Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus)

Marine mammals rely on maintaining sufficient blood oxygen levels while diving to prevent drowning. Generally, oxygen is cognitively imperceptible to mammals that instead sense rising carbon dioxide as a proxy for low oxygen. Not perceiving oxygen, however, is risky for diving mammals. We argue that any ability to alter dives based upon direct perception of oxygen should have been strongly selected for. We exposed diving seals to inhaled gas mixes that were experimentally altered to affect circulating levels of oxygen and carbon dioxide. Dive duration was positively correlated with circulating oxygen levels but unaffected by carbon dioxide levels and pH. These results suggest that seals do cognitively perceive circulating oxygen and use this to alter dive behavior.

Ecophysiology is a relatively recent interdisciplinary field, and although active prior to the 75th anniversary of the American Society of Mammalogists (ASM), it has grown in breadth since then. This growth is in part a result of advances in technology that have reduced the size and improved the portability of key instrumentation, and also made sequencing of proteins and nucleic acids faster and less expensive. Here, we demonstrate the breadth of recent research on the ecophysiology of mammals, quantify the research activity of the past 25 years, and consider future research needs. Some of the most active areas of research have related to maintenance of homeostasis, associations of physiological traits with the evolution of varied life styles and life histories, and reproductive physiology. Key findings involve allometry and scaling, energetics and thermoregulation, phenotypic plasticity and epigenetics, and the importance of microbial symbionts. With respect to predictions of trends in mammalian ecophysiology, the strongest themes relate to conservation biology, in large part related to rapid climate change, habitat destruction, and other anthropogenic factors. How our mammalian fauna adapts (or not) to these changes will be of great interest, and has the potential to affect the science of mammalogy in the future. La ecofisiología es un campo interdisciplinario relativamente reciente, y a pesar de que era un área vigorosa antes del septuagésimo quinto aniversario de la Sociedad Americana de Mastozoología (ASM), desde entonces ha crecido en envergadura. Este crecimiento es en parte el resultado de los avances tecnológicos en donde el tamaño de los instrumentos se ha reducido facilitando de ese modo la portabilidad de instrumentos claves, y al mismo tiempo, permitiendo que la secuenciación de proteínas y ácidos nucleicos sea más rápida y menos costosa. En este trabajo, se demuestra el alcance de la investigación reciente sobre la ecofisiología de los mamíferos, se cuantifica la actividad científica de las investigaciones de las últimas dos décadas y se hacen predicciones para los próximos 25 años. Algunas de las áreas más activas de investigación se han asociado con el mantenimiento de la homeostasis, la relación entre los rasgos fisiológicos con la evolución de diferentes estilos e historias de vida, y la fisiología reproductiva. Los hallazgos clave incluyen la alometría y la escala, la energética y la termorregulación, la plasticidad fenotípica y la epigenética, y la importancia de los simbiontes microbianos. Con respecto a las predicciones sobre las tendencias de la ecofisiología de mamíferos, los temas más comunes están relacionados con la biología de la conservación, en su mayor parte a consecuencia del rápido cambio climático, la destrucción del hábitat y otros factores antropogénicos. La forma en que nuestra fauna de mamíferos se adapta (o no) a estos cambios, a nivel fisiológico y de otro tipo, será de gran interés en el futuro, teniendo el potencial de influir a la ciencia de la mastozoología.

1Marine mammals are characterised as having physiological specializations that 2 maximize use of oxygen stores to prolong time spent under water. However, it has 3 been difficult to undertake controlled studies to determine the physiological 4 limitations and trade-offs that marine mammals face while diving in the wild under 5 varying environmental and nutritional conditions. For the past decade, Steller sea 6 lions (Eumetopias jubatus) trained to swim and dive in the open ocean away from 7 the physical confines of pools participated in studies that investigated the 8 interactions between diving behaviour, energetic costs, physiological constraints, 9and prey availability. Many of these studies measured the costs of diving to 10 understand how they vary with behaviour and environmental and physiological 11 conditions. Collectively, these studies show that the type of diving (dive bouts or 12 single dives), the level of underwater activity, the depth and duration of dives, and 13 the nutritional status and physical condition of the animal affect the cost of diving 14 and foraging. They show that dive depth, dive and surface duration, and the type of 15 dive result in physiological adjustments (heart rate, gas exchange) that may be 16 independent of energy expenditure. They also demonstrate that changes in prey 17 abundance and nutritional status causes sea lions to alter the balance between time 18 spent at the surface acquiring oxygen (and offloading CO2 and other metabolic by-19 products) and time spent at depth acquiring prey. These new insights into the 20 physiological basis of diving behaviour furthers understanding of the potential 21 scope for behavioural responses of marine mammals to environmental changes, the 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Diving metabolism of Steller sea lions 3 energetic consequences of these adjustments, and the consequences of approaching 23 physiological limits. 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Diving metabolism of Steller sea lions 4The need to study diving metabolism 25Marine mammals are well known for being able to remain submerged for extended 26 durations. Early studies of marine mammals (mainly phocid or "true" seals) 27 investigated the anatomical features by which they managed to do so. These include 28 adaptations for withstanding the intense pressures experienced at depth and 29 greater relative on-board oxygen stores than their terrestrial counterparts, which 30 allows them to remain active during submergence breath-holding. For example, 31 elevated oxygen storage is present in both circulating haemoglobin and the 32 myoglo...