Bats have unique immunological characteristics among vertebrates. Understanding how the immune system responds during the life cycle of bat populations is crucial for grasping how their defenses against infection vary during critical life events. We used the lesser long-nosed bat (Leptonycteris yerbabuenae) as a model to assess adaptive humoral immunity (total immunoglobulin G concentration, tIgG) at different periods during the year involving different ages, reproductive conditions and migratory habits (females migrate seasonally to give birth while males are resident). We also determined the relationship of tIgG with previously published values of plasma bacterial killing ability against Escherichia coli (BKA). tIgG did not vary among reproductive periods in males and females, although higher values were measured in non-reproductive males in 2019 than in 2020. In contrast, tIgG was lower in young individuals than in non-reproductive and lactating females. The tIgG had a positive relationship with BKA in non-reproductive females, but no relationship was observed in non-reproductive males and reproductive females. These results suggest that tIgG is resilient to physiological and ecological changes experienced by adults throughout their reproductive and migratory cycle. The low tIgG during early life suggests that young individuals have not developed an IgG pool comparable to that of adults. The positive relationship between tIgG and BKA in non-reproductive females suggests that E. coli clearance is enhanced by the presence of antibodies that facilitate its elimination.

The immune system and gut microbiota are interconnected components that play a key role in host health. The nature of this relationship in wildlife is under-explored, especially in migratory animals, which are exposed to diverse microorganisms that can impact their immune system, microbiota and health. This study explored the relationship between systemic humoral immunity (bacterial killing-ability, BKA, and total immunoglobulin G concentration, tIgG) and the diversity and composition of gut microbiota (16 S rRNA gene amplicons) in the lesser long-nosed bat Leptonycteris yerbabuenae. Males of this bat form resident populations whereas some females migrate to complete their reproductive cycle. No significant relationships were found between BKA and microbiota diversity (except for beta diversity in females), but tIgG was negatively correlated with Shannon index in females. Humoral immunity in both sexes was significantly related to fecal bacterial genera known to harbor immunostimulatory species, species linked to intestinal mucosa integrity, and infection-associated species. Other free-living and unclassified bacterial genera were associated with immunity in a sex-specific manner. These findings suggest that gut microbiota composition, and to a lesser extent diversity, are linked to systemic humoral immunity in L. yerbabuenae. The distinct relationship exhibited by each sex suggests that migration and other sex-associated traits may be crucial to understanding the natural variation of immunity in wildlife.

The immune system and gut microbiota are interconnected components that play a key role in host health. The nature of this relationship in wildlife is under-explored, especially in migratory animals, which are exposed to diverse microorganisms that can impact their immune system, microbiota and health. This study explored the relationship between constitutive humoral immunity (bacterial killing-ability, BKA, and total immunoglobulin G concentration, tIgG) and the diversity and composition of gut microbiota (16S rRNA gene amplicons) in the lesser long-nosed bat Leptonycteris yerbabuenae. Males of this bat form resident populations whereas some females migrate to complete their reproductive cycle. Each sex harbored a distinctive fecal microbiota, yet no significant relationships were found between BKA and microbiota diversity, but tIgG was negatively correlated with Shannon's and Simpson's inverse indices in females and positively with the Shannon's index in males. Humoral immunity in both sexes was significantly related to fecal bacterial genera known to harbor immunostimulatory species, species linked to intestinal mucosa integrity, and infection-associated species. Other free-living and unclassified bacterial genera were associated with immunity in a sex-specific manner, highlighting the importance of novel and uncommon bacteria for immune activity in wildlife. These findings suggest that gut microbiota composition, and to a lesser extent diversity, are linked to constitutive humoral immunity in L. yerbabuenae. The distinct relationship exhibited by each sex suggests that migration and other sex-associated traits may be crucial to understanding the natural variation of immunity in wildlife.

