The lonely walk to extinction

Our very species is an oxymoron. When Linnaeus added us to the taxonomic ranks of life, he dubbed humans Homo sapiens: literally meaning ‘wise man’. Sometimes I wonder how ‘wise’ we are. We can send people to live in space, talk to another person instantly across the world; yet we can chop down a rain forest, hunt rare animals for fun, and have seemingly little awareness of our disastrous impact on the planet. So many species have disappeared in the last 10,000 years that our planet is only a fraction of the beautiful diversity that once was. And despite cloning and sexy ‘resurrection’ stories, once a species has become extinct, that’s it, it’s gone.

What is perhaps most frustrating is that through social media we can instantly see another dozen acres of rainforest lost to logging, or we count down the last surviving individuals of a species. Frustrating because we know it is happening yet there is little we can do. And sad because we are witnessing animals disappear right before our eyes with literally the last few numbers counting down to zero.

Even before a species number reaches zero, there is a point where that animal is functionally extinct. Less than a dozen animals left and it is unlikely for that group to grow in numbers and recover. Despite a few individuals still alive in zoos today, the Western black rhino (Diceros biceros lonipes) was declared extinct in 2013. The Javan rhinoceros (Rhinoceros sondaicus) has less than 70 individuals left in the wild: this species is already on its way to extinction.

The Javan rhinoceros has been hunted for centuries. This painting by W.F.A. Zimmermann, Library of Congress is from 1861. (Image Public Domain)

Animals need space to roam and numbers to survive. Genetic diversity makes a species strong: larger populations means healthier individuals because there is more genetic diversity. For example if there is an odd mutation in the genes passed on to offspring and that offspring doesn’t survive, in large populations, this gene may only be given to a few individuals; whereas in a small population, this gene could kill the species.

The last surviving woolly mammoths showed signs of a ‘genetic meltdown’. On the small desolate Wrangel Island in Siberia, the woolly mammoths survived until just 3,700 years ago (that’s a whopping 5000 years longer than woolly mammoths anywhere else on Earth). Here a small population of these shaggy giants survived, but research examining the DNA has shown that they were full of mutations. Without other mammoths to breed with bringing in fresh DNA, this population was riddled with mutated genes. There isn’t any evidence that these mutations were especially harmful: these Wrangel Island mammoths were a little smaller than their mainland relatives (an example of island dwarfism, which we have seen before with mammoths). It appears that the last of these iconic beasts disappeared shortly after humans arrived on Wrangel Island.

The iconic, shaggy, woolly mammoth. The last surviving species of the great Mammuthus genus vanishing just 3,700 years ago.

DNA is a pretty fragile bunch of chemicals. It breaks apart pretty soon after an organism dies.  For some extinct animals it is difficult to see if mutations dominated a genome, or even had an effect on the last members of a species. Sometimes odd features in an animal might be preserved as a fossil, which provide evidence of a species struggle for survival. A recent study investigating woolly rhinoceros bones looked at just that.

Researchers examined 32 woolly rhino cervical vertebra for abnormalities, specifically looking for signs of rib growth. This may seem for a strange thing to look for, but it can tell us quite a lot. Apart from the wonderfully placid manatees, and the equally docile sloths, all mammals, from giraffes to mice, have seven cervical vertebrae (the neck vertebrae). A genetic mutation can alter the growth in an embryo changing one neck vertebra to grow into a thoracic vertebra (the vertebrae that hold ribs). The research found a particularly high number of ribs which showed this change: 5 out of the 32 specimens. This is an odd result, because it is particularly high for such a small sample. It does and can happen in animals, including humans, and can shorten the life span of the individual with the abnormality. It does happen, but it is rare. So something must have been going on to cause the high incidence of this genetic mutation.

