Porter, Marianne

Among vertebrates, whole-body movement is centered around the vertebral column. The bony vertebral column primarily consists of trabecular (spongy) bone that adapts in vivo to support mechanical demands respective to region, ontogeny, ecology, and locomotion. Previous work has extensively investigated the formfunction relationships of vertebral trabecular bone in terrestrial mammals, who use limb contact with a substrate as the primary support against gravity. However, we lack data from obligate swimming mammals whose locomotor ecology diverged from their terrestrial counterparts in two major ways: (1) body mass is supported by water’s uplifting buoyant forces and (2) swimmers power movement through dorsoventral loading of the axial body. This study examined vertebral trabecular bone mechanical properties and micoarchitecture from fully aquatic mammals, specifically sirenians (i.e. manatees) and cetaceans (i.e. dolphins and whales). We compression tested bone from several regions of the vertebral column among developmental stages in Florida manatees (Trichechus manatus latirostris) and among 10 cetacean species (Families Delphinidae and Kogiidae) with various swimming modes and diving behaviors. In addition, we microCT scanned a subset of cetacean vertebrae before subjecting them to mechanical tests. We demonstrated that in precocial manatee calves, vertebrae were the strongest and toughest in the posterior vertebral column, which may support rostrocaudal force propagation and increasing bending amplitudes towards the tail tip during undulatory swimming. Among cetaceans, we showed that greatest strength, stiffness, toughness, bone volume fraction, and degree of anisotropy were in rigidtorso shallow-divers, while properties had the smallest values in flexible-torso deep-divers. We propose that animals swimming in shallower waters actively swim more than species that conduct habitual glides during deep descents in the water column, and place comparatively greater loads on their vertebral columns. We found that cetacean bone volume fraction was the best predictor for mechanical properties. Due to the shared non-weight bearing conditions of water and microgravity, we present these data as a contribution to the body of work investigating bone adaptations in mammals that live in weightless conditions throughout life and evolutionary history.

Manatees, who use their vertebral column to propel themselves in swimming, are the product of a major evolutionary shift from land to water. This project explores the structure of trabecular (spongy) bone, which changes with force direction and magnitude, from the vertebral column of manatees. The goal of this research is to investigate the structural properties of manatee vertebral trabecular bone to better understand this animal's development and swimming mechanics. Vertebrae were dissected from four regions of the vertebral column and scanned with micro-computed tomography. Images were analyzed in BoneJ to quantify trabecular width, number, length, bone volume fraction (amount of bone/total area) and degree of anisotropy (orientation bias). Results from this project will be paired with mechanical data in future work to better understand forces on the vertebral column in a swimming mammal throughout development, and how these properties may have diverged from those found in their terrestrial counterparts.

Quantifying swimming kinematics of fishes often occurs in a lab setting using flumes, water treadmills, to examine movement. These methods rely on researchers to pick the animals swimming speed. We have been focusing on volitional kinematics in the lab where we quantify swimming as determined by the fish. However, our volitional swimming experiments are still limited to the space available in a lab setting. In this study, we examine swimming kinematics of black tip sharks (Carcharhinus limbatus) during their annual winter aggregations in South Florida. Using an aerial drone, video of sharks can be obtained through noninvasive methods in the wild, and examined frame-by-frame using the Loggerpro software. We track points along the shark’s midline to examine body curvature, tailbeat frequency, tailbeat amplitude, and whole-body swimming velocity. These data represent the first time we have been able to quantify kinematics in a free-swimming shark in the wild.