Economics of energy

30 November 2019 - By: Alex Evans

Economics of energy

For all creatures great and small, energy is the currency of life – but survival doesn’t often come cheap.  At the 2019 SEB Annual Meeting, animal physiologists and ecologists gathered together during the “How do animals manage their energy expenditure?” session to discuss the diverse strategies used by animals to balance their energetic budgets.


There are few phenomena in nature more physically demanding than the long-distance flights of migratory birds. For some avian species, these flights can span multiple continents and last for weeks on end, meaning that sleep deprivation and lack of food pose serious challenges to their survival. However, when mid-migration stopovers are a scarce commodity, how should birds partition their time and energy to maximise their chances of survival? This is a question that Andrea Feretti, PhD student at the University of Vienna, Austria and University of Veterinary Medicine, Konrad Lorenz Institue of Ethology in Austria, is focused on solving. Through collaboration with a team of researchers that included his PhD supervisor, Leonida Fusani, Andrea set out to uncover how small trans-continental migratory birds are able to balance their needs for both sleep and sustenance.

Due to their migratory nature and their prior roles in research, garden warblers (Sylvia borin) of the Italian island of Ponza quickly became the focus of the team’s investigation [1]. “During spring migration, Ponza is used as stopover site by thousands of migratory species after crossing the Mediterranean – one of the largest ecological barriers for many European passerine birds,” Andrea explains. “These species migrate at night and when they reach a stopover site, they need to rest but also to replenish their fat stores.” To assess how the warblers handled this energetic balancing act, they first caught the birds and took baseline physiological measurements such as their current fat reserves. Next, they recorded videos of the warblers while they slept to identify how sleep behaviors differed between birds with smaller and larger fat stores, leading to some surprising results.

“We found that the sleep posture was in fact affected by the physiological condition,” explains Andrea. “Birds with a low amount of fat reserves slept mainly with their head tucked in the scapular feathers, whereas birds that were in good condition preferred to sleep untucked!” To understand exactly why the hungry warblers preferred to sleep tucked, the team conducted further experiments with respirometry equipment and simulated predator noises that revealed sleeping tucked in was energetically cheaper than sleeping untucked by reducing heat loss. However, tucking postures tended to leave the birds more vulnerable to approaching predators – creating a trade-off between energy efficiency and safety. “We did not expect to find such a strong difference between the two sleeping postures in terms of metabolic rate,” says Andrea, “Although there were good reasons to think that birds reduce heat loss by tucking their heads in their feathers, we were surprised to see that they actually reduce their alertness when sleeping in this position.”


Since migrations are such a fundamental aspect of life for many bird species, improving our understanding of how birds manage their energy reserves through sleep can provide conservationists with a better idea of what constitutes good stopover sites for maximising species survival. “From a conservation standpoint, our findings indicate that migratory songbirds may benefit from stop-over habitats that not only provide sufficient food, but that are also conducive to undisturbed sleep,” adds Andrea. “We will continue to study the physiological bases of decision making in migratory birds, which is a field that has received far less attention than navigation or migratory routes,” concludes Andrea, adding that “the interface between the ecology and physiology of sleep seems to be a very promising avenue of research for the future.”


 Weight management is not just a human problem – it is something that many animals struggle with on a daily basis to maintain the energy stores required for life’s many challenges. While diet and physical activity certainly play a role in an organism’s ability to control these energy stores, what other factors are at play? Kim Mathot, behavioural ecologist and physiologist at the University of Alberta, is working to improve our understanding of how energy storage strategies vary across both free-living and captive birds. “I started studying birds purely by chance,” recalls Kim. “As an undergraduate student, I took a summer research position studying the foraging behaviour of shorebirds and while I didn't know much about birds at that time, I was soon hooked.” From her time working with free-living birds, Kim learned first-hand about the limitations of experimental research in the wild, but she was keen on further investigating how foraging decisions were influenced by physiological and energetic demands. “Therefore, experiments with captive birds were the logical next step,” she adds.

