Catalan Institute for Research and Advanced Studies, Catalan Institute of Paleontology, Autonomous University of Barcelona, 08193 Bellaterra, Spain.
Proc Natl Acad Sci U S A. 2009 Dec 1;106(48):20354-8. doi: 10.1073/pnas.0813385106. Epub 2009 Nov 16.
Because of their physiological and life history characteristics, mammals exploit adaptive zones unavailable to ectothermic reptiles. Yet, they perform best in energy-rich environments because their high and constant growth rates and their sustained levels of resting metabolism require continuous resource supply. In resource-limited ecosystems such as islands, therefore, reptiles frequently displace mammals because their slow and flexible growth rates and low metabolic rates permit them to operate effectively with low energy flow. An apparent contradiction of this general principle is the long-term persistence of certain fossil large mammals on energy-poor Mediterranean islands. The purpose of the present study is to uncover the developmental and physiological strategies that allowed fossil large mammals to cope with the low levels of resource supply that characterize insular ecosystems. Long-bone histology of Myotragus, a Plio-Pleistocene bovid from the Balearic Islands, reveals lamellar-zonal tissue throughout the cortex, a trait exclusive to ectothermic reptiles. 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 somatic maturity extremely late by approximately 12 years. This developmental pattern denotes that Myotragus, much like extant reptiles, synchronized its metabolic requirements with fluctuating resource levels. Our results suggest that developmental and physiological plasticity was crucial to the survival of this and, perhaps, other large mammals on resource-limited Mediterranean Islands, yet it eventually led to their extinction through a major predator, Homo sapiens.
由于其生理和生活史特征，哺乳动物利用了变温爬行动物无法利用的适应区。然而，它们在能量丰富的环境中表现最佳，因为它们高而稳定的生长率和持续的静息代谢水平需要持续的资源供应。因此，在资源有限的生态系统中，如岛屿，爬行动物经常取代哺乳动物，因为它们缓慢而灵活的生长率和低代谢率使它们能够在低能量流的情况下有效地运作。这一普遍原则的一个明显矛盾是，某些化石大型哺乳动物在能量匮乏的地中海岛屿上长期存在。本研究的目的是揭示允许化石大型哺乳动物应对以岛屿生态系统为特征的低资源供应水平的发育和生理策略。来自巴利阿里群岛的上新世至更新世牛科动物 Myotragus 的长骨组织学显示，皮质中存在板层-带组织，这是一种仅存在于变温爬行动物中的特征。骨骼微观结构表明，Myotragus 的生长方式与任何其他哺乳动物都不同，但与鳄鱼相似，生长缓慢且灵活，周期性地停止生长，并在大约 12 岁时极其晚地达到体成熟。这种发育模式表明，Myotragus 像现存的爬行动物一样，将其代谢需求与波动的资源水平同步。我们的研究结果表明，发育和生理可塑性对于这种动物以及其他可能在资源有限的地中海岛屿上生存的大型哺乳动物的生存至关重要，但它最终通过主要捕食者智人导致了它们的灭绝。