These two woolly rhinoceros cervical vertebrae are almost completely fused, indicating a genetic abnormality in this individual. (Image van der Geer & Galis)

This study looked at specimens from the North Sea and the Netherlands. (The North Sea was once dry and was a rich ecosystem for Pleistocene fauna. Lots of bones of many Twilight Beasts have been dredged up by fishermen over the years, including mammoths, giant deer, and even a Neanderthal skull fragment.) The time span for the 32 specimens is pretty large: they haven’t been radiocarbon dated, but are from deposits dating between 115,000 to 36,000 years old. The results indicate to the authors that the last populations of woolly rhinoceros were under tough conditions, leading to high prevalence of genetic mutations.

Towards the end of the Pleistocene a lot was changing. Temperatures were fluctuating, eventually warming rapidly. The Steppe environment that sustained so many of the familiar giant European mammals was shrinking: removing low lying grasses and shrubs the thick lips of the woolly rhinoceros were adapted to feasting on. Trees and woodlands grew instead. A quick change in environment like this can put stresses on animals (such as lack of food) causing foetuses to be aborted or grow abnormally. A rapidly changing environment would lead to lower numbers of woolly rhinos,  which likely subsequently increases inbreeding (again causing more mutations). Humans did hunt and eat woolly rhino, and may have had an impact on an already shrinking species.

The glorious Woolly Rhinoceros. One of the most underappreciated Twilight Beasts.

What this new research shows is that species are under more pressures to survive than we might think. We know about the effects of climatic fluctuations and habitat loss, but the damage this does internally is devastating to a species genome: Additional stresses on an animal can cause mutations in genes to become fixed and if a population is too small, then it may be the end of that species. We are an oxymoron. Our species is wise and we have a very good understanding of why species have gone extinct in the past, and even see what happens to the genes of animals when the species is put under pressure. Yet…

The last woolly rhinos may have had the loneliest end out of all the Pleistocene mega-fauna. Like rhinos today, they were solitary animals. To find a mate they may have slowly trekked for miles across frozen desolate land, howling wind, and an endless expanse of nothingness. Those last few may have trekked for years, warm breath puffing out of their nostrils as they slowly trundled along, never finding their mate.

Written by Jan Freedman (@JanFreedman)

*******Postscript: In this post I have said ‘we’ throughout when referring to humans. I know that many of us are very proactive and donate and sign petitions to help save a plethora of animals. I have purposefully used ‘we’ for this post, simply because you will relate to it more. You are not someone who is hunting animals or destroying ecosystems. But together we can all do something about it. **************

Further Reading:

Gunthrie, R. D. (2004), ‘Radiocarbon evidence of mid-Holocene mammoths stranded on an Alaskan Bering Sea island’, Nature. 429. (6993). 746-9. [Abstract only]

Jacobi, R. M. et al. (2009), ‘Revised radiocarbon ages on woolly rhinoceros (Coelodonta antiquitatis) from western central Scotland: significance for timing the extinction of woolly rhinoceros in Britain and the onset on the LGM in Central Scotland’, Quaternary Science Reviews. 28. 2551-56. [Abstract only]

Kurten, B. (1968), ‘Pleistocene mammals of Europe’, The World Naturalist. [Book]

Lister, A, & Bahn, P. (2007), ‘Mammoths – Giants of the Ice Age’, (3rd Edition). London: Frances Lincoln. [Book]

Markova, A. K. et al. (2013), ‘New data on changes in the European distribution of the mammoth and woolly rhinoceros during the half of the Late Pleistocene and the Early Holocene’, Quaternary International. 292. 4-14. [Full article]

Stuart, A. J, & Lister, A, M. (2012), ‘Extinction chronology of the woolly rhinoceros Coelodonta antiquitatis in the context of late Quaternary megafaunal extinctions in northern Eurasia’, Quaternary Science Reviews. 51. 1-17. [Full article]

Stuart, A. J. (1982), ‘Pleistocene vertebrates in the British Isles’, Longman Group Limited. [Book]

Sutcliffe, A. J. (1985), ‘On the track of Ice Age mammals’, British Museum (Natural History) [Book]

Van der Geer & Galis, F. (2017). ‘High incidence of cervical ribs indicates vulnerable condition in Late Pleistocene woolly rhinoceroses.’ PeerJ. [Full Article]

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