Field energetics and lung function in wild bottlenose dolphins, Tursiops truncatus, in Sarasota Bay Florida

1. Introduction

Climate change is causing perturbations in oceanic and coastal circulation and water temperature, which can alter the spatial and temporal distribution of animals and their food resources [1]. Increased utilization of the planet's oceans by a growing human population has impacted marine ecosystems. For marine mammals known effects include increased ocean noise, increased incidence of human interactions like fishing gear entanglements and ship strikes, exposure to toxic chemicals (e.g. organochlorines such as DDT, PCBs, heavy metals, etc.), and eutrophic waste from agriculture, expanding industry and developing urban areas. Previous studies have suggested that marine predators may be valuable bio-indicators of environmental health [2]. As top-level predators, marine mammals may be particularly prone to effects of climate change and deteriorating environmental conditions; serving as the proverbial canary in the coal-mine, indicating significant changes at lower trophic levels and showing the impact those changes could have on other mammalian species, including humans, that share the same water, air and fish.

An essential aspect of understanding the ecology and distribution of any species is to define their resource and energy requirements. While considerable work has been done on the bioenergetics and respiratory physiology of seals and sea lions, technical and logistical limitations have prevented comparable measurements in wild cetaceans. One approach to estimate field metabolic rate in large whales has been to measure their breathing frequency [3–8]. The assumption is that increased breathing, as occurs during active foraging or travel, is tightly linked to the energy requirements of the activity state and can be used to assess the field metabolic rate for different activities, e.g. resting or swimming at the surface and diving. While this method is attractive in its simplicity, it assumes that the average tidal volume (V_T) and O₂ exchange fractions are known, and that they remain constant over the measurement period. This is not always the case [9], and improved knowledge of the cardiorespiratory physiology, and the temporal patterns of respiration and gas exchange following exercise or diving would significantly improve the estimate of metabolic cost [10–12]. There are also other techniques that can provide proxy estimates of field metabolic rate, such as the dilution of doubly labelled water in the body water pool, activity records or heart rate monitoring, but these techniques require careful validation to assess how changes in their relationships may change over different seasons, years, activities, life stage, etc. Still, the resting metabolic rate (RMR) is often used as a tool to provide an estimate of the minimum energy requirements for a species.

Our objective was to measure lung function and estimate RMR for common bottlenose dolphins (Tursiops truncatus), using breath-by-breath respirometry in wild animals. Understanding metabolic and respiratory requirements is of particular interest because it is vital to identifying and understanding the physiological limitations for survival of a species. In addition, respiratory disease is a major cause for morbidity and mortality in wild cetaceans, and increasing environmental stressors, such as exposure to pollutants, is likely to exacerbate this problem [13,14]. Despite significant advances in technology, we still know very little about the respiratory physiology of marine mammals. Because respiratory physiology is difficult to study in free-ranging dolphins, normal lung function data for wild animals are limited [15,16]. We therefore compared data collected from wild animals with data from healthy dolphins and cetaceans managed under human care [17–21] to assess if respiratory physiology measurements may provide information that is relevant to both populations.

3. Results

Data from 2475 spontaneous breaths were collected over 4 years (2014–2017) from 32 male and female juvenile and adult bottlenose dolphins (table 1), living in and around Sarasota Bay, Florida. Measurements on the deck occurred over 20–60 min, and in water 15–20 min, with continuous measurements for at least 3 min.

3.1. Respiratory flows and timing

The average respiratory frequency (f_R) during trials was 2.7 ± 1.3 breaths min⁻¹ (range: 0.8–6.5), and did not differ among years, sex, with body mass (M_b) or position (whether held in or out of the water; all p > 0.1, t-value: 0.8).

The average maximum inspiratory flow of spontaneous breaths was 13.8 ± 3.8 l s⁻¹ (range: 6.4–21.9 l s⁻¹), which was significantly lower than the maximum spontaneous expiratory flow of 20.0 ± 7.5 l s⁻¹ (range: 7.5–39.4 l s⁻¹, t-ratio = 10.9, p < 0.01, paired-t test). The log₁₀-transformed inspiratory (Inspflow, χ²= 20.6, 1 d.f., p < 0.01) and expiratory flow (Expflow, χ²= 14.2, 1 d.f., p < 0.01) correlated with log₁₀-transformed M_b (log₁₀ [M_b], figure 1), but neither sex nor position (in-water versus out of water) affected inspiratory or expiratory flow. When separating animals into juveniles and adults, M_b did not correlate with respiratory flow (χ²= 2.9, 1 d.f., p > 0.05).

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Figure 1.

