Animals at the extremes: Polar biology

# Course questions

## Question 1

Which of the factors (a)-(g) is/are a valid reason(s) for the fact that penguins are numerous and diverse around Antarctica but absent from the Arctic?

- (a) The climate in the Arctic is too severe for penguins.

- (b) There are not enough fish in Arctic waters to sustain a population of penguins.

- (c) Penguins are excluded from the Arctic by the presence of bears.

- (d) Penguins are excluded from the Arctic by the presence of seals.

- (e) Penguins evolved in the Southern Hemisphere and have never occurred naturally in or around Europe, Asia or North America.

- (f) In general, there are fewer birds in the Arctic than in the Antarctic.

- (g) In general, there are fewer vertebrate animals in the Arctic than in the Antarctic.

## Answer

Only (e) is correct. In general, the climate is harsher in Antarctica than in the Arctic. There are plenty of fish in the Arctic Ocean (although they are of different species from those in the Southern Ocean). Polar bears eat seals, not birds although they might eat a penguin if the species ever met). Leopard seals are common in coastal waters of the Southern Ocean and penguins are their principal prey, but the two species have coexisted successfully for millions of years. Many migratory birds breed in the Arctic and a few, notably ptarmigan, are resident. With bears, reindeer, arctic foxes, birds, walruses and numerous fish, as well as seals, the Arctic has more species of vertebrates than Antarctica, which is without any fully terrestrial vertebrates.

## Question 2

What is the evidence that endogenous factors as well as daylength control food intake and breeding? Why are such dual control mechanisms adaptive for polar animals?

## Answer

Svalbard and Norwegian reindeer ate different amounts even when they had unlimited access to food and were exposed to the same environmental conditions, so the appetites of the two subspecies must be at least partially under endogenous control (see Figure 3).

The effects of exposure to continuous light on food intake and body mass depends upon the season at which the treatment is started. At very high latitudes, there is continuous light or continuous darkness for several months (see Figure 1), so daylength would not, by itself, be a sufficiently precise clue to seasonal breeding that must be accurately timed to favourable weather and food supply, e.g. for ptarmigan (see Figure 6).

## Question 3

Which of the statements (a)–(g) about the food intake and metabolism of polar mammals and birds is/are generally true?

1. There are many more animals relative to the food supply in polar regions than in the tropics.

2. The food supply in polar regions is highly seasonal and/or irregular.

3. Being obese makes animals lethargic.

4. Very lean animals are incapable of strenuous physical activity.

5. Polar animals have fewer predators than tropical animals so they can afford to be fat and lazy.

6. The only food available in polar regions is less nutritious than that available in the tropics.

7. Polar animals have a higher metabolic rate and so need more food than those in the tropics.

## Answer

Only (b) is generally true. It would be impossible to sustain permanently more animals than the food supply could support. At the beginning of fasting, bears are both fat and lethargic, but one does not cause the other: male penguins are least active during incubation, and walk briskly back to the open sea at the end of the fast when they are thin. Bears and penguins, like large animals everywhere, have few predators, but smaller birds, lemmings and fish have plenty of predators. Since many fish and terrestrial animals are fat while in arctic regions, it cannot be true that the available food is less nutritious. There is no clear evidence for (g): BMR actually falls below normal for weeks at a time in bears and penguins and is less in winter- than summer-acclimatized foxes (Figure 4).

## Question 4

Explain in a few sentences why measurements of the concentration of ICTP and PICP in blood samples taken during peak activity and in the middle of the dormant period failed to explain how black bears avoided increased risk of bone fracture at emergence from dormancy.

## Answer

The marker for bone breakdown, ICTP, was more abundant during dormancy, but the differences between denning and activity for the same bears were not significant for PICP, the marker for bone formation (Figure 13). These observations explain how the skeleton is weakened during dormancy, but not how it is reformed during activity. PICP has a brief peak at the start of the active period (Figure 14), probably indicating rapid strengthening of the skeleton that was not detected by this sampling procedure.

