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Hearing mediates many behaviours critical for survival in echolocating bats, including foraging and navigation. Although most mammals are susceptible to progressive age-related hearing loss, the evolution of biosonar, which requires the ability to hear low-intensity echoes from outgoing sonar signals, may have selected against the development of hearing deficits in bats. Many echolocating bats exhibit exceptional longevity and rely on acoustic behaviours for survival to old age; however, relatively little is known about the ageing bat auditory system. In this study, we used DNA methylation to estimate the ages of wild-caught big brown bats (Eptesicus fuscus) and measured hearing sensitivity in young and ageing bats using auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). We found no evidence for hearing deficits in bats up to 12.5 years of age, demonstrated by comparable thresholds and similar ABR and DPOAE amplitudes across age groups. We additionally found no significant histological evidence for cochlear ageing, with similar hair cell counts, afferent and efferent innervation patterns in young and ageing bats. Here, we demonstrate that big brown bats show minimal evidence for age-related hearing loss and therefore represent informative models for investigating mechanisms that may preserve hearing function over a long lifetime. [ABSTRACT FROM AUTHOR]

Social structure can emerge from hierarchically embedded scales of movement, where movement at one scale is constrained within a larger scale (e.g. among branches, trees, forests). In most studies of animal social networks, some scales of movement are not observed, and the relative importance of the observed scales of movement is unclear. Here, we asked: how does individual variation in movement, at multiple nested spatial scales, influence each individual's social connectedness? Using existing data from common vampire bats (Desmodus rotundus), we created an agent-based model of how three nested scales of movement—among roosts, clusters and grooming partners—each influence a bat's grooming network centrality. In each of 10 simulations, virtual bats lacking social and spatial preferences moved at each scale at empirically derived rates that were either fixed or individually variable and either independent or correlated across scales. We found that numbers of partners groomed per bat were driven more by within-roost movements than by roost switching, highlighting that co-roosting networks do not fully capture bat social structure. Simulations revealed how individual variation in movement at nested spatial scales can cause false discovery and misidentification of preferred social relationships. Our model provides several insights into how nonsocial factors shape social networks. [ABSTRACT FROM AUTHOR]

Experimental evolution provides an integrative method for revealing complex interactions among evolutionary processes. One such interaction involves sex-linked selfish genetic elements and sexual selection. X-linked segregation distorters, a type of selfish genetic element, influence sperm transmission to increase in frequency and consequently alter the population sex ratio and the opportunity for sexual selection, while sexual selection may impact the spread of X-linked distorters. Here we manipulated sexual selection by controlling female mating opportunities and the presence of a distorting X chromosome in experimental lines of the stalk-eyed fly, Teleopsis dalmanni, over 11 generations. We find that removal of sexual selection leads to an increase in the frequency of the X-linked distorter and sex ratio across generations and that post-copulatory sexual selection alone is sufficient to limit the frequency of distorters. In addition, we find that male eyestalk length, a trait under pre-copulatory sexual selection, evolves in response to changes in the strength of sexual selection with the magnitude of the response dependent on X chromosome type and the frequency of distorting X chromosomes. These results reveal how a selfish X can interact with sexual selection to influence the evolution of sexually selected traits in multiple ways. [ABSTRACT FROM AUTHOR]