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Philosophical Transactions Of The Royal Society Of London. Series B, Biological Sciences[JOURNAL]
Philosophical Transactions Of The Royal Society Of London. Series B, Biological Sciences[JOURNAL]
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200 papers
Rethinking mitochondrial heteroplasmy: selection, conflict and adaptation.
Kannangara JR, Ma H, Dowling DK
Philos Trans R Soc Lond B Biol Sci · 2026 Apr · PMID 41923601 · Publisher ↗
Mitochondrial DNA (mtDNA) variation is increasingly recognized for its role in shaping evolutionary changes at the species and population levels. Yet, its evolutionary relevance within individuals remains less explored....
Mitochondrial DNA (mtDNA) variation is increasingly recognized for its role in shaping evolutionary changes at the species and population levels. Yet, its evolutionary relevance within individuals remains less explored. Eukaryotic cells typically carry multiple copies of mtDNA. When these copies differ in sequence, heteroplasmy arises-a form of intra-organismal genetic diversity with potentially profound biological implications. To elucidate the evolutionary significance of heteroplasmy in animals, we first review how natural selection shapes adaptive mtDNA dynamics at broader biological levels, via cases of mito-nuclear coadaptation, environmental-mediated and sex-specific selection and balancing selection. We then explore whether analogous selective pressures may operate at the intra-individual level. Heteroplasmy introduces the potential for multi-level selection-from the genome to the organism-potentially yielding synergistic or antagonistic evolutionary outcomes. This framework encompasses both the selfish transmission of certain mtDNA variants and emerging evidence for adaptive shifts in heteroplasmy levels under environmental stress. These findings are supported by theoretical models suggesting that paternal mtDNA transmission-historically viewed as a stochastic anomaly-may confer adaptive benefits under specific ecological and evolutionary contexts by introducing intra-individual mtDNA diversity. Collectively, these insights suggest that heteroplasmy may act as an underappreciated reservoir of adaptive potential, enhancing the evolutionary capacity of organisms in a changing world. This article is part of the theme issue 'Evolutionary genetics of mitochondria: on diverse and common evolutionary constraints across eukarya'.
The functional importance of mitochondrial sequences with dual functions: overlapping protein-protein and protein-RNA gene sequences as test cases.
Shtolz N, Horovitz L, Caruchero Y … +1 more , Mishmar D
Philos Trans R Soc Lond B Biol Sci · 2026 Apr · PMID 41923600 · Publisher ↗
Human mitochondrial DNA (mtDNA) contains 13 protein-coding subunits of the oxidative phosphorylation pathway, 22 tRNA and two rRNA genes. However, accumulating evidence suggests that mtDNA encodes additional overlapping...
Human mitochondrial DNA (mtDNA) contains 13 protein-coding subunits of the oxidative phosphorylation pathway, 22 tRNA and two rRNA genes. However, accumulating evidence suggests that mtDNA encodes additional overlapping genetic elements, including mitochondrial-derived peptides (MDPs) and alternative reading frames. Here, we assessed signatures of selection across 66 328 human mtDNAs and studied the potential impact of disease-causing mutation within these mtDNA overlapping sequences. By employing frame-specific dN/dS analysis for the overlapping reading frames, and codon position-specific diversity calculations we found that SHLP6 and SHLP3 (within 16S rRNA) display significant signatures of purifying selection. Mutational asymmetry analysis revealed purifying selection in both frames and strands of GAU/COX1, while other alternative reading frames show asymmetric patterns, supporting negative selection primarily on the encompassing canonical gene. Analysis of mito-ribosome profiling (HEK293 cells) revealed translation initiation signatures only for SHLP6 and ALTND4, providing functional support for their translation in HEK293 cells. Analysis of disease-causing mutations revealed that several such mutations have predicted deleterious effects on both canonical and alternative sequences, though canonical genes tend to be more frequently affected. Taken together, we provide evolutionary and functional evidence supporting biological relevance of certain MDPs and underline the need to re-evaluate the functionality of mutations in such sequences. This article is part of the theme issue 'Evolutionary genetics of mitochondria: on diverse and common evolutionary constraints across eukarya'.
