Fatness and fitness in seals and humans
Obesity in humans is bad news. Being too fat is associated with a range of diseases from cardiovascular problems to many forms of cancer; from sleep disturbance to arthritis and diabetes.
For seals, on the other hand, being fat is essential for health, fitness and survival. Fatter pups are more likely to survive their first year than their thinner friends. Pups of fatter mums are bigger and put on weight faster. Fatter adult males can defend territory or females against rivals during the breeding season.
Why is fat so important for these animals? To start with, seals need a thick blubber layer to insulate themselves against the cold water they spend so much of their time in. They also need fat as a metabolic fuel to sustain them when they come ashore to moult and rest and breed. There are no fish to eat on a seal haul out! Their blubber fat can keep seals going for weeks (in the case of grey seals) or even months (in the case of elephant seals) without suffering any ill- effects of going without food or water.
So for physiologists, understanding how seals regulate their fat stores and remain healthy despite having body content of up to 45% may provide useful insights into management of diabetes and obesity. For ecologists, a better understanding of how seals regulate their blubber fat will allow us to predict the consequences of stress or altered diet or food availability on individual energy balance and future population size.
But how can we find out how these big, wild, and often inaccessible animals regulate their energy balance?
The typical approach to investigate the control of energy balance in seals is to relate estimates of body fatness or the duration of a fasting period to measurements of hormones and metabolites in the blood. Fatness can be estimated either by measuring blubber depth using ultrasound or by using a labelled water dilution technique to calculate percentage body fatness. This approach has provided ideas that can be tested about the roles of different hormones in fat regulation in seals. However, a snap shot measurement is very limited for a number of reasons. Hormones and metabolite levels in the blood change much more rapidly than body fatness, which means there can be a lag between a signal in the blood stream and a change in the fat tissue. Short-lived signals may be missed and the role of hormones can be misinterpreted. A change in two measurements does not mean that one necessarily causes the other. Without experimental data, observational information can be misinterpreted.
An experimental approach
Experiments in seals have provided important insights into the regulation of their fat. Feeding seal pups extra fish before they start their period of fasting after weaning causes them to use more fat to fuel their metabolism and spare their precious protein. Dexamethasone, an artificial hormone that mimics the effects of the stress hormone, cortisol, causes fasting pups to use a greater proportion of protein to meet their energy demands. However, experiments in seals can be logistically challenging. They must be simple and the risk to the animals justified and minimised. Dose responses and time courses can be hard to achieve and it is impossible to perform a control and a treatment in the same animal at the same time, which can limit experimental design and statistical power.
Seals in a dish: adapting an established method for use in wildlife
In biomedical science, small pieces of tissue from biopsies obtained from patients or volunteers can be kept alive for short periods of time and used in experiments to investigate how fat works. We’ve borrowed this method and applied it to seal blubber. By using this in vitro experimental method for the first time, rather than working at the whole animal level, we can do a greater range of experiments, perform simultaneous controls and treatments and dig into mechanisms much more easily to investigate local regulation of fat stores in seals.
The method involves taking a blubber biopsy and mincing it into small pieces, called explants. The explants are kept alive in a nutrient mix in a dish or tube placed in an incubator that keeps the levels of oxygen, carbon dioxide and temperature similar to levels that the tissue experiences in the body. One biopsy can provide lots of explants, which allows us to run several experimental treatments at once on one tissue that all came from one animal. This helps us to identify more subtle effects of the treatments we are interested in. Explants could also be used in situations where the experiments on whole animals would not be ethically justified, too logistically difficult to perform or involve a species of conservation concern.
Insights from seal blubber explants
In our experiments we found that blubber from male seals has a higher rate of fat breakdown than blubber from females, which might explain why females can start to lay down fat depots earlier in the year and sooner after they finish moulting than males. We’ve also shown that blubber alters the way it handles glucose and mobilises fat in response to external glucose and treatment with an artificial stress hormone, hydrocortisone. Blubber from female seals seems to hang onto fat more when exposed to hydrocortisone. These results help us to predict how fat tissue and therefore whole animal energy balance might respond to changes in food intake or short term stress, including whether males and females will differ in their vulnerability.
Seals have relatively high levels of blood glucose and make more glucose in the liver than expected for their size, which is odd because they don’t use glucose much as a metabolic fuel. This unusual metabolic characteristic hasn’t been explained, but has led to the suggestion that seals might be useful to better understand diabetes. Our data from the explant experiments show that blubber can produce a lot of lactate. Lactate production causes a drop in pH, which can cause metabolic problems involved in diabetes and obesity. Lactate can be recycled back into glucose by the liver. Our observation from the explants led us to speculate that seals may produce a lot of glucose to avoid the big drop in pH that would otherwise result from lactate made by their large fat depots. This could explain the mysterious high glucose production and show how seal metabolism is similar to or different from metabolism in diabetic and obese people.
Challenges of working with fat tissue
Getting fat tissue from seals is not that easy! It requires a lot of infrastructure (fast boats; gear to safely catch and handle the animals), expertise (skilled seal handlers; trained boat crew and skipper) and personnel (people to haul nets, take notes and monitor animals). The field work needs to be planned carefully for days when the weather is calm enough and when there is a big tidal range to give us a long working window while the seals are hauled out. Even with careful planning and kit checks, engines can fail, propellors can hit sand banks, seals can haul out in inaccessible places, the weather can change fast for the worse or we can sometimes just be too slow to get to the haul out in time.
Once we have the biopsy, the centre of the tissue can quickly run out of oxygen and nutrients so it’s crucial to keep it in a warm bath of nutrients and get it back to the lab to process it as fast as possible. Oh, and everything has to be sterile. Sterile? On a seal haul out? On a sand bank in the North Sea? Or a filthy seal colony? Yup. And we manage it. Amazingly!
Now we have the explant approach up and running we are using it in a NERC funded project to investigate the impact of persistent organic pollutants, or POPs, on blubber function. POPs are contaminants that accumulate in fat tissue and become more concentrated in animals further up the food chain. That means, along with whales, dolphins and polar bears, that seals have the highest POP loads of any animals because they are so fat and because they are top predators. POPs have been implicated as contributing factors in the development of obesity and diabetes in humans. Recent work in polar bears shows POPs can alter signalling pathways in their fat cells. In Arctic seals, POP levels in blubber have been linked to changes in the activation of fat regulating genes. Our project will show if and how POPs alter blubber tissue function and hormone responses. The data will help to predict the relative risk to both human and seal energy balance from POP levels in fat tissue.