Green Daniel R, Olack Gerard, Colman Albert S
Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge MA 02138, USA.
Forsyth Institute, 245 First Street, Cambridge MA 02142, USA.
Chem Geol. 2018 May 10;485:32-43. doi: 10.1016/j.chemgeo.2018.03.034. Epub 2018 Mar 26.
Mammalian body, blood and hard tissue oxygen isotope compositions ( O values) reflect environmental water and food sources, climate, and physiological processes. For this reason, fossil and archaeological hard tissues, which originally formed in equilibrium with body chemistry, are a valuable record of past climate, landscape paleoecology, and animal physiology and behavior. However, the environmental and physiological determinants of blood oxygen isotope composition have not been determined experimentally from large herbivores. This class of fauna is abundant in Cenozoic terrestrial fossil assemblages, and the isotopic composition of large herbivore teeth has been central to a number of climate and ecological reconstructions. Furthermore, existing models predict blood water, or nearly equivalently body water, O values based on environmental water sources. These have been evaluated on gross timescales, but have not been employed to track seasonal variation. Here we report how water, food, and physiology determine blood water O values in experimental sheep () subjected to controlled water switches. We find that blood water O values rapidly reach steady state with environmental drinking water and reflect transient events including weaning, seasons, and snowstorms. Behavioral and physiological variation within a single genetically homogenous population of herbivores results in significant inter-animal variation in blood water O values at single collection times (1 s.d. = 0.1-1.4 ‰, range = 3.5 ‰) and reveals a range of water flux rates ( = 2.2-2.9 days) within the population. We find that extant models can predict average observed sheep blood O values with striking fidelity, but predict a pattern of seasonal variation exactly opposite of that observed in our population for which water input variation was controlled and the effect of physiology was more directly observed. We introduce to these models an evaporative loss term that is a function of environmental temperatures. The inclusion of this function produces model predictions that mimic the observed seasonal fluctuations and match observations to within 1.0 ‰. These results increase the applicability of available physiological models for paleoseasonality reconstructions from stable isotope measurements in fossil or archaeological enamel, the composition of which is determined in equilibrium with blood values. However, significant blood O variation in this experimentally controlled population should promote caution when interpreting isotopic variation in the archaeological and paleontological record.
哺乳动物的身体、血液和硬组织氧同位素组成(δ¹⁸O值)反映了环境水源、食物来源、气候以及生理过程。因此,最初与身体化学处于平衡状态下形成的化石和考古硬组织,是过去气候、景观古生态学以及动物生理和行为的宝贵记录。然而,尚未通过对大型食草动物进行实验来确定血氧同位素组成的环境和生理决定因素。这类动物群在新生代陆地化石组合中很丰富,大型食草动物牙齿的同位素组成一直是许多气候和生态重建的核心。此外,现有模型根据环境水源预测血液水或几乎等效的身体水的δ¹⁸O值。这些模型已在总体时间尺度上进行了评估,但尚未用于追踪季节变化。在此,我们报告了水、食物和生理因素如何在经历受控水源切换的实验绵羊(Ovis aries)中决定血液水的δ¹⁸O值。我们发现血液水的δ¹⁸O值会迅速与环境饮用水达到稳态,并反映断奶、季节和暴风雪等瞬态事件。在单一基因同质的食草动物群体中,行为和生理变化导致在单次采集时间时动物间血液水的δ¹⁸O值存在显著差异(标准差 = 0.1 - 1.4‰,范围 = 3.5‰),并揭示了群体内一系列的水通量率(τ = 2.2 - 2.9天)。我们发现现有模型能够以惊人的保真度预测观察到的绵羊血液平均δ¹⁸O值,但预测的季节变化模式与我们群体中观察到的完全相反,在我们的群体中,水输入变化受到控制,生理效应得到更直接的观察。我们在这些模型中引入了一个作为环境温度函数的蒸发损失项。包含此函数后,模型预测结果模仿了观察到的季节性波动,并且与观察值的匹配精度在1.0‰以内。这些结果提高了现有生理模型在通过化石或考古牙釉质中的稳定同位素测量进行古季节重建中的适用性,牙釉质的组成与血液值处于平衡状态下确定。然而,在这个实验控制的群体中显著的血液δ¹⁸O变化,在解释考古和古生物学记录中的同位素变化时应谨慎对待。