Why selection for mitochondrial quality drives the evolution of sexes.
Lane N, Pomiankowski A
Philos Trans R Soc Lond B Biol Sci · 2026 Apr · PMID 41923599 · Publisher ↗
The evolution of sexes is closely tied to uniparental inheritance (UPI) of mitochondrial DNA (mtDNA), where only females transmit mtDNA. Unlike nuclear DNA, mtDNA is highly polyploid and never evolved to be part of meiot...
The evolution of sexes is closely tied to uniparental inheritance (UPI) of mitochondrial DNA (mtDNA), where only females transmit mtDNA. Unlike nuclear DNA, mtDNA is highly polyploid and never evolved to be part of meiotic sex. Modelling shows that UPI increases mtDNA mutational variance, enhancing selection for high-quality mtDNA and promoting the emergence of sexes from mating types in unicellular eukaryotes. Paternal control of mitochondrial transfer favours some degree of mtDNA leakage, whereas maternal control favours strict UPI, leading to sexual conflict driving turnover in transmission mechanisms. In multicellular organisms, mitotic segregation of mtDNA increases variance in gametes, again facilitating selection. Surprisingly, germline evolution seems to reflect mtDNA mutation rates: plants and sessile metazoans have low rates and produce gametes from somatic cells, while bilaterians and ctenophores with higher rates sequester germlines with restricted cell division. High mtDNA ploidy in oocytes allows early embryonic cell division without replication, reducing mutational variance across tissues and enhancing somatic fitness. Germline mtDNA quality is maintained by mitotic over-proliferation of germ cells and the selective transfer of mtDNA into primordial oocytes linked with massive apoptotic germ-cell atresia. Overall, selection for mtDNA quality elucidates the evolution of sexes and the architecture of the female germline. This article is part of the theme issue 'Evolutionary genetics of mitochondria: on diverse and common evolutionary constraints across eukarya'.
Segregation of mtDNA mutations protects mitochondria across kingdoms, with impacts from longevity to agriculture and evolvability.
Johnston IG
Philos Trans R Soc Lond B Biol Sci · 2026 Apr · PMID 41923598 · Publisher ↗
Mitochondrial DNA (mtDNA) encodes essential bioenergetic and metabolic machinery across eukaryotes, but it is susceptible to mutational damage. The high copy number, physical location and inheritance patterns of mtDNA me...
Mitochondrial DNA (mtDNA) encodes essential bioenergetic and metabolic machinery across eukaryotes, but it is susceptible to mutational damage. The high copy number, physical location and inheritance patterns of mtDNA mean that specialist approaches to mitigate such damage are needed. A common theme across many species is segregation or 'sorting out' of different mtDNA types-generating variance in mutant frequencies within and between generations, so that multiscale selection can act to remove deleterious mutations. Eukaryotes with different physiologies and ecologies use different strategies for this segregation. This article attempts to review and-with the aid of some bioinformatics and new modelling results-synthesize the ways that this segregation is achieved across different eukaryotic organisms. In parallel, the importance of segregation in human disease, longevity, agriculture and for biology on a rapidly changing planet is discussed. This article is part of the theme issue 'Evolutionary genetics of mitochondria: on diverse and common evolutionary constraints across eukarya'.
Size-based competition explains large inter- and intra-individual variation in both intake and growth.
Johne AS, Richards SA, Carter CG … +2 more , Symonds J, Blanchard JL
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852225 · Publisher ↗
Size-based intraspecific competition can lead to high variation in both feed intake and growth. However, isolating the effects of competition on fish physiology and growth from other environmental factors is challenging....
