Comparison of the Phenotypic Flexibility of Muscle and Body Condition of Migrant and Resident White-Crowned Sparrows

AbstractSeasonally breeding birds express variations of traits (phenotypic flexibility) throughout their life history stages that represent adaptations to environmental conditions. Changes of body condition during migration have been well studied, whereas alterations of skeletal and cardiac muscles, body mass, and fat scores have yet to be characterized throughout the spring or fall migratory stages. Additionally, we examined flexible patterns of muscle, body mass, and fat score in migrant white-crowned sparrows (Zonotrichia leucophrys gambelii) in comparison with those in a resident subspecies (Zonotrichia leucophrys nuttalli) during the stages they share to evaluate the influence of different life histories. Migrants showed hypertrophy of the pectoralis muscle fiber area on the wintering grounds in late prealternate molt, yet increased pectoralis muscle mass was not detected until birds readied for spring departure. While pectoralis profile and fat scores enlarged at predeparture in spring and fall, pectoralis, cardiac, and body masses were greater only in spring stages, suggesting seasonal differences for migratory preparation. Gastrocnemius mass showed little change throughout all stages, whereas gastrocnemius fiber area declined steadily but rebounded in fall on the wintering grounds, where migrants become more sedentary. In general, residents are heavier birds with larger leg structures, while migrants sport longer wings and greater heart mass. Phenotypic flexibility was most prominent among residents with peaks of pectoralis, gastrocnemius, and body masses during the winter stage, when local weather is most severe. Thus, the subspecies express specific patterns of phenotypic flexibility with peaks coinciding with the stages of heightened energy demands: the winter stage for residents and the spring stages for migrants.

Sarcolipin relates to fattening, but not sarco/endoplasmic reticulum Ca2+-ATPase uncoupling, in captive migratory gray catbirds

In order to complete their energetically demanding journeys, migratory birds undergo a suite of physiological changes to prepare for long-duration endurance flight, including hyperphagia, fat deposition, reliance on fat as a fuel source, and flight muscle hypertrophy. In mammalian muscle, SLN is a small regulatory protein which binds to sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and uncouples Ca2+ transport from ATP hydrolysis, increasing energy consumption, heat production, and cytosolic Ca2+ transients that signal for mitochondrial biogenesis, fatigue resistance and a shift to fatty acid oxidation. Using a photoperiod manipulation of captive gray catbirds (Dumetella carolinensis), we investigated whether SLN may play a role in coordinating the development of the migratory phenotype. In response to long-day photostimulation, catbirds demonstrated migratory restlessness and significant body fat stores, alongside higher SLN transcription while SERCA2 remained constant. SLN transcription was strongly correlated with h-FABP and PGC1α transcription, as well as fat mass. However, SLN was not significantly correlated with HOAD or CD36 transcripts or measurements of SERCA activity, SR membrane Ca2+ leak, Ca2+ uptake rates, pumping efficiency or mitochondrial biogenesis. Therefore, SLN may be involved in the process of storing fat and shifting to fat as a fuel, but the mechanism of its involvement remains unclear.

White-throated sparrow (Zonotrichia albicollis) liver and pectoralis flight muscle transcriptomic changes in preparation for migration

Migratory songbirds undertake challenging journeys to reach their breeding grounds each spring. They accomplish these nonstop flapping feats of endurance through a suite of physiological changes, including the development of substantial fat stores and flight muscle hypertrophy and an increased capacity for fat catabolism. In addition, migratory birds may show large reductions in organ masses during flight, including the flight muscle and liver, which they must rapidly rebuild during their migratory stopover before replenishing their fat stores. However, the molecular basis of this capacity for rapid tissue remodeling and energetic output has not been thoroughly investigated. We performed RNA-sequencing analysis of the liver and pectoralis flight muscle of captive white-throated sparrows in experimentally photostimulated migratory and nonmigratory condition to explore the mechanisms of seasonal change to metabolism and tissue mass regulation that may facilitate these migratory journeys. Based on transcriptional changes, we propose that tissue-specific adjustments in preparation for migration may alleviate the damaging effects of long-duration activity, including a potential increase to the inflammatory response in the muscle. Furthermore, we hypothesize that seasonal hypertrophy balances satellite cell recruitment and apoptosis, while little evidence appeared in the transcriptome to support myostatin-, insulin-like growth factor 1-, and mammalian target of rapamycin-mediated pathways for muscle growth. These findings can encourage more targeted molecular studies on the unique integration of pathways that we find in the development of the migratory endurance phenotype in songbirds.NEW & NOTEWORTHY Migratory songbirds undergo significant physiological changes to accomplish their impressive migratory journeys. However, we have a limited understanding of the regulatory mechanisms underlying these changes. Here, we explore the transcriptomic changes to the flight muscle and liver of white-throated sparrows as they develop the migratory condition. We use these patterns to develop hypotheses about metabolic flexibility and tissue restructuring in preparation for migration.

