Insular dwarfism is the evolutionary phenomenon that large species tend to decrease in size when confined to an area of limited space, most frequently an island. This is also called the island effect. The body size of large animals often adapts to the confined environment by shrinking down. The reason for that is that the island provides only limited space and limited resources, so that smaller individuals have a selective advantage over large ones due to the fact that they need less space and resources. Also, the smaller the individuals the higher is number of individuals that can survive on the given area, and the higher the number of individuals, the higher is the potential for genetic diversity and thus the chance of survival. Therefore, shrinking in size provides evolutionary advantages for large animals on islands, which is why there are plenty of examples for insular dwarfism in many vertebrate groups.
The Mediterranean Sea has numerous islands, some of them large, such as Sicily, others small, such as Crete. And consequently, there were plenty of mammal species endemic to these islands that are examples for island dwarfism until the late Pleistocene. There were pygmy hippopotamuses on Crete, Sicily, Malta and Cyprus. Famous are also the dwarf elephant species Palaeoloxodon falconeri, P. cypriotes and Mammuthus creticus, to name a few.
Sicily was home to several pygmy mammal species. Hippopotamuses, elephants, a dwarf deer (Cervus siciliae), a dwarf steppe bison (Bos priscus siciliae) and also a dwarf aurochs. It was already noted by van Vuure (2005) that aurochs on Sicily were 20% smaller than on the mainland. To be precise, the Sicilian dwarf aurochs had a withers height of only 130 cm [1]. This dwarf aurochs was described as a subspecies on its own that is barely recognized in the literature, Bos primigenius siciliae. There also was a dwarf aurochs on the small island of Pianosa, which had a shoulder height of only 100 to 120 cm and was described as Bos primigenius bubaloides [1]. Authors have suggested that this aurochs population evolved adaptions for moving on rocky uneven grounds [1], which would be unique for an aurochs.
Both B. p. siciliae and B. p. bubaloides are barely recognized in the aurochs-related literature. The very small body size found in these populations seems to justify the subspecies status, and assuming that both taxa are not nomina nuda but have been described properly, they are valid. Thus, the aurochs actually was not divided into only three subspecies, but actually six: Bos primigenius primigenius (the European aurochs), Bos primigenius africanus (the North African aurochs), Bos primigenius namadicus (the Indian aurochs), Bos primigenius suxianensis (the East-Asian aurochs, go here for details), Bos primigenius siciliae (the Sicilian dwarf aurochs) and Bos primigenius bubaloides (the Pianosa dwarf aurochs). It would be interesting to see the osteologic material of these two subspecies. Island dwarfism often goes hand in hand with other morphological changes, such as paedomorphy and changes in proportions. Also, it would be relevant to know if the material assigned to the taxa was from males or females, because of the considerable sexual dimorphism we find in the aurochs.
In contrast to other aurochs populations, the extinction of these insular forms does not necessarily have to be of anthropogenic cause. Small populations are particularly vulnerable to inbreeding and the resulting loss of genetic diversity, what makes them less able to adapt against abiotic and biotic factors. The Sicilian aurochs probably died out because it was outcompeted by the normal-sized aurochs when a land bridge between the Italian mainland and the island emerged about 20.000 years ago [2]. The same also happened to the other pygmy species [2]. In this case, human activities were not the cause of the extinction of the endemic mammal fauna.
The island effect is also relevant for rewilding and “breeding-back”. Reserves are of limited space and if the population is reproductively isolated, it is basically an island from an evolutionary perspective. This implicates that in order to avoid an inbreeding depression individuals have to be exchanged between the reserves (which is, as far as I know, common practise in African reserves), and to avoid the island effect. Chillingham cattle, which have been living reproductively isolated in the Chillingham park for several centuries, apparently decreased in body size over time (go here for my article on Chillingham cattle). Thus, while natural selection seems to replicate many of the wildtype aurochs traits in feral cattle, we cannot expect the Heck cattle at Oostvaardersplassen to become larger by natural selection – if anything, they will get smaller or already got smaller. In order to avoid this as much as an inbreeding depression in other semi-feral herds of aurochs-like cattle, individuals between the reserves would have to be exchanged on a regular basis.
Literature
[1] Masini, Palombo, Rozzi: A reappraisal of the Early to Middle Pleistocene Italian Bovidae. 2012.
[2] Sondaar, van der Geer: Evolution and extinction of Plio-Pleistocene Island ungulates. 2005.
No comments:
Post a Comment