Size-based intraspecific competition can lead to high variation in both feed intake and growth. However, isolating the effects of competition on fish physiology and growth from other environmental factors is challenging. To address this, we analysed a longitudinal dataset of individual-level feed intake and growth measurements of 543 fish. We used relative body size as a proxy for competitive ability and integrated it into a bioenergetic model to estimate body mass-dependent scaling of feed intake and energetic costs of living. Including relative body size effects in the model improved the model fit and replicated the observed size variation. Our results demonstrate that size-dependent competitive interactions can be an important driver of size-at-age variation even under controlled conditions. The findings also suggest that suppressed growth in smaller fish was more strongly associated with increased energetic costs of living than with a reduction in feed intake. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Diel variability and decoupled pH-oxygen dynamics drive metabolic plasticity in kelp crabs from an upwelling seascape.
Vargas CA, Garces K, Cuevas LA … +4 more , Castillo N, Jorquera E, Felez-Bernal J, Urbina MA
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852224 · Publisher ↗
Coastal upwelling zones, shaped by global change and human impacts, naturally impose hypoxia and acidification on marine species, creating selective pressures that influence physiological responses and drive phenotypic v...
Coastal upwelling zones, shaped by global change and human impacts, naturally impose hypoxia and acidification on marine species, creating selective pressures that influence physiological responses and drive phenotypic variability. Understanding these responses is crucial for predicting marine biodiversity dynamics in heterogeneous seascapes. We explored diel cycles of pH and dissolved oxygen (DO) and their influence on the metabolic performance of the kelp crab Taliepus dentatus, a species with limited larval dispersal. Kelp crabs from two environmentally contrasting sites along an upwelling seascape in central Chile-an upwelling shadow and an active upwelling zone-were studied using field sensor data and laboratory experiments. Active upwelling disrupted the regular diel pH cycle, resulting in persistently low pH (pHT ≈ 7.5) decoupled from oxygen dynamics. Experimental simulations of diel pH-DO fluctuations revealed that nocturnal low DO/low pH conditions (DO = 1 and 5 mg l⁻¹; pH = 7.5 and 7.8 for 'upwelling' and 'downwelling' conditions, respectively) reduced metabolic rates and respiratory quotient in crabs. Individuals from the active upwelling zone exhibited elevated metabolic rates, haemolymph pH and lactate accumulation under extremely low pH/low DO conditions compared with those from the upwelling shadow, suggesting site-specific physiological adjustments. These findings underscore the importance of incorporating natural variability into experimental designs and management frameworks aimed at predicting species resilience under climate change. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Unpredictability and adaptive responses to dietary variability in Japanese quail.
Reda GK, Ndunguru SF, Csernus B … +4 more , Knop R, Szabó C, Czeglédi L, Lendvai ÁZ
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852223 · Publisher ↗
Understanding animals' molecular mechanisms of adaptation to unpredictable changes in dietary availability provides insights into how organisms optimize resource utilization and ensure survival under such conditions. The...
Understanding animals' molecular mechanisms of adaptation to unpredictable changes in dietary availability provides insights into how organisms optimize resource utilization and ensure survival under such conditions. The mechanistic target of rapamycin (mTOR) is a candidate nutrient-sensing pathway underpinning a form of adaptive plasticity when individuals encounter constraints in their energy budget. We exposed adult Japanese quails (Coturnix japonica) of both sexes to three dietary conditions: daily unpredictable feeding, constant restriction and full feeding for 16 days. Our study revealed that unpredictability reduced the body mass of female birds, whereas males remained unaffected. Egg production traits were not affected by unpredictable feeding. However, constant dietary restriction had a significant negative impact, suggesting resource reallocation to reproduction under unpredictable feeding conditions. We found sex-dependent gene expression patterns of mTOR pathway genes: while the unpredictable treatment did not affect gene expression in males, it downregulated MTOR and insulin-like growth factor-1 (IGF1) and its receptor (IGF1R) and upregulated Unc-51-like autophagy activating kinase-1 (ULK1) and autophagy-related genes (ATG9A and ATG5) in females. Additionally, variation in mTOR pathway gene expression was significantly associated with the effect of treatments on body mass and egg mass. Our study highlighted molecular mechanisms and adaptive responses towards dietary unpredictability and scarcity. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Natural variation in temperature drives thermal preference in a freshwater fish, independent of infection status.