Ghrelin, not corticosterone, is associated with transitioning of phenotypic states in a migratory Galliform

In both captive and free-living birds, the emergence of the migratory phenotype is signalled by rapid and marked increases in food intake and fuelling, as well as changes in amount of nocturnality or migratory restlessness. The metabolic hormone corticosterone and, as more recently suggested, the gut-derived hormone ghrelin have been suggested to play a role in mediating such phenomenal phenotypic flexibility given that they both regulate fuel metabolism and locomotion across vertebrate taxa. Here, using the Common quail (Coturnix coturnix) as our study species, we induced autumn migration followed by a non-migratory wintering phase through controlled changes in daylight. We thus compared plasma corticosterone and ghrelin concentrations between the two sampling phases and assessed whether these hormones might reflect the migratory state. While we found no differences in plasma corticosterone between the two sampling phases and no link of this hormone with changes in body mass, levels of food intake or migratory restlessness, the migratory birds had substantially higher levels of plasma ghrelin relative to the non-migratory birds. Furthermore, while ghrelin did not correlate with the gain in body mass over the entire pre-migratory fuelling phase (over an average of nine weeks preceding blood sampling), plasma ghrelin did positively correlate with the gain in body mass observed during the final fattening stages (over an average of three weeks preceding blood sampling). Again, variation in plasma ghrelin also reflected the amount of body mass depleted over both the long- and short-time frame as birds returned to their non-migratory baseline - lower levels of plasma ghrelin consistently correlated with larger losses in body mass. Thus, while our data do not highlight a role of the hormone corticosterone in sustaining pre-migratory fattening as shown in other bird species, they do add evidence for a potential role of ghrelin in mediating migratory behaviour and further suggest that this hormone might be important in regulating the transitioning of migratory states, possibly by promoting fuel mobilisation and usage.

Epigenomics as a paradigm to understand the nuances of phenotypes

Quantifying the relative importance of genomic and epigenomic modulators of phenotype is a focal challenge in comparative physiology, but progress is constrained by availability of data and analytic methods. Previous studies have linked physiological features to coding DNA sequence, regulatory DNA sequence, and epigenetic state, but few have disentangled their relative contributions or unambiguously distinguished causative effects ('drivers') from correlations. Progress has been limited by several factors, including the classical approach of treating continuous and fluid phenotypes as discrete and static across time and environment, and difficulty in considering the full diversity of mechanisms that can modulate phenotype, such as gene accessibility, transcription, mRNA processing and translation. We argue that attention to phenotype nuance, progressing to association with epigenetic marks and then causal analyses of the epigenetic mechanism, will enable clearer evaluation of the evolutionary path. This would underlie an essential paradigm shift, and power the search for links between genomic and epigenomic features and physiology. Here, we review the growing knowledge base of gene-regulatory mechanisms and describe their links to phenotype, proposing strategies to address widely recognized challenges.

Breeding at higher latitude is associated with higher photoperiodic threshold and delayed reproductive development in a songbird

Many seasonally breeding animals exhibit a threshold day length (critical photoperiod; CPP) for gonadal growth, and populations breeding at higher latitudes typically have a higher CPP. Much less is known about latitudinal variation in CPP in migratory population that winter away from their breeding range and must time their reproduction to match favorable conditions at their destination. To address the relationship between migration, breeding latitude, and CPP, we held two closely related songbird populations in a common environment. One population is resident (Junco hyemalis carolinensis), the other winters in sympatry with the residents but migrates north to breed (Junco hyemalis hyemalis). We gradually increased photoperiod and measured indices of readiness to migrate (fat score, body mass) and breed (cloacal protuberance volume, baseline testosterone, and gonadotropin releasing hormone challenged testosterone). To estimate breeding latitude, we measured hydrogen isotopes in feathers grown the preceding year. As we predicted, we found a higher CPP in migrants than residents, and a higher CPP among migrants deriving from higher as opposed to lower latitudes. Migrants also terminated breeding earlier than residents, indicating a shorter breeding season. To our knowledge, this is a first demonstration of latitudinal variation in CPP-dependent reproductive timing in bird populations that co-exist in the non-breeding season but breed at different latitudes. We conclude that bird populations appear to exhibit local adaptation in reproductive timing by relying on differential CPP response that is predictive of future conditions on the breeding ground.

Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry

The initiation of reproduction in many seasonally breeding animals is controlled by photoperiod and tends to be clinal: populations at higher latitudes breed later than those at lower latitudes, often reflecting a higher photoperiodic threshold. Migratory animals presumably time reproduction to match conditions at their breeding grounds, at least in part, by cues perceived on their wintering grounds. We asked how closely related dark-eyed junco (Junco hyemalis) populations that overwinter in sympatry but breed in allopatry respond to their shared winter environment by comparing early spring indices of readiness to migrate (fat and muscle condition) and breed (baseline and elevated testosterone). We measured stable hydrogen isotopes from feathers grown the preceding year and claws grown during winter to estimate breeding and wintering latitudes, respectively. We predicted that if reproductive initiation is adapted to the emergence of resources at their respective breeding destinations, then birds migrating to higher latitudes (slate-coloured junco; J. h. hyemalis) should delay breeding as compared with those migrating to lower latitudes (pink-sided junco; J. h. mearnsi) despite a common overwinter environment. We found higher testosterone in pink-sided juncos, consistent with earlier reproductive initiation, suggesting local adaptation in reproductive phenology is achieved through differential responses to predictive environmental cues.

What Do Ecology, Evolution, and Behavior Have in Common? The Organism in the Middle

Biologists who publish in The American Naturalist are drawn to its unifying mission of covering research in the fields of ecology, evolution, behavior, and integrative biology. Presented here is one scientist's attempt to straddle these fields by focusing on a single organism. It is also an account of how time spent in the field stimulates a naturalist to wonder "why did that animal just do that?" and how research is guided by chance and intention interacting with the scientific literature and the people one meets along the way. With respect to the science, the examples come from bird migration, hormones and their connection to phenotypic integration, sexual and natural selection, and urban ecology. They also come from research on the impact of environmental change on the timing of reproduction and the potential for allochrony in migratory species to influence population divergence.

Dietary antioxidants attenuate the endocrine stress response during long-duration flight of a migratory bird

Glucocorticoids (GCs) are metabolic hormones that promote catabolic processes, which release stored energy and support high metabolic demands such as during prolonged flights of migrating birds. Dietary antioxidants (e.g. anthocyanins) support metabolism by quenching excess reactive oxygen species produced during aerobic metabolism and also by activating specific metabolic pathways. For example, similar to GCs' function, anthocyanins promote the release of stored energy, although the extent of complementarity between GCs and dietary antioxidants is not well known. If anthocyanins complement GCs functions, birds consuming anthocyanin-rich food can be expected to limit the secretion of GCs when coping with a metabolically challenging activity, avoiding the exposure to potential hormonal detrimental effects. We tested this hypothesis in European starlings (Sturnus vulgaris) flying in a wind tunnel. We compared levels of corticosterone, the main avian GC, immediately after a sustained flight and at rest for birds that were fed diets with or without an anthocyanin supplement. As predicted, we found (i) higher corticosterone after flight than at rest in both diet groups and (ii) anthocyanin-supplemented birds had less elevated corticosterone after flight than unsupplemented control birds. This provides novel evidence that dietary antioxidants attenuate the activation of the HPA axis (i.e. increased secretion of corticosterone) during long-duration flight.

Declining food availability, corticosterone, and migratory response in a nomadic, irruptive migrant

While obligate migrants time their movements to respond to predictable changes in the environment, facultative migration is characterized by more variable movements that are driven by unpredictable changes in resource availability. The proximate cues that trigger facultative migrations and the endocrine mechanisms involved in these responses remain poorly understood, though corticosterone may be a key mediator of facultative migration due to its effects on activity and metabolic processes. We conducted experiments in the fall and spring to examine the response of pine siskins (Spinus pinus), a facultative migrant, to a two-stage food restriction. Our goals were to examine whether declining food availability stimulated behavioral and/or physiological changes consistent with a migratory response, whether anticipatory behavioral and physiological adjustments occurred when birds were initially presented with changing food availability, and if observed changes corresponded to changes in circulating corticosterone levels. We found no evidence of preparatory physiological changes for migration, but food-restricted birds in the spring had increased daytime activity indicative of a migratory response. Corticosterone increased at each stage of the restriction and the change in corticosterone corresponded to the magnitude of decline in body condition. Increased corticosterone was also correlated with larger increases in activity during the initial stage of food restriction, but only during the spring, when birds also displayed higher levels of initial body condition. These results suggest that initial energetic state and corticosterone response may interact to determine an individual's behavioral and physiological response to declining food availability and ultimately the facultative migratory response.

