Big-Brained Mammals at Greatest Risk of Extinction

A big brain can be a liability instead of an asset for many mammals, finds new research that identifies a reversal of a 40-million-year-old trend.

Bigger is not always better when it comes to brains, finds a surprising new study that links brain size to endangerment status in mammals.

The findings, published in the Proceedings of the Royal Society B, completely reverse a trend seen over millions of years.

"For the past 40 million years, carnivore species with larger relative brain sizes were less likely to become extinct, but in mammalian species alive today, we find the opposite trend," author Eric Abelson told Discovery News.

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"Modern mammals with large relative brain sizes are more likely to be endangered rather than less," added Abelson, who is a research wildlife biologist at the Pacific Southwest Research Station of the U.S. Forest Service.

Abelson examined the relationship between brain size and endangerment status for 1679 individual animals representing 160 species of different types of mammals. He controlled for body size, since larger animals inherently tend to have larger brains, just as smaller animals tend to have smaller brains.

The analysis determined that mammals with larger brains relative to their overall body mass were more likely to be near threatened, vulnerable, threatened, endangered and even critically endangered versus those with brains that are smaller relative to their body size.

The short-eared dog (Atelocynus microtis), for example, was found to have a large relative brain size and is now listed as near threatened by the International Union for Conservation of Nature (IUCN). The tiger cat (Leopardus tigrinus), despite its big brain, is additionally listed as being near threatened. The wily Channel Island fox (Urocyon littoralis), known by locals for its intelligence, also rated very high for its relative brain size, and yet is now also near threatened.

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The Cozumel raccoon, also called the pygmy raccoon (Procyon pygmaeus), was nearly off the charts in terms of its relative brain size, which surpassed that of all other mammals included in the study. This raccoon, however, is critically endangered in its habitat on Cozumel Island off the coast of the Yucatan Peninsula, Mexico.

Clearly these and many other mammals are therefore often unable to think their way out of the daily challenges that they are now facing.

"There is no cognitive solution for the problems faced by non-human mammals living in a forest that is being bulldozed over, or for those that are in a highly polluted stream," Abelson said.

He added, "A larger brain can even be a liability in some cases."

Abelson explained that neural tissue is "very expensive," in terms of the nutritional energy that it requires, not to mention all of its other needed metabolic maintenance.

The correlation between relative brain size and endangerment status is not as strong for very large mammals, however. Abelson is not at present sure why, but he quickly added that the new findings apply to the vast majority of all mammals.

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It could be that only species with the ability to travel easily over long distances and adapt to new environments benefit the most from their costly big brains. Earlier research led by Daniel Sol of the Spanish National Research Council found that birds with big brains, relative to their body mass, show higher survival than smaller brained bird species.

Sol explained that "larger brains help birds respond to novel conditions by enhancing their innovation propensity ... enlarged (bird) brains function, and hence may have evolved, to deal with changes in the environment."

The rate of change affecting animals today is so fast, though, that it appears to be overwhelming many mammals' ability to react and adapt. Time will tell if even our own species is able to cope with such changes in future if current trends, such as rapid climate change, continue.

For now, Abelson hopes that his research will help estimate extinction vulnerability in mammals for which accurate population numbers are not available. As he said, "Brain size holds promise for helping to understand species endangerment because it is an easily measured, heritable trait and has behavioral implications."

New research shows that mammals with big brains relative to their body size, such as the Channel Island fox, are more likely to be endangered than mammals with smaller brains.

The world's sixth mass extinction began about 40 years ago, with humans contributing to the loss of biodiversity, according to a new study in the journal Science Advances. The authors call for fast action to conserve remaining threatened species, populations and habitat, but warn that the window of opportunity is rapidly closing. "We have maybe a decade to turn the boat in the right direction," co-author Anthony Barnosky of the University of California at Berkeley told Discovery News. "If we don't start cranking the rudder hard now, we won't make the turn in time." He and lead author Gerardo Ceballos explained that a mass extinction is a relatively sudden, global decrease in the diversity of life forms. Barnosky added that it's "when three out of four familiar species disappear from the face of the earth in a short time period." Already extinct is the Pinta Island tortoise, famously represented by a single male individual, "Lonesome George," for many years. He died on June 24, 2012, without siring any offspring. With him went an entire subspecies of tortoise.

