A ‘slow pace of life’ in Australian old-endemic passerine birds is not accompanied by low basal metabolic rates

Metabolic rate and ecological traits of ectoparasites: a case study with seven flea species from the Negev Desert

We studied the relationship between fleas' metabolic rate and their ecological traits, using data on standard metabolic rate (SMR), mean abundance, host specificity, and geographic range size in males and females of seven desert flea species. SMR was measured via mass-specific CO2 emission, whereas host specificity was measured as (a) the mean number of host species used by a flea per region in regions where this flea was recorded; (b) the total number of host species a flea exploited across its geographic range; and (c) the phylogenetic diversity of the flea's hosts. To control for confounding effects of phylogeny when analysing data on multiple species, we applied the Phylogenetic Generalised Least Squares (PGLS) model. We found that the only ecological trait significantly correlating with flea SMR was the phylogenetic diversity of hosts utilized by a flea across its geographic range. The strength of the association between SMR and host phylogenetic diversity was higher in male than in female fleas. We explain the relationship between flea SMR and their host specificity by the necessity of host-opportunistic species to compensate for the high energetic cost of neutralizing multiple defences from multiple hosts by increased SMR.

AnimalTraits - a curated animal trait database for body mass, metabolic rate and brain size

Trait databases have become important resources for large-scale comparative studies in ecology and evolution. Here we introduce the AnimalTraits database, a curated database of body mass, metabolic rate and brain size, in standardised units, for terrestrial animals. The database has broad taxonomic breadth, including tetrapods, arthropods, molluscs and annelids from almost 2000 species and 1000 genera. All data recorded in the database are sourced from their original empirical publication, and the original metrics and measurements are included with each record. This allows for subsequent data transformations as required. We have included rich metadata to allow users to filter the dataset. The additional R scripts we provide will assist researchers with aggregating standardised observations into species-level trait values. Our goals are to provide this resource without restrictions, to keep the AnimalTraits database current, and to grow the number of relevant traits in the future.

Measurement(s)	metabolic rate quantification • body mass • brain size
Technology Type(s)	metabolic rate measurement • body mass quantification • brain mass brain volume

Short- and long-distance avian migrants differ in exercise endurance but not aerobic capacity

Background

Migratory birds differ markedly in their migration strategies, particularly those performing short- versus long-distance migrations. In preparation for migration, all birds undergo physiological and morphological modifications including enlargement of fat stores and pectoral muscles to fuel and power their flights, as well as cardiovascular and biochemical adjustments that improve lipid and oxygen delivery and uptake by flight muscles. While the magnitude of these changes varies in relation to migration strategy, the consequence of these variations on aerobic performance is unknown. We tested whether the aerobic performance of four Old-world flycatcher species (Muscicapidae) varied according to migration strategy by comparing minimum resting metabolic rates (RMRmin), exercise-induced maximum metabolic rates (MMR), and exercise endurance times of short-distance and long-distance migratory birds.

Results

As expected, RMRmin did not vary between short-distance and long-distance migrants but differed between the species within a migration strategy and between sexes. Unexpectedly, MMR did not vary with migration strategy, but MMR and blood haemoglobin content were positively related among the birds tested. Exercise endurance times differed substantially between migration strategies with long-distance migrants sustaining exercise for > 60% longer than short-distance migrants. Blood haemoglobin content had a significant positive effect on endurance among all birds examined.

Conclusions

The lack of difference in RMRmin and MMR between long- and short-distance migrants during this stage of migration suggests that the attributes favouring the greater aerobic endurance of long-distance migrants did not come at the expense of increased maintenance costs or require greater aerobic capacity.

Somatotropic Axis, Pace of Life and Aging

Mice with genetic growth hormone (GH) deficiency or GH resistance live much longer than their normal siblings maintained under identical conditions with unlimited access to food. Extended longevity of these mutants is associated with extension of their healthspan (period of life free of disability and disease) and with delayed and/or slower aging. Importantly, GH and GH-related traits have been linked to the regulation of aging and longevity also in mice that have not been genetically altered and in other mammalian species including humans. Avai+lable evidence indicates that the impact of suppressed GH signaling on aging is mediated by multiple interacting mechanisms and involves trade-offs among growth, reproduction, and longevity. Life history traits of long-lived GH-related mutants include slow postnatal growth, delayed sexual maturation, and reduced fecundity (smaller litter size and increased intervals between the litters). These traits are consistent with a slower pace-of-life, a well-documented characteristic of species of wild animals that are long-lived in their natural environment. Apparently, slower pace-of-life (or at least some of its features) is associated with extended longevity both within and between species. This association is unexpected and may appear counterintuitive, because the relationships between adult body size (a GH-dependent trait) and longevity within and between species are opposite rather than similar. Studies of energy metabolism and nutrient-dependent signaling pathways at different stages of the life course will be needed to elucidate mechanisms of these relationships.

