The research, published in Frontiers in Marine Science, relied on the doubly labeled water method, a technique that has been used for decades to measure energy expenditure in humans, birds and seals but has rarely been applied to cetaceans. The approach hinges on the different fates of two isotopes of water in the body: oxygen-18-labeled water leaves as both carbon dioxide and water, while deuterium-labeled water exits only as water. The difference in their turnover rates reveals how much carbon dioxide the animal produced — and therefore how much energy it burned — over the measurement period.

Applying this method to dolphins required a rare combination of circumstances. The Sarasota Bay community, roughly 170 animals, is one of the best-studied cetacean populations on Earth, with individuals of known age, sex and reproductive status. Researchers selected 14 dolphins aged 6 to 23 years — four females and ten males — that were not pregnant and were not nursing young calves. During routine health assessments, the animals were caught in a shallow-water net, dosed with isotopes via stomach tube, fitted with a small VHF radio tag on the dorsal fin, and released. Three to ten days later, the same animals were recaptured and resampled.

The results revealed a pronounced seasonal pattern. In summer, when water temperatures reached about 28 degrees Celsius, the dolphins burned energy at 5.66 watts per kilogram on average, compared with 3.98 watts per kilogram in winter, when water temperatures dropped to roughly 17 degrees. Statistical models confirmed the seasonal effect was robust, explaining nearly half the variation in metabolic rate. Total water influx followed the same pattern, averaging 207 milliliters per kilogram per day in summer versus 117 in winter — a difference that carries important clues about what the animals were eating.

Perhaps the most surprising finding was that the dolphins were heavier and fatter in winter. Their body fat content, estimated from total body water dilution, averaged about 22 percent in winter but only around 5 percent in summer, and mean body mass was slightly higher in the colder months. Because blubber serves as insulation, this makes sense: the animals build up energy reserves in autumn that help them weather the winter thermal gradient, when the temperature difference between their core and the surrounding water can reach 23 degrees Celsius.

Why would dolphins burn more energy in warm water than in cold? The authors consider several explanations. One possibility — increased swimming to shed heat — seems unlikely, because behavioral observations showed no significant seasonal difference in activity budgets. A more probable driver is skin biology. Dolphin skin cells proliferate rapidly in warmer water, and the cost of repairing and rejuvenating the epidermis rises with temperature. Thermal imaging studies show that dolphin skin temperature closely tracks ambient water, and in summer the animals’ bellies turn pink from increased peripheral blood flow, while in winter they appear pale as circulation is restricted to conserve heat.

Prey dynamics offer a third explanation. Winter dolphins had greater body mass, lower water influx and lower metabolic rates, suggesting they were foraging more efficiently on energy-dense fish. Striped mullet, a staple of the Sarasota dolphins’ diet, can range from about 3.8 to 8.4 kilojoules per gram depending on fat content, and local fishers report that mullet fatten in autumn before moving offshore to spawn. The data support this: dolphins with higher metabolic rates gained more mass or lost less, and the relationship between metabolic rate and preformed water intake indicates that individuals burning more energy were consuming more prey.

The water flux measurements also allowed the team to estimate seawater ingestion — a long-debated question in marine mammal physiology. By subtracting the water contained in prey and the water produced metabolically from total water influx, the researchers calculated that summer dolphins may have taken in roughly 16 liters of seawater per day and winter dolphins about 7 liters, representing 44 and 33 percent of their total water influx respectively. These estimates align with laboratory studies showing that dolphin skin is permeable to water, and that the animals can maintain water and salt balance thanks to their highly efficient, multi-lobed kidneys.

Placed in a broader context, the measurements suggest that bottlenose dolphins lead an energetically expensive lifestyle comparable to that of sea lions and fur seals — surface-swimming predators with high costs of transport. This contrasts sharply with the conservative energy strategies of deep divers such as beaked whales and elephant seals, which travel mostly underwater at low cost. The comparison underscores how lifestyle, not just body size, shapes metabolic intensity across marine mammals.

The findings also carry a warning for a warming ocean. If dolphin metabolic rates rise as waters warm — whether through direct thermal effects on skin and peripheral tissues or indirectly through changes in prey availability and quality — the animals may have limited capacity to compensate, especially when resident communities are constrained by the geography of bays and estuaries. Range shifts have already been documented for bottlenose dolphins during El Niño events and in the warming western North Atlantic. For the dolphins of Sarasota Bay, already operating at a high energetic baseline, climate change may leave little room to maneuver.

Subject of Research: Seasonal field metabolic rates and water flux of free-ranging bottlenose dolphins measured with doubly labeled water

Article Title: Metabolic rates and water influx of free-ranging bottlenose dolphins in Sarasota Bay, Florida

Article References: Costa, D. P., Wells, R. S., Worthy, G. A. J., Holser, R. R., Waples, D. M., Read, A. J., Scott, M. D., & Irvine, A. B. (2026). Metabolic rates and water influx of free-ranging bottlenose dolphins in Sarasota Bay, Florida. Frontiers in Marine Science, 13, Article 1917705. https://doi.org/10.3389/fmars.2026.1917705

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1917705

Keywords: bottlenose dolphins, field metabolic rate, doubly labeled water, Sarasota Bay, marine mammal energetics, thermoregulation, blubber, seawater ingestion, climate change, cetacean physiology, water influx, body condition