The design of experiments to investigate the combined effects of multiple stressors requires exposing target organisms to multiple combinations of stressor doses. Concurrent manipulation of stressors is often infeasible with wildlife, but long-lasting health effects allow individual health to be used as an integrator of prior stressor exposure. The population of bottlenose dolphins in Barataria Bay, Louisiana, experienced long-lasting health effects after exposure to oil from the Deepwater Horizon spill. We investigated whether compromised health status affects dolphins' ability to respond to other stressors, specifically vessel traffic, potentially leading to increased collision risk. We used this case study to develop a hypothesis-driven experimental approach to assess the combined effects of multiple stressors in a large, long-lived vertebrate species. We conducted controlled vessel approaches to test whether the health status of targeted dolphins affected their behavioral responses. Our results highlighted some effects of health, suggesting that oil spills may exacerbate the effects of other stressors in coastal populations. For example, lung disease was associated with a delayed dive response, which could affect the ability to vertically avoid vessels and lead to increased collision risk. However, health effects on response probability were overall smaller than anticipated, and other contextual variables (e.g., sex, age, calf presence, prior experience, and exposure context) contributed to response variability. Our work demonstrated the value of formalizing stressor interactions as multidimensional dose-response functions and showed the feasibility of an experimental, multiple-stressor study in a wild system in which individual health status can be used as an integrator of prior stressor exposure. This approach has broad implications for other species that are difficult to handle experimentally. The quantification and management of the cumulative risk from multiple stressors on wildlife will require a combination of empirical and mechanistic approaches to inform long-term, population consequences.
The dolphin's ability to detect changes in the phase of complex echo highlights was investigated by training two bottlenose dolphins to detect a phase "jitter" applied to the second highlight of a two-highlight "phantom" echo. One dolphin had normal hearing and the other had high-frequency hearing loss. The phase jitter changed the complex echo temporal and spectral fine structure without altering the echo energy, temporal and spectral envelopes, or spectral notch spacing. Over the course of testing, the inter-highlight interval (IHI) and frequency content of the echoes were varied. When echo IHIs were less than 300 μs, phase jitter thresholds for the two dolphins were equal, despite large differences in high-frequency hearing ability. At larger time separations, the perceptual cue appeared to change, presumably from spectral to temporal. High-pass filtering of echoes suggested that the lower range of echolocation frequencies are most useful for detecting echo highlight phase shifts. Simple models for across-channel spectral profile differences and differences in repetition pitch provided mixed results: both models matched the experimental data well for some conditions but poorly for other conditions, especially as the IHI and high-pass cutoff frequencies increased.
An exceptionally large blood O2 store underlies the remarkable dive performance of elephant seals. However, elevated carboxyhemoglobin (COHb) concentrations in these seals complicate estimations of blood O2 content and O2 depletion rates, and may also affect monitoring of brain oxygenation in seals with new non-invasive near-infrared (NIR) recorders. Using hemoximetry analyses of blood samples during sleep apneas of juvenile northern elephant seals (Mirounga angustirostris), we constructed in vivo Hill plot equations and O2–Hb dissociation curves (ODCs). We found: (a) COHb and methemoglobin (both of which do not bind O2 and increase hemoglobin (Hb) affinity for O2) comprised 8
Seasonal changes in metabolic rate have been studied to varying degrees in terrestrial mammals, but relatively little is known about this phenomenon in odontocetes. Seasonal changes in thyroid hormone levels in bottlenose dolphins suggest potential seasonal effects on resting metabolic rate (RMR). To explore this potential, the RMR of nine adult and four adolescent bottlenose dolphins were measured monthly for 13 months using open-flow respirometry. Oxygen consumption was measured while dolphins were post-absorptive and during the morning to avoid diurnal hormone fluctuations. Blood samples were collected from six of the adult dolphins immediately following oxygen consumption measurements and assessed for levels of total and free T3 (fT3), reverse T3 (rT3), total and free T4 (tT4 and fT4, respectively), and cortisol. Resting metabolic rate varied seasonally in mature dolphins; RMRs were lower in the spring and higher in the summer and fall. Suppression of RMR in the spring was followed by a progressive increase in RMR through the summer and fall. The mean mass-specific RMR of adolescent dolphins (5.49 ml O2 min-1 kg-1) was higher than that of adult dolphins (4.38 O2 min-1 kg-1), and the RMR of adults did not vary between sexes. The RMR of adult dolphins was significantly related to serum concentrations of tT3 and fT3, whereas rT3, fT3 and tT3 were all significantly related to photoperiod. The entrainment of seasonal changes in dolphin RMR is probably driven by photoperiod, although the signaling mechanism mediated by melatonin is likely altered in this species because it lacks a defined pineal gland.
Pneumonia is the leading cause of morbidity and mortality of bottlenose dolphins Tursiops truncatus . We investigate a series of rare and opportunistic samples of pulmonary surfactant membranes (PSMs) extracted from lungs of seven dolphins in the care of the U.S. Navy Marine Mammal Program. We found a striking correlation between PSM structure, lipidome, and mechanical properties with the severity of lung injury. Specifically, lipidomics reveals exacerbated contents of cardiolipins, confirming a result obtained for terrestrial mammals afflicted by experimental pneumonia. Employing a battery of X-ray scattering, atomic force, and electron microscopy, we evaluate how the altered lipid composition impairs the structural integrity of the PSM and leads to dehydration and enhanced rigidity. Our findings demonstrate that the function of pulmonary surfactant membranes goes far beyond lowering alveolar surface tension, regulating their biochemical and biophysical properties with lung pathology progression. This knowledge will be useful to the development of future diagnostics and therapeutics of respiratory diseases targeting lung membranes.
