Browsing by Subject "dolphins"

The ocean environment poses several adversities to usual mammalian function. Perhaps most consequential to life is the lack of air underwater. For marine mammals, like whales and dolphins, that are required to perform breath-hold dives to forage for prey, this necessitates a unique set of adaptations to efficiently manage oxygen resources while diving. In an era of global environmental change, this hostile habitat is expected to become increasingly challenging for air-breathing mammals; warming waters will necessitate deeper foraging trips and noisier oceans may compel unplanned dives to evade perceived threats. An understanding of marine mammals’ solutions to the physiological challenges of a dually-constrained lifestyle is therefore important not only to reveal how marine mammals are built to thrive where other mammals, particularly humans, falter but also the extent to which these adaptations may scale in a changing ocean environment. In this dissertation, I explore the physiological adaptations, particularly those of the cardiovascular and respiratory systems, that this taxon has evolved to mediate the challenges associated with breath-hold diving. I take a multi-scale approach to investigating these physiological traits, exploring hypotheses at the molecular, tissue-specific, and organismal scales. Accordingly, I leverage both familiar and emerging methodologies in the field of marine mammal physiology to examine adaptations that support the extended dive capacities of whales and dolphins. Cellular and molecular responses to environmental stimuli influence tissue-specific and organismal physiological responses. Despite the inextricable link between molecular and organismal physiology, studies of the molecular adaptations of marine mammals for diving are limited, in part due to the logistical complexity of obtaining molecular samples from this difficult-to-study group. To fill this gap, my collaborators and I deployed RNA-seq and enzymatic assays to examine the molecular-level changes that occur in bottlenose dolphins (Tursiops truncatus) performing extended breath-holds (Chapter 1). We demonstrated that dolphins exhibit transcriptomic and proteomic changes that occur in a time-dependent fashion during breath-holding that could support their ability to maintain selective perfusion during diving. The upregulation of ALOX5, a gene targeted for the treatment of eosinophilic asthma in humans, and lipoxygenase suggest a mechanism by which differential gene regulation could contribute to sustained vasoconstriction during the dive response. These findings illustrate the importance of responses at the molecular level for supporting the unique physiology of marine mammals. Coordinated, tissue-specific physiological changes are central to the mammalian dive response. During dives marine mammals drastically reduce their heart rate (fH) while narrowing the blood vessels that supply their peripheral tissues, thereby slowing oxygen consumption of the heart itself as well as reducing the supply of oxygen-rich blood to non-essential tissues. The factors that modulate fH and contribute to diving bradycardia are complex, largely because they are numerous and often linked, but are crucial to understanding oxygen consumption patterns and, ultimately, whole-organism physiology and behavior. Using simultaneous electrocardiographic (ECG) recordings and respirometry, I show that whales and dolphins exhibit a strong cardiorespiratory coupling that may support the conservation of blood oxygen for hypoxia-intolerant tissues during a breath-hold. This variation in fH with breathing, or respiratory sinus arrhythmia (RSA), is modulated by breathing rate (fR) in bottlenose dolphins such that slow breathing results in larger fluctuations in fH (Chapter 2). Following a breath, fH increases rapidly to a maximum and then decreases through the end of the inter-breath interval (IBI). Notably, some of the minimum fH’s of the RSA were comparable to reported diving fH’s for this species suggesting the importance of apnea alone in modulating the fH of a diving marine mammal. I also demonstrate that this cardiorespiratory coupling scales with body size and fR across five cetacean species suggesting both physical scaling laws and dynamic physiological needs play a role in determining the magnitude of the RSA (Chapter 3). These studies highlight the complexity of tissue-specific responses and the need to contextualize physiological rates. Ultimately, it is the interactions of tissues that determine organismal physiology – the fundamental constraint on an organism’s behavior. To investigate the connection between organismal physiology and behavior, I developed a novel method for extracting fR from free-ranging whale biologging tag data (Chapter 4). I found that the high-flow rate and large tidal volume breaths of cetaceans generate movement signals which are captured by the accelerometers of biologging tags, enabling respiration event detection from historical biologging tag datasets. I applied this tool to movement data collected from short-finned pilot whales in Cape Hatteras, North Carolina using digital acoustic recording tags (DTAGs) and examined variation in respiratory patterns associated with diving (Chapter 5). I found that whales vary their pre- and post-dive surface duration and post-dive fR in proportion to the duration and activity of upcoming dives illustrating the physiological challenge of preparing for and recovering from breath-hold diving and highlighting optimization of surface behavior required to support breath-holds. Physiological responses are coordinated across multiple levels of biological organization necessitating the use of various tools and techniques to fully elucidate the adaptations that support marine mammals’ capacity to dive for minutes to hours without a breath. The findings of this dissertation underscore that the physiological function of breath-holding whales and dolphins is coordinated across scales, the physiological responses of cardiovascular and respiratory systems are linked, and sensing vital rates can provide insights into the physiological demands of a dive. Future studies should continue to focus on integrating methods across scales to better understand the physiological function of these animals and its plasticity in a changing ocean.

