No abstract

Some features of emotional prosody in human speech may be traced back to affect cues in mammalian vocalizations. The present study addresses the question whether affect intensity, as expressed by the intensity of behavioral displays, is encoded in vocal cues, i.e., changes in the structure of associated calls, in bats, a group evolutionarily remote from primates. A frame-by-frame video analysis of 109 dyadic agonistic interactions recorded in approach situations was performed to categorize displays into two intensity levels based on a cost-benefit estimate. M. lyra showed graded visual displays accompanied by specific calls and response calls of the second bat. A sound analysis revealed systematic changes of call sequence parameters with display level. At the high intensity level, total call duration, number of syllables within a call, and the number of calls within a sequence were increased, while intervals between call syllables were decreased for both call types. In addition, the latency of the response call was shorter, and its main syllable-type durations and fundamental frequency were increased. These systematic changes of vocal parameters with affect intensity correspond to prosodic changes in human speech, suggesting that emotion-related acoustic cues are a common feature of vocal communication in mammals.

Phenotypes of distantly related species may converge through adaptation to similar habitats and/or because they share biological constraints that limit the phenotypic variants produced. A common theme in bats is the sympatric occurrence of cryptic species that are convergent in morphology but divergent in echolocation frequency, suggesting that echolocation may facilitate niche partitioning, reducing competition. If so, allopatric populations freed from competition, could converge in both morphology and echolocation provided they occupy similar niches or share biological constraints. We investigated the evolutionary history of a widely distributed African horseshoe bat, Rhinolophus darlingi, in the context of phenotypic convergence. We used phylogenetic inference to identify and date lineage divergence together with phenotypic comparisons and ecological niche modelling to identify morphological and geographical correlates of those lineages. Our results indicate that R. darlingi is paraphyletic, the eastern and western parts of its distribution forming two distinct non-sister lineages that diverged ~9.7 Mya. We retain R. darlingi for the eastern lineage and argue that the western lineage, currently the sub-species R. d. damarensis, should be elevated to full species status. R. damarensis comprises two lineages that diverged ~5 Mya. Our findings concur with patterns of divergence of other co-distributed taxa which are associated with increased regional aridification between 7-5 Mya suggesting possible vicariant evolution. The morphology and echolocation calls of R. darlingi and R. damarensis are convergent despite occupying different biomes. This suggests that adaptation to similar habitats is not responsible for the convergence. Furthermore, R. darlingi forms part of a clade comprising species that are bigger and echolocate at lower frequencies than R. darlingi, suggesting that biological constraints are unlikely to have influenced the convergence. Instead, the striking similarity in morphology and sensory biology are probably the result of neutral evolutionary processes, resulting in the independent evolution of similar phenotypes.

Communication is a fundamental component of evolutionary change because of its role in mate choice and sexual selection. Acoustic signals are a vital element of animal communication and sympatric species may use private frequency bands to facilitate intraspecific communication and identification of conspecifics (acoustic communication hypothesis, ACH). If so, animals should show increasing rates of misclassification with increasing overlap in frequency between their own calls and those used by sympatric heterospecifics. We tested this on the echolocation of the horseshoe bat, Rhinolophus capensis, using a classical habituationedishabituation experiment in which we exposed R. capensis from two phonetic populations to echolocation calls of sympatric and allopatric horseshoe bat species (Rhinolophus clivosus and Rhinolophus damarensis) and different phonetic populations of R. capensis. As predicted by the ACH, R. capensis from both test populations were able to discriminate between their own calls and calls of the respective sympatric horseshoe bat species. However, only bats from one test population were able to discriminate between calls of allopatric heterospecifics and their own population when both were using the same frequency. The local acoustic signalling assemblages (ensemble of signals from sympatric conspecifics and heterospecifics) of the two populations differed in complexity as a result of contact with other phonetic populations and sympatric heterospecifics. We therefore propose that a hierarchy of discrimination ability has evolved within the same species. Frequency alone may be sufficient to assess species membership in relatively simple acoustic assemblages but the ability to use additional acoustic cues may have evolved in more complex acoustic assemblages to circumvent misidentifications as a result of the use of overlapping signals. When the acoustic signal design is under strong constraints as a result of dual functions and the available acoustic space is limited because of cooccurring species, species discrimination is mediated through improved sensory acuity in the receiver.

The skulls of animals have to perform many functions. Optimization for one function may mean another function is less optimized, resulting in evolutionary trade-offs. Here, we investigate whether a trade-off exists between the masticatory and sensory functions of animal skulls using echolocating bats as model species. Several species of rhinolophid bats deviate from the allometric relationship between body size and echolocation frequency. Such deviation may be the result of selection for increased bite force, resulting in a decrease in snout length which could in turn lead to higher echolocation frequencies. If so, there should be a positive relationship between bite force and echolocation frequency. We investigated this relationship in several species of southern African rhinolophids using phylogenetically informed analyses of the allometry of their bite force and echolocation frequency and of the three-dimensional shape of their skulls. As predicted, echolocation frequency was positively correlated with bite force, suggesting that its evolution is influenced by a trade-off between the masticatory and sensory functions of the skull. In support of this, variation in skull shape was explained by both echolocation frequency (80%) and bite force (20%). Furthermore, it appears that selection has acted on the nasal capsules, which have a frequency-specific impedance matching function during vocalization. There was a negative correlation between echolocation frequency and capsule volume across species. Optimization of the masticatory function of the skull may have been achieved through changes in the shape of the mandible and associated musculature, elements not considered in this study.

No abstract

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.

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