According to Kim, the long-lived red knots (Calidris canutus) make for a very solid choice when it comes to examining the energetics of captive birds. “Their diet makes quantifying their intake very easily, and we can measure their stomach size using ultrasound, which lets us ask lots of interesting questions about digestive physiology.” Interestingly, the research that Kim presented at SEB was not the result of a single focused study, but rather a bonus feature achieved by analysing intriguing sets of data from two previous experiments. “The first was an experiment where we looked at how perceived predation danger affected activity budgets in these birds [2],” explains Kim. “The second was designed to study how behaviour and physiology change as birds age [3].” Together, the data from these experiments provided some interesting insights into how these red knots were managing their masses.

The main finding of the study was that to lose weight - birds eat less and exercise more. “Probably not a surprise to anyone who has ever tried to manage their own weight,” explains Kim, “but across individuals, food and exercise were not actually the best predictors of body mass.” Instead, she found that individual differences in metabolic rates were the major drivers of body weight, as the heaviest birds not necessarily those that ate the most or exercised the least. “Even though these results are consistent with the advice given to dieters, the simplicity of it, or maybe the universality, was a bit surprising.” Kim suggests that an interesting, albeit messy, direction for further research could be to examine a deeper suite of physiological and dietary variables, including food intake, activity and digestive efficiency, which can be calculated by measuring the amount of metabolisable energy in a bird’s droppings. “It would be a bit tedious to collect, but I recently learned about diapers designed for messenger pigeons that could potentially be modified for such a study on knots,” Kim adds.

Despite the constraints of working with free-living birds, Kim is eager to not dismiss the importance of studying wild behaviours. Controlled lab experiments allow for precise manipulation of variables, but these scenarios are often much more simplified than the complexities of wild populations. “Field studies are great because they are the only way we can study what goes on with birds under the full suite of ecological conditions they have evolved to cope with,” she concludes. “By comparing field and lab studies to better understand particular observations and patterns, we can get the best of both worlds”.


For more than a decade, Vincent Careau at the University of Ottowa, has been exploring many aspects of animal energetics, but a recent trend in physiological data collection from wild animals has opened up new opportunities to analyse and compare patterns of energy expenditure across a wide range of lifeforms. “Recently, other researchers have been extracting information about the energy management strategies of animals by looking at the relationships between basal metabolic rates and daily energy expenditure,” explains Vincent. “Measuring these traits can be challenging, but while working with my collaborator Lewis Halsey from the University of Roehampton, I realised that published datasets of heart rate could be used to estimate and compare these variables.”

Vincent and Lewis started this project by reaching out to fellow researchers that had published datasets of heart-rates in wild animals using modern biologging techniques, as well as using their own heart-rate data collected from species including king penguins [4] and even humans [5]. After collating all the information, they then extracted key information such as the average daily heart rate and lowest daily heart rate, which were used as proxies for daily energy expenditure and resting metabolic rate, respectively. To maximise both the intra- and inter-specific scope of the project, the team collected data from a diverse range of vertebrates comprising nine bird species, six mammal species and one fish species, which accounted for a staggering total of 46,539 days of recorded data across these species. From this pool of data, the team were then able to start looking for patterns at the individual and taxonomic group levels. “The assumption that animals exhibit discrete energy management patterns without variation seems simplistic,” says Vincent. “Instead, we suggest that animals can exhibit gradations of different energy management patterns and that they will fluctuate as their environmental context changes.”  Indeed, one of the main findings of their study was that these energy management strategies were not necissarily fixed all year round, but instead varied over time in response to key life cyclce events such as reproduction. These fluctuations suggest that some animals are able to adapt their energy allocations to meet their needs at specific times.

“We also found that the prevalent models of energy management are different at the taxonomic group level than at the within-individual level,” explains Vincent. “At the group level, either the independent or performance model apply, whereas the allocation model mainly applies at the individual level.” Across their taxonomically diverse data pool, the team found that some indviduals within a species were more able to partition more energy to all aspects of their life than others. “This means that some individuals have more energy to invest in both the active and resting components of their energy expenditure,” explains Vincent, “but for a given indivudal, an increase in one of these components is linked to a decrease in the other, which suggests some form of trade-off occuring by these individuals.”