Migratory birds inhabit diverse geographic regions during their reproductive and wintering periods; however, information regarding the climatic conditions that enable migratory birds, particularly hummingbirds, to thrive in these regions is limited. This study aimed to characterize the geographic distribution and seasonal climatic niche of the Nearctic-Neotropical Rufous Hummingbird (Selasphorus rufus)-classified as near threatened-across its reproductive and wintering areas in North America. We obtained monthly occurrence records for the Rufous Hummingbird during its reproductive (May to June) and wintering (November to January) periods. We generated seasonal ecological niche models to reconstruct the climatic space occupied by the Rufous Hummingbird during these periods and evaluated climatic differences through niche similarity analysis. Our results indicate that the Rufous Hummingbird is a longitudinally migratory species that engages in niche-switching, occupying distinct climatic conditions in its reproductive and wintering areas across Mexico and the eastern United States. Changes in the climatic niche of the Rufous Hummingbird may be linked to its adaptive plasticity in response to varying climatic conditions, competition for resources, and recent climate change. These findings have biogeographic, evolutionary, and conservation implications for the Rufous Hummingbird and other migratory birds with similar movement patterns.

Immunological variations of bats throughout their life cycle represent an underexplored and controversial topic that has great potential for understanding their immunology. This gap is particularly large regarding the state of immunity during early life and migration, and how different immune components interact with each other. In this study, the partial migratory long-nosed bat ( Leptonycteris yerbabuenae ) was used as a model to assess the state of acquired humoral immunity (total IgG concentration, tIgG) during early life, during the reproductive cycle and at the extremes of the migratory range of females. We also determined the relationship of tIgG with plasma bactericidal activity against Escherichia coli . The concentration of tIgG was stable throughout the reproductive cycle of adults, and no significant changes were detected at the extremes of the migratory range of females. However, tIgG concentration was reduced during early life (2 to 3 months of age) and in non-reproductive males in March-May 2019 relative to non-reproductive males in March 2020. The tIgG concentration had a positive relationship with plasma bactericidal ability in non-reproductive females, but no relationship was observed in non-reproductive males and reproductive females. These results suggests that tIgG concentration is resilient to physiological and ecological changes experienced by adults throughout their reproductive and migratory cycle. The reduction of tIgG during early life suggests that young individuals have not developed an IgG pool comparable to that of adults. Finally, the positive relationship observed between tIgG and BKA in non-reproductive females suggests that E. coli clearance is enhanced by the presence of antibodies that facilitate the elimination of this bacteria. ### Competing Interest Statement The authors have declared no competing interest.

Antibiotics are pharmaceutical products with the potential to affect the immune performance of wildlife. Wildlife species might incorporate antibiotic residues in their system when feeding on livestock treated with these chemicals. One of the most studied interactions of livestock with wildlife is that established with the common vampire bat (Desmodus rotundus), which predates on livestock. We tested the effect of clindamycin on the humoral and cellular acquired immune responses of common vampire bats captured in the wild in Mexico. We hypothesized that both cellular and humoral acquired immune responses would be negatively affected after bats were exposed to clindamycin for 15 days. We measured local inflammation (swelling index = SI) and serum immunoglobulin concentration (IgG) after the repeated application of phytohemagglutinin (PHA) in bats treated with clindamycin and in a control group that did not receive the antibiotic. PHA is a plant lectin that induces mitosis in the effector cells of cellular-mediated immunity. We expected that antibiotic-exposed bats would present a weaker inflammatory response and that their IgG serum levels would not increase at the same rate after three consecutive injections of PHA than bats in the control group. Mean SI after the second injection of PHA was higher than that after the first injection only in bats in the control group. No significant difference was found in SI after the third injection with respect to the first injection in both the control and the experimental group. Mean IgG concentration was higher after the third injection than before the first injection only in vampire bats not treated with antibiotic. Our study shows the negative effect that exposure to anthropogenic chemicals generates on wildlife capacity to maintain a healthy immune system.

