Vocal Communication in a Fission-Fusion Society: Do Spider Monkeys Stay in Touch With Close Associates?

In fission-fusion societies, in which animals from the same group may spend long periods of time apart from each other, individuals could use long-distance vocalizations to maintain contact with others. This could be achieved with vocalizations that simply carried information about the caller's identity and location. I explored this possibility using observations and experiments from a 3-year field study of spider monkeys (Ateles geoffroyi) in Punta Laguna, México, analyzing the use of the species' most frequent vocalization, the whinny. By following 2 subgroups simultaneously, I found that subgroups that were within the active space of the whinny approached each other more often than subgroups that were farther apart. Individual adults in these subgroups also emitted more whinnies when they were within hearing range of another subgroup than when farther apart. I used a paired playback design to determine whether whinnies could influence the behavior of close associates as opposed to nonassociated individuals. Although nonassociates were as likely as close associates to respond vocally to playbacks of whinnies, only a close associate ever approached the speaker. Collectively, the results suggest that whinnies are used by spider monkeys to achieve flexibility in spacing while maintaining specific social relationships.     

Grouping in auditory temporal perception and vocal production is mutually adapted: the case of wriggling calls of mice

Auditory Gestalt perception by grouping of species-specific vocalizations to a perceptual stream with a defined meaning is typical for human speech perception but has not been studied in non-human mammals so far. Here we use synthesized models of vocalizations (series of wriggling calls) of mouse pups (Mus domesticus) and show that their mothers perceive the call series as a meaningful Gestalt for the release of instinctive maternal behavior, if the inter-call intervals have durations of 100–400 ms. Shorter or longer inter-call intervals significantly reduce the maternal responsiveness. We also show that series of natural wriggling calls have inter-call intervals mainly in the range of 100–400 ms. Thus, series of natural wriggling calls of pups match the time-domain auditory filters of their mothers in order to be optimally perceived and recognized. A similar time window exists for the production of human speech and the perception of series of sounds by humans. Neural mechanisms for setting the boundaries of the time window are discussed.     

Geographic variation in female preference functions and male songs of the field cricket Teleogryllus oceanicus

Male crickets (Teleogryllus oceanicus) produce a complex call consisting of two elements, the long chirp (three to eight sound pulses) followed by a series of short chirps (each with two sound pulses). There is significant geographic variation in the temporal structure of calls, and the long chirp is selected against by acoustically orienting parasitoids in some populations. Here we examine geographic variation in female preference functions for the amount of long chirp. In general, females prefer calls with greater proportions of long chirp, although the strength and nature of selection varied across populations. Variation in preference functions did not match variation in call structure. There was a mismatch between the proportion of long chirp produced by males in a population and the proportion of long chirp preferred by females. The convergent preferences of predators and females are likely to maintain genetic variation in song traits in parasitized populations. The apparent mismatch between preference and trait is discussed in relation to theoretical models of preference evolution.     

Variation in the frequency of the echolocation calls of Hipposideros ruber in the Gulf of Guinea: an exploration of the adaptive meaning of the constant frequency value in rhinolophoid CF bats

This study describes variation patterns in the constant frequency (CF) segment of echolocation calls of the bat Hipposideros ruber within and among populations across the region of the Gulf of Guinea. Correlations of variation in CF with variation in body size, body condition, environmental humidity and presence of ecologically similar species are studied in an attempt to identify the forces driving the evolution of CF. We found that bats may adapt the frequency to humidity, and that CF may evolve under interspecific interactions, either of ecological or of social nature. The results support an adaptive value for the high values of CF, and challenge the ‘Allotonic Frequency Hypothesis’. We found correlation between frequency and a body condition index, which may trigger social selection processes in this species sexually dimorphic in CF. Combined social and environmental selection on CF could trigger diversification of bats along ecotones separating habitats with contrasting air humidity.     

Female preference functions based on call duration in the gray tree frog (Hyla versicolor)

Preference functions, which quantify preference strength relativeto variation in male traits or signals, are central to understandingmechanisms and consequences of female choice. Female tree frogs(Hyla versicolor) choose mates on the basis of advertisementcalls and prefer long calls to short calls. Here we show, intwo experimental designs, that preference strength increasedsignificantly as the difference in call duration was increasedonly if the absolute durations of alternative stimuli were below average. Hence preference strength was a non-linear functionof duration, and females did not base preferences solely onthe percentage difference in duration. In experiments simulatingcostly choice (unequal playback levels), non-linear effectswere more pronounced than in the conventional design (equal playback levels). Repeated estimates of preference strengthusing the unequal-playback design revealed significant among-femalevariation. These patterns of preference suggest that selectionby female choice for males producing calls of average durationover males producing very short calls is stronger than selectionfor males producing very long calls over males producing callsof average duration. Female preferences, especially in tests simulating a potentially costly choice, could reflect differencesin the net benefits to females of mating with males producingcalls of different duration.     

Patterns of contact call differentiation in the panmictic East African Abyssinian White‐eye Zosterops abyssinicus (Aves: Passeriformes)

Species distribution patterns range from highly disjunct to continuous, depending on their ecological demands and the availability of respective habitats. East African savannahs are mostly interconnected and ecologically comparatively homogenous and thus provide a prerequisite for a rather panmictic distribution pattern for species occurring in this habitat. The Abyssinian white‐eye Zosterops abyssinicus is a savannah inhabiting bird species, representing such a continuous distribution. This species occurs in high abundances and is very mobile, and past population genetic studies have suggested that gene flow is high and genetic differentiation is low even across relatively large geographic distances. Further, only little morphological differences were found. In order to test for potential divergence in acoustic traits despite its interconnected geographic distribution, we analyzed 2795 contact calls of Z. abyssinicus, which were recorded at 19 sites across Kenya. Our data indicate weak, but significant differentiation in call characteristics across latitudinal gradients. We found strong changes in call characteristics in populations where Z. abyssinicus occurs in sympatry with its highland congener, Zosterops poliogaster. However, the changes in call characteristics in sympatry were in different directions and lead to strong differentiation of the sympatric populations to other conspecific populations potentially representing a case of cascade reinforcement. The detected spatial gradients likely result from ecological differences and balancing effects of natural and sexual selection.     

Does vocal learning accelerate acoustic diversification? Evolution of contact calls in Neotropical parrots

Learning has been traditionally thought to accelerate the evolutionary change of behavioural traits. We evaluated the evolutionary rate of learned vocalizations and the interplay of morphology and ecology in the evolution of these signals. We examined contact calls of 51 species of Neotropical parrots from the tribe Arini. Parrots are ideal subjects due to their wide range of body sizes and habitats, and their open‐ended vocal learning that allows them to modify their calls throughout life. We estimated the evolutionary rate of acoustic parameters of parrot contact calls and compared them to those of morphological traits and habitat. We also evaluated the effect of body mass, bill length, vegetation density and species interactions on acoustic parameters of contact calls while controlling for phylogeny. Evolutionary rates of acoustic parameters did not differ from those of our predictor variables except for spectral entropy, which had a significantly slower rate of evolution. We found support for correlated evolution of call duration, and fundamental and peak frequencies with body mass, and of fundamental frequency with bill length. The degree of sympatry between species did not have a significant effect on acoustic parameters. Our results suggest that parrot contact calls, which are learned acoustic signals, show evolutionary rates similar to those of morphological traits. This is the first study to our knowledge to provide evidence that change through cultural evolution does not necessarily accelerate the evolutionary rate of traits acquired through life‐long vocal learning.