Locomotor muscle morphology of three species of pelagic delphinids

Building Cetacean Locomotor Muscles throughout Ontogeny to Support High-Performance Swimming into Adulthood

The demands on the locomotor muscles at birth are different for cetaceans than terrestrial mammals. Cetacean muscles do not need to support postural costs as the neonate transitions from the womb because water's buoyant force supports body weight. Rather, neonatal cetacean muscles must sustain locomotion under hypoxic conditions as the neonate accompanies its mother swimming underwater. Despite disparate demands at birth, cetaceans like terrestrial mammals require postnatal development to attain mature musculature. Neonatal cetaceans have a low proportion of muscle mass, and their locomotor muscles have lower mitochondrial density, myoglobin content (Mb), and buffering capacity than those found in the adult locomotor muscle. For example, the locomotor muscle of the neonatal bottlenose dolphin has only 10 and 65% of the Mb and buffering capacity, respectively, found in the adult locomotor muscle. The maturation period required to achieve mature Mb and buffering capacity in the locomotor muscle varies across cetacean species from 0.75 to 4 and 1.17 to 3.4 years, respectively. The truncated nursing interval of harbor porpoises and sub-ice travel of beluga whales may be drivers for faster muscle maturation in these species. Despite these postnatal changes in the locomotor muscle, ontogenetic changes in locomotor muscle fiber type seem to be rare in cetaceans. Regardless, the underdeveloped aerobic and anaerobic capacities of the locomotor muscle of immature dolphins result in diminished thrusting capability and swim performance. Size-specific stroke amplitudes (23-26% of body length) of 0-3-month-old dolphins are significantly smaller than those of >10-month-olds (29-30% of body length), and 0-1-month-olds only achieve 37 and 52% of the mean and maximum swim speed of adults, respectively. Until swim performance improves with muscle maturation, young cetaceans are precluded from achieving their pod's swim speeds, which could have demographic consequences when fleeing anthropogenic disturbances.

Scaling of fibre area and fibre glycogen concentration in the hindlimb musculature of monitor lizards: implications for locomotor performance with increasing body size

A considerable biomechanical challenge faces larger terrestrial animals as the demands of body support scale with body mass (Mb), while muscle force capacity is proportional to muscle cross-sectional area, which scales with Mb2/3. How muscles adjust to this challenge might be best understood by examining varanids, which vary by five orders of magnitude in size without substantial changes in posture or body proportions. Muscle mass, fascicle length and physiological cross-sectional area all scale with positive allometry, but it remains unclear, however, how muscles become larger in this clade. Do larger varanids have more muscle fibres, or does individual fibre cross-sectional area (fCSA) increase? It is also unknown if larger animals compensate by increasing the proportion of fast-twitch (higher glycogen concentration) fibres, which can produce higher force per unit area than slow-twitch fibres. We investigated muscle fibre area and glycogen concentration in hindlimb muscles from varanids ranging from 105 g to 40,000 g. We found that fCSA increased with modest positive scaling against body mass (Mb0.197) among all our samples, and ∝Mb0.278 among a subset of our data consisting of never-frozen samples only. The proportion of low-glycogen fibres decreased significantly in some muscles but not others. We compared our results with the scaling of fCSA in different groups. Considering species means, fCSA scaled more steeply in invertebrates (∝Mb0.575), fish (∝Mb0.347) and other reptiles (∝Mb0.308) compared with varanids (∝Mb0.267), which had a slightly higher scaling exponent than birds (∝Mb0.134) and mammals (∝Mb0.122). This suggests that, while fCSA generally increases with body size, the extent of this scaling is taxon specific, and may relate to broad differences in locomotor function, metabolism and habitat between different clades.

Comparative morphology of the spinal cord and associated vasculature in shallow versus deep diving cetaceans

The cetacean vertebral canal houses the spinal cord and arterial supply to and venous drainage from the entire central nervous system (CNS). Thus, unlike terrestrial mammals, the cetacean spinal cord lies within a highly vascularized space. We compared spinal cord size and vascular volumes within the vertebral canal across a sample of shallow and deep diving odontocetes. We predicted that the (a) spinal cord, a metabolically expensive tissue, would be relatively small, while (b) volumes of vascular structures would be relatively large, in deep versus shallow divers. Our sample included the shallow diving Tursiops truncatus (n = 2) and Delphinus delphis (n = 3), and deep diving Kogia breviceps (n = 2), Mesoplodon europaeus (n = 2), and Ziphius cavirostris (n = 1). Whole, frozen vertebral columns were cross-sectioned at each intervertebral disc, scaled photographs of vertebral canal contents acquired, and cross-sectional areas of structures digitally measured. Areas were multiplied by vertebral body lengths and summed to calculated volumes of neural and vascular structures. Allometric analyses revealed that the spinal cord scaled with negative allometry (b = 0.51 ± 0.13) with total body mass (TBM), and at a rate significantly lower than that of terrestrial mammals. As predicted, the spinal cord represented a smaller percentage of the total vertebral canal volume in the deep divers relative to shallow divers studied, as low as 2.8% in Z. cavirostris. Vascular volume scaled with positive allometry (b = 1.2 ± 0.22) with TBM and represented up to 96.1% (Z. cavirostris) of the total vertebral canal volume. The extreme deep diving beaked whales possessed 22-35 times more vascular volume than spinal cord volume within the vertebral canal, compared with the 6-10 ratio in the shallow diving delphinids. These data offer new insights into morphological specializations of neural and vascular structures that may contribute to differential diving capabilities across odontocete cetaceans.

Myoglobin Concentration and Oxygen Stores in Different Functional Muscle Groups from Three Small Cetacean Species

Simple Summary

Marine mammals display several physiological adaptations to their marine environment. Higher myoglobin concentrations in their muscles compared to terrestrial mammals allow them to increase their onboard oxygen stores, enhancing the time available to dive. Most previous studies have calculated cetaceans’ onboard oxygen stores by assuming the myoglobin concentration of a single muscle to be representative of all the muscles in the body. In this study, we analyzed this assumption by comparing it to a more precise method that weighs all body muscles and measures myoglobin concentration in different functional groups.

Abstract

Compared with terrestrial mammals, marine mammals possess increased muscle myoglobin concentrations (Mb concentration, g Mb · 100g⁻¹ muscle), enhancing their onboard oxygen (O₂) stores and their aerobic dive limit. Although myoglobin is not homogeneously distributed, cetacean muscle O₂ stores have been often determined by measuring Mb concentration from a single muscle sample (longissimus dorsi) and multiplying that value by the animal’s locomotor muscle or total muscle mass. This study serves to determine the accuracy of previous cetacean muscle O₂ stores calculations. For that, body muscles from three delphinid species: Delphinus delphis, Stenella coeruleoalba, and Stenella frontalis, were dissected and weighed. Mb concentration was calculated from six muscles/muscle groups (epaxial, hypaxial and rectus abdominis; mastohumeralis; sternohyoideus; and dorsal scalenus), each representative of different functional groups (locomotion powering swimming, pectoral fin movement, feeding and respiration, respectively). Results demonstrated that the Mb concentration was heterogeneously distributed, being significantly higher in locomotor muscles. Locomotor muscles were the major contributors to total muscle O₂ stores (mean 92.8%) due to their high Mb concentration and large muscle masses. Compared to this method, previous studies assuming homogenous Mb concentration distribution likely underestimated total muscle O₂ stores by 10% when only considering locomotor muscles and overestimated them by 13% when total muscle mass was considered.