Environmental adaptation has long been a central focus in the comparative physiology of animals, as survival and reproduction depend on their ability to adapt to both abiotic and biotic environmental conditions. While comparative physiology has since branched into two or more subfields, the study of environmental adaptation remains a key topic across them all. Additionally, researchers from diverse disciplines outside traditional physiology are now actively investigating the mechanisms underlying environmental adaptation and its evolutionary significance. This special issue brings together papers that explore environmental adaptation in animals from a broad perspective, featuring a diverse array of subject organisms, ranging from protists to vertebrates.

To ensure survival and reproduction, organisms must continually adapt to environmental fluctuations, such as temperature, humidity, oxygen level, and salinity. Particularly, temperature profoundly influences biochemical reactions crucial for survival. Here, we present the mechanisms employed by the nematode Caenorhabditis elegans to anticipate and respond to cold temperatures. Our findings reveal that temperature is detected by specific neurons linked to various physiological processes in the gut, spermatheca, and muscles. Notably, the gut, a primary fat storage organ in C. elegans, regulates fat mobilization and accumulation in a temperature-dependent manner, thereby contributing to temperature adaptation. Furthermore, normal spermatogenetic mechanisms influence cold tolerance by modulating the responsiveness of thermosensory neurons to temperature changes. Considering our results together with recent reports, we suggest that a polyU-specific endoribonuclease expressed in muscle cells plays a role in cold tolerance through a non-cell-autonomous mechanism, possibly involving transportation intertissues. Thus, understanding cold tolerance and temperature acclimation in C. elegans can provide valuable insights on systemic physiological regulation in response to temperature fluctuations. Moreover, they could help elucidate the actions of thermosensory neurons and their downstream neuronal circuits or neuropeptides on the peripheral organs.

Among various environmental factors, temperature is one of the critical factors for organisms since it can affect most, if not all, biological processes. Therefore, animals precisely sense ambient and body temperatures and physiologically and behaviorally respond to temperature changes. Taking such nature into consideration, alteration of thermal perception should have played a pivotal role in adaptation to diverse thermal niches. Temperature as well as other physical and chemical stimuli are perceived by the primary afferent neurons where transient receptor potential (TRP) channels are expressed, and these channels serve as multimodal receptors in the somatosensory system. To understand the roles of TRP channels in the evolution of sensory perception, comparative analyses have been performed using various animal species, and their functional diversity has been well documented over the past 2 decades. Furthermore, in recent years, species differences in the thermal responses of TRP channels have been found among closely related species inhabiting different thermal niches, which have uncovered the contributions of TRP channels to environmental adaptation in various vertebrate species. The purpose of this review is to summarize the studies that addressed the functional evolution of TRP channels associated with sensory diversification and environmental adaptation.

Yawning is a stereotyped behavior widely observed in vertebrates, serving as an adaptation to the environment. Previous research has highlighted the correlations between yawning and physiological arousal or temperature regulation. However, the majority of those studies have primarily focused on endothermic animals. Thus far, the function of yawning in ectothermic animals remains unclear. In this study, we observed the behavior of leopard geckos, Eublepharis macularius, ectothermic reptiles, over a period of 3 days under constant ambient temperatures of 25°C, 30°C, or 35°C. By investigating the relationship between temperature, spontaneous yawning, and activity levels, we found that yawning frequency is affected by ambient temperature, and also observed a significant increase in post-yawning activity particularly under the 30°C and 35°C conditions. Furthermore, a near 24-hour periodicity in yawning was detected under all temperature conditions. These results align with previous studies conducted on endothermic animals, suggesting the conservation of primitive functions of yawning across vertebrate species.

The present study investigates the physiological aspects of overwintering in an exposed microhabitat in the lime seed bug, Oxycarenus lavaterae. We found that the overwintering lime seed bugs do not survive freezing of their body fluids, but instead rely on supercooling (freeze avoidance). The seasonal modulation of the supercooling capacity was very limited, with the midwinter mean supercooling point reaching –15.5°C, but the individual variability was very high (– 6°C to – 22°C). Most of the other physiological parameters of overwintering lime seed bugs (utilization of energy substrates, changes in hydration, and metabolite composition [although metabolite levels were low]) were consistent with the general knowledge gathered for other freeze-avoiding insects. A significant exception was found in the amount of osmotically active water (“freezable” water), which constituted up to 95% of the lime seed bug body water. Such a proportion is unusually high, as it typically ranges from 59% to 86% in other insects and invertebrates. At present, we have no plausible explanation for this anomaly or its possible relationship to the lime seed bug's overwintering microhabitat.