Expiratory or inspiratory respiratory flow (l s⁻¹) versus body mass (M_b, kg) for dolphin either in air or in water.

There were no differences in inspiratory (5.3 ± 1.8 l, range: 2.2–10.4) or expiratory (5.2 ± 1.8 l, range: 2.3–10.4) V_T (p > 0.05, t-stat: 1.9). For all age groups, only M_b correlated with V_T (χ²= 22.8, 1 d.f., p < 0.01, figure 2). When separating animals into juveniles (less than 10 years) and adults (greater than or equal to 10 years), log₁₀ [M_b] warranted inclusion for adults (p < 0.05, χ²= 5.2, 1 d.f.): log10[VT]=−0.18 (±0.22)+0.41 (±0.17)⋅log10[Mb], 3.1

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Figure 2.

Inspiratory tidal volume (V_T, l) versus body mass (M_b, kg) for dolphins either in air or in water. Solid line is the estimated V_T for adult dolphins (equation (3.1)) and the dotted line the regression equation for terrestrial mammals [24].

The duration of the expiratory phase (0.437 ± 0.136 s) was significantly shorter than that of the inspiratory phase (0.512 ± 0.112 s), and neither inspiratory nor expiratory durations changed with sex, M_b, or whether animals were in water or out of water (p > 0.1 for all).

3.3. Metabolic rates

Resting V˙O2 (RMR) was estimated for periods during which the dolphin had remained calm for a minimum of 20 min. The sample duration used for calculating the V˙O2 ranged from 3 to 15 min (figure 3a). The estimated RMR varied substantially within and among animals and the mass-specific V˙O2 (sV˙O2) ranged from 0.76 ml min⁻¹kg⁻¹ to 9.45 ml O₂min⁻¹ kg⁻¹ (figure 3a). There was a correlation between log₁₀-transformed V˙O2 (log₁₀[V˙O2]), M_b (log₁₀[M_b]) and whether animals were positioned in (position = 1) or out of the water (position = 0, figure 3a, p < 0.05, χ²= 5.7, 1 d.f.) log10[V˙O2]=−1.15 (±0.40)+0.43 (±0.18)⋅log10[Mb]−0.13 (±0.05) ⋅position. 3.2

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Figure 3.

Rate of O₂ consumption (V˙O2, l O₂min⁻¹) versus body mass (M_b, kg) for (a) dolphins in air and in water from the current study. (b) For a range of cetaceans from the following studies: harbour porpoise [25,26], Pacific white-sided dolphin [27,28], bottlenose dolphin [17–20], beluga whale [29], killer whale [30,31].

but neither year nor sex were significant.

When separating animals into juveniles and adults, both position and log₁₀[M_b] warranted inclusion for juveniles (p < 0.01, χ²= 9.0, 1 d.f.) log10[V˙O2]=−2.75 (±0.71)+1.25 (±0.35) ⋅log10[Mb]−0.23 (±0.08)⋅ position. 3.3 but neither position or M_b warranted inclusion for adults (p > 0.1, χ²= 2.1, 1 d.f.).

4. Discussion

We estimated RMR and measured lung function parameters across 2 age classes of bottlenose dolphins (juvenile and adult) over a 4-year period. When both age groups were analysed together, RMR and V_T rate scaled with M_b. When the groups were separated into juveniles and adults, RMR only correlated with M_b for juveniles. V_T scaled with M_b for adults with an allometric mass-exponent of 0.41 ± 0.17. These results are similar to those measured in bottlenose dolphins under human care, suggesting that data from the latter group provide physiological information of relevance for wild dolphins.

The measured V_T collected in this study agree well with previous measurements in cetaceans [16]. The average mass-specific V_T (sV_T) recorded in the current study was 30.9 ml kg⁻¹ (range: 13.6–67.5 ml kg⁻¹), which is similar to previous data for the bottlenose dolphin (27.5–58.8 ml kg⁻¹, [9,19,32]), the harbour porpoise (39.3–52.6 ml kg⁻¹ in [25]) and grey whale (12.0–35.3 ml kg⁻¹, [33]). Results from these past studies are from a range of experiments with animals positioned on land or in water, and from voluntarily participating animals in human care to restrained free-ranging animals. For all data, both the respiratory flow and V_T scaled with M_b, but when separated into juveniles and adults, only V_T from the latter group correlated with size. Unlike terrestrial mammals, where the mass-exponent is close to 1 (table 1 in [24]), V_T scaled allometrically with a mass-exponent of 0.41 (equation (3.1)). Consequently, the sV_T decreased with M_b and for a 150 kg and a 300 kg dolphin was 34 ml kg⁻¹ and 23 ml kg⁻¹, respectively. Similarly, the V_T as a percentage of the estimated total lung capacity (TLC_est) decreased from 38% for a 150 kg dolphin, to 27% for a 300 kg animal [16,34]. The V_T in the dolphin was greater when compared with terrestrial mammals of comparable size, but the relative difference decreased with size.

