Mass extinctions are often typified by the catastrophic loss of charismatic animals. Even though ammonites, pterosaurs, many forms of marine reptiles, and even some lineages of mammals all succumbed during the great dying at the end of the Cretaceous, that event will always be cast as the unexpected curtain-fall on the Age of the Dinosaurs. The same pattern holds for the world’s most recent mass extinction, which occurred between the end of the Pleistocene and the dawn of the Holocene around 12,000 years ago. This was the extinction which wiped out the mammoths, mastodons, saber-toothed cats, and giant ground sloths, but, as with any extinction, the pruning back of ancient lineages is only part of the story.
No mass extinction has ever entirely extinguished life on earth. There have always been survivors, and, thanks to the contingent nature of evolution, these were the creatures which set the stage for the succeeding radiations of life in a world stripped of its previous ecological richness. Yet species and lineages which survived mass extinction events have not always emerge unscathed. In the world’s latest mass extinction, horses and camels were extirpated from the continent of their origin – North America – and predators which once prowled much of the northern hemisphere, such as lions, had their ranges severely restricted. Among tattered remnants of the Pleistocene world are spotted hyenas (Crocuta crocuta), and in a new study published in Quaternary Science Reviews scientists Sara Varela, Jorge Lobo, Jesús Rodríguez, and Persaram Batra have presented an analysis of climate’s role in the disappearance of the bone-crushing mammals from the European continent.
In terms of global ecology, the waning days of the Pleistocene were marked by at least two major events. The world’s vast ice sheets retreated as the global climate became warmer and wetter, and by 50,000 years ago our species had begun to spread out of Africa to continents beyond. Both events have been taken as triggers for the extinction of the world’s megafauna, and both likely had an influence on populations of spotted hyenas in Europe. What Varela and colleagues wanted to find out was whether the disappearance of the European hyenas could be attributed to climate change, and, if climate was not solely responsible, what else might have pushed them over the edge.
As reviewed by the authors, spotted hyenas were present in Europe for about 1 million years and ranged from the western coast of the Iberian Peninsula to the Ural Mountains. To determine the particular climatic niche of these hyenas over the span of space and time, Varela and co-authors combined the known distribution of hyena fossils with climate data to estimate the existence of preferred hyena habitats at five time periods from 126,000 years ago to the present. During this time the earth went from interglacial to the last “ice age” and back to an interglacial again, and so the biogeography of hyenas during the waxing and waning of the glaciers has the potential to illustrate the effects of climate change on European hyenas.
As it turned out, the spotted hyena is a hardy species which was widespread during the oscillating climate shifts. Even though many of the Pleistocene hyenas endured colder and drier conditions than their living counterparts in Africa, some of the prehistoric fossil sites had conditions comparable to areas where spotted hyenas live today. Hence, it cannot be said that the European hyenas were strictly adapted to either cold or warm climates and were a victims of a change in temperature alone. Spotted hyenas were clearly capable of living in both glacial and interglacial conditions – by all accounts there should still be European hyenas – but just because hyenas survived into the height of the ice age does not mean that conditions were ideal for them.
While spotted hyenas ranged over almost the whole of Europe during much of the past 126,000 years, at the 21,000 year mark the intense cold restricted them to a band of habitats south of present-day Lithuania. In this situation, Varela and colleagues hypothesize, the hyenas could have been placed under intense climatic stress and experienced the fragmentation of their populations. This would have been a critical time for the European hyenas in which they were especially vulnerable to changes in the abundance of prey, perhaps further altered by competition with humans, but this hypothesis has yet to be examined in full detail. For now it is clear that changes in temperature and precipitation alone cannot account for the extirpation of the European hyenas, but exactly what ushered them off the evolutionary stage remains unknown.
Varela, S., Lobo, J., Rodríguez, J., & Batra, P. (2010). Were the Late Pleistocene climatic changes responsible for the disappearance of the European spotted hyena populations? Hindcasting a species geographic distribution across time Quaternary Science Reviews, 29 (17-18), 2027-2035 DOI: 10.1016/j.quascirev.2010.04.017
Image: A spotted hyena (Crocuta crocuta) in the January cold at Blijdorp Zoo in the Netherlands. Photo from Flickr user Silvain de Munck.