For the survival and efficient breeding of wild-living animals, it is crucial to predict seasonal changes and prepare appropriate physiological functions and neurobehavioral mechanisms. In mammals, photoperiod serves as a reliable cue for seasonal changes in the environment, primarily transmitted by melatonin. This review focuses on the seasonal adaptation of mammalian development, specifically the effect of early-life photoperiod on reproductive, somatic, and neurobehavioral development in small- and large-sized mammals. Prediction of seasons through early-life photoperiod is particularly important for small mammals, which have relatively short longevity, to adjust their maximum growth and breeding ability in appropriate seasons during the birth year or the following round. Brain plasticity, as well as cognitive and emotional behaviors, are also highly modulated by early-life photoperiods for successful mating and spatial memory for foraging. This review first summarizes the basic knowledge and recent progress in the programming and epigenetic regulatory mechanisms of reproductive and neurobehavioral development in small mammals, including C57BL/6J mice, which cannot produce detectable amounts of melatonin. The review then focuses on the influence of perinatal environmental conditions or birth season on adult phenotypes in large livestock and humans. Studies have advanced on the concept of the developmental origins of health and disease (DOHaD). Evidence from large mammals suggests that the prediction of seasons is crucial for high-fitness functions over several years. Finally, this review discusses the association of the season of birth with life course physiology and diseases in humans, and the possible mechanisms.

Animals possess many light-sensitive molecules. They exist as dedicated photoreceptors, or as byproducts of biochemical reactions. Their numbers are often high even in species that live in environments that humans would consider dark, as well as in species that are considered comparably simple (e.g., worms, cnidarians).

But why are there so many photoreceptors? We provide some considerations on this question. Light conveys a significant amount of information to animals, through complex spectral and intensity changes, often specific to the spatial and temporal ecological niches a species inhabits. We discuss that the large number of opsins and cryptochromes, often also present outside the eyes and partially co-expressed, represent adaptation mechanisms to the highly complex light environment within a given niche. While theoretical, it is a plausible hypothesis given that most experimentally tested opsins and cryptochromes have been shown to be functional photoreceptors. The example of lunar and solar timing of the marine annelid Platynereis dumerilii provides insight on how animals use the biochemical and cellular properties of different photoreceptors to decode solar versus lunar light, and their different adaptations in Drosophila melanogaster. We suggest that the future understanding of biological processes will strongly benefit from comparative lab and field work on the same species, and provide a first example for such work in P. dumerilii. Finally, we point out that work on animal light detection systems and their adaptability is crucial to understand the impact of anthropogenic changes on species and ecosystems.

Most organisms can sense and adapt to a wide range of light intensities. Although animals commonly use opsins for light detection, the nematode Pristionchus pacificus lacks conserved photoreceptors. The cyclic GMP signaling pathway and G protein-coupled receptor kinase are essential for light-avoidance behavior in P. pacificus. Although the mechanism of light sensing in P. pacificus has been partially elucidated, it remains unclear whether, and how, P. pacificus adapts to light. Here, we found that prior exposure to light reduced the frequency of light-avoidance behavior in P. pacificus, indicating its ability to adapt to light. To reveal the mechanism of light adaptation in P. pacificus, we used CRISPR/Cas9 genome editing to generate Gβ and Gγ subunit mutants, as these subunits are involved in chemosensory adaptation in the nematode Caenorhabditis elegans. Gβ and Gγ subunit mutants exhibited light-avoidance behavior similar to that of the wild type, but light adaptation was impaired in the Gβ mutants. Similarly, the Gγ and arrestin mutants showed minor abnormalities in light adaptation. These findings suggest that these proteins play a role in sensory adaptation beyond that in chemosensation and could contribute to light response mechanisms in nematodes.