The breathing strategy of adult terrestrial mammals involves a high respiration frequency f_R, low V_T, and a brief respiratory pause following expiration. Aquatic mammals, on the other hand, have a lower f_R, a greater V_T and a respiratory pause that often lasts for several seconds to minutes following inhalation [16,35]. It has been hypothesized that the aquatic breathing strategy may enhance buoyancy management [35], or help keep the arterial partial pressure of CO₂ (PACO₂) at levels similar to that of land mammals [36]. Minute ventilation, the product of V_T and f_R, is regulated to deliver O₂ to and remove CO₂ from the alveoli. In terrestrial mammals, V_T (1.04) scales isometrically while f_R (−0.26) is inversely related to M_b, resulting in a mass-exponent for minute volume that scales with the metabolic rate [24]. Thus, the allometric changes in minute ventilation in terrestrial mammals are governed by an f_R that decreases with M_b. The data reported in the current study suggest that the decrease in mass-specific minute ventilation with size is governed by a reduction in V_T, while f_R seems to remain constant. This agrees with other data reported for cetaceans, where the mass-exponent for f_R was not different from 0 (see data on cetaceans in table 1 in [35]). Thus, as mass-specific RMR decreases with size, the reduction in minute ventilation is caused by a reduction in mass-specific V_T. Variation in V_T is more efficient to alter alveolar ventilation as it reduces the dead space ventilation. Consequently, the allometric changes in cetaceans may be an evolutionary consequence that helps reduce the work of breathing and enhances gas exchange in the aquatic environment.

The basal metabolic rate (BMR) of an animal is an estimate of the energetic cost of the basic functions required to sustain life, and it is measured under highly specific and controlled conditions, e.g. adult animals in a thermoneutral environment. In field situations, it is logistically challenging to meet these conditions, and RMR often serves as a proxy for BMR. Relatively few measurements of RMR have been made in marine mammals, and most of these have been made on animals in managed care or with animals held under quasi-natural conditions [17–21,25,37–43]. Many of the older studies concluded that the mass-specific RMR is considerably higher in aquatic species than in their terrestrial counterparts. Possible explanations for the higher RMR in dolphins include increased energetic demands due to thermoregulation or a high protein diet [44–47]. More recent studies, where the sample techniques have been refined for marine mammals, have measured RMRs that are at or near basal levels [18,21,27,29,30]. Thus, there is currently considerable controversy whether estimated BMR/RMR in marine mammals is truly elevated as compared to terrestrial species, and criticisms include limitations with experimental design (e.g. duration of measurements) and analysis of collected data [48–50]. Studies have shown that variation in RMR is affected by body condition, the thermal environment, desensitization through training and psychological state [18,19,29,30,51,52]. These past studies indicate that experimental design and conditions may significantly alter the results and could explain the large variability in RMR reported across published studies. In past studies, the mass-specific RMR for bottlenose dolphins in water ranged from 3.0 to 7.0 ml O₂min⁻¹ kg⁻¹ (M_b range 125–250 kg, [10,17–20,53]), and were in the same range as the values reported in the current study (figure 3b).

In the current study, the data from all age groups indicate that RMR scaled allometrically with a mass-exponent that was significantly lower than those reported for terrestrial mammals (equation (3.2), [54,55]. In land mammals, BMR in adults is well known to scale allometrically with body size over several orders of magnitude, but may vary greatly within species or a narrow weight range. For example, the mass-specific value may vary 2- to 3-fold, and the variation explained by M_b is often low [56,57]. When the data were separated into juveniles and adults, RMR only correlated with M_b for the juveniles (figure 3a). The mass-exponent for juveniles was greater than the adult value in terrestrial mammals (equation (3.3)).