Levet M, Killen SS, Gradito M … +4 more , Manata E, Gazda MA, L'heureux E, Binning SA
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852222 · Publisher ↗
Aquatic environments are often thermally variable, and ectotherms may select thermal habitats based on their physiological state. However, it remains unclear whether immune reactions and sickness behaviours, including be...
Aquatic environments are often thermally variable, and ectotherms may select thermal habitats based on their physiological state. However, it remains unclear whether immune reactions and sickness behaviours, including behavioural fever or chill, drive temperature preference and what role natural temperature shifts play in thermal choice. We used a shuttle box system to determine temperature preference in wild-caught pumpkinseed sunfish, Lepomis gibbosus, that were: (i) naturally co-infected by parasitic helminths; (ii) experimentally infected with trematodes, or (iii) immune challenged through lipopolysaccharide (LPS) endotoxin injection. We found no significant differences in preferred temperatures or activity levels based on parasite infection intensity, experimental trematode exposure or LPS injection, despite significant among-individual variation in temperature preference. However, LPS-injected fish caught in 2021 preferred higher temperatures compared with those caught in 2022, reflecting warmer water temperatures experienced in June and July of 2021. Additionally, fish from the natural infection experiment captured over three weeks in July 2021 showed increased preferred temperatures that paralleled rising water temperatures in their natural habitat. Our results suggest that variability in temperature preference reflects natural temperature shifts rather than infection status. Ultimately, this finding enhances our understanding of individual microhabitat choice while challenging the concept of behavioural fever/chill as a thermoregulatory strategy in sunfish. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Temperature influences inter-individual variability in morphological and physiological traits of Baja California chorus frog tadpoles.
Mueller CA, St Onge CJ
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852221 · Publisher ↗
Inter-individual variability is the range of phenotypes within a population and can shape adaptive responses to environmental change. However, how temperature influences this variability remains unclear despite its impor...
Inter-individual variability is the range of phenotypes within a population and can shape adaptive responses to environmental change. However, how temperature influences this variability remains unclear despite its importance for predicting population resilience to warming. We tested how rearing temperature (15°C and 22°C) affects both trait means and variability in time to stage, size, oxygen consumption rate (V˙O2) and swim speed in Baja California chorus frog (Pseudacris hypochondriaca) tadpoles. Mean time to stage and morphological traits decreased at 22°C, while mass-specific V˙O2 and swim speed increased. Variance (s2, absolute variation) was lower at 22°C for time to stage and mass, higher for mass-specific V˙O2, and similar for length and swim speed. Coefficient of variation (CV, relative variation) was higher at 22°C for mass, tail length and depth, and mass-specific V̇O2, indicating trait-specific changes in canalization. Log-transformed Q10 response ratios for trait means (lnRRQ10), variance (lnVRQ10) and CV (lnCVRQ10) revealed strong thermal sensitivity for means but more limited and trait-specific responses in variability. Thus, temperature effects on variability are trait-specific and variability does not always change with, or in the same direction as, trait means. These phenotypic changes may have ecological consequences that influence physiological performance during subsequent life stages. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Locomotor variation of fishes: connecting energetics and kinematic modulation.
Zhang Y, Ramesh D, Ko H … +1 more , Lauder GV
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852220 · Publisher ↗
Analyses of vertebrate locomotion have frequently revealed variations in locomotor energetics and movement both among individuals and through time within an individual. This variation is often collapsed into mean values...
