How much food energy does an adult dolphin need from fish to survive and thrive? How much does a young animal need, and how much milk energy does a mother dolphin transfer to her nursing calf? These are the types of questions we hope to answer for the wild dolphins resident to Sarasota Bay and nearby areas.
To do this, we measured the metabolism (field metabolic rate) of two adult female bottlenose dolphins and their calves using a technique called the doubly-labeled water method. Doubly-labeled water measures field metabolic rates by comparing the amounts of particular isotopes in the blood before and after a few days have passed. To do this, animals receive a solution of water (H2O) with extra isotopes of heavy hydrogen and oxygen – these heavier isotopes occur naturally in the environment and are harmless when administered at higher concentrations like this. Then, a blood sample is taken a few days later to measure how much doubly-labeled water remains, to determine how quickly the water was used by the body. This rate of water use is a measure of how fast or slow that animal’s metabolism was during that time, and therefore how many food calories it utilized. For lactating mothers, the turnover rate of heavy water is also an indicator of milk-energy transfer to nursing calves.
This is our second field season collecting this type of information, enabling us to build upon our sample of four adult females from 2015. Particularly exciting are our measurements of milk transfer rates between moms and calves – the first of their kind for a wild cetacean! We already know something about the content of dolphin milk from other studies: it is higher in calories than human breast milk, containing about 3-4 times as much fat. What we do not yet know is just how much milk mother dolphins need to produce for their calves. Our measurements this year will allow us to not only understand the amount of extra energy mother dolphins need to acquire from fish to support their calves, but also the overall energy needs of their calves and how much of that they are able to acquire for themselves while foraging for fish alongside mom. Funding for this project has been provided by the Joint Industry Program.
This article appeared on page 15 of the 2017 SDRP Annual Report, Nicks n Notches.