Developmental instability (DI) is a phenomenon whereby organisms are unable to buffer developmental disturbances, resulting in asymmetric variation of paired traits. Previous research has demonstrated a negative relationship between DI, measured as forearm asymmetry, and survival in the bat Carollia perspicillata. This study aims to test the hypothesis that individuals with higher DI exhibit a lower immune response. We measured a delayed-type hypersensitivity to the antigen phytohemagglutinin (PHA) on 74 males and 65 females of C. perspicillata before and after the breeding season (BS). Linear models were used to predict the immunological response based on body mass (BM), forearm asymmetry, sex, BS, and testicle length. The best-fitting model accounted for 29% of the variation in immune response and included asymmetry, BM, sex, and BS as predictors. The immune response was negatively associated with asymmetry and testicle length in males but positively related to asymmetry in females. Both sexes showed a reduced immune response in the late BS. Additionally, the association between immune response and BM changed direction seasonally, with heavier individuals showing weaker responses early in the BS and stronger responses later. Individual variation in male immunity was predicted by individual attributes, whereas variation in immune response in females was mostly seasonal. Our results support the link between DI, survival, and immune response in short-tailed bats, and suggest that the immunological component measured by the PHA response may be under finer selection in males due to its stronger correlation with individual traits.

Resilience of mammals to anthropogenic climate and land-use changes is associated with the maintenance of adequate responses of several fitness-related traits such as those related to immune functions. Isolated and combined effects of decreased food availability and increased ambient temperature can lead to immunosuppression and greater susceptibility to disease. Our study tested the general hypothesis that decreased food availability, increased ambient temperature and the combined effect of both factors would affect selected physiological and behavioral components associated with the innate immune system of fruit-eating bats (Carollia perspicillata). Physiological (fever, leukocytosis and neutrophil/lymphocyte ratio) and behavioral (food intake) components of the acute phase response, as well as bacterial killing ability of the plasma were assessed after immune challenge with lipopolysaccharide (LPS: 10 mg/kg) in experimental groups kept at different short-term conditions of food availability (ad libitum diet or 50% food-deprived) and ambient temperature (27 and 33°C). Our results indicate that magnitude of increase in body temperature was not affected by food availability, ambient temperature or the interaction of both factors, but the time to reach the highest increase took longer in LPS-injected bats that were kept under food restriction. The magnitude of increased neutrophil/lymphocyte ratio was affected by the interaction between food availability and ambient temperature, but food intake, total white blood cell count and bacterial killing ability were not affected by any factor or interaction. Overall, our results suggest that bacterial killing ability and most components of acute phase response examined are not affected by short-term changes in food availability and ambient temperature within the range evaluated in this study, and that the increase of the neutrophil/lymphocyte ratio when bats are exposed to low food availability and high ambient temperature might represent an enhancement of cellular response to deal with infection.

Bats use olfactory signals to provide information of species and individual identity and quality. Males of two species of long-nosed bats, Leptonycteris curasoae and L. yerbabuenae , display an odoriferous dorsal patch during the mating season that is involved in mate choice. The dorsal patch is documented to serve as an indicator of male quality; for instance, males with dorsal patch present larger testes and lower ectoparasite loads. We evaluated the role of the dorsal patch in males of L. curasoae and L. yerbabuenae as an indicator of inflammatory response 6 and 12 h after being injected with phytohemagglutinin (PHA). We found that the inflammatory response of males of both species was not different between reproductively active bats with or without dorsal patch 6 h after PHA injection, but the response was higher in bats with dorsal patch 12 h after the injection. Our study adds to the evidence that the dorsal patch exhibited by males of long-nosed bats indicates their immune quality for mating females. Further work including other branches of the immune system and the potential role of microbiota in the strength of the inflammatory response is warranted.

Bat populations are dwindling worldwide due to anthropogenic activities like agriculture, however the role that pesticide exposure plays on these declines is unclear. To address these research gaps, we first need to develop reliable methods to detect and monitor exposure to environmental pollutants and its effects on free-living bats. The use of biomarkers is a sensitive and informative tool to study sublethal effects in wildlife, however it requires laboratory validation and integrative approaches to be applicable to free-living species. In this study, we propose a set of non-destructive biomarkers to evaluate pesticide exposure in free-ranging bats and validated their suitability with dose-exposure experiments in captivity. We selected three biomarkers that have been widely used in vertebrate ecotoxicology and that combined represent sensitive, specific, and ecologically relevant responses to pollutants: DNA damage, AChE activity, and leukocyte profiles. We used two insectivorous bat species as model species Eptesicus fuscus (laboratory) and Pteronotus mexicanus (field). We found that micronuclei frequency (genotoxicity) and AChE activity (exposure and neurotoxicity) were robust indicators of toxicant exposure. The validity of this set of endpoints was supported by their consistent performance in laboratory and field experiments as well as by the significant correlation among them. Leukocyte profile (systemic stress) results were not consistent between laboratory and field studies, suggesting further evaluation of its suitability is needed. Integrative approaches, like the one we used here, maximize the insights about toxicant effects by combining the information of single biomarkers into more meaningful inferences, which can be applied to environmental risk assessments in wildlife. Furthermore, the use of non-destructive, cost-effective biomarkers is imperative when assessing toxicant exposure and effects in vulnerable wildlife and it should be a priority in the field of wildlife toxicology.

