Saturday 5 May 2012

Did Spotted Hyenas colonize Britain once or twice?

Spotted Hyaenas (Crocuta crocuta) are now restricted to sub-Saharan Africa, but in the Pleistocene were far more widespread, roaming across much of Eurasia. The oldest known fossil Spotted Hyaenas in the UK come from Westbury-sub-Mendip in Somerset and Joint Mitnor in Devon, and are thought to be about 700 000 years old, dating from the Cromerian Interglacial (there are older British Hyaena fossils belonging to another species, Pachycrocuta brevirostris, the Giant Pleistocene Hyaena). After these fossils there is a gap in the fossil record of Hyenas in the UK, with the next specimens coming from 320 000 year old deposits at Grays Thurrock in Essex. Spotted Hyaenas then persist in the fossil record till around 26 000 years ago, when they appear to have finally died out during the Devensian Glaciation.

Spotted Hyaenas have previously been shown to have invaded Europe from Africa on two separate occasions. The first group, known as Clade B (a clade is a group of animals with a common ancestry) arrived around 800 000 years ago, the second group, known as Clade A arrived about 360 000 years ago; these two groups can be distinguished by mitochondrial DNA (mDNA) analysis. It has been unclear whether the later British Hyaenas were derived from this second invasion, or were descended a relict population of Hyaenas that had survived in Britain since the original colonization. The later specimens have been shown to have been larger and more robust than the earlier ones, but this could be a local adaptation to conditions in Britain rather than evidence of separate ancestry.

In a paper published in the journal Archaeological and Anthropological Sciences on 25 April 2012, a team of scientists lead by Danae Dodge of the Department of Archaeology at the University of Sheffield discuss the results of mDNA analysis of a number of Hyaena teeth from the Late Pleistocene Creswell Crags in Derbyshire and Kent's Cavern in Devon.

Spotted Hyaena tooth from Creswell Crags. Dodge et al. (2012).

Mitochondrial DNA is found in the mitochondria, organelles outside the nucleus of the cell. Unlike regular DNA it is inherited only from the mother, and is not recombined in each generation. This makes it ideal for this sort of study, as it is passed from generation to generation with little variation, making it excellent for tracing lineages.

Spotted Hyaena Tooth from Kent's Cavern. Dodge et al. (2012).

All the teeth studied by Dodge et al. showed markers associated with Clade A, suggesting that these animals were descended from animals that arrived in Britain some time after 360 000 years ago. No mDNA analysis of the earliest Spotted Hyaenas in the UK, but since this lived before the arrival of Clade A in Europe, it is logical to assume they were members of Clade B, making it unlikely that they were the ancestors of the later animals. This does not rule out the possibility of some Hyaenas surviving from the earlier invasion to the Late Pleistocene, but makes it much less likely.

This is not altogether surprising; during the period from 700 000 to 320 000 years ago the UK underwent considerable climate change, as well as colonization by another large, pack hunting predator (humans) with whom interactions do not always go well for Hyaenas. Hyaenas are apparently able to cope well with wide a range of temperatures, the animals used in this study lived during a period when the UK was considerably colder than today, but are restricted by vegetation cover. They are predators of open grasslands, able to pursue prey animals at speed over long distances, but poorly equipped for hunting in woodland, where prey animals are able to escape by hiding. The spread of woodlands across Britain, which happened several times as the climate changed in the Pleistocene, would have made it a very poor environment for Hyaenas.

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