Vertebrate animals undergo a constellation of physiological responses when they experience submersion. These responses, collectively known as the dive response, include apnea (breath-hold), bradycardia (a reduction in heart rate), and peripheral vasoconstriction (the restriction of oxygenated blood to organs critical to life). Cetaceans, the order of mammals that includes whales, dolphins, and porpoise, are obligate air-breathing mammals and one of the few mammalian taxa to become fully aquatic. Given this evolutionary trajectory, cetaceans are an excellent model for investigating the physiological extremity of the dive response.

One limiting factor in dive response research involving cetaceans is the relative lack of non-invasive physio-logging devices that can be attached in free-swimming animal contexts. To address this gap, my collaborators and I invented a new multi-sensor, suction cup-attached device called the FaunaTag. The FaunaTag was custom-built to enable non-invasive collection of cardiovascular physiological data in cetacean species. Equipped with a novel contact sensor column that interfaces with the body surface of the tagged animal, the FaunaTag's near-infrared spatially-resolved diffuse reflectance bio-optical sensor and its accelerometer and gyroscope sensors were used to investigate aspects of the dive response in bottlenose dolphins (Tursiops truncatus), the most accessible and well-studied member of the cetacean order.

In the first set of experimental trials, I used the FaunaTag and a new methodological approach to investigate the extent to which dolphin cardiac heart rate changes during alternating bouts of stationary surface free-breathing and submerged apnea. In these trials, the FaunaTag and its unique contact sensor measured the vibrations associated with the cardiac cycle at the dolphin's chest wall. These vibrations were used to compute instantaneous heart rate and instantaneous kinetic energy associated with cardiac contractility. During these trials, we also tested the efficacy of the FaunaTag's near-infrared bio-optical sensor to measure dolphin heart rate before, during, and after apnea, with the FaunaTag placed at a variety of body locations, and the extent to which optically-computed heart rate estimates matched the cardiac frequency estimates calculated from cardiac vibrations.

I found that instantaneous heart rate estimates measured in this study were consistent with the heart rates computed using electrocardiography in previous studies involving these same animals. I also observed expected patterns of bradycardia during extended apneas, respiratory sinus arrhythmias following respiration events, and a return to a baseline heart rate shortly after respiration. I also found that instantaneous kinetic energy of cardiac contraction varies between free-breathing and breath-holding trial phases, with a decline to a stable apneic baseline during submerged breath-holds, followed by a steep rise following cessation of apnea and an eventual return to a variable but reduced post-apnea baseline. The FaunaTag's near-infrared spectroscopy performed poorly at dorsal body locations, detected 60% of the matched heartbeats while attached to the cardiac window of the bottlenose dolphin, and achieved a match rate exceeding 90% in the best trial. Future efforts involving the FaunaTag will feature an improved bio-optical sensing module which may resolve poor optical cardiography at the dorsal surfaces of the dolphin body and other cetacean species.