The adult body size variation of Dryas iulia (Lepidoptera,Nymphalidae, Heliconiinae) in different populations is more influenced by temperature variation than by host‐plant availability during the seasons

The body size of insects is affected by environmental conditions during development and can present considerable intraspecific variations, which can be seen as an ultimate consequence/adaptation to environmental conditions. This paper evaluated whether the body size of the butterfly Dryas iulia from subtropical populations was influenced by changing climate conditions and food source availability during the seasons. The likely reasons behind body size variation were also investigated. First, field data on body size variation, host‐plant availability and climate fluctuation throughout the seasons were recorded. Then, the effects of host‐plant species and temperature on body size were analyzed by controlled experiments. Field data revealed that body size and host‐plant availability varied significantly through the seasons. Populations had the smallest body size during the spring and the biggest size during summer, whereas host‐plant availability was lower during winter and higher during spring. The controlled experiments revealed that both temperature and host‐plant had significant effect on the plasticity of body size. Larvae subjected to winter temperature treatment led to smaller butterflies when compared to immatures reared under summer temperature treatment, and larvae fed with Passiflora misera produced bigger adults when compared to larvae reared on Passiflora suberosa. The combination of data gathered in the field and in the laboratory suggests that seasonal body size variation in D. iulia is related mainly to differences in the temperatures to which larvae are subjected during development, while host‐plant shifts caused by differential availability of food through the seasons had slightly effects on the variation observed.     

Energy metabolism of Inuit sled dogs

We explored how seasonal changes in temperature, exercise and food supply affected energy metabolism and heart rate of Inuit dogs in Greenland. Using open flow respirometry, doubly labeled water, and heart rate recording, we measured metabolic rates of the same dogs at two different locations: at one location the dogs were fed with high energy food throughout the year while at the other location they were fed with low energy food during summer. Our key questions were: is resting metabolic rate (RMR) increased during the winter season when dogs are working? Does feeding regime affect RMR during summer? What is the proportion of metabolic rate (MR) devoted to specific dynamic action (SDA), and what is the metabolic scope of working Inuit sled dogs? The Inuit dogs had an extremely wide thermoneutral zone extending down to ?25°C. Temperature changes between summer and winter did not affect RMR, thus summer fasting periods were defined as baseline RMR. Relative to this baseline, summer MR was upregulated in the group of dogs receiving low energy food, whereas heart rate was downregulated. However, during food digestion, both MR and HR were twice their respective baseline values. A continuously elevated MR was observed during winter. Because temperature effects were excluded and because there were also no effects of training, we attribute winter elevated MR to SDA because of the continuous food supply. Working MR during winter was 7.9 times the MR of resting dogs in winter, or 12.2 times baseline MR.     

Seasonal variation in the metabolic rate and body composition of female grey seals: fat conservation prior to high-cost reproduction in a capital breeder?

Many animals rely on stored energy through periods of high energy demand or low energy availability or both. A variety of mechanisms may be employed to attain and conserve energy for such periods. Wild grey seals demonstrate seasonal patterns of energy storage and foraging behaviour that appear to maximize the allocation of energy to reproduction—a period characterized by both high energy demand and low food availability. We examined seasonal patterns in resting rates of oxygen consumption as a proxy for metabolic rate (RMR) and body composition in female grey seals (four adults and six juveniles), testing the hypothesis that adults would show seasonal changes in RMR related to the reproductive cycle but that juveniles would not. There was significant seasonal variation in rates of resting oxygen consumption of adult females, with rates being highest in the spring and declining through the summer months into autumn. This variation was not related to changes in water temperature. Adults increased in total body mass and in fat content during the same spring to autumn period that RMR declined. RMR of juveniles showed no clear seasonal patterns, but did increase with increasing mass. These data support the hypothesis that seasonal variation in RMR in female grey seals is related to the high costs of breeding.     