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This colorful little toad from Costa Rica was declared extinct in the past few decades. A single male was spotted in 1989, but no one has seen this species since. Even many surviving amphibian species have dropped to dangerous population lows in recent years, with habitat loss, climate change, viruses and other factors theorized to contribute to the losses.

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A camera trapping study conducted from 2000–2004 hoped to capture a Formosan clouded leopard in a photo, but came up empty. No one has seen these big cats for decades, so it is presumed that the species -- native to the island of Taiwan -- is extinct. Ceballos told Discovery News that past mass extinctions have been driven by sudden climate change, meteorite hits, cataclysmic volcanic eruptions and other major natural disasters. Now, with human activity adding to the existing "backdrop" of natural disasters, yet another mass extinction is underway, he said. For the study, he and his colleagues compared a highly conservative estimate of current extinctions with this "backdrop" rate estimate. They even doubled the latter to bring the two rates as close to each other as possible. Despite this adjustment, they found that species are disappearing up to about 100 times faster than the normal rate. Human activity that continues to alter or destroy natural habitats includes the following: land clearing for farming, logging and settlement; introduction of invasive species; carbon emissions that drive climate change and ocean acidification; toxins that alter and poison ecosystems.

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Japanese sea lions are thought to have become extinct in the 1970s. This species, and all others that have gone extinct, once played vital roles in their ecosystems. "To me, the most important loss is that we'd be wiping out in one fell swoop what took millions of years for evolution to create," Barnosky said. "It's like going into the Louvre with a razor blade and slashing up at least three out of every four great works of art on display there. Others would argue that the tremendous monetary losses would be as important. (There would be a) loss of fisheries, forests, and other ways nature serves us, which scientists call 'ecosystem services.'"

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The Javan tiger is a tiger subspecies that once inhabited the Indonesian island of Java until it was declared extinct in the mid 1970s. Ceballos points out that "75 percent of all medicine initially derived from plants and animals," so extinctions deprive us of potential cures for countless diseases and illnesses.

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The last known dusky seaside sparrow, a bird once found in Florida, died in June of 1987. The subspecies was declared extinct in December 1990. Discoveries of new species happen each year, but unfortunately many of these newly found species are already highly endangered at the time of their discovery. There is no question that humans led to the demise of the dusky seaside sparrow. In an attempt to reduce the mosquito population where the birds lived, workers flooded the bird's habitat, devastating the sparrow's nesting grounds. Later, a highway was constructed at the site. Pollution and pesticides were also introduced into the area. Only six or seven birds survived the onslaught, and they have all since died, presumably without producing any offspring.

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Once common in parts of Spain and France, the Pyrenean ibex was declared extinct in January 2000. There have been attempts to clone them using the DNA from one of the last surviving females, but so far those efforts have proven to be mostly unsuccessful. People basically hunted this animal to death, but habitat loss and other factors played a role in their demise too.

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The Northern white rhino and the following two species on this list represent animals that are not extinct yet, but are endangered. Hunting, primarily for the rhino's ivory tusk, is driving the demise of this particular species. The situation is so dire that the rhino shown here, along with three others, is under 24-hour armed guard in Kenya.

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According to Ceballos, orangutan numbers in the wild "are so very low now." The Sumatran orangutan was once distributed over the entire island of Sumatra and further south into Java. Now, the species in the wild is restricted to just two sections in the north of the island. Of the nine known groups of these primates, only two are thought to have prospects for long-term viability. A factor in this and other losses is human population growth. "Each one of us has an ecological footprint that contributes to stomping out other species," Barnosky said. "Even conservative estimates of population growth indicate there will be 2–3 more billion people on the planet by 2050."

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There are two types of African elephants still in existence: the African bush elephant and the African forest elephant. Both are listed as "vulnerable" by the IUCN. "Poaching at current rates would cause the extinction of all wild elephants within 20 years," Barnosky said. As co-author Paul Ehrlich of Stanford University said, "There are examples of species all over the world that are essentially the walking dead."

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