity for years now, but today’s study has shown that our ancestors started this trend millions of years ago. Interestingly, this was likely not due to direct hunting of large carnivores, but instead due to kleptoparasitism. Over time, hominins moved beyond stealing the food from large carnivores to actively hunting the same kind of prey animals. The large carnivores were out-competed, and most of them went extinct as a result.

These results support a hypothesis whereby early hominins were able to exploit more and more resources over time, due to their growing brains and advancing tool use and behavior. While it was on a much smaller scale than what we see today, it indicates that we as a group of organisms have been using our brains to exploit the natural world since the beginning.

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