SummaryTwo theories of how energy metabolism should be associated with longevity, both mediated via free-radical production, make completely contrary predictions. . We sought associations between longevity and individual variations in energy metabolism in a cohort of outbred mice. We found a positive association between metabolic intensity (kJ daily food assimilation expressed as g/body mass) and lifespan, but no relationships of lifespan to body mass, fat mass or lean body mass. Mice in the upper quartile of metabolic intensities had greater resting oxygen consumption by 17% and lived 36% longer than mice in the lowest intensity quartile. Mitochondria isolated from the skeletal muscle of mice in the upper quartile had higher proton conductance than mitochondria from mice from the lowest quartile. The higher conductance was caused by higher levels of endogenous activators of proton leak through the adenine nucleotide translocase and uncoupling protein-3. Individuals with high metabolism were therefore more uncoupled, had greater resting and total daily energy expenditures and survived longestsupporting the 'uncoupling to survive' hypothesis.

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Microsatellites were used to investigate fine-scale spatial and temporal genetic structure of a grey seal breeding colony, using samples collected throughout the colony (1997) and at a fine scale (2000-2002). Behaviour of breeding seals on North Rona, including philopatry and high breeding site fidelity, suggested female kin may cluster together. However, low F ST-scores indicated no genetic differentiation between the major breeding aggregations, categorised as regions in the colony. Nevertheless, the pairwise relatedness of mothers within regions was significantly higher than the relatedness of mothers between regions. There was evidence of kin clustering within only one region in the colony. Therefore, sustained philopatry appears sufficient to produce differences in relatedness at a within-colony scale (> 500 m), but not at finer scales. However, within one region, mothers' pairwise relatedness decreased significantly with increasing distance between pupping sites. Pairwise relatedness of neighbouring females within this region was also higher than expected. Conversely, in the other regions mothers that were considered likely to have social interactions, based on their spatial and temporal proximity, were not significantly more related to each other than random. This suggests the social associations of mothers on North Rona detected previously are unlikely to be influenced by kin selection.

We studied factors influencing daily energy expenditures (DEE) of male least weasels (Mustela nivalis) using the doubly labelled water technique. The relationship between ambient temperature and DEE formed a triangular pattern, characterized by invariance of the maximum DEE and an inverse relationship between minimum DEE and temperature. A simple energetic model relating the DEE of male weasels to activity time (AT) and ambient temperature predicted that, across seasons, less than 10 per cent of measurements approach the upper bound of observed DEE. Male weasels were able to maintain a relatively constant maximum energy output across varying temperatures by adjusting their AT to changes in temperature. They achieved maximum energy expenditures in winter due to high thermoregulatory costs, and in spring and summer due to high levels of physical activity. This pattern exemplifies a 'metabolic niche' of a small mammal having extremely high energy expenditures primarily driven by ambient temperature.

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