Analyses of vertebrate locomotion have frequently revealed variations in locomotor energetics and movement both among individuals and through time within an individual. This variation is often collapsed into mean values for broad comparative analyses of function. However, kinematic patterns of locomotion, even when animals move at a near-constant mean speed, frequently vary with both the physical and biological context. Here, we demonstrate, using analyses of fish locomotion and energetics, how variation among individuals in kinematic gaits can manifest as changes in dynamics of metabolic rate (estimated from oxygen uptake). We present kinematic data from a small school of giant danio (Devario aequipinnatus) to show that fishes within a school frequently modulate their kinematics and change position, even when the school moves at an overall constant mean speed. We show that rainbow trout (Oncorhynchus mykiss), swimming over a range of speeds, exhibit considerable variation in tail beat frequency and metabolic rate among speeds. By experimentally altering the fluid dynamic environment, we demonstrate that brook trout (Salvelinus fontinalis) show correlated modulation of both kinematics and metabolic rate. Simultaneous measurement of energetic and biomechanical characteristics can unveil the physiological, biomechanical and fluid dynamic mechanisms that underlie dynamic changes in vertebrate locomotor gaits. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Exploring metabolism-growth relationships within and across individual fish.
Hoots EC, Kuchenmüller LL, Biro PA … +1 more , Clark TD
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852219 · Publisher ↗
Differences in growth rate and metabolism between individual animals can influence survival, reproduction and competitive social dynamics during vulnerable life stages. Acknowledging this, among-individual variability ha...
Differences in growth rate and metabolism between individual animals can influence survival, reproduction and competitive social dynamics during vulnerable life stages. Acknowledging this, among-individual variability has received growing research attention. Less attention has been given to temporal variability within individuals, which can also be substantial and influence individual-level performance through time. Here, we used the estuarine fish Galaxias maculatus to understand among- and within-individual variabilities in metabolism, their relative contributions to growth and their thermal dependence. We collected five repeated measurements of metabolism (standard, routine and maximum) and mass across five months and 160 fish assigned to either 18℃ (typical summer temperature) or 23℃ (challenging summer temperature). Among-individual variability in metabolic parameters was generally low, generating low trait repeatability, and did not differ between temperature treatments. The percentage of fish achieving reproductive maturity was higher at 18℃ (98%) than 23℃ (62%). The growth rate was also higher at 18℃ and varied significantly among individuals. Individual growth rate was not correlated with standard or routine metabolic rates, but was positively correlated with maximum metabolic rate. We conclude that growth and metabolic rates are highly plastic over time, responding to a suite of influences including temperature, life stage and, in some circumstances, each other. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Temperature and predation alter metabolic scaling without changing the size-based community structure of freshwater macroinvertebrates.
Gjoni V, Glazier DS, Pomeranz JPF … +6 more , Junker JR, Smith A, Woelber J, Reynolds S, Welch T, Wesner JS
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852218 · Publisher ↗
Body size is a key trait that influences ecological processes such as metabolism, abundance and species interactions. While the metabolic theory of ecology (MTE) proposes a universal scaling of metabolic rate with body m...
Body size is a key trait that influences ecological processes such as metabolism, abundance and species interactions. While the metabolic theory of ecology (MTE) proposes a universal scaling of metabolic rate with body mass, recent evidence shows that this relationship is not fixed. Environmental factors like temperature and predation can alter the metabolic scaling exponent, potentially reshaping size distributions. However, most research has examined these patterns within individual species, leaving open questions about how environmental drivers affect scaling at the community level. To address this, we performed a mesocosm experiment manipulating both temperature and fish predator presence in freshwater macroinvertebrate communities. We found that warming altered metabolic rates in a predation-dependent way: without fish, metabolic rates increased in large individuals but decreased in small individuals, whereas the opposite occurred with fish present. This suggests that larger individuals reduce their baseline metabolic rates under predation risk, especially at higher temperatures. Interestingly, the slope of the community size distribution remained stable across treatments, indicating that shifts in metabolic scaling occurred independently of changes in size structure. Together, these findings highlight the environmental sensitivity of metabolic scaling and suggest that links between metabolism scaling and size distributions may be more complex than MTE predicts. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
'Variation ... a central attribute of living systems': patterns and implications of variability in the development of molluscan heart function.