In contrast to birds, the relationship between migration and immunity has been scarcely studied in bats. We examined how the expression of the humoral portion of the constitutive immunity varied in a bat with partial, sex-biased migration: the lesser long-nosed bat (Leptonycteris yerbabuenae (Phyllostomidae)). The lesser long-nosed bat is a nectarivorous species distributed in the arid and semi-arid regions of North and Central America. We evaluated the bacteria-killing abilities (BKAs) of the plasma of male and female lesser long-nosed bats on the Pacific coast in different periods of the year. Because adult males are resident, they were used to explore the effect of reproductive activity on BKA, and we predicted higher values in mating males (i.e., individuals presenting scrotal testicles and a fresh dorsal patch). In contrast to males, most females migrate to cactus deserts in northern Mexico during pregnancy and lactation, and then return to the dry forests of west-central Mexico to mate. We predicted that the combined effect of breeding and migration would have an adverse effect on BKA; therefore, migratory pregnant and lactating females were expected to exhibit a lower BKA than mating females in west-central Mexico. We compared the BKAs of females captured in October and December in central Mexico, and we predicted that migratory females that had recently arrived in October should exhibit a lower BKA than females captured two months later. In addition, we compared the BKAs between lactating females and young in northern Mexico and predicted lower values in recently born individuals. We found that the BKAs of males were higher in reproductive individuals than in non-reproductive individuals. We found a significant difference in the BKAs between females at the two extremes of their migratory range: the values of pregnant females in Sonora and females in December were higher than those of females captured in October. Finally, we found no difference in BKAs between lactating females and young individuals. Our findings indicate that the basal levels of the innate humoral component are heightened in mating males, that this response is reduced in females that recently returned to their mating grounds, and that the constitutive immunity of young individuals matures early, probably in anticipation of the potential to encounter pathogens during their migration to west-central Mexico.

Tissue samples are frequently collected to study various aspects of avian biology, but in many cases these samples are not used in their entirety and are stored by the collector. The already collected samples provide a largely overlooked opportunity because they can be used by different researchers in different biological fields. Broad reuse of samples could result in multispecies or large‐scale studies, interdisciplinary collaborations, and the generation of new ideas, thereby increasing the quality and impact of research. Sample reuse could also reduce the number of new samples needed for a study, which is especially pertinent to endangered species where sample collection is necessarily limited. Importantly, reusing samples may be mutually beneficial for both the researchers providing samples and those reusing them. Here, we identify the benefits of sample reuse, describe currently available sources of already collected samples and their limitations, and highlight the wide range of potential applications in a single research field – avian isotopic ecology. To facilitate the reuse of avian samples worldwide and across research fields, we introduce the AviSample Network metadata repository. The main aims of this metadata repository are to collate and provide access to descriptions of available avian tissue samples. We contend that the creation of the AviSample Network metadata repository will provide the opportunity for new collaborations and studies. Moreover, we believe that this will help create research connections between ornithologists across the globe and encourage sample reuse in other fields.

Abstract In contrast to birds, the relationship between migration and immune response has been scarcely studied in bats. We examined how the expression of the humoral portion of the constitutive immune response varied in a bat with partial and differential migration: the Lesser longed-nosed bat Leptonycteris yerbabuenae (Phyllostomidae). The Lesser longed-nosed bat is a nectarivorous species in which pregnant females migrate ~ 1,500 km along the Pacific Coast to northern Mexico and southern USA in spring-summer where they have their young, while males and some females remain throughout the year in west-central Mexico. We measured bacterial killing ability (BKA) in plasma of males and females throughout the year and along its geographic distribution in the Pacific coast. We also examined if BKA varied with sex, age category and reproductive activity. We found that BKA values did not vary significantly with reproductive activity, migratory behavior, sex and age. However, big interindividual variation indicates that other intrinsic factors not examined in our study might play a role in bactericidal activity. Our findings and those of previous bat studies indicate that, in contrast to other migratory vertebrates, migratory bats do not regulate the humoral portion of the constitutive immunity during migration.

