Habitat acoustics and primate communication

The acoustic characteristics of three tropical habitats were investigated to determine how they might constrain the structure of primate signals. Ambient noise was measured, along with signal attenuation and aspects of signal degradation (reverberation, amplitude fluctuations, and pulse train modulation depth). These measures allowed estimation of the effects of habitat acoustics on the distances over which calls would be audible (the “active space”) and over which primates could reliably transmit amplitude-modulated or pulse-coded information.     

The effect of body size and habitat on the evolution of alarm vocalizations in rodents

When confronted with a predator, many mammalian species emit vocalizations known as alarm calls. Vocal structure variation results from the interactive effects of different selective pressures and constraints affecting their production, transmission, and detection. Body size is an important morphological constraint influencing the lowest frequencies that an organism can produce. The acoustic environment influences signal degradation; low frequencies should be favoured in dense forests compared to more open habitats (i.e. the ‘acoustic adaptation hypothesis’). Such hypotheses have been mainly examined in birds, whereas the proximate and ultimate factors affecting vocalizations in nonprimate mammals have received less attention. In the present study, we investigated the relationships between the frequency of alarm calls, body mass, and habitat in 65 species of rodents. Although we found the expected negative relationship between call frequency and body mass, we found no significant differences in acoustic characteristics between closed and open‐habitat species. The results of the present study show that the acoustic frequencies of alarm calls can provide reliable information about the size of a sender in this taxonomic group, although they generally do not support the acoustic adaptation hypothesis.     

Habitat complexity and the structure of vocalizations: a test of the acoustic adaptation hypothesis in three rail species (Rallidae)

The acoustic adaptation hypothesis is based on the assumption that senders are directionally selected to maximize transmission and minimize degradation; however, the two aims are not necessarily convergent. In complex habitats, where more effects that might potentially cause attenuation and degradation co‐occur and longer transmission must incur a higher cost, signals should attenuate faster and have shorter transmission ranges. At the same time, such signals should be more resistant to degradation in order to preserve their communicatory function. Based on a sound transmission experiment, we tested the evidence for these predictions using territorial calls of three sympatric species of rails, inhabiting habitats with increasing complexity: Corncrake Crex crex, Spotted Crake Porzana porzana and Water Rail Rallus aquaticus. In the experiment, the calls were broadcasted with similar amplitudes through a heterogeneