2017年5月17日，国际顶尖学术期刊《Science》旗下《Science Advances》杂志上在线发表了澳大利亚格里菲斯大学Tanya M. Smith研究员的一篇研究论文，论文通过研究猩猩的牙齿，揭示了猩猩长时间吸奶的情况。

研究人员研发了一种方法，它能对灵长类中特别难以捉摸的喂奶方式进行了追踪，他们用它发现，某些发育未全的猩猩吸奶时间长达8年或以上--这超过了其它非人类灵长动物中所报告的最长断奶年龄。这些发现对管理生殖率缓慢低下的濒危灵长动物(如苏门答腊和婆罗洲猩猩)颇具意义，因为人们对这些动物的生活史了解甚少。对野生猩猩的断奶年龄几无所知，但有些科学家提出，它们在出生后的断奶所需时间会持续6年之久(或更长)。钡是一种非必需元素，它在体内的代谢通路常常类似于钙;当钡随母乳一同被摄取后，它很容易进入像牙齿等骨骼组织之中。因此，对这一元素进行测试会是一种提示婴儿或发育不成熟幼儿是否还在吸奶的有效方法。

在本研究中，Tanya M. Smith和同事对4个猩猩从其出生头一年便开始分析其牙齿中的钡含量，从而提供了首个如此详尽的野猩猩吸奶史。他们发现，在出生第一年后，牙齿中的钡浓度通常会降低，但在其8岁和9岁时钡会一直存在于牙齿之中。此外，作者发现，牙齿中钡浓度会年复一年地出现高低周期性变化。他们指出，牙齿中钡浓度下降时期可能对应于水果获取量大的时候，这时，正在生长的猩猩对奶汁的依赖度较小，而牙齿中钡浓度的增加则是因为奶摄取量增加的结果，它发生在食物稀少时，那时正在长身体的猩猩需要多吸奶才能维持生长。对猩猩的喂奶和断奶模式有所了解是有价值的，因为要观察生活在树上且不显眼的猩猩的吸奶情况总是具有挑战性的。

原文摘要：Nursing behavior is notoriously difficult to study in arboreal primates, particularly when offspring suckle inconspicuously in nests. Orangutans have the most prolonged nursing period of any mammal, with the cessation of suckling (weaning) estimated to occur at 6 to 8 years of age in the wild. Milk consumption is hypothesized to be relatively constant over this period, but direct evidence is limited. We previously demonstrated that trace element analysis of bioavailable elements from milk, such as barium, provides accurate estimates of early-life diet transitions and developmental stress when coupled with growth lines in the teeth of humans and nonhuman primates. We provide the first detailed nursing histories of wild, unprovisioned orangutans (Pongo abelii and Pongo pygmaeus) using chemical and histological analyses. Laser ablation inductively coupled plasma mass spectrometry was used to determine barium distributions across the teeth of four wild-shot individuals aged from postnatal biological rhythms. Barium levels rose during the first year of life in all individuals and began to decline shortly after, consistent with behavioral observations of intensive nursing followed by solid food supplementation. Subsequent barium levels show large sustained fluctuations on an approximately annual basis. These patterns appear to be due to cycles of varying milk consumption, continuing until death in an 8.8-year-old Sumatran individual. A female Bornean orangutan ceased suckling at 8.1 years of age. These individuals exceed the maximum weaning age reported for any nonhuman primate. Orangutan nursing may reflect cycles of infant demand that relate to fluctuating resource availability.