The immune response is affected by aridity, but it has been rarely examined in desert-dwelling bats. For two consecutive years, we examined the seasonal variation in the innate immune response of an insular desert bat, the fish-eating myotis (Myotis vivesi), in relation to its reproductive activity and ectoparasite load. We evaluated the reproductive activity based on external morphological traits and testosterone levels in the plasma for males and progesterone and estradiol for females. We injected phytohemagglutinin (PHA) into the footpads of the bats to estimate the innate cellular response, and we measured the bacterial killing ability (BKA) of the blood plasma to determine the innate humoral response. Both the external morphological traits and hormone levels indicate that the females were pregnant in spring and lactating in summer, and that the males were reproductively active in autumn, when mating probably occurred. The swelling response of the female and male bats was lower in spring. The BKA in the males did not vary seasonally; the BKA in the females varied seasonally but only in the first year of the study, with lower values in spring and summer. The BKA in spring was lower in the first year of the study, when the females appeared to be in early pregnancy, compared to the second year, when the females were in advanced pregnancy. The swelling increased as the body mass and body conditions of the males increased, but the BKA was not correlated with body mass or body condition in either sex. Ectoparasite abundance and prevalence did not vary among seasons. Ectoparasite abundance was not correlated with the PHA response in both sexes; it was not correlated with the BKA in females, but it was inversely correlated in males. Of the three hormones measured, only estradiol was correlated with the immune response: females with higher estradiol levels had a higher PHA response and BKA. Our findings indicate that the cellular and humoral innate immune responses of the fish-eating myotis varied throughout the year, following the seasonal reproductive pattern of the species. Our evaluation of the proximal factors affecting the expression of the immune response points to the potential immunoregulatory role of sex hormones and body mass.

The acute phase response (APR) is a core component of the innate immune response and represents the first line of immune defense used in response to infections. Although several studies with vertebrates reported fever, a decrease in food intake and body mass, and an increase in neutrophil/lymphocyte ratio and total white blood cell count after lipopolysaccharide (LPS) inoculation, there was great variability in the magnitude of these responses. Some of these differences might reflect, to some extent, differences in the time of endotoxin inoculation (during active or rest periods) and dose. Therefore, our study tested the interplay between LPS dose and time of injection on selected physiological ( fever and increase in total white blood cell count and neutrophil/lymphocyte ratio) and behavioral (food intake) components of the APR using a Neotropical fruit-eating bat (Carollia perspicillata) as a model organism. We predicted that LPS would trigger a dose- and time-dependent response in APR components. APR components were assessed in rest and active periods after injection of three doses of LPS (5, 10 and 15 mg kg(-1) LPS). The results indicate a more robust decrease in food intake at higher doses during the active period, while increased neutrophil/lymphocyte ratio was more robust during the active period regardless of dose. Furthermore, the skin temperature increase lasted longer at higher doses regardless of the timing of injections. Our study offers important insights into the dependence of time as well as the LPS dosage effect in the APR of bats, and how they deal with the magnitude of infections at different times of day.

Lead (Pb) is one of the most common metals found in ecosystems in elevated concentrations derived mainly from anthropogenic activities. Pb toxicity is of special concern in birds due to its capacity for bioaccumulation in the liver, bones, and kidneys causing physiological disruptions. Such disruptions can be lethal in a few days after Pb acute intoxication and they are associated with several million deaths of birds. Moreover, Pb may work as an immunosuppressant as it affects the cell-mediated and humoral immune responses, including components of the acute-phase response (APR). We (1) examined the effects of Pb contamination on the innate immune system, body mass, and food intake of Japanese quails (Coturnix coturnix japonica), and (2) evaluated the effects of Pb on its APR after exposing the animals to Pb acetate in drinkable water during 7 days. We found that Pb contamination increased the number of circulating white blood cells (WBCs), but no effect was found on body mass, food intake, the heterophil/lymphocyte (H/L) ratio, and haptoglobin (Hp) concentration. When Pb-exposed birds were injected with lipopolysaccharide from Escherichia coli to activate the APR, they had a negative body mass ratio, reduced food intake, and increased the number of WBCs, the H/L ratio, and the Hp concentration. We conclude that Pb exposure at this dose did not affect baseline values of the constitutive response and that it did not affect the APR of quails, but commend for further studies testing the effect of different Pb doses.