The lung of marine snakes varies in structure and function related to diversity of phylogeny, behavior, and environment. All species are secondarily marine and retain dependence on aerial breathing, although some also exchange gases across the skin. Generally, there is an elongated functional right lung, and the left lung is vestigial or absent. Respiratory gases are exchanged in the ‘vascular lung’ segment, which may include the trachea, whereas the more distal ‘saccular lung’ has a poorly vascularized muscular wall, terminates blindly, and facilitates storage of oxygen. In terrestrial scansorial species of snakes, the vascularized segment of the lung is relatively short to prevent gravity-induced edema when the body is vertical or upright. In contrast, the vascular lung in marine snakes is relatively long. In the file snake Acrochordus granulatus, an exceptionally elongated vascular lung is shown to maximize oxygen storage when the snake is submerged at neutral buoyancy in shallow-water habitats. In deeper diving sea snakes, the entire lung is shorter, and the saccular segment functions to store oxygen. Movement of air within the lung is possible by means of body movements, but such behavior in naturally diving snakes is not well understood. Marine snakes avoid decompression sickness (the bends) by ‘metering’ the transfer of nitrogen from lung to seawater by varying pulmonary bypass of blood flow via intraventricular shunts and simultaneously varying blood flow to the skin. These complex cardiovascular actions are also influenced by the need of metabolizing tissues for oxygen uptake from lung stores or ambient water.

The deep sea, which encompasses the largest habitat on Earth, presents a set of extreme and unique environmental conditions, including high hydrostatic pressure, near-freezing temperatures, and perpetual darkness. These conditions pose significant challenges to the survival and energy management of its inhabitants. Deep-sea organisms have evolved a range of bioenergetic adaptations to negotiate these harsh conditions, ensuring efficient energy acquisition and utilization. This review examines the multifaceted strategies employed by deep-sea animals, focusing on three key areas: energy input, digestive and absorptive efficiency, and energy consumption. We examine the physical environment of the deep sea, highlighting vertical profiles of temperature, salinity, and dissolved oxygen, which contrast sharply with surface conditions. Physiological adaptations of deep-sea species, such as specialized digestive systems and enzyme modifications that function optimally under high pressure, are explored in detail. Furthermore, we discuss behavioral adaptations, including diurnal vertical migration, which optimize energy intake and reduce metabolic costs. Comparative analyses with shallow-water species provide insights into the evolutionary pressures that have shaped these adaptations. This review also addresses the concept of “power budgeting”, in which energy expenditures for specific dynamic actions (SDAs) must be balanced with other metabolic demands. This comprehensive examination of bioenergetic adaptation in deep-sea organisms enhances our understanding of their resilience and adaptability, offering glimpses into the complex interplay between environmental constraints and biological processes in one of the most challenging habitats on the planet.

Resting cyst formation is a strategic aspect of the life cycle of some eukaryotes such as protists, and particularly ciliates, that enables adaptation to unfavorable environmental conditions. The formation of resting cysts involves large scale morphological and physiological changes that provide tolerance of extreme environmental stresses. The resting cyst shows suppression of normal features of life such as eating, moving, proliferation, and even mitochondrial metabolic activity, and appears lifeless. This review discusses resting cyst formation in the ciliates Colpoda as a representative model of cyst-forming organisms, and focusses on morphogenesis, molecular events, tolerances, and metabolic activities in resting cysts.

