Metabolic rates determine the resources needed to maintain healthy animals and the potential impacts that these animals have on the world around them.
Bryde’s Whale by Paphop/Pixabay
The ocean is a home, playground, and hunting zone for a wide variety of creatures, including marine mammals. From dolphins and walruses to manatees and sea otters, the classification “marine mammals” umbrellas four different groups: cetaceans, pinnipeds, sirenians, and marine fissipeds. These diverse animals all have unique features that help them adapt to the watery world around them.
One of those features is their metabolic rate — the rate of energy expenditure over a specific period of time. A recent study published in Conservation Physiology, using only “gold standard” data, provided an overview of over a century’s worth of knowledge about marine mammal metabolism and the factors and behaviours that influence metabolic rates.
“We’re interested in this because we need to understand how much these animals need to survive when they are in a changing environment,” said Dr. Shawn Noren, lead author and research scientist and educator at the UC Santa Cruz. “We need to know how those calorie requirements might change with increased human disturbance or changes in the climate that makes them have to look harder for their food.”
Funded by the Office of Naval Research, Noren was tasked with reviewing the metabolism of marine mammals due to concerns with disturbances on beaked whales during routine sonar tests by the American navy. The beaked whales were observed leaving the area during these tests, often during important feeding times. These offshore, deep diving giants are elusive and mysterious, and scientists have been unable to measure their metabolic rates or gather much data on them in general. This is true for many marine mammals, especially larger species that are not easily captured or trained.
This review is particularly vital for understanding how marine mammals respond to different changes in their environment, either natural or man-made.
“There’s certainly a lot of human disturbances for a wide variety of animals,” said Noren. “For example, dolphins in the eastern tropical Pacific travel with tuna fish, but these fish are also chased by tuna fisheries. Because this disturbs the dolphins’ normal behaviours, this results in a huge energy expenditure for the mammals.”
Monica Max West/Pixabay
“In other parts, walruses are being impacted because melting ice means that they can’t use the ice as a base for their foraging,” said Noren. “They have to sit on land, and that requires them to swim further to their favorite food resource — clams. So, we are trying to figure out how much harder these animals are working, and to do that, we needed reliable data to figure out these extra costs (calories).”
Collecting metabolic data is a complicated task. There are many different ways to document marine mammal metabolism, each with varying degrees of reliability. One gold standard method measures places animals in a tent-like apparatus where their oxygen and carbon dioxide levels can be monitored. Other methods require the repeated capture and handling of individuals. This is nearly impossible to do in the wild. Marine mammals who live their entire lives at sea or remain submerged for long periods of time are almost impossible to measure, creating large gaps in the data.
“Shawn Noren was initially asked to condense what we already knew about cetaceans — whales, dolphins, and porpoises,” said David Rosen, study author and assistant professor in the Marine Mammal Research Unit at UBC’s Institute for the Oceans and Fisheries. “What she found out was that we don’t really know a whole lot because they’re extremely difficult to measure under controlled circumstances. There are whole groups of whales that we don’t have measurements for simply because, right now, it’s technologically impossible to do that.”
Pixabay
There are also different types of metabolic rates that can be measured: basal metabolic rate (BMR) which is the normal base rate of an adult animal; resting metabolic rate (RMR) which measures the rate of an animal at rest; and field metabolic rate (FMR) which measures free-living individuals. While all three rates are extremely important and all use well-defined measurement factors, FMR is considered more ecologically relevant because it includes baseline energy consumption (growing and resting) while also containing the energy expenditure as the animal moves and survives.
For applying and interpreting field metabolic rates, underlying factors become even more restricted and complex. Even within one reproductive season for one species, FMR can vary greatly depending on the behaviour performed. For example, breeding adult Antarctic fur seals rates are very different depending on if you are measuring fasting territorial males or measuring lactating females.
“When you’re talking about the metabolic rate, or daily energy expenditure, it’s hugely dependent on the life stage of the animal,” said Rosen. “For example, when you capture a female sea lion nursing a pup onshore, it’s metabolic rate will be very different if you catch it when it has been ashore for a while versus if it has just returned from a long foraging trip.”
Out in the wild, calories are what every animal is trying to collect if they are not reproducing or resting. During the colder months when food may not be as readily available, calories are even more precious as survival is dependent on how much blubber an animal can put on. What animals eat, and how they expend this energy, is essential for understanding how animals may adapt to future conditions. Metabolic rates allow scientists to measure and track the rates of calorie consumption and expenditure, allowing us to understand how climate change and other factors are currently impacting the survival of wild marine mammals.
TheOtherKev/Pixabay
But one clear result of the study is that there is no single estimate of energy expenditure for “a marine mammal”. There are significant differences among major groups (such as whales versus sea lions) and even differences between species within each group. As climate change and human impact increase, estimates of energy expenditure are crucial to marine mammal conservation efforts. Metabolic rates determine the resources needed to maintain healthy animals and the potential impacts that these animals have on the world around them.
What are the Metabolic Rates of Marine Mammals and What Factors Impact this Value: A review was published in Conservation Physiology
Tags: Cetaceans, David Rosen, dolphins, energetics, Faculty, killer whales, Marine mammals, metabolic rates, MMean Lab, MMRU, sea lions, Sea otters, tuna, walrus, whales