Con algunas adaptaciones morfológicas, fisiológicas y conductuales, y con una alimentación casi exclusivamente marina, el murciélago pescador enfrenta las altas temperaturas del verano, las temperaturas muy frías y los fuertes vientos del invierno, y la aridez de las islas del Golfo de California en las que habita.

Mercury is a toxic element acquired by animals through feeding which can accumulate within food chains through biomagnification. This possesses particular risks to higher trophic levels and may unduly impact marine foraging species or individuals. The fish-eating bat (Myotis vivesi Menegaux, 1901) inhabits islands in the Gulf of California and can act as a predator in the marine environment. A predominantly marine diet and a high trophic position increase the risk of mercury exposure owing to increased bioaccumulation. Using molecular techniques to reconstruct diet, we show that M. vivesi regularly feeds on small fishes and crustaceans, particularly on the Californian anchovy (Engraulis mordax Girard, 1854) and a krill species (Nyctiphanes simplex Hansen, 1911). Additionally, we identify significant interannual variation in diet composition within this population, but measured levels of total mercury in faecal samples were not related to dietary diversity or trophic level.

Along with its many advantages, social roosting imposes a major risk of pathogen transmission. How social animals reduce this risk is poorly documented. We used lipopolysaccharide challenge to imitate bacterial infection in both a captive and a free-living colony of an extremely social, long-lived mammal-the Egyptian fruit bat. We monitored behavioral and physiological responses using an arsenal of methods, including onboard GPS to track foraging, acceleration sensors to monitor movement, infrared video to record social behavior, and blood samples to measure immune markers. Sick-like (immune-challenged) bats exhibited an increased immune response, as well as classic illness symptoms, including fever, weight loss, anorexia, and lethargy. Notably, the bats also exhibited behaviors that would reduce pathogen transfer. They perched alone and appeared to voluntarily isolate themselves from the group by leaving the social cluster, which is extremely atypical for this species. The sick-like individuals in the open colony ceased foraging outdoors for at least two nights, thus reducing transmission to neighboring colonies. Together, these sickness behaviors demonstrate a strong, integrative immune response that promotes recovery of infected individuals while reducing pathogen transmission inside and outside the roost, including spillover events to other species, such as humans.

In the Neotropical region, euphonias (Euphonia spp., Fringillidae) are the quintessential example of specialized bird frugivores, making the bulk of feeding visits to certain mistletoes (Phoradendron spp., Santalaceae) and epiphytes in the genus Rhipsalis (Cactaceae), whose fruits have high water and low sugar and protein concentrations. Surprisingly, a mechanistic explanation for such specialized, otherwise rare, relationships is lacking. Using captive birds and artificial diets, we contrasted euphonias with frugivorous tanagers in the genus Thraupis (Thraupidae), which rarely eats Rhipsalis fruits, to test the hypothesis that the digestive capacity of euphonias entails them to exploit such low‐energy fruits. We expected that compensatory feeding in response to decreasing energy density would occur only in euphonias, whose higher reliance on fruits would entail a lower nitrogen requirement than the tanagers. Euphonias and tanagers were both able to compensate energy intake as sugar density decreased, and both species had the same mass‐corrected energy intake at any given sugar concentration. Similarly, euphonias and tanagers did not differ in mass‐corrected maintenance nitrogen requirement. Therefore, the physiological traits we investigated do not explain euphonias' specialization on Rhipsalis fruits. The fast rates of fruit passage typical of specialized avian frugivores as euphonias that entail the processing of a large volume of fruits and the putative better abilities of such birds to deal with secondary compounds likely present in Rhipsalis fruits are other possible mechanisms that should be considered in future studies to unveil the mechanisms underlying the intriguing specialized relationships between euphonias and certain fruits.