Preparing to migrate: expression of androgen signaling molecules and insulin-like growth factor-1 in skeletal muscles of Gambel's white-crowned sparrows

Migratory birds, including Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), exhibit profound modifications of skeletal muscles prior to migration, notably hypertrophy of the pectoralis muscle required for powered flight. Muscle growth may be influenced by anabolic effects of androgens; however, prior to spring departure, circulating androgens are low in sparrows. A seasonal increase in local androgen signaling may occur within muscle to promote remodeling. We measured morphological parameters, plasma and tissue levels of testosterone, as well as mRNA expression levels of androgen receptor, 5α-reductase (converts testosterone to 5α-dihydrotestosterone), and the androgen-dependent myotrophic factor insulin-like growth factor-1. We studied the pectoralis muscle as well as the gastrocnemius (leg) muscle of male sparrows across three stages on the wintering grounds: winter (February), pre-nuptial molt (March), and pre-departure (April). Testosterone levels were low, but detectable, in plasma and muscles at all three stages. Androgen receptor mRNA and 5α-reductase Type 1 mRNA increased at pre-departure, but did so in both muscles. Notably, mRNA levels of insulin-like growth factor-1, an androgen-dependent gene critical for muscle remodeling, increased at pre-departure in the pectoralis but decreased in the gastrocnemius. Taken together, these data suggest a site-specific molecular basis for muscle remodeling that may serve to enable long-distance flight.

Increasing photoperiod stimulates the initiation of spring migratory behaviour and physiology in a facultative migrant, the pine siskin

The transition to a migratory state involves coordinated changes in physiology and behaviour. In species with regular, predictable (obligate) migrations, increasing day length triggers the expression of a spring migratory state and androgens play an important role in stimulating its development. By contrast, we know little about the environmental cues and endocrine mechanisms that regulate migration in species with less predictable (facultative) migrations. Here, we tested whether photoperiod stimulates a migratory state in a facultative nomadic migrant, the pine siskin (Spinus pinus). We exposed wintering birds to either a naturally increasing or short-day photoperiod and measured physiological and behavioural changes indicative of a migratory state. We also examined changes in circulating hormones that may play a role in the migratory transition. Natural-day, but not short-day, birds displayed physiological preparations for migration, including increases in fat deposition, and showed increased levels of migratory restlessness. We found no evidence for a role of corticosterone in the migratory transition, but testosterone may be important. This study is the first experimental test of the role of photoperiod in regulating facultative migration and demonstrates that the predictive cue used by many obligate migrants to time spring migration is also important in a facultative migrant.

Effects of a social cue on reproductive development and pre-alternate molt in seasonally breeding migrant and resident female songbirds (Zonotrichia leucophrys)

To time reproduction optimally, birds have evolved diverse mechanisms by which they respond to environmental changes that help them anticipate and prepare for the breeding season. While residents initiate reproductive preparation and breed in the same geographic location, migrant birds simultaneously prepare for breeding and migration far from their breeding grounds. As a result, it is hypothesized that migrant and resident birds use environmental cues differently to prepare to breed and that there is adaptive specialization in mechanisms regulating reproductive preparation. Specifically, residents are expected to rely more on non-photic cues (e.g. food, temperature, social cues) than migrants. We tested this general prediction using a social cue manipulation. First, we compared the effects of subspecies-appropriate recorded male song on reproductive development in migrants and residents on a naturally increasing photoperiod. Second, we tested the sensitivity of migrant-specific life history events (fattening and pre-alternate molt) to song treatment. After 82 days, residents had higher luteinizing hormone and greater ovarian development than migrants, but song treatment had no effect on these metrics in either subspecies. Song advanced pre-alternate molt but had no effect on fattening in migrants. While our study does not support specialization in social cue use in migrants and residents, it is consistent with findings in the literature of specialization in photoperiodic response. It also demonstrates for the first time that social cues can influence molt in a migrant species. Additional findings from a pilot study looking at responses to a live male suggest it is important to test other kinds of social cues.

Rapid adaptive response to a Mediterranean environment reduces phenotypic mismatch in a recent amphibian invader

Invasive species frequently cope with ecological conditions that are different from those to which they adapted, presenting an opportunity to investigate how phenotypes change across short time scales. In 2000, the guttural toad Sclerophrys gutturalis was first detected in a peri-urban area of Cape Town, where it is now invasive. The ability of the species to invade Cape Town is surprising as the area is characterized by a Mediterranean climate significantly drier and colder than that of the native source area. We measured field hydration state of guttural toads from the invasive Cape Town population and a native source population from Durban. We also obtained from laboratory trials: rates of evaporative water loss and water uptake, sensitivity of locomotor endurance to hydration state, critical thermal minimum (CT_min) and sensitivity of CT_min to hydration state. Field hydration state of invasive toads was significantly lower than that of native toads. Although the two populations had similar rates of water loss and uptake, invasive toads were more efficient in minimizing water loss through postural adjustments. In locomotor trials, invasive individuals noticeably outperformed native individuals when dehydrated but not when fully hydrated. CT_min was lower in invasive individuals than in native individuals, independent of hydration state. Our results indicate that an invasive population that is only 20 years old shows adaptive responses that reduce phenotypic mismatch with the novel environment. The invasion potential of the species in Cape Town is higher than we could infer from its characteristics in the native source population.