Ecology of Storage and Allocation of Resources: Animals

Abstract

Most organisms store lipids and/or carbohydrates for energy production during fasting. Lipids store much more energy per unit mass. Long‐chain fatty acids are absorbed and stored almost unaltered, so serve as indicators of natural diets and food chains. Vertebrates and higher arthropods have tissues specialised for lipid storage and management. Adipocytes are 40–85% triacylglycerols and occur in various intra‐abdominal and superficial sites in all tetrapods and some fish. Some mammalian adipose depots have site‐specific properties specialised to local, paracrine interactions with adjacent cells and tissues. In mammals, adipocyte volume is determined by anatomical location, body size and natural diet as well as fatness. The anatomical patterns of relative sizes of adipocytes are similar in all terrestrial mammals but species differ in the relative abundance of adipocytes in each depot. The chemical compositions of storage molecules are adapted to the ecological processes they serve, including migration, hibernation, lactation or unpredictable food supply. Storage of energy and other materials is essential to many aspects of animals' ecology. Adipose tissue can reach 50% body mass before migration or breeding fasts with superficial depots expanding most, especially in large vertebrates.

Key Concepts:

  • Storage is essential unless nutrients are continuously available.

  • Storage materials must minimise weight, volume and toxicity; some vitamins and minerals are too toxic to be stored in more than small quantities.

  • In animals, glycogen and acylglycerols can be safely stored in large quantities and metabolised to produce energy and/or tissues.

  • Much more energy can be stored as lipid than as glycogen, though the latter is more quickly mobilised.

  • The chemical composition of storage lipids is adapted to body temperature and the ecological processes that they support. Triacylglycerols of cold‐adapted poikilotherms and hibernating mammals have more unsaturated fatty acids than those of homeotherms.

  • Some migratory birds maximises energy density and efficient mobilisation of the storage lipids during prolonged flight by adaptive desaturation.

  • Specialised lipid storage tissues include the arthropod fat body and vertebrate adipose tissue that sequesters large quantities of triacylglycerols.

  • Vertebrate adipocytes are nearly spherical cells, 0.01–4 nL in volume, specialised to take up, hold and release storage lipids and to the regulation of appetite and metabolism.

  • In mammals, the mean volume of adipocytes is determined by anatomical location, body size and natural diet as well as by fatness.

  • Some mammalian adipocytes have site‐specific properties specialised to paracrine interactions.

Keywords: adipose tissue; adipocyte; fatty acid; triacylglycerol; hibernation; migration; fat indices; secondary compound; lymph node

Figure 1.

A thick section of fixed white adipose tissue from an obese adult rabbit. The storage triacylglycerols have dissolved away in the preparation process so most adipocytes appear ‘empty’. The thin layer of cytoplasm near the cell membrane and extracellular mesh of collagen are stained pink, and none of the small nuclei are visible. The small bright red cells are erythrocytes (red blood cells) in blood capillaries that permeate the tissue. A larger blood vessel is visible at the top left. The adipocytes are roughly spherical but only a few are sectioned through the centre thus revealing their actual diameter of about 100 μm (0.1 mm). Copyright © Pond CM.

Figure 2.

The effects of body size and natural diet on the structure of adipose tissue in mammals. Upper sets of figures are the mean volume of adipocytes in nL (10−9 L); lower figures are the proportions of the total dissectible adipose tissue found in each major depot indicated by dotted lines (small intermuscular depots shaded). (a) Stoat (Mustela erminea), figures are the means of five specimens, body mass 0.19–0.4 kg, 3.2–6.3%. (b) Two‐humped camel (Camelus bactrianus), a pregnant female, body mass 650 kg, 6.1% fat. (c) A pregnant female lion (Panthera leo), body mass 167 kg, 13.3% fat. (d) Domesticated horse (Equus caballus); figures are means of measurements from two geldings, body mass, 444 and 570 kg, 5.2% and 4.0% fat. Less than 1% of the camel's adipose tissue was in the inguinal depots (on the front of the thighs and the sides of the abdomen) compared with over a third in stoats. 37% of all adipose tissue was in the camel's humps, compared a maximum of 1% in the homologous depot of stoats. The camel and stoats were similar in total fatness, although the camel's adipocytes range <0.4→1.0 nL and the stoats' <0.1→0.3 nL. Adipocytes in the lion and the horses were of similar size ranges, although the carnivore was twice as fat as the nonruminant herbivore. Nonetheless, site‐specific differences in the relative sizes of their adipocytes form the same general pattern. Reproduced from Pond . Copyright © Pond CM.

Figure 3.

Mean and 95% confidence intervals of the average volume of adipocytes in a sample of adipose tissue and total body mass of wild polar bears in northern Canada, measured in females including pregnant and lactating mothers (pale grey), cubs accompanied by their mothers (mid grey) and unaccompanied males (dark grey). Data from Ramsay et al.. Copyright © Pond CM.

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Further Reading

Fantuzzi G and Mazzone T (2007) Adipose Tissue and Adipokines in Health and Disease. Totowa, NJ: Humana Press Inc.

Gurr MI (1992) Role of Fats in Food and Nutrition. London/New York: Elsevier Applied Science.

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Piersma T and van Gils JA (2010) The Flexible Phenotype: A Body‐Centred Integration of Ecology, Physiology, and Behaviour. Oxford: Oxford University Press.

Pond CM (2011) The evolution of mammalian adipose tissue, chap. 8. In: Symonds ME (ed.) Adipose Tissue Biology. ISBN 978‐1‐4614‐0964‐9. Heidelberg: Springer Science.

Speakman JR (2001) Body Composition Analysis in Animals: A Handbook of Non‐destructive Methods. Cambridge: Cambridge University Press.

Wells JCK (2010) The Evolutionary Biology of Human Body Fatness: Thrift and Control. Cambridge: Cambridge University Press.

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Pond, Caroline M(Oct 2011) Ecology of Storage and Allocation of Resources: Animals. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003207.pub2]