METHODS

Animals and study site

The study was carried out near the town of Hotazel (open semi-desert) (27o17’S 22o58’E) in the Northern-Cape, South Africa. Rainfall patterns are unpredictable; annually there may be 200-300mm, which occurs in isolated thunderstorms. We trapped mole-rats1 in early March 2003 when conditions were dry (no rainfall for at least a month) and in early March 2004 after approximately 40mm of rain. It took approximately 3 days to capture all individuals in their colony groups. Captured animals were maintained in their colonies in large cages (80x40 cm wide x50 cm high) with dry sawdust provided as bedding. Cages were shaded from direct sunlight but were otherwise exposed to ambient conditions. Sweet potatoes were offered ad libitum. We then injected individuals with DLW (below) and released them at the site of capture. Of the 55 animals initially captured and injected during the dry period, we re-captured 41 from 8 different colonies (14 infrequent workers, 21 frequent workers and 6 queens; recapture rates were 78%, 72% and 88%, respectively). Similarly, of the 60 individuals initially captured and injected during the wet period, we re-captured 37 from 7 colonies (13 infrequent workers, 18 frequent workers and 6 queens; recapture rates were 50%, 64% and 83%, respectively). We trapped at colonies during the wet period where we had not previously trapped during the dry period because we could not guarantee to capture the same individuals if we trapped the same colonies; hence we avoided the possibility of the data potentially including a mixture of repeated and non-repeated measures. Queens were readily distinguished by the presence of extended nipples, a perforate vagina and/or signs of pregnancy. Infrequent workers were distinguished by their morphologically distinct large heads and long bodies2,3. On completion of experiments, animals were returned to their original capture sites. The total number of animals measured is among the largest sample sizes of the DEE of free-ranging mammals ever collected.

Daily energy expenditure (DEE)

We used the DLW technique4,5 to measure the DEE (kJ/day) of captured animals. Briefly, animals were blood sampled, weighed, and then injected IP with a known mass of DLW [100g 95% APE enriched 18O water (Rotem Industries Ltd, Beer Sheva, Israel) and 50g 99.9% APE enriched 2H water (Isotec Inc. Miamisburg OH, USA) mixed with 342g 1H216O; 0.3g/100g]. Blood samples were taken after 1 h to estimate initial enrichments. Animals were then released at the site of capture. After 48 h, traps were set and animals were recaptured over the next 72 h. Final blood samples were taken after whole 24-h periods to estimate isotope elimination rates, prior to calculation of DEE6. After blood sampling, animals were taken to a field laboratory where their RMR was measured (below). Percent body fat7 and Sustained Metabolic Scope (SusMS) (an independent index of how hard an animal is working8) were also determined for each animal.

Resting metabolic rate (RMR)

We used an open circuit system9 in which a metabolic chamber (1610 cm3) was immersed in a temperature-controlled water bath maintained at 28-29oC (within the thermoneutral zone10). Dried air was pumped into the chamber at 500 ml/min, controlled by an upstream flow regulator. Oxygen concentration was measured with an oxygen analyser (S-2A Applied Electrochemistry). We determined RMR as minimal oxygen consumption when animals were seen to be at rest, after an initial hour in which they were familiarised to the chamber.

Statistical analyses

We used generalised linear modelling (GLM) to examine differences in DEE, RMR and SusMS between the sexes and between the seasons (wet and dry)11. Body mass was included as a covariate with sex, season and caste as factors. We fitted one model for each variate and determined all interaction terms. Post-hoc tests were used to determine differences between specific groups.


REFERENCES

1. Hickman, G.C. 1979 A live trap and trapping technique for fossorial mammals. S. Afr. J. Zool. 14, 9-12.

2. Bennett, N.C. & Jarvis, J.U.M. (1988). The social structure and reproductive biology of colonies of the mole-rat Cryptomys damarensis (Rodentia: Bathyergidae). J. Mamm. 69, 293-302.

3. Jacobs, D.S., Bennett, N.C., Jarvis, J.U.M. & Crowe, T.M. 1991 The colony structure and dominance hierarchy of the Damaraland mole-rat, Cryptomys damarensis (Rodentia: Bathyergidae) from Namibia. J. Zool., Lond. 224, 553-576.

4. Speakman, J.R. 1997 Doubly Labelled Water, Theory and Practice. Chapman and Hall, London.

5. Lifson, N. & McClintock, R. 1966 Theory of the turnover rates of body water for measuring energy and material balance. J. Theoret. Biol. 12, 46-74.

6. Lemen, C. & Speakman, J.R. 1997 DLWprogram and DLWuserguide. http://www.abdn.ac.uk/zoology/speakman.htm.

7. Scantlebury, M., Oosthuizen, M.K., Speakman, J.R., Jackson, C.R. & Bennett, N.C. 2005 Seasonal field energetics of the Hottentot golden mole (Ambysomus hottentotus longiceps) at 1500m altitude. Physiol. Behav. 84, 739-745.

8. Hammond, K.A., Diamond, J. 1997 Maximal sustained energy budgets in humans and animals. Nature. 386, 457-462.

9. Hill, R.W. 1972 Determination of oxygen consumption by use of the paramagnetic oxygen analyzer. J. Appl. Physiol. 33, 261-263.

10. Bennett, N.C., Clarke, B.C. & Jarvis, J.U.M. 1992 A comparison of metabolic acclimation in two species of social mole-rats (Rodentia: Bathyergidae) in southern Africa. J. Arid Environ. 22, 189-198.

11. McKenzie, J. & Goldman, R.N. 1998 Minitab Handbook for Windows 1995 and Windows NT. Addison-Wesley. Boston, MA.

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