Lower body mass and higher metabolic rate enhance winter survival in root voles,Microtus oeconomus

Although the biological significance of individual variation in physiological traits is widely recognized, studies of their association with fitness in wild populations are surprisingly scarce. We investigated the effect of individual phenotypic variation in body mass, resting (RMR) and peak metabolic rates (PMR) on mortality of the root vole Microtus oeconomus. Body mass and metabolic rates varied significantly among consecutive years and were also age dependent, as individuals born in late summer and autumn were characterized by significantly lower body mass and metabolic rates than animals born earlier. At the beginning of winter voles born in spring and early summer exhibited reduced body mass and metabolic rates, whereas animals born later maintained lower body mass and RMR, which may be interpreted as phenotypic plasticity enhancing the probability of survival. Body mass had no significant effect on vole survival during summer. In contrast, smaller individuals were characterized by lower mortality during early winter, whereas higher body mass was positively associated with survival later in the season. High body‐mass‐corrected RMR positively affected survival in both summer and winter. The effect of PMR was apparent only during winter, though its direction (and correlation with RMR) varied among years. Deep snow cover negatively affected the survival of voles in both early and late winter. Ambient temperature was positively associated with winter survival, except for late winter, when rising temperature caused flooding of vole habitat. We conclude that the lack of consistency in the directionality and strength of the effects of body mass and metabolic rates on winter survival does not undermine their importance, but rather demonstrates the ability of individuals to adjust metabolic rate to changing environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 297–309.     

The relative influence of temperature and food on the metabolism of a marine invertebrate

Many benthic marine invertebrates exhibit a seasonal cycle in activities such as feeding, growth and reproduction. In temperate regions, this seasonality is typically correlated with coincident cycles in photoperiod, temperature and food availability and it can be difficult to determine which of these environmental factors is the key driver. Polar regions are characterised by greatly reduced seasonal variation in temperature, and an enhanced seasonality of food availability; they therefore form a natural laboratory for distinguishing the ecological effects of food from those of temperature. Here, we report a study of the common shallow water urchin Sterechinus neumayeri from Rothera Point, Antarctica. This species exhibits a marked seasonal variation in metabolic rate and feeding activity (which ceases completely in winter). In this study the metabolic rate of urchins collected in late winter and held in the laboratory without food was compared with that of wild urchins undertaking the transition to summer feeding and growth. Starved urchins showed a small rise in metabolic rate in summer which could be explained entirely by the small increase in temperature (Q(10)=2.5). At the same time, the wild population showed a much larger increase in metabolic rate related largely to the costs of feeding and growth. Rates of nitrogen excretion were also much larger in wild urchins, and the O:N atomic ratio indicated that starved urchins were depending to a greater extent on lipid and carbohydrate. Gut mass and test organic content showed no change in starved urchins, indicating that metabolic substrate was being provided by the gonad. The data suggest that in wild S. neumayeri only 15-20% of the summer increase in metabolism is caused directly by the temperature rise whereas 80-85% is caused by increased physiological activity associated with feeding, growth and spawning.     

Seasonal effects on metabolism and thermoregulation abilities of the Red-winged Starling (Onychognathus morio)

Basal metabolic rate (BMR) of birds is beginning to be viewed as a highly flexible physiological trait influenced by environmental fluctuations, and in particular changes in ambient temperatures (T_a). Southern Africa is characterized by an unpredictable environment with daily and seasonal variation. This study sought to evaluate the effects of seasonal changes in T_a on mass-specific resting metabolic rate (RMR), BMR and body temperature (T_b) of Red-winged Starlings (Onychognathus morio). They have a broad distribution, from Ethiopia to the Cape in South Africa and are medium-sized frugivorous birds. Metabolic rate (VO₂) and T_b were measured in wild caught Red-winged Starlings after a period of summer and winter acclimatization in outdoor aviaries. RMR and BMR were significantly higher in winter than summer. Body mass of Starlings was significantly higher in winter compared with summer. The increased RMR and BMR in winter indicate improved ability to cope with cold and maintenance of a high T_b. These results show that the metabolism of Red-winged Starlings are not constant, but exhibit a pronounced seasonal phenotypic flexibility with maintenance of a high T_b.     

Seasonal variation in larval host plants and consequences for Heliconius erato (Lepidoptera: Nymphalidae) adult body size

