Sex-Specific Seasonal Change in Body Mass in Two Species of Vespertilionid Bats

The big brown bat (Eptesicus fuscus) reduces its body mass during winter in a tropical montane ecosystem of central Mexico

Most animals face changes in the availability of food and the environmental conditions in the places where they live. In response, they need to adjust their behavioral, physiological, and morphological traits. In temperate zones and high latitudes, bats increase their body mass (M b ) in autumn to store fat reserves and use them during hibernation. However, other small mammals decrease their M b prior to winter to reduce the energetic requirements of individuals. These changes are unknown for bats inhabiting other highly energetic demanding environments. We measured changes in M b of 84 non-reproductive males of Eptesicus fuscus inhabiting a tropical montane ecosystem in central Mexico over seasons. We also examined the relationship of bats’ M b with the minimum ambient temperature (T a , °C) and mean precipitation (mm). Bats presented an increase in M b from March to June, followed by a decrease from September to November and presented the lowest M b from November to March, in the dry-cold season. The results suggest that the pattern of changes in M b could be the result of two non-exclusive components related to the bats’ energy budget, the energetic demands experienced by the bats throughout the year and the morphological adaptations animals could display to reduce their energy requirements during the winter.

Recovery of southern bent-winged bats (Miniopterus orianae bassanii) after PIT-tagging and the use of surgical adhesive

Surgical adhesive is widely used to prevent shedding of injected PIT-tags, but the effect of this adhesive on individuals has not been documented. We recaptured 52 southern bent-winged bats up to 741 days after PIT-tagging. All recaptured individuals were in good body condition, with no signs of infection or skin irritation; however, temporary fur loss occurred at the injection site. We advocate that researchers consider the impact this may have on study species, particularly when PIT-tagging small mammals during cooler months.

Live fast, don't die young: Survival-reproduction trade-offs in long-lived income breeders

Trade-offs between survival and reproduction are at the core of life-history theory, and essential to understanding the evolution of reproductive tactics as well as population dynamics and stability. Factors influencing these trade-offs are multiple and often addressed in isolation. Further problems arise as reproductive states and survival in wild populations are estimated based on imperfect and potentially biased observation processes, which might lead to flawed conclusions. In this study, we aimed at elucidating trade-offs between current reproduction (both pregnancy and lactation), survival and future reproduction, including the specific costs of first reproduction, in long-lived, income breeding small mammals, an under-studied group. We developed a novel statistical framework that encapsulates the breeding life cycle of females, and accounts for incomplete information on female pregnancy and lactation and imperfect and biased recapture rates. We applied this framework to longitudinal data on two sympatric, closely related bat species (Myotis daubentonii and M. nattereri). We revealed the existence of several, to our knowledge previously unknown, trends in survival and breeding of these closely related, sympatric species and detected remarkable differences in their age and costs of first reproduction, as well as their survival-reproduction trade-offs. Our results indicate that species with this type of life history exhibit a mixture of patterns expected for long-lived and short-lived animals, and between income and capital breeders. Thus, we call for more studies to be conducted in similar study systems, increasing our ability to fully understand the evolutionary origin and fitness effects of trade-offs and senescence.

High Duty Cycle Echolocation May Constrain the Evolution of Diversity within Horseshoe Bats (Family: Rhinolophidae)

The phenotype of organisms is the net result of various evolutionary forces acting upon their lineages over time. When an innovative trait arises that confers a substantial advantage in terms of survival and reproduction, the evolution of adaptive complexes between such an innovation and other traits may constrain diversification of that lineage. The specialized echolocation system of the Rhinolophidae may represent such an innovation which affects other parts of the phenotype. We investigated a potential constraint on the diversity of phenotypes of several species of horseshoe bats within a phylogenetic framework. If phenotypic convergence stems from stasis as a result of the specialized echolocation system, phenotypes should converge not only among members of the same species and between sexes but also among species. We analyzed the phenotypic diversity of >800 individuals of 13–16 species. The phenotypes in the horseshoe bats did indeed converge. There was no sexual size dimorphism in mass, forearm length and wingspan within species and there was marked interspecific similarity in both wing and echolocation variables but marked variability in body mass. Furthermore, correlations of wing and echolocation variables with mass suggest that variability within horseshoe bats was largely the result of selection on body size with allometric responses in wing and echolocation parameters, a potential consequence of constraints imposed by their specialized echolocation.

Testing the Sensory Drive Hypothesis: Geographic variation in echolocation frequencies of Geoffroy's horseshoe bat (Rhinolophidae: Rhinolophus clivosus)

Geographic variation in sensory traits is usually influenced by adaptive processes because these traits are involved in crucial life-history aspects including orientation, communication, lineage recognition and mate choice. Studying this variation can therefore provide insights into lineage diversification. According to the Sensory Drive Hypothesis, lineage diversification may be driven by adaptation of sensory systems to local environments. It predicts that acoustic signals vary in association with local climatic conditions so that atmospheric attenuation is minimized and transmission of the signals maximized. To test this prediction, we investigated the influence of climatic factors (specifically relative humidity and temperature) on geographic variation in the resting frequencies of the echolocation pulses of Geoffroy's horseshoe bat, Rhinolophus clivosus. If the evolution of phenotypic variation in this lineage tracks climate variation, human induced climate change may lead to decreases in detection volumes and a reduction in foraging efficiency. A complex non-linear interaction between relative humidity and temperature affects atmospheric attenuation of sound and principal components composed of these correlated variables were, therefore, used in a linear mixed effects model to assess their contribution to observed variation in resting frequencies. A principal component composed predominantly of mean annual temperature (factor loading of -0.8455) significantly explained a proportion of the variation in resting frequency across sites (P < 0.05). Specifically, at higher relative humidity (around 60%) prevalent across the distribution of R. clivosus, increasing temperature had a strong negative effect on resting frequency. Climatic factors thus strongly influence acoustic signal divergence in this lineage, supporting the prediction of the Sensory Drive Hypothesis. The predicted future increase in temperature due to climate change is likely to decrease the detection volume in echolocating bats and adversely impact their foraging efficiency.

Using sutures to attach miniature tracking tags to small bats for multimonth movement and behavioral studies

Determining the detailed movements of individual animals often requires them to carry tracking devices, but tracking broad-scale movement of small bats (<30 g) has been limited by transmitter technology and long-term attachment methods. This limitation inhibits our understanding of bat dispersal and migration, particularly in the context of emerging conservation issues such as fatalities at wind turbines and diseases. We tested a novel method of attaching lightweight global positioning system (GPS) tags and geolocating data loggers to small bats. We used monofilament, synthetic, absorbable sutures to secure GPS tags and data loggers to the skin of anesthetized big brown bats (Eptesicus fuscus) in Colorado and hoary bats (Lasiurus cinereus) in California. GPS tags and data loggers were sutured to 17 bats in this study. Three tagged bats were recaptured 7 months after initial deployment, with tags still attached; none of these bats showed ill effects from the tag. No severe injuries were apparent upon recapture of 6 additional bats that carried tags up to 26 days after attachment; however, one of the bats exhibited skin chafing. Use of absorbable sutures to affix small tracking devices seems to be a safe, effective method for studying movements of bats over multiple months, although additional testing is warranted. This new attachment method has the potential to quickly advance our understanding of small bats, particularly as more sophisticated miniature tracking devices (e.g., satellite tags) become available.