The Forearm Musculature of the Gray Mouse Lemur (Microcebus murinus): An Ontogenetic Study

Abstract

Although studies have sought to characterize variation in forearm muscular anatomy across the primate order, none have attempted to quantify ontogenetic changes in forearm myology within a single taxon. Herein, we present muscle architecture data for the forearm musculature (flexors and extensors of the wrist and digits) of Microcebus murinus, a small Lemuroid that has been the focus of several developmental studies. A quadratic curvilinear model described ontogenetic changes in muscle mass and fascicle length; however, fascicle lengths reached peak levels at an earlier age and showed a stronger decline during senescence. Conversely, physiological cross-sectional area followed a more linear trend, increasing steadily throughout life. As previous studies into the functional role of the primate forelimb emphasize the importance of long muscle fascicles within arboreal taxa in order to maximize mobility and flexibility, the early attainment of peak fascicle lengths may consequently reflect the importance of agility within this mobile and highly arboreal species. Similarly, observed myological trends in forearm strength are supported by previous in vivo data on grip strength within M. murinus in which senescent individuals showed no decline in forearm force relative to prime age individuals. This trend is interpreted to reflect compensation for the previously reported decline in hind limb grip strength in the hind limb with age, such that older individuals are able to maintain arboreal stability. Interestingly, the ontogenetic trajectory of each architectural variable mirrored previous observations of the masticatory musculature in M. murinus, suggesting that ontogenetic trends are relatively conserved between anatomical regions.

Anatomical and ontogenetic influences on muscle density

“…We procured our highly controlled sample from a commercial meat farm where they do not have animals that reach senescence because they would no longer be reproductively efficient. In a future extension of this work, it would be valuable to find and incorporate some data from senescent individuals perhaps from show rabbit breeders or to replicate this density research on a taxon for which a broader age range is available (e.g., the mouse lemurs used in 22 , 23 , though a larger taxon would probably yield more accurately measured densities). Additionally, further studies are needed to determine the species-specificity of these values of muscle density presented here.…”

“…For this assumption to be true, muscle mass and muscle volume would have to change proportionally because density is defined as the mass per unit of volume. However, given that other architectural properties of muscle have been demonstrated to be dynamic and change throughout the lifespan of animals (see, for example 21 – 23 ), this is also likely to be the case for muscle density. For example, muscle mass has been shown to decline with increasing age—a condition that has been termed as sarcopenia 24 , 25 .…”

Effects of long‐term ethanol storage on muscle architecture

“…For a subset of our sample (n = 75 muscles from 25 specimens), we chemically digested the muscles to break them down into individual fascicles (following Herrel et al, 2008;Boettcher et al, 2020;Leonard et al, 2019) to evaluate the effects of long-term ethanol storage on average fascicle length-a key variable in the study of muscle fiber architecture. To do this, once the mass and densities had been obtained for each of these muscles, we submerged them into 35% nitric acid until the muscle fascicles could be separated without tearing them.…”

“…and correlations have been found between these properties and, for instance, dietary (Curtis & Santana, 2018;Deutsch et al, 2019;Eng, Ward, Vinyard, & Taylor, 2009;Fabre, Herrel, Fitriana, Meslin, & Hautier, 2017;Hartstone-Rose, Deutsch, Leischner, & Pastor, 2018;Hartstone-Rose, Hertzig, & Dickinson, 2019;Hartstone-Rose, Perry, & Morrow, 2012;Herrel, De Smet, Aguirre, & Aerts, 2008;Perry & Hartstone-Rose, 2010;Perry, Hartstone-Rose, & Wall, 2011;Perry, Macneill, Heckler, Rakotoarisoa, & Hartstone-Rose, 2014;Perry & Wall, 2008;Santana, 2018;Santana & Cheung, 2016;Taylor, Jones, Kunwar, & Ravosa, 2006;, and locomotor adaptations (Berbel-Filho, Pereira, & Martinez, 2013;Ogihara et al, 2017;Oishi, Ogihara, Endo, Ichihara, & Asari, 2009). While studies investigating these architectural properties of musculature ideally do so using fresh specimens (e.g., Deutsch et al, 2019;Hartstone-Rose et al, 2012;Hartstone-Rose et al, 2018;Leischner et al, 2018), the inclusion of rare or endangered specimens which can often only be obtained from the fluid collections of museums can necessitate the use of previously preserved specimens (e.g., Boettcher et al, 2020;Leonard et al, 2019), or require both fresh and preserved specimens to be combined within one sample (e.g., Hartstone-Rose et al, 2019). Museums commonly use formalin fixation followed by ethanol storage as a means to preserve soft-tissue specimens (Simmons, 2014).…”

Myological variation in the forearm anatomy of Callitrichidae and Lemuridae

“…Once muscle fascicles became separable, muscles were removed from acid and placed in an aqueous 50% glycerol solution to neutralize residual acid and prevent overdigestion. Following Boettcher et al, (2020b) and Leonard et al, (2020), muscle fascicles were then separated manually with forceps into a representative sample of at least 40 fascicles to be photographed alongside a scale bar for subsequent digital data collection. The photographs of the fascicles were measured using the ImageJ 1.52a software package to record the average lengths of individual fascicles for each muscle.…”