Heterothermy in a Small Passerine: Eastern Yellow Robins Use Nocturnal Torpor in Winter

Torpor is a controlled reduction of metabolism and body temperature, and its appropriate use allows small birds to adapt to and survive challenging conditions. However, despite its great energy conservation potential, torpor use by passerine birds is understudied although they are small and comprise over half of extant bird species. Here, we first determined whether a free-living, small ∼20 g Australian passerine, the eastern yellow robin (Eopsaltria australis), expresses torpor by measuring skin temperature (Ts) as a proxy for body temperature. Second, we tested if skin temperature fluctuated in relation to ambient temperature (Ta). We found that the Ts of eastern yellow robins fluctuated during winter by 9.1 ± 3.9°C on average (average minimum Ts 30.1 ± 2.3°C), providing the first evidence of torpor expression in this species. Daily minimum Ts decreased with Ta, reducing the estimated metabolic rate by as much as 32%. We hope that our results will encourage further studies to expand our knowledge on the use of torpor in wild passerines. The implications of such studies are important because species with highly flexible energy requirements may have an advantage over strict homeotherms during the current increasing frequency of extreme and unpredictable weather events, driven by changing climate.

Basal metabolic rate in free-ranging tropical birds lacks long-term repeatability and is influenced by ambient temperature

Tropical birds live longer, have smaller clutches and invest more resources into self-maintenance than temperate species. These "slow" life-histories in tropical birds are accompanied by low basal metabolic rate (BMR). It has recently been suggested that the low BMR of tropical species may be related not to their slow "pace of life" or high ambient temperatures (T_a ) in tropical latitudes, but to the stability of environmental conditions in tropics. Since the repeatability of metabolic traits is higher in stable environments, such as laboratory conditions, we predicted that long-term repeatability of BMR in a tropical climate should be higher than in a temperate one. Contrary to our predictions, the repeatability of mass-independent BMR in 64 individuals of free-living tropical birds from Vietnam was low and insignificant after the species affiliation was taken into account. It indicates that BMR cannot be used as an individual long-term characteristic of tropical birds. On the other hand, tropical birds showed consistent differences in their mass-independent BMR at the interspecific level. Using BMR measurements from 1543 individuals of 134 species, we also found that different characteristics of T_a within the week preceding BMR measurements had a significant impact on the mass-independent BMR of tropical birds. The most significant effect was the difference between the absolute maximum and minimum T_a within a single week. Our results indicate that the physiology of tropical birds is more subject to changes than would be expected based on the notion of the stability of climatic conditions in the tropics.

Sex-Specific Population Differences in Resting Metabolism Are Associated with Intraspecific Variation in Sexual Size Dimorphism of Brown Anoles

AbstractSexual size dimorphism can vary in direction and magnitude across populations, but the extent to which such intraspecific variation is associated with sex and population differences in underlying metabolic processes is unclear. We compared resting metabolic rates (RMRs) of brown anole lizards (Anolis sagrei) from two island populations in the Bahamas (Eleuthera and Great Exuma) that differ in the magnitude of male-biased sexual size dimorphism. Whereas females from each population exhibit similar growth rates and body sizes, males from Great Exuma grow more quickly and attain larger body sizes than males from Eleuthera. We found that these population differences in growth of males persisted in captivity. Therefore, we predicted that males from each population would differ in RMR, whereas females would not. Consistent with this prediction, we found that RMR of males from Eleuthera was higher than that of males from Great Exuma, particularly at higher temperatures. As predicted, RMR of females did not differ between populations. Despite this apparent sex-specific trade-off between growth rate and RMR at the population level, we found a positive relationship between growth rate and RMR at the individual level. The fact that Great Exuma males maintain lower RMR than Eleuthera males, despite their greater absolute growth rates and the positive relationship between RMR and growth rate across individuals, suggests that Great Exuma males may have lower baseline metabolic demands and/or greater growth efficiency than Eleuthera males. Our results call attention to sex-specific divergence in metabolism as a potential mechanism for intraspecific divergence in sexual size dimorphism.

The Ecology of the Zebra Finch Makes It a Great Laboratory Model but an Outlier amongst Passerine Birds

Zebra Finches have become the most widely researched bird species outside of those used in agricultural production. Their adoption as the avian model of choice is largely down to a number of characteristics that make them easy to obtain and use in captivity. The main point of our paper is that the very characteristics that make the Zebra Finch a highly amenable laboratory model species mean that it is by definition different from many other passerine birds, and therefore not a good general model for many research areas. The Zebra Finch is likely to be particularly resilient to the effects of stress early in life, and is likely to show great flexibility in dealing with a wide variety of conditions later in life. Whilst it is tempting for researchers to turn to species such as the Zebra Finch, that can be the focus of manipulative work in the laboratory, we caution that the findings of such studies may confound our understanding of general avian biology. The Zebra Finch will remain an excellent species for laboratory work, and our paper should help to direct and interpret future work in the laboratory and the field.

