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Researchers from diverse disciplines, including organismal and cellular physiology, sports science, human nutrition, evolution and ecology, have sought to understand the causes and consequences of the surprising variation in metabolic rate found among and within individual animals of the same species. Research in this area has been hampered by differences in approach, terminology and methodology, and the context in which measurements are made. Recent advances provide important opportunities to identify and address the key questions in the field. By bringing together researchers from different areas of biology and biomedicine, we describe and evaluate these developments and the insights they could yield, highlighting the need for more standardisation across disciplines. We conclude with a list of important questions that can now be addressed by developing a common conceptual and methodological toolkit for studies on metabolic variation in animals.

The non-breeding season presents significant energetic challenges to birds that breed in temperate or polar regions, with clear implications for population dynamics. In seabirds, the environmental conditions at non-breeding sites drive food availability and the energetic cost of regulatory processes, resulting in variation in diet, behaviour and energetics; however, very few studies have attempted to understand if and how these aspects vary between populations. We investigated whether non-breeding location influenced diet, behaviour and energetics in the common guillemot Uria aalge. We studied guillemots from four UK breeding colonies, two located on the west coast of Scotland and two on the east. We quantified non-breeding distribution, foraging behaviour and activity budgets of 39 individuals from July to March, using geolocation–immersion loggers and time-depth recorders, and used feather stable isotope signatures to infer diet during the post-breeding moult. We calculated energy expenditure and investigated whether the peak (an indicator of the potential vulnerability to marine threats) varied between colonies. Individuals were spatially segregated according to the coastline they breed on, with west coast guillemots distributed off the west coast of the UK and east coast guillemots distributed off the east coast. Diet and behaviour were more similar in guillemots that shared a breeding coastline than those that did not, as west coast guillemots foraged at a lower trophic level, spent less time diving and engaged in more pelagic foraging than east coast guillemots. However, energy expenditure was remarkably similar between colonies, peaking during late February/early March, indicating that, during our study period, there was high synchrony between colonies in the timing of potential vulnerability to marine threats. Therefore, any anthropogenic changes that result in decreased food availability or increased energy expenditure during late winter may have greater im

1. The ability of individual animals to balance their energy budgets throughout the annual cycle is important for their survival, reproduction and population dynamics. However, the annual cycles of many wild, mobile animals are difficult to observe and our understanding of how individuals balance their energy budgets throughout the year therefore remains poor. 2. We developed a hierarchical Bayesian state-space model to investigate how key components of animal energy budgets (namely individual energy gain and storage) varied in space and time. Our model used biologger-derived estimates of time-activity budgets, locations and energy expenditure to infer year-round time series of energy income and reserves. The model accounted for seasonality in environmental drivers such as sea surface temperature and daylength, allowing us to identify times and locations of high energy gain. 3. Our study system was a population of common guillemots Uria aalge breeding at a western North Sea colony. These seabirds manage their energy budgets by adjusting their behaviour and accumulating fat reserves. However, typically during severe weather conditions, birds can experience an energy deficit over a sustained period, leading to starvation and large-scale mortality events. 4. We show that guillemot energy gain varied in both time and space. Estimates of guillemot body mass varied throughout the annual cycle and birds periodically experienced losses in mass. Mass losses were likely to have either been adaptive, or due to energetic bottlenecks, the latter leading to increased susceptibility to mortality. Guillemots tended to be lighter towards the edge of their spatial distribution. 5. We describe a framework that combines biologging data, time-activity budget analysis and Bayesian state-space modelling to identify times and locations of high energetic reward or potential energetic bottlenecks in a wild animal population. Our approach can be extended to address ecological and conservation

Information on seabird foraging behaviour outside the breeding season is currently limited. This knowledge gap is critical as this period is energetically demanding due to post‐fledging parental care, feather moult and changing environmental conditions. Based on species’ body size, post‐fledging parental strategy and primary moult schedule we tested predictions for key aspects of foraging behaviour (Maximum Dive Depth (MDD), Daily Time Submerged (DTS) and Diurnal Dive Activity (DDA)) using dive depth data collected from three seabird species (common guillemot Uria aalge, razorbill Alca torda and Atlantic puffin Fratercula arctica) from the end of the breeding season (July) to mid‐winter (January). We found partial support for predictions associated with body size; guillemots had greater MDD than razorbills but MDD did not differ between razorbills and puffins, despite the former being 35% heavier. In accordance with sexual monomorphism in all three species, MDD did not differ overall between the sexes. However, in guillemots and razorbills there were sex‐specific differences, such that male guillemots made deeper dives than females, and males of both species had higher DTS. In contrast, there were no marked sex differences in dive behaviour of puffins in July and August in accordance with their lack of post‐fledging parental care and variable moult schedule. We found support for the prediction that diving effort would be greater in mid‐winter compared to the period after the breeding season. Despite reduced daylight in mid‐winter, this increase in DTS occurred predominantly during the day and only guillemots appeared to dive nocturnally to any great extent. In comparison to diving behaviour of these species recorded during the breeding season, MDD was shallower and DTS was greater during the non‐breeding period. Such differences in diving behaviour during the post‐breeding period are relevant when identifying potential energetic bottlenecks, known to be key drivers