Tills O, Ibbini Z, Rundle S … +1 more , Spicer J
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852217 · Publisher ↗
Diversity in the ontogeny of physiological systems is a central, too often overlooked tenet of biodiversity. Here, we use intensive screening of the development of heart function in embryos of the gastropod Radix balthic...
Diversity in the ontogeny of physiological systems is a central, too often overlooked tenet of biodiversity. Here, we use intensive screening of the development of heart function in embryos of the gastropod Radix balthica to investigate (i) variability in the physiological development of this system; (ii) how variability is associated with the hierarchical levels of population, egg mass and individual; and (iii) whether such variability links to hatch size. Despite considerable between-individual variability in heart function, a biphasic model was effective in modelling how heart activity changed through time. Heart rate variability analysis revealed marked reduced beat-to-beat variability in the period spanning the second slope of the biphasic pattern. The total number of heartbeats during development was positively correlated with hatch time, suggesting no detectable energetic trade-off between growth and heart function. Variability in the timing of first heart function was greatest at the egg mass level, whereas variation in the timing of the break in the biphasic pattern was greatest within egg masses at the individual level, with the least variability in both traits explained at the level of population, indicating that such physiological diversity is an intrinsic property apparent at every level of biological organization. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Thermal variability: how realistic temperature fluctuations alter physiological performance in intertidal mussels.
Nishizaki MT, Leuchtenberger S, Na W … +1 more , Armstrong M
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852216 · Publisher ↗
Understanding how fluctuating temperatures impact ectothermic physiological performance is critical for predicting species' responses to climate change. Many laboratory studies use simplified thermal designs that typical...
Understanding how fluctuating temperatures impact ectothermic physiological performance is critical for predicting species' responses to climate change. Many laboratory studies use simplified thermal designs that typically alternate between fixed minimum and maximum temperatures to simulate environmental conditions. However, these designs may not accurately reflect the rate, timing and duration of natural temperature fluctuations. We investigated the effects of two fluctuating temperature regimes on mussel respiration: an alternating treatment that cycled between the daily minimum and maximum temperatures and a field-based treatment constructed from continuous temperature measurements collected from our field site. Using temperature-controlled chambers, we exposed mussels to thermal conditions representing natural seasonal variation and measured oxygen consumption. Nonlinear thermal performance curves (TPCs) were fitted to estimate the optimal temperature (Topt), maximum metabolic rate (Rmax) and critical thermal maximum (CTmax). We found that respiration patterns differed between treatments, and TPC parameter estimates (Rmax, Topt, CTmax) showed non-overlapping bootstrap confidence intervals, indicating treatment-level differences supported by bootstrap inference. These results suggest that thermal performance is strongly influenced by heating rate and exposure duration. Our findings underscore the importance of using ecologically realistic thermal regimes in laboratory studies and have implications for predicting organismal resilience to climate change. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Harnessing variability in temperament to improve outcomes in experimental biology.
Wu F, Ting S, Ding L … +2 more , Blache D, Maloney SK
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852215 · Publisher ↗
Animal personality, also sometimes called temperament, has been investigated in domesticated and non-domesticated species, mammal and non-mammal animals, from foxes and marsupials to horses and sheep. Standardized behavi...
Animal personality, also sometimes called temperament, has been investigated in domesticated and non-domesticated species, mammal and non-mammal animals, from foxes and marsupials to horses and sheep. Standardized behavioural tests can be used to assess the phenotype of temperament and have identified large variability within populations of many species. Here, we present three examples that illustrate the importance of temperament variability in animal biology. The first example shows that while there were differences in temperament between five breeds of sheep that were raised and flocked together since weaning, there was a similar variability in temperament phenotype within each breed. The second example demonstrates that extreme temperament phenotypes, labelled calm or nervous, can affect how sheep perceive, habituate and anticipate a stressful situation to which they have been exposed previously. Our last example shows how the inclusion of individual temperament score can improve the predictive power of Bayesian models of the response of cortisol to an immune challenge. Classifying individuals based on their temperament, based on their phenotypic traits or genotyping, could offer a way to include the variability in temperament in experimental design. We conclude that the inclusion of temperament in experimental design could improve our understanding of animal biology. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Thermal variability during development facilitates enduring physiological tolerance.