Nest predation and adult mortality relationships with post-natal metabolic rates and growth among songbird species

Metabolism is thought to mediate the connection between environmental selection pressures and a broad array of life history tradeoffs, but tests are needed. High juvenile predation correlates with fast growth, which may be achieved via fast juvenile metabolism. Fast offspring metabolism and growth can create physiological costs later in life that should be minimized in species with low adult mortality. Yet, relationships between juvenile metabolism and mortality at offspring versus adult stages are unexplored. We found that post-natal metabolism was positively correlated with adult mortality but not nest predation rates among 43 songbird species on three continents. Nest predation, but not adult mortality, explained additional variation in growth rates beyond metabolism. Our results suggest that metabolism may not be the mechanism underlying the relationships between growth and mortality at different life stages.

Metabolic rate is negatively linked to adult survival but does not explain latitudinal differences in songbirds

Survival rates vary dramatically among species and predictably across latitudes, but causes of this variation are unclear. The rate-of-living hypothesis posits that physiological damage from metabolism causes species with faster metabolic rates to exhibit lower survival rates. However, whether increased survival commonly observed in tropical and south temperate latitudes is associated with slower metabolic rate remains unclear. We compared metabolic rates and annual survival rates that we measured across 46 species, and from literature data across 147 species of birds in northern, southern and tropical latitudes. High metabolic rates were associated with lower survival but survival varied substantially among latitudinal regions independent of metabolism. The inability of metabolic rate to explain latitudinal variation in survival suggests (1) species may evolve physiological mechanisms that mitigate physiological damage from cellular metabolism and (2) extrinsic rather than intrinsic sources of mortality are the primary causes of latitudinal differences in survival.

Resolving Phylogenetic Relationships within Passeriformes Based on Mitochondrial Genes and Inferring the Evolution of Their Mitogenomes in Terms of Duplications

Mitochondrial genes are placed on one molecule, which implies that they should carry consistent phylogenetic information. Following this advantage, we present a well-supported phylogeny based on mitochondrial genomes from almost 300 representatives of Passeriformes, the most numerous and differentiated Aves order. The analyses resolved the phylogenetic position of paraphyletic Basal and Transitional Oscines. Passerida occurred divided into two groups, one containing Paroidea and Sylvioidea, whereas the other, Passeroidea and Muscicapoidea. Analyses of mitogenomes showed four types of rearrangements including a duplicated control region (CR) with adjacent genes. Mapping the presence and absence of duplications onto the phylogenetic tree revealed that the duplication was the ancestral state for passerines and was maintained in early diverged lineages. Next, the duplication could be lost and occurred independently at least four times according to the most parsimonious scenario. In some lineages, two CR copies have been inherited from an ancient duplication and highly diverged, whereas in others, the second copy became similar to the first one due to concerted evolution. The second CR copies accumulated over twice as many substitutions as the first ones. However, the second CRs were not completely eliminated and were retained for a long time, which suggests that both regions can fulfill an important role in mitogenomes. Phylogenetic analyses based on CR sequences subjected to the complex evolution can produce tree topologies inconsistent with real evolutionary relationships between species. Passerines with two CRs showed a higher metabolic rate in relation to their body mass.

Nocturnal torpor by superb fairy-wrens: a key mechanism for reducing winter daily energy expenditure

Many passerine birds are small and require a high mass-specific rate of resting energy expenditure, especially in the cold. The energetics of thermoregulation is, therefore, an important aspect of their ecology, yet few studies have quantified thermoregulatory patterns in wild passerines. We used miniature telemetry to record the skin temperature ( T_skin) of free-living superb fairy-wrens ( Malurus cyaneus, 8.6 g; n = 6 birds over N = 7-22 days) and determine the importance of controlled reductions in body temperature during resting to their winter energy budgets. Fairy-wrens routinely exhibited large daily fluctuations in T_skin between maxima of 41.9 ± 0.6°C and minima of 30.4 ± 0.7°C, with overall individual minima of 27.4 ± 1.1°C (maximum daily range: 14.7 ± 0.9°C). These results provide strong evidence of nocturnal torpor in this small passerine, which we calculated to provide a 42% reduction in resting metabolic rate at a T_a of 5°C compared to active-phase T_skin. A capacity for energy-saving torpor has important consequences for understanding the behaviour and life-history ecology of superb fairy-wrens. Moreover, our novel field data suggest that torpor could be more widespread and important than previously thought within passerines, the most diverse order of birds.