There are a number of possible reasons for our findings. First, the measurements in the current study do not fulfil the criteria for BMR; we studied juvenile and adult animals and had no control over whether the animals were pre- or post-prandial. This may increase the variation, but we should also expect the mass-specific RMR to be higher as younger animals tend to have greater energetic requirements as they allocate considerable energy for growth. Second, breath-by-breath respirometry was used in the current study, which is experimentally challenging [19]. These limitations have been discussed previously, and it is unlikely that limitations in the experimental design would have significantly affected the results. Factors such as stress and personality are known to alter metabolic rate, and while stress often causes hypermetabolism, some species respond with a reduction in metabolic rate [56,58,59]. This may explain the low RMR in the current study, and wild dolphins may respond by reducing metabolic rate in response to any stress associated with capture. Fourth, in the current study the dolphins had been restrained for at least 20 min before measurements began, followed by repeated measurements ranging from 3 to 15 min. In some previous studies, the resting period ranged from 40 s to 4.5 min [17,19,20]. As the duration of measurement of RMR is important [21,27,60], it is possible that these extended measurement periods helped calm the animals and reduce the RMR. In fact, studies where the dolphins were resting for 10–20 min in the respirometer have reported values that are similar to those predicted by Kleiber [18,21,29,52]. Consequently, appropriately designed metabolic studies on marine mammals managed under human care provide RMR values that are similar to those in wild populations.

Studies indicate that RMR is about 30–40% of daily energy requirement [61,62], and understanding a species' metabolic cost is important for assessing energy flow within populations and ecosystems. For marine mammals, estimated or measured RMR values have been used in bioenergetics models [63–67]. Results vary greatly among models with varying assumptions, and the largest variation is generally caused by uncertainty in energy requirements and diet [64]. The ingestion rates for animals under human care have been used to provide estimates of energy requirements, but these studies are likely to overestimate metabolic rates (see references in [50]). A number of studies have measured RMR in dolphins under human care [17–21,68], but the current study is the first to provide estimates for wild animals. Our results indicate that RMR overlaps (RMR in current study for all animals: 3.7 ± 1.8 ml O₂ min⁻¹ kg⁻¹, range: 0.8–9.4 ml O₂ min⁻¹ kg⁻¹) with those in previous studies for animals under human care (3.0–7.4 ml O₂ min⁻¹ kg⁻¹) [17–19,21]. These results are similar to the values estimated by Kleiber's equation and provide a measured value that can be used in bioenergetics models for dolphins [69]. As RMR did not vary with M_b (range: 141–291 kg) in adult dolphins, the average V˙O2 (592 ml O₂ min⁻¹) can be used for an estimate of the RMR for this M_b range. For a 150 kg dolphin under the seasonal water temperature conditions of our study (spring in Sarasota Bay, FL), the RMR would be 3.9 ml O₂ min⁻¹ kg⁻¹. Assuming a multiplier of 3–6 for the daily metabolic rate of active animals [63,70], the metabolic rate would range from 11.7 to 23.4 ml O₂ min⁻¹ kg⁻¹ (126–252 MJ kg⁻¹ yr⁻¹). This value is similar to the field metabolic rates estimated from a bioenergetics model in dolphins [63].

5. Conclusion

Numerous studies have tried to identify the energetic requirements of marine mammals, and recently interest as to how anthropogenic disturbances may alter these requirements has increased. For models that attempt to predict how disturbances may alter population levels, an understanding of the eco-physiology of a study species is crucial. These data are important for helping ecologists understand the flow of energy between different trophic levels. However, there is limited understanding about the physiology of marine mammals and how physiological constraints limit survival. The data presented in this paper provide estimates on the energy requirements and respiratory physiology in two age classes (3–34 years of age) of briefly restrained wild bottlenose dolphins that had been free-ranging just prior to measurement. These data will help improve estimates from bioenergetics models and contribute to our understanding of how a changing environment may alter survival in this species.

Ethics

All work was approved by the IACUC at Texas A&M University Corpus Christi (TAMUCC-IACUC AUP#04-11) and by a research permit issued by the National Marine Fisheries Service (Scientific Research Permit No. 15543).

Data accessibility

The model and datasets used in this study are freely available at the following link: https://osf.io/6wjh8/ [71].

Authors' contributions

A.F. conceived of the study, designed the experiments, collected and analysed the data, carried out the statistical analysis and drafted the paper. R.W. coordinated the dolphin health assessments, arranged for funding, and holds the National Marine Fisheries Service Scientific Research Permit (No. 15543) and Mote Marine Laboratory IACUC approvals for the work. M.B. designed and built components of the testing equipment. M.M., J.C.S., R.W. and M.B. helped develop the testing methodology. K.M., J.A. and A.B. assisted in coordinating sampling, handling, and collection and verification of data from health assessments. J.C.S. and D.F. provided veterinary care for dolphins sampled in the field, and helped coordinate sampling in the field. All authors helped revise the paper and gave final approval for publication.

Competing interests

We have no competing interests.

Funding

Funding for this project was provided by the Office of Naval Research (ONR YIP Award # N000141410563, Dolphin Quest, Inc., and Woods Hole Oceanographic Institution.