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.

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.

Assessments of specific virus-host interactions via experimental infection has shown that bats do not show clinical symptoms of disease, including some hallmarks of acute phase response (APR), upon specific viral infections. Despite these findings, the metabolic cost of APR upon viral challenge has not yet been studied in bats. Therefore, we evaluated the energy cost of APR in Artibeus lituratus upon challenge with polyinosinic-polycytidylic acid (Poly(I:C)), a synthetic analog of double-stranded RNA that induces APR in mammals. To do this, we compared the resting metabolic rates of bats before and after the Poly(I:C) challenge and evaluated skin temperature and body mass loss. Our results revealed that the innate immune response elicited by Poly(I:C) involves a resting metabolic rate increase of 20%, equivalent to 0.28 Kj/h, associated with a body temperature increase of 1.2 °C. In principle, this increase is approximately three times lower than that previously reported in bat species challenged with a bacterial molecular pattern and represents 0.25% of the daily energy expenditure calculated for A. lituratus . The reasons underlying the differing APR energy costs induced by bacterial and viral molecular patterns as well as whether this energy investment, which is trivial in appearance, impacts the daily energy expenditure of A. lituratus remain to be determined.

Bats play a key role as host for multiple microorganism and virus without showing clinical manifestations of disease. After recognition of a potential threat, innate immunity triggers acute phase response, a systemic reaction that contributes to restrain microbial and viral growth. APR is characterized by fever, leukocytosis, and production of acute phase proteins, but also by behavioral changes, including somnolence, lethargy, and anorexia. Deploying immune responses, such as acute phase response, represents an energetic cost for vertebrates. In bats, it has been suggested that higher metabolic rates reached during flight might subsidize any inherent cost of raising metabolism to activate an immune response. Therefore, a central question is whether immune response represents a significant cost to bats and, if so, how much is the metabolic cost of these responses. Here, we assess the resting metabolic rate of Artibeus lituratus in response to challenge with LPS. In addition, we assessed parameters of acute phase response including fever, body mass loss, and leukocytosis in this specie. We found that challenge with LPS leads to an increase of 40% in resting metabolic rate of A. lituratus, concomitant with body mass loss and an increase in body temperature of 1.5 °C.