Abstract Adult body size, a key life history component, varies strongly within and between Heliconius erato phyllis (Lepidoptera: Nymphalidae) populations. In the present study, we determined whether seasonal variation in adult body size is temperature related and/or determined by seasonal changes of host plants (Passifloraceae) used by the larval stage. A population of H. erato phyllis located in a Eucalyptus plantation (Barba Negra Forest, Barra do Ribeiro County, Rio Grande do Sul State, Brazil) was sampled every 45 days from March 1997 to October 1998 to quantify seasonal variation in adult body size and use of larval host plants. In the laboratory, the effects of the following factors on adult body size were quantified: (i) host plant species (Passiflora misera or Passiflora suberosa); (ii) food quantity consumed by larvae (experimentally manipulated for each passion vine species); (iii) winter and summer temperatures (15 and 25°C, respectively); and (iv) the interaction between host plant species and temperature. Adults emerging during summer were larger than those emerging in other seasons. Female butterflies oviposited selectively on P. misera even when the dominant passion vine was P. suberosa. They only switched from using P. misera to P. suberosa during later autumn and winter, when P. misera vines were completely defoliated. The laboratory feeding trials with both passion vines showed a strong positive association between food quantity consumed by larvae and adult size. They also confirmed that adults are larger when their larvae are reared on P. misera than on P. suberosa. Temperature during larval development had no effect on H. erato phyllis adult size. Thus, seasonal variation of H. erato phyllis adult size in a given place is primarily determined by the availability and quality of host plant species used by the larval stage.     

Influence of food,body size,and fragmentation on metabolic rate in a sessile marine invertebrate

Metabolic rates vary among individuals according to food availability and phenotype, most notably body size. Disentangling size from other factors (e.g., age, reproductive status) can be difficult in some groups, but modular organisms may provide an opportunity for manipulating size experimentally. While modular organisms are increasingly used to understand metabolic scaling, the potential of feeding to alter metabolic scaling has not been explored in this group. Here, we perform a series of experiments to examine the drivers of metabolic rate in a modular marine invertebrate, the bryozoan Bugula neritina. We manipulated size and examined metabolic rate in either fed or starved individuals to test for interactions between size manipulation and food availability. Field collected colonies of unknown age showed isometric metabolic scaling, but those colonies in which size was manipulated showed allometric scaling. To further disentangle age effects from size effects, we measured metabolic rate of individuals of known age and again found allometric scaling. Metabolic rate strongly depended on access to food: starvation decreased metabolic rate by 20% and feeding increased metabolic rate by 43%. In comparison to other marine invertebrates, however, the increase in metabolic rate, as well as the duration of the increase (known as specific dynamic action, SDA), were both low. Importantly, neither starvation nor feeding altered the metabolic scaling of our colonies. Overall, we found that field‐collected individuals showed isometric metabolic scaling, whereas metabolic rate of size‐manipulated colonies scaled allometrically with body size. Thus, metabolic scaling is affected by size manipulation but not feeding in this colonial marine invertebrate.     

Seasonal trends in body mass, food intake and resting metabolic rate, and induction of metabolic depression in arctic foxes (Alopex lagopus) at Svalbard

 Post-absorptive resting metabolic rates (RMRs), body mass and ad libitum food intake were recorded on an annual cycle in captive arctic foxes (Alopex lagopus) at Svalbard. During the light season in May and in the dark period in November, RMR during starvation and subsequent re-feeding were also measured. In contrast to earlier findings, the present study indicated a seasonal trend in post-absorptive RMR (in W · kg⁻¹ and W · kg^−0.75). The values in the light summer were 15% and 11% higher than the values in the dark winter, suggesting a physiological adaptation aiding energy conservation during winter in arctic foxes. Body mass and ad libitum food intake varied inversely through the year. A significant reduction in RMR (in W and W · kg^−0.75) with starvation (metabolic depression) was recorded both in May and November, indicating an adaptation to starvation in arctic foxes. The lack of metabolic depression during a period of starvation that was concomitant with extremely cold ambient temperatures in November 1994 indicates that metabolic responses to starvation may be masked by thermoregulatory needs. At very low ambient temperatures, arctic foxes may require increased heat production which cannot be achieved via below-average rates of metabolism. Accepted: 7 June 1999     

Seasonal variation in thermal energetics of the Australian owlet-nightjar (Aegotheles cristatus)

Many birds living in regions with seasonal fluctuations in ambient temperatures (T_a) typically respond to cold by increasing insulation and adjusting metabolic rate. Seasonal variation in thermal physiology has not been studied for the Caprimulgiformes, an order of birds that generally have basal metabolic rates (BMR) lower than predicted for their body mass. We measured the metabolic rate and thermal conductance of Australian owlet-nightjars (Aegotheles cristatus) during summer and winter using open-flow respirometry. Within the thermoneutral zone (TNZ; 31.3 to 34.8 °C), there was no seasonal difference in BMR or thermal conductance (C), but body temperature was higher in summer- (38.2 ± 0.3 °C) than winter-acclimatized (37.1 ± 0.5 °C) birds. Below the TNZ, resting metabolic rate (RMR) increased linearly with decreasing T_a, and RMR and C were higher for summer- than winter-acclimatized birds. The mean mass-specific BMR of owlet-nightjars (1.27 mL O₂ g^− 1 h^− 1) was close to the allometrically predicted value for a 45 g Caprimulgiformes, but well below that predicted for birds overall. These results suggest that owlet-nightjars increase plumage insulation to cope with low winter T_a, which is reflected in the seasonal difference in RMR and C below the TNZ, rather than adjusting BMR.     