Massey MD, Fredericks MK, Hutchings JA … +1 more , Dalziel AC
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852214 · Publisher ↗
Many organisms in aquatic ecosystems experience physiological stress owing to hypoxic events, warm temperatures and high thermal variability. Within-lifetime phenotypic plasticity might act to buffer, or exacerbate, the...
Many organisms in aquatic ecosystems experience physiological stress owing to hypoxic events, warm temperatures and high thermal variability. Within-lifetime phenotypic plasticity might act to buffer, or exacerbate, the negative impacts of these stressors, but long-term experiments using ecologically realistic conditions are needed to understand and predict organismal responses in adults. In this eight month experiment, we investigate responses in multiple physiological traits in response to diel thermal variability (27 ± 5°C) or a constant optimal temperature (27°C). Following zebrafish (Danio rerio) from fertilization through to sexual maturity, we use a factorial design to isolate the effects of developmental plasticity in early ontogeny (0-29 days post-fertilization (dpf)) from effects of later life acclimation (30+ dpf to testing). We show that developmental plasticity in response to thermal variability can facilitate enduring, beneficial plasticity in multiple whole-organism physiological traits, including thermal tolerance (CTmax), and, to a greater degree, hypoxia tolerance (Pcrit), resting metabolic rates and oxygen supply capacity (α). However, only thermal tolerance is additively improved by further acclimation to thermal variability later in life. These results highlight the early developmental environment as a significant modulator of adult phenotypes and emphasize the importance of testing environmentally variable treatments across life stages. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Daily maximum temperatures affect recovery of Brook char (Salvelinus fontinalis) metabolic rate from an acute thermal challenge.
Andrew S, Currie S, Morash AJ
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852213 · Publisher ↗
The intensity and frequency of warm events are projected to increase with climate warming, imposing significant challenges for ectothermic species. Daily maximum temperature, or Tmax, in salmonid habitats is exceeding th...
The intensity and frequency of warm events are projected to increase with climate warming, imposing significant challenges for ectothermic species. Daily maximum temperature, or Tmax, in salmonid habitats is exceeding the purported upper temperature limits for these species. Recovery from thermal challenges has been thought to be facilitated by cool daily thermal minima (Tmin), with Tmin above 20°C currently triggering the closure of some recreational fisheries. Here, we sought to investigate the relatively poorly understood effect of Tmax on salmonid metabolic rate and recovery. To this end, we acclimated Brook char (Salvelinus fontinalis) to an ecologically relevant diel thermal cycle with Tmin fixed at 19°C and Tmax set to 24°C for a minimum of one month. We then monitored oxygen consumption rate over 4 days throughout the diel thermal cycle. On the 5th day, Tmax was either 24, 26 or 27 °C, and we continued to monitor oxygen consumption during an 18-h recovery period. After exposure to a Tmax of 26 or 27°C and recovery at 19°C, oxygen consumption rate did not recover to resting levels. Tmax appears to have the potential to adversely affect aerobic metabolism and thus is worth considering in the management of salmonid recreational fisheries. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Beyond the mean: adaptive variability in Amazonian aquatic systems.
Val AL, Almeida-Val VMF
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852212 · Publisher ↗
Conventional ecological and conservation approaches often rely on average-based strategies, which obscure the complexity of biological systems. This article revisits the concept of the 'Tyranny of the Golden Mean' to arg...