A genetics-based approach confirms immune associations with life history across multiple populations of an aquatic vertebrate (Gasterosteus aculeatus)

Understanding how wild immune variation covaries with other traits can reveal how costs and trade‐offs shape immune evolution in the wild. Divergent life history strategies may increase or alleviate immune costs, helping shape immune variation in a consistent, testable way. Contrasting hypotheses suggest that shorter life histories may alleviate costs by offsetting them against increased mortality, or increase the effect of costs if immune responses are traded off against development or reproduction. We investigated the evolutionary relationship between life history and immune responses within an island radiation of three‐spined stickleback, with discrete populations of varying life histories and parasitism. We sampled two short‐lived, two long‐lived and an anadromous population using qPCR to quantify current immune profile and RAD‐seq data to study the distribution of immune variants within our assay genes and across the genome. Short‐lived populations exhibited significantly increased expression of all assay genes, which was accompanied by a strong association with population‐level variation in local alleles and divergence in a gene that may be involved in complement pathways. In addition, divergence around the eda gene in anadromous fish is likely associated with increased inflammation. A wider analysis of 15 populations across the island revealed that immune genes across the genome show evidence of having diverged alongside life history strategies. Parasitism and reproductive investment were also important sources of variation for expression, highlighting the caution required when assaying immune responses in the wild. These results provide strong, gene‐based support for current hypotheses linking life history and immune variation across multiple populations of a vertebrate model.

Basal metabolic rate in free-living tropical birds: the influence of phylogenetic, behavioral, and ecological factors

The majority of our knowledge of avian energetics is based on studies of birds from temperate and high latitudes. Using the largest existing sample of wild-caught Old World tropical species, we showed that birds from Southern Vietnam had lower basal metabolic rate (BMR) than temperate species. The strongest dissimilarity between tropical and temperate species was the low scaling exponent in the allometric relation between BMR and body mass in tropical birds (the regression slope was 0.573). The passerine migrants to temperate and high latitudes had higher BMR than tropical sedentary passerines. Body mass alone accounted for 93% of the variation in BMR (body mass ranged from 5 to 252 g). Contrary to some other studies, we did not find evidence besides the above mentioned that phylogeny, taxonomy, behavior, or ecology have a significant influence on BMR variation among tropical birds.

Metabolic rate evolves rapidly and in parallel with the pace of life history

Metabolic rates and life history strategies are both thought to set the “pace of life”, but whether they evolve in tandem is not well understood. Here, using a common garden experiment that compares replicate paired populations, we show that Trinidadian guppy (Poecilia reticulata) populations that evolved a fast-paced life history in high-predation environments have consistently higher metabolic rates than guppies that evolved a slow-paced life history in low-predation environments. Furthermore, by transplanting guppies from high- to low-predation environments, we show that metabolic rate evolves in parallel with the pace of life history, at a rapid rate, and in the same direction as found for naturally occurring populations. Together, these multiple lines of inference provide evidence for a tight evolutionary coupling between metabolism and the pace of life history.

The ‘pace of life’ depends on both metabolic rate and life history traits; however, whether these evolve similarly in response to the environment is not clear. Here, Auer et al. show parallel evolution of metabolic rate and a suite of life history traits in response to predator environment in Trinidadian guppies.

Energetics of communal roosting in chestnut-crowned babblers: implications for group dynamics and breeding phenology

For many endotherms, communal roosting saves energy in cold conditions, but how this might affect social dynamics or breeding phenology is not well understood. Using chestnut-crowned babblers (Pomatostomus ruficeps), we studied the effects of nest use and group size on roosting energy costs. These 50 g cooperatively breeding passerine birds of outback Australia breed from late winter to early summer and roost in huddles of up to 20 in single-chambered nests. We measured babbler metabolism at three ecologically relevant temperatures: 5°C (similar to minimum nighttime temperatures during early breeding), 15°C (similar to nighttime temperatures during late breeding) and 28°C (thermal neutrality). Nest use alone had modest effects: even for solitary babblers at 5°C, it reduced nighttime energy expenditures by <15%. However, group-size effects were substantial, with savings of up to 60% in large groups at low temperatures. Babblers roosting in groups of seven or more at 5°C, and five or more at 15°C, did not need to elevate metabolic rates above basal levels. Furthermore, even at 28°C (thermoneutral for solitary babblers), individuals in groups of four or more had 15% lower basal metabolic rate than single birds, hinting that roosting in small groups is stressful. We suggest that the substantial energy savings of communal roosting at low temperatures help explain why early breeding is initiated in large groups and why breeding females, which roost alone and consequently expend 120% more energy overnight than other group members, suffer relatively higher mortality than communally roosting group mates.