Tardigrades are small metazoans renowned for their exceptional tolerance against various harsh environments in a dehydrated state. Some species exhibited an extraordinary tolerance against high-dose irradiation even in a hydrated state. Given that natural sources of high radiation are rare, the selective pressure to obtain such a high radiotolerance during evolution remains elusive. It has been postulated that high radiation tolerances could be derived from adaptation to dehydration, because both dehydration and radiation cause similar damage on biomolecules at least partly, e.g., DNA cleavage and oxidation of various biomolecules, and dehydration is a common environmental stress that terrestrial organisms should adapt to. Although tardigrades are known for high radiotolerance, the radiotolerance records have been reported only for desiccation-tolerant tardigrade species and nothing was known about the radiotolerance in desiccation-sensitive tardigrade species. Hence, the relationship between desiccation-tolerance and radiotolerance remained unexplored. To this end, we examined the radiotolerance of the desiccation-sensitive tardigrade Grevenius myrops (formerly known as Isohypsibius myrops) in comparison to the well-characterized desiccation-tolerant tardigrade, Ramazzottius varieornatus. The median lethal dose (LD50) of G. myrops was approximately 2240 Gy. This was much lower than those reported for desiccation tolerant eutardigrades. The effects of irradiation on the lifespan and the ovipositions were more severe in G. myrops compared to those in R. varieornatus. The present study provides precise records on the radiotolerance of a desiccation-sensitive tardigrade and the current data supported the correlation between desiccation tolerance and radiotolerance at least in eutardigrades.

Teleost fishes have independently colonized polar regions multiple times, facing many physiological and biochemical challenges due to frigid temperatures. Although increased gene copy numbers can contribute to adaptive evolution in extreme environments, it remains unclear which categories of genes exhibit increased copy numbers associated with polar colonization. Using 104 species of ray-finned fishes, we systematically identified genes with a significant correlation between copy number and polar colonization after phylogenetic correction. Several genes encoding extracellular glycoproteins, including zona pellucida (ZP) proteins, which increase their copy number in Antarctic notothenioid fishes, exhibited elevated copy numbers across multiple polar fish lineages. Additionally, some genes reported to be highly expressed under cold stress, such as cold-inducible RNA-binding protein (CIRBP), had significantly increased copy numbers in polar fishes. Further analysis will provide a fundamental basis for understanding the role of gene duplication in polar adaptations.

Life-history traits such as growth, reproduction, and lifespan in animals are shaped by both genetic and environmental factors, with nutrition being one of the most important environmental factors. However, it remains unclear how and to what extent changes in the nutritional environment affect animals and what molecular mechanisms they employ to adapt to these varying conditions. In recent years, the fruit fly Drosophila melanogaster and related species have been developed as model systems for studying the effects of nutrition and microbes on animals at the molecular level. This review summarizes recent findings on nutritional adaptation in Drosophila species, focusing on nutrition-dependent neuronal developmental mechanisms, carbohydrate-responsive systems that generate differences in adaptabilities among species, and animal-associated microbes that support host growth.

Daphnia switches between asexual and sexual reproductive strategies, depending on environmental conditions. For sexual reproduction, unfavorable environmental signals induce production of males and formation of meiotic eggs. Induction of both these phenotypes is strongly dependent upon the arthropod endocrine factor juvenile hormone (JH). This review presents the current state of research on regulatory mechanisms of reproductive strategy alteration in Daphnia, focusing on studies related to JH signaling conducted during the past several decades. Additionally, it discusses what is needed in future research to fully understand these mechanisms and evolution of complicated life cycle and environmental adaptation systems in Daphnia.

Animals organize their time so that their behaviors do not conflict with each other and align well with environmental conditions. In species with parental care, adults must also accommodate offspring needs into their temporal allocation of resources and activities. Avian parents face harsh constraints on their time budget during incubation, when they must sustain themselves but also transfer heat to eggs. During day-time, their shuttling between incubating and foraging is well studied. At night, birds usually rest on the nest and provide stable incubation. However, the stability of night rest depends on parental physiology and environmental conditions, and its patterns and consequences are poorly understood. We propose that stable parental night rest enhances the chances of embryos to hatch and might shorten incubation time, but that, in an urbanizing world, night rest may be compromised. We recorded nocturnal incubation restlessness, defined as variation in nest temperature, by placing thermal loggers into nest boxes of urban (25 clutches) and forest (70 clutches) great tits, where only females incubate. We found that with increasing nocturnal restlessness, hatching success dropped by ca. 60% per unit of increase in incubation restlessness in both habitats, despite higher hatching success in the forest. One putative driver of unstable incubation was artificial light at night, which for urban nest boxes was associated with increased nocturnal restlessness. Restlessness did not affect time to hatching. We conclude that sitting tight at night provides fitness pay-offs for incubating birds, but is influenced by environmental conditions, including those shaped by human activities.