Conventional ecological and conservation approaches often rely on average-based strategies, which obscure the complexity of biological systems. This article revisits the concept of the 'Tyranny of the Golden Mean' to argue that variability and plasticity are not sources of disorder, but essential components of ecological robustness, particularly in the Amazon. The Amazon's extraordinary heterogeneity shapes physiological, biochemical and genetic diversity. At genetic, biochemical and physiological levels, Amazonian aquatic species exhibit resilience, enabled by traits like low oxygen tolerance, metabolic flexibility and microanatomical adaptations. Examples span to amphibians, reptiles, aquatic insects and floodplain trees, illustrating that such adaptive strategies are widespread across taxa. These responses are context-specific and often missed by average values. As environmental changes intensify, conservation strategies based on means become insufficient. Recognizing variability is essential, as it underpins resilience to rapid environmental changes. We advocate for adaptive management, in which biological variability is recognized as a source of strength rather than noise. This approach enables the identification of critical thresholds and tipping points, enhancing the capacity to anticipate and respond to disruption. Ultimately, embracing variability improves conservation effectiveness and aligns with the evolutionary and ecological realities of the Amazon. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
Within-individual variation in glucocorticoid and metabolic rate plasticity: a mechanism of phenotypic fine-tuning to complex environmental challenges.
Malkoc K
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852211 · Publisher ↗
Complex environmental variation underlying phenomena like climate change can be represented as a continuum between gradual, predictable (i.e. global warming) and sudden, unpredictable components (i.e. extreme weather eve...
Complex environmental variation underlying phenomena like climate change can be represented as a continuum between gradual, predictable (i.e. global warming) and sudden, unpredictable components (i.e. extreme weather events). Reversible phenotypic plasticity in key physiological traits like glucocorticoids and metabolic rates, as measured by the reaction norm slope, represents the most informative metric of animal capacity to respond at any point along this continuum. In this article, I first describe which patterns of variation in physiological plasticity within and among individuals may favour populations' persistence to climate change. In doing so, I argue that within-individual variation in plasticity, i.e. the capacity of single individuals to change the steepness of their physiological reaction norm depending on the context, represents an overlooked, yet potentially crucial mechanism of phenotypic fine-tuning of animal responses to sudden and unpredictable environmental challenges. Second, I review currently published physiological reaction norm work in search of evidence for the above patterns of variation in glucocorticoid and metabolic rate plasticity. Finally, I remark that for understanding populations' coping capacity to ongoing climate change, future reaction norm research should investigate multilevel, multidimensional and multivariate physiological plasticity, its causes, consequences, temporal dynamics and covariation patterns. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
A decision matrix to better identify repeatable physiological variation within individuals.
Zhang Y, Wood CM, Brauner CJ … +1 more , Farrell AP
Philos Trans R Soc Lond B Biol Sci · 2026 Mar · PMID 41852210 · Publisher ↗
The performance of an individual has remained at the heart of evolutionary biology since the time of Darwin. Physiologists are equally drawn to the implications of individual variation for health and sporting endeavours,...
The performance of an individual has remained at the heart of evolutionary biology since the time of Darwin. Physiologists are equally drawn to the implications of individual variation for health and sporting endeavours, and specifically, whether or not a physiological trait is repeatable within an individual. Experimental biologists are especially interested in temporally stable physiological traits that are relevant to an individual's lifetime fitness for natural selection to act upon. Experimental noise, however, confounds the measurement of such repeatability, even though validated protocols exist for measuring many meaningful physiological performance traits. Missing is a decision matrix that helps distinguish individual variation from experimental noise. We propose a Precision-&-Repeatability Assessment Matrix (PRAM) that integrates established assessments of individual variability and repeatability. This matrix places metrics that are more repeatable and precise in the quadrant closest to the origins of Cartesian coordinates; those farthest away are less acceptable in terms of both repeatability and precision. As a case study, PRAM is applied to whole-organism aerobic and non-aerobic metabolic performance metrics from fish that were measured with the same protocols. The analysis illustrates that aerobic metabolic metrics can be more repeatable and precise than non-aerobic ones. Consequently, PRAM helps physiologists to better understand whether the observed variability is owing to non-repeatable metrics or true individual variation. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.
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