## Question 5

What information relevant to the metabolism of energy and/or proteins can be obtained from measurements of:

1. body temperature

2. RER

3. the composition of the blood

## Answer

(a) Body temperature is a crude and somewhat indirect measure of activity. In both bears and penguins, body temperature falls slightly at the start of fasting (see Figure 11). Since there is no evidence that the body insulation has changed, the rate of production of heat (i.e. BMR) must have decreased. Body temperature rises abruptly during egg-laying in female penguins; the synthesis of components of the relatively large egg and the effort of laying would raise BMR.

(b) RER provides information about the chemical composition of the fuel broken down by respiration. An RER close to 1 indicates that carbohydrates are being used; a lower RER means that proteins and/or lipids are being used.

(c) Chemical analysis of blood plasma provides detailed information about the concentrations of different fuels available to cells (e.g. fatty acids, glucose, T-hydroxybutyrate) and about the rates of formation and accumulation of breakdown products of metabolism (e.g. urea, creatinine, T-hydroxybutyrate).

## Question 6

Which of the statements (a)-(h) about the structure and arrangement of adipose tissue is/are true?

1. All polar mammals have thick subcutaneous adipose tissue.

2. Birds, whether polar or not, do not have thick subcutaneous adipose tissue.

3. Polar bears have thick subcutaneous adipose tissue because they live in very cold climates and swim in the sea.

4. In Carnivora, the partitioning of adipose tissue between superficial and intra- abdominal depots depends upon body size.

5. In Carnivora, the partitioning of adipose tissue between superficial and intra- abdominal depots depends upon habits and habitat.

6. In naturally obese mammals, fatter individuals always have more adipocytes than thinner individuals.

7. In naturally obese mammals, as an individual gets fatter, its adipocytes enlarge.

8. The number and size of adipocytes in relation to fatness are quite variable in naturally obese mammals and in humans.

## Answer

Only (d), (g) and (h) are true. Many arctic mammals have subcutaneous adipose tissue for part of the year, because many are large and they naturally become obese because the food supply is seasonal. However, in mammals, having subcutaneous adipose tissue is not an essential or unique feature of living in an arctic climate. In Carnivora, including bears, the partitioning of adipose tissue is related to body size, not to habits or habitat (see Figure 16). In any one animal, the adipocytes enlarge with increasing fatness, but the total adipocyte complement, and hence the relationship between mean adipocyte size and fatness, differs from individual to individual.

## Question 7

Explain in a few sentences:

1. The effects of the absence of red blood cells on delivery of oxygen to the muscles of Chaenocephalus.

2. Why most fish living in very cold water remain on or very near the bottom, but there are plenty of invertebrates in mid- and surface waters.

## Answer

(a) Without haemoglobin in red blood cells, oxygen is carried in solution in the blood, but the solubility of oxygen in water increases with decreasing temperature. The enormous reduction in density of cells in the blood offsets the increase in its viscosity at low temperature, so the blood flows faster for the same work of pumping by the heart. These factors mean that at −1.5 °C, the muscles of an icefish receive nearly as much oxygen as those of a fish with haemoglobin, but such fish would be unable to swim efficiently in warm water.

(b) The salt concentration of the body fluids of fish is less than that of seawater so their supercooled body fluids are at risk from freezing if they come into contact with ice crystals (Figure 24). Because ice is less dense than water, it floats at the surface. The salt concentration of the body fluids of most invertebrates is similar to that of seawater, so they are not vulnerable to freezing until the seawater itself freezes. There are also more diving birds such as penguins and skuas, and marine mammals, all of which eat fish, in surface waters.

## Question 8

Explain in a few sentences why:

1. The diet of large fish and marine mammals changes seasonally, and may differ greatly for members of the same species found at different sites.

2. The relative abundance of the fatty acids in adipose tissue triacylglycerols of herbivores such as reindeer would reveal little about the diets of animals found in different areas.

## Answer

(a) The abundance of marine plankton, and hence of all the animals that eat it, changes greatly with the season, and with currents and ice movements. The climate, and hence the presence and availability of prey species, change erratically and may differ greatly between apparently similar sites. So large predatory fish and mammals (and seabirds such as albatross) range over a wide area and have a varied diet.

(b) The composition of fatty acids in the storage lipids is determined by the metabolism of the microbes in the rumen and by that of the reindeer tissues, more than by the composition of fatty acids in the diet.

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