November 16, 2009
Islands, especially nutrient-poor islands, are usually home to reptiles. Mammals are endotherms, meaning that they have steady and high growth rates, and therefore require rich environments to sustain them. However reptiles, ectotherms, grow slowly and are able to change their growth rates when resources fluctuate, making them perfectly adaptable to resource-poor islands, like the Balearic islands of Spain. So, given the environmental conditions of these islands, how did a species of goat thrive there for millions of years?
Bone tissue records the physiological and life history strategies of fossil organisms. Ectotherms are characterized by zonal bone with cyclical growth rings (above); endotherms typically have azonal, highly vascularized bone (below). The presence of ectotherm bone tissue in a fossil insular bovid (middle) denotes that under certain environmental conditions mammals may evolve reptile-like developmental and physiological plasticity. Image courtesy of Institut Català de Paleontologia ICP (Catalan Institute of Paleontology).
"The bone microstructure indicates that Myotragus grew unlike any other mammal but similar to crocodiles at slow and flexible rates, ceased growth periodically, and attained [physical] maturity extremely late by 12 years," Kohler and Moya-Sola write in a paper published in the Proceedings of the National Academy of Science.
While the Balearic Island cave goat is the world's first mammal known to have adapted reptilian growth strategies, Kohler and Moya-Sola believe there were probably others. The Balearic Island cave goat was a dwarf species: standing only 50 centimeters (19.5 inches) at the shoulder, smaller than a medium-size dog. Only a few thousands years ago, Mediterranean islands were also home to a variety of dwarf mammals, including elephants, hippos, and deer. Kohler and Moya-Sola hypothesize that these other dwarf mammals must have evolved traits similar to the Balearic Island cave goat—and reptiles—in order to survive in low nutrient environments.
Yet, as Kohler and Moya-Sola write it was likely these same traits that led to the species' downfall.
"The reptile-like physiological and life history traits found in Myotragus were certainly crucial to their survival on a small island for the amazing period of 5.2 million years, more than twice the average persistence of continental species. […] However, precisely because of these traits (very tiny and immature neonates, low growth rate, decreased aerobic capacities, and reduced behavioral traits), Myotragus did not survive the arrival of a major predator, Homo sapiens, some 3,000 years ago," they write.
Citation: Meike Kohler, Salvador Moya-Sola. Physiological and life history strategies of a fossil large mammal in a resource-limited environment. Proceedings of the National Academy of Science.
Reconstruction of Myotragus balearicus at Cosmo Caixa, Barcelona. Photo: Xavier Vázquez.
Skeleton of Myotragus balearicus. Photo: Francisco Valverde.
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