Seasonal variations in feeding range and flock structure of the Rook Corvus frugilegus in eastern Ireland

The feeding range and flock structure of Rooks showed temporal variations caused mainly by the dispersion of food and reproductive behaviour.
Feeding range was restricted in spring, autumn and early winter when food availability was high and Rooks were reproductively active. A large feeding range occurred in late summer and late winter, when food availability was either generally low or locally distributed and when Rooks were reproductively inactive. The occurrence of flocks common to several rookeries mirrored variations in feeding range; mixed rookery flocks were more common in late summer and late winter. Similarly, the size of the rookery (as measured by the number of nests) was related to feeding range only when Rooks were reproductively inactive and at such times larger rookeries had greater feeding ranges.
Flock structure showed similar seasonal variations; small widely spaced flocks predominated in summer, autumn, early winter and spring, whereas large dense flocks occurred in late winter. Diurnal variations in flock structure occurred within any one season. Both seasonal and diurnal variations in flock structure may be determined by the dispersion of the prey and the feeding strategy used to obtain it, reproductive behaviour and the risk of predation.     

Onset of seasonal metabolic depression in the Antarctic midge Belgica antarctica appears to be independent of environmental cues

Seasonal progression is tracked in most animals by changes in daylength, thus allowing reliable synchrony with abundant food and favourable developmental conditions. In polar regions, daylength varies extensively, fluctuating at the highest latitudes from persistent light to persistent dark. The Antarctic midge Belgica antarctica has a narrow seasonal window in which to feed and develop, and previous work shows that this insect, despite having the elements of a circadian clock, remains continuously active when temperatures are permissive. The present study aims to clarify seasonal tracking in B. antarctica during the austral summer by monitoring oxygen consumption rates in a field population and in experimental groups exposed to shortened daylength, dehydration and chilling. Remarkably, during March, coordinated decreases in oxygen consumption are observed, ranging from 18% to 42%, in all treatment groups, indicating an anticipatory response to seasonal change regardless of the environmental cues. These results suggest that B. antarctica relies on an intrinsic mechanism to program metabolic depression at the onset of the long austral winter.     

Seasonal acclimatization of metabolism in Eurasian tree sparrows (Passer montanus)

Acclimatization to winter conditions is an essential prerequisite for survival of small passerines of the northern temperate zone. Changes in photoperiod, ambient temperature and food availability trigger seasonal acclimatization in physiology and behavior of many birds. In the present study, seasonal adjustments in several physiological, hormonal, and biochemical markers were examined in wild-captured Eurasian tree sparrows (Passer montanus) from the Heilongjiang Province in China. In winter sparrows had higher body mass and basal metabolic rate (BMR). Consistently, the dry mass of liver, heart, gizzard, small intestine, large intestine and total digestive tract were higher in winter than in that in summer. The contents of mitochondrial protein in liver, and state-4 respiration and cytochrome c oxidase (COX) activity in liver and muscle increased significantly in winter. Circulating level of serum triiodothyronine (T₃) was significantly higher in winter than in summer. Together, these data suggest that tree sparrows mainly coped with cold by enhancing thermogenic capacities through increased organ masses and heightened activity of respiratory enzymes activities. The results support the view that prominent winter increases in BMR are manifestations of winter acclimatization in tree sparrows and that seasonal variation in metabolism in sparrows is similar to that in other small temperate-wintering birds.     

