Cold-blooded Goats?

The fossil remains of a prehistoric goat-like artiodactyl that once lived on Mediteranean islands over 1.5 million years ago provides evidence of how some large mammals may have survived in resource-poor ecosystems. Myotragus, a Plio-Pleistocene bovid from the Balearic Islands off the eastern coast of Spain have a particular bone structure that is shared with cold-blooded reptiles. Not cold blooded, but having a variable growth rate according to changing environmental conditions:

The Press Release from Institut Català de Paleontologia:

Mammal or reptile? Researchers from the ICP discovered that the bovidae Myotragus balearicus‘ metabolism was more similar to a reptile metabolism than a mammal one.

Researchers Meike Köhler and Salvador Moyà, from the Institut Català de Paleontologia (ICP-UAB) have released the results of a highly innovative research in the field of paleontology. Using paleohistological techniques, they were able to infer the physiology of extinct species. The study, based on the goat-like bovidae Myotragus from the Balearic Islands, has been published in the prestigious scientific journal Proceedings of the National Academy of Science USA (PNAS).

The Myotragus balearicus, is an extinct mammal, similar to a goat, that belongs to the family Bovidae. All the Myotragus fossil data has been found in Mallorca, Menorca, Cabrera and Dragonera. The first Myotragus balearicus was discovered in the beginning of S. XX.(sic) and since those early days, it has been center of many paleontological studies. All thanks to its remarkable characteristics, fruit of an evolution in terms of insularity.

The islands and their fossil legacy are authentic laboratories of evolution for paleontologists. In fact, we can say that they are experiments in evolution, due to the islands have many environmental factors determined: small geographic area, limited resources, and trophic (ecosystem) limited by the absence of large mammals predators. The fossil islands also give us a proper time perspective to study the evolution, thanks to the scale of geological time. A variable that the study of the actual islands can not provide us, today.

Myotragus lived more than 5 million years in an island. A very long time for a species survival, especially in an area as small as an island, which means limited resources and fluctuating. Therefore, researchers were asked how this species had been able to survive more than double the time than their mainland relatives.

The results of histological thin section of a large sample of Myotragus’ bones, with which we study the bone tissue and its growth pattern, show that Myotragus were closer to a physiological crocodile (reptile) that to a mammal. The histology showed that Myotragus’ bone grew unlike any other mammal, at a slow pace and flexible, stopping periodically growing, reaching maturity at very advanced ages which meant a significant increase in longevity; similar to existing crocodiles. The explanation of why this happened is simple. Mammals are endotherms (warm-blooded) and, unlike cold-blooded (cold blood) spend a lot of energy to maintain body temperature constant. When resources are scarce, some individuals chose burning less energy, ie, individuals with a lower metabolism. This implies, however, grewing at a much slower pace, reaching maturity later and living longer.

The results of the study published in the article “Physiological and life history strategies of large mammal fossils in a resource-limited environment” in the renowned scientific journal Proceedings of the National Academy of Science (PNAS) suggest that although Myotragus were an endothermic mammal, they could experience lethargic seasons when weather conditions brought a lack of resources and made life difficult for them. The abundance of Myotragus’ fossil found in caves suggests the possibility of these mammals using the caves to pass the difficult seasons, lowering metabolism and reducing vital functions.

Currently, some deer or goat-like bovidae can leave the expensive maintenance of a constant body temperature (endothermy) in a very critical period, generating a temporary fluctuation of body temperature (heterotèrmia). However, the only large mammal that hibernates is the bear. Bears are dangerous animals that suffer little or no predation. In addition, during their hibernation, they can wake up at any time and their temperature never falls below 20 º C. Bears and Myotragus balearicus shared the absence of threats from predators, because Myotragus lived in an island. This enabled them, not only to reduce their metabolism in difficult moments, but also to get to stop their growth, delayed age reproduction, extend their longevity (life years) or cease feeding at critical moments.

Today, scientific experiments are conducted to study the rhythms of life and longevity. Researchers study the reactions of invertebrates and small mammals (mice) under conditions of stress and lack of resources. Observations conducted with by this type of research point to the increased longevity or duration of the life of these animals. The results presented by the ICP provide an example which supports this hypothesis and explain the evolutionary reasons for this increase in longevity. Reducing the pace of life and increased longevity are favored under conditions of stress and lack of resources. Thus, research in paleontology supports research in other fields such as medicine. Thanks to this inter-disciplinary, the study of a goat-like bovidae that evolved into a natural laboratory conditions (islands) could reveal the mechanisms that influence the physiology of humans and causes of longevity.

This scientific study is free online:
Physiological and life history strategies of large mammal fossils in a resource-limited environment. Meike Köhler and Salvador Moyà-Solà. Proceedings of the National Academy of Science (PNAS)

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