Department of Basic Science, College of Osteopathic Medicine, Touro University, Vallejo, CA, 94592, USA.
Physician Assistant Program, Department of Family Medicine and Community Health, Duke University School of Medicine, 800 S. Duke Street, Durham, NC, 27701, USA.
J Hum Evol. 2021 Feb;151:102938. doi: 10.1016/j.jhevol.2020.102938. Epub 2021 Jan 22.
Skeletal muscle fiber types are important determinants of the contractile properties of muscle fibers, such as fatigue resistance and shortening velocity. Yet little is known about how jaw-adductor fiber types correlate with feeding behavior in primates. Compared with chimpanzees and bonobos, gorillas spend a greater percentage of their daily time feeding and shift to herbaceous vegetation when fruits are scarce. We thus used the African apes to test the hypothesis that chewing with unusually high frequency is correlated with the expression in the jaw adductors of a high proportion of type 1 (slow, fatigue-resistant) fibers at the expense of other fiber types (the Frequent Recruitment Hypothesis). We used immunohistochemistry to determine the presence and distribution of the four major myosin heavy chain (MHC) isoforms in the anterior superficial masseter (ASM), superficial anterior temporalis, and deep anterior temporalis of four Gorilla gorilla, two Pan paniscus, and four Pan troglodytes. Serial sections were stained against slow (MHC-1/-α-cardiac) and fast (MHC-2/-M) fibers. Fibers were counted and scored for staining intensity, and fiber cross-sectional areas (CSAs) were measured and used to estimate percentage of CSA of each MHC isoform. Hybrid fibers accounted for nearly 100% of fiber types in the masseter and temporalis of all three species, resulting in three main hybrid phenotypes. As predicted, the gorilla ASM and deep anterior temporalis comprised a greater percentage of CSA of the slower, fatigue-resistant hybrid fiber type, significantly so for the ASM (p = 0.015). Finally, the results suggest that fiber phenotype of the chewing muscles contributes to behavioral flexibility in ways that would go undetected in paleontological studies relying solely on morphology of the bony masticatory apparatus.
骨骼肌纤维类型是肌肉纤维收缩特性的重要决定因素，例如抗疲劳性和缩短速度。然而，人们对颌内肌纤维类型与灵长类动物摄食行为的相关性知之甚少。与黑猩猩和倭黑猩猩相比，大猩猩在一天中花费更多的时间进食，并且在果实稀缺时会转向草本植被。因此，我们使用非洲猿类来检验咀嚼频率异常高与颌内肌中高比例的 1 型（慢，抗疲劳）纤维表达相关的假设，而牺牲了其他纤维类型（高募集假说）。我们使用免疫组织化学方法来确定在四个大猩猩（Gorilla gorilla）、两个倭黑猩猩（Pan paniscus）和四个普通黑猩猩（Pan troglodytes）的前浅层咬肌（ASM）、前浅层颞肌和前深层颞肌中四种主要肌球蛋白重链（MHC）同工型的存在和分布。对慢（MHC-1/-α-心脏）和快（MHC-2/-M）纤维进行连续切片染色。计数纤维并对染色强度进行评分，并测量纤维横截面积（CSA）并用于估计每种 MHC 同工型的 CSA 百分比。杂交纤维占所有三种物种的咬肌和颞肌中纤维类型的近 100%，导致三种主要的杂交表型。如预测的那样，大猩猩的 ASM 和前深层颞肌包含更大比例的较慢、抗疲劳的杂交纤维类型 CSA，ASM 显著（p=0.015）。最后，结果表明咀嚼肌肉的纤维表型以仅依赖于骨性咀嚼器官形态的古生物学研究无法检测到的方式为行为灵活性做出贡献。