A metabolic theory of ecology applied to temperature and mass dependence of N and P excretion by common carp

Our study used a metabolic theory of ecology (MTE) to explore scaling of metabolic rates by body size and temperature, and to predict nutrient excretion by common carp (Cyprinus carpio). At high biomasses, common carp have negative impacts on water quality, and one mechanism is excretion of the nutrients N and P. We measured whole-body and mass-specific excretion rates during summer and winter for fish of different sizes (wet mass range 28–1,196 g) to produce an allometric scaling model capable of predicting excretion at different temperatures. We found positive relationships between both dissolved and total nutrient concentrations and fish wet mass in summer and winter, with greater excretion rates in summer (mean water temperature 24.2°C) than in winter (mean water temperature 9.2°C). Mass-specific excretion rates decreased with increasing fish size, consistent with the MTE, and the temperature-adjusted model explained more variation for N excretion than for P. The proportion of dissolved nutrients (NH₄ and PO₄) to total nutrients increased with increasing fish size. The significance of these models is that they can be used to predict population-based nutrient excretion by common carp when thermal history, fish density and size distribution in a water body are known.     

Seasonal changes in body mass and thermogenesis in tree shrews (Tupaia belangeri): The roles of photoperiod and cold

Environmental factors play an important role in the seasonal adaptation of body mass and thermogenesis in small, wild mammals. To determine the contributions of photoperiod and cold on seasonal changes in energy metabolism and body mass, the resting metabolic rates (RMR), nonshivering thermogenesis (NST), energy intake and gut morphology of the tree shrews were determined in winter and summer and in laboratory acclimated animals. Body mass, RMR and NST increased in winter, and these changes were mimicked by exposing animals to short-day photoperiod or cold in the animal house. Energy intake and digested energy also increased significantly in winter, and also during exposure of housed animals to both short-day photoperiod and cold. The lengths and weights of small intestine increased in winter. These results indicated that Tupaia belangeri overcomes winter thermoregulatory challenges by increasing energy intake and thermogenesis, and adjusted gut morphology to balance the total energy requirements. Short-day photoperiod and cold can serve as environmental cues during seasonal acclimatization.     

Seasonal variation in the metabolism-temperature relation of House Sparrows (Passer domesticus) in KwaZulu-Natal,South Africa

Many birds exhibit considerable phenotypic flexibility in metabolism to maintain thermoregulation or to conserve energy. This flexibility usually includes seasonal variation in metabolic rate. Seasonal changes in physiology and behavior of birds are considered to be a part of their adaptive strategy for survival and reproductive success. House Sparrows (Passer domesticus) are small passerines from Europe that have been successfully introduced to many parts of the world, and thus may be expected to exhibit high phenotypic flexibility in metabolic rate. Mass specific Resting Metabolic Rate (RMR) and Basal Metabolic Rate (BMR) were significantly higher in winter compared with summer, although there was no significant difference between body mass in summer and winter. A similar, narrow thermal neutral zone (25–28 °C) was observed in both seasons. Winter elevation of metabolic rate in House Sparrows was presumably related to metabolic or morphological adjustments to meet the extra energy demands of cold winters. Overall, House Sparrows showed seasonal metabolic acclimatization similar to other temperate wintering passerines. The improved cold tolerance was associated with a significant increase in VO₂ in winter relative to summer. In addition, some summer birds died at 5 °C, whereas winter birds did not, further showing seasonal variation in cold tolerance. The increase in BMR of 120% in winter, compared to summer, is by far the highest recorded seasonal change so far in birds.     

Disruption of seasonality in growth hormone-transgenic coho salmon (Oncorhynchus kisutch) and the role of cholecystokinin in seasonal feeding behavior

Seasonal variation in daily food intake is a well-documented phenomenon in many organisms including wild-type coho salmon where the appetite is noticeably reduced during periods of decreased day length and low water temperature. This reduction may in part be explained by altered production of cholecystokinin (CCK) and growth hormone (GH). CCK is a hormone produced in the brain and gut that mediates a feeling of satiety and thus has an inhibitory effect on food intake and foraging behaviour. Growth hormone (GH) enhances feeding behaviour and consequently growth, but its production is reduced during winter. The objectives of this study were: first, to compare the seasonal feeding behaviour of wild and GH-transgenic coho salmon; second, to determine the behavioural effect of blocking the action of CCK (by using devazepide) on the seasonal food intake; and third, to measure CCK expression in brain and gut tissues between the two genotypes across seasons. We found that, in contrast to wild salmon, food intake in transgenic salmon was not reduced during winter indicating that seasonal control of appetite regulation has been disrupted by constitutive production of GH in transgenic animals. Blocking of CCK increased food intake in both genotypes in all seasons. The increase was stronger in wild genotypes than transgenic fish; however blocking CCK in wild-type fish in winter did not elevate appetites to levels observed in the summer. The response to devazepide was generally faster in transgenic than in wild salmon with more rapid effects observed during summer than during winter, possibly due to a higher temperature in summer. Overall, a seasonal effect on CCK mRNA levels was observed in telencephalon with levels during winter being higher compared to the summer in wild fish, but with no seasonal effect in transgenic fish. No differences in seasonal CCK expression were found in hypothalamus. Higher levels of CCK were detected in the gut of both genotypes in winter compared to summer. Thus, CCK appears to mediate food intake among seasons in both wild-type and GH-transgenic salmon, and an altered CCK regulation may be responsible at least in part for the seasonal regulation of food intake.     