Diet and exercise often come hand-in-hand for humans; or for our fishy cousins, fin-in-fin. Many species of fish swim in collective groups, and as one of life’s most fundamental activities, feeding plays a significant role in how these groups behave. Lucy Cotgrove, a PhD student and behavioural physiology at the University of Glasgow, is working to find out more about the relationship between social behaviours and feeding in fish. “We are becoming increasingly interested in the influence of physiology on collective behaviours. Previous research by Steph McLean [6] found that in schooling fish, the front-most fish tended to have higher metabolic rates and would move to rear positions in the school after feeding,” explains Lucy. “However, this research only looked at fish continuously swimming in the flow of a swim-tunnel, so we wanted to investigate these behaviours in free-swimming fish.”

Through a collaboration with Chongqing University in China, Lucy was able to investigate the interesting interactions between the behaviour and physiology of Qingbo carp, a schooling fish that is a common and economically relevant species for the region. “I had to build the arena with food tubes in a way that would allow bloodworm food items to be delivered into the arena at random places and intervals unpredictably for the fish,” she explains. Using a flow-through respirometry setup and a behavioural arena tank, Lucy and her team were able to measure the standard metabolic rate (basal metabolic rate) and maximum metabolic rate (highest rate of oxygen consumption) of the schooling fish, as well as recording behavioural metrics with video cameras. “We also looked at specific dynamic actionb, which is a measure of how much oxygen is consumed per food particle,” she adds.

While Lucy’s analysis of her data is still ongoing, she has made two important findings so far. Firstly, the fish at the front of free-swimming groups tend consume more food items, which supports the earlier findings from Steph’s swim-tunnel experiments. Secondly, the fish that ate more food were generally consuming more oxygen, which again supports the theory that fish with higher metabolic rates maintain more dominant positions within the school. The next steps of Lucy’s analysis will focus on the inter-individual relationships between fish in feeding groups. “For example, we want to know if individual fish prefer to swim next to the same fish, even if this means moving positions within the group,” she explains.

As Lucy’s primary interests are focused on the collective behaviours of the fish, she has kept her eye out for interesting feeding strategies that have occurred within her experimental schools. “One surprising feeding behaviour occurs when a leading fish overshot a food item, as there is usually a smaller fish at the back of the school that will speed up to gobble it,” explains Lucy. “This is more of an opportunistic strategy that avoids any conflict of jostling for dominant positions at the front!” Finally, although she is not currently planning to work with other animal taxa just yet, Lucy is curious to know if this relationship extends beyond fishkind. “It would be interesting to see if these group feeding behaviours are similar in mammal and bird species that also have strong social dynamics,” she adds.



[1] Ferretti, Andrea, et al. "Sleeping unsafely tucked in to conserve energy in a nocturnal migratory songbird." Current Biology 29.16 (2019): 2766-2772.

[2] Mathot, K. J., P. J. Van den Hout, and T. Piersma. 2009. Differential responses of red knots, Calidris canutus, to perching and flying sparrowhawk, Accipiter nisus, models. Animal Behaviour 77:1179-1185. 

[3] Kok., E. M., J. B. Burant, A. Dekinga, P. Manche, D. Saintonge, T. Piersma, and K. J. Mathot. 2019. Within-individual canalization contributes to age-related increases in trait repeatability: a longitudinal experiment in red knots. American Naturalist 194: 455-469.

[4] Halsey, L. G., et al. "Changes in the foraging dive behaviour and energetics of king penguins through summer and autumn: a month by month analysis." Marine Ecology Progress Series 401 (2010): 279-289.

[5] Halsey, Lewis G., et al. "Flexibility, variability and constraint in energy management patterns across vertebrate taxa revealed by long‐term heart rate measurements." Functional ecology 33.2 (2019): 260-272.

[6] McLean, Stephanie, et al. "Metabolic costs of feeding predictively alter the spatial distribution of individuals in fish schools." Current Biology 28.7 (2018): 1144-1149.