Adaptive thermoregulation in golden spiny mice: the influence of season and food availability on body temperature

We studied the effect of food supplementation during summer and winter in seminatural field conditions on thermoregulation of a desert rodent, the golden spiny mouse Acomys russatus. We hypothesized that (a) under natural food availability (control conditions), mice will use less precise thermoregulation (i.e., an increase in the variance of body temperature [T(b)]) during winter because of low ambient temperatures (T(a)'s) and low food availability and during summer because of low food and water availability; (b) food supplementation will result in more precise thermoregulation during winter, but the effect will be smaller during summer because variation in T(b) in summer is also driven by water availability during that period. We found that under natural food availability, spiny mice thermoregulated more precisely during summer than during winter. They spent more time torpid during summer than during winter even when food was supplemented (although summer nights are shorter), allowing them to conserve water. Supplementing food resulted in more precise thermoregulation in both seasons, and mice spent less time torpid. In summer, thermoregulation at high T(a)'s was less precise, resulting in higher maximum T(b)'s in summer than in winter and when food was supplemented, in accord with the expected effect of water shortage on thermoregulation. Our results suggest that as expected, precise thermoregulation is beneficial when possible and is abandoned only when the costs of homeothermy outweigh the benefits.     

Home range variation of two different-sized groups of golden snub-nosed monkeys(Rhinopithecus roxellana) in Shennongjia,China: implications for feeding competition

Knowledge on the home range size of a species or population is important for understanding its behavioral and social ecology and improving the effectiveness of conservation strategies. We studied the home range size of two different-sized groups of golden snub-nosed monkeys(Rhinopithecus roxellana) in Shennongjia, China. The larger group(236 individuals)had a home range of 22.5 km2 from September2007 to July 2008, whereas the smaller group(62 individuals) occupied a home range of 12.4 km2 from November 2008 to July 2009. Both groups exhibited considerable seasonal variation in their home range size, which was likely due to seasonal changes in food availability and distribution. The home range in any given season(winter, spring, summer, or winter+spring+summer) of the larger group was larger than that of the smaller group. As the two groups were studied in the same area, with the confounding effects of food availability thus minimized, the positive relationship between home range size and group size suggested that scramble feeding competition increased within the larger group.     

Seasonal variation in body composition in an Afrotropical passerine bird: increases in pectoral muscle mass are,unexpectedly, associated with lower thermogenic capacity

Phenotypic flexibility in avian metabolic rates and body composition have been well-studied in high-latitude species, which typically increase basal metabolic rate (BMR) and summit metabolism (M_sum) when acclimatized to winter conditions. Patterns of seasonal metabolic acclimatization are more variable in lower-latitude birds that experience milder winters, with fewer studies investigating adjustments in avian organ and muscle masses in the context of metabolic flexibility in these regions. We quantified seasonal variation (summer vs winter) in the masses of organs and muscles frequently associated with changes in BMR (gizzard, intestines and liver) and M_sum (heart and pectoral muscles), in white-browed sparrow-weavers (Plocepasser mahali). We also measured pectoral muscle thickness using a portable ultrasound system to determine whether we could non-lethally estimate muscle size. A concurrent study measured seasonal changes in BMR and M_sum in the same population of sparrow-weavers, but different individuals. There was no seasonal variation in the dry masses of the gizzard, intestines or liver of sparrow-weavers, and during the same period, BMR did not vary seasonally. We found significantly higher heart (~ 18% higher) and pectoral muscle (~ 9% higher) dry mass during winter, although ultrasound measurements did not detect seasonal changes in pectoral muscle size. Despite winter increases in pectoral muscle mass, M_sum was ~ 26% lower in winter compared to summer. To the best of our knowledge, this is the first study to report an increase in avian pectoral muscle mass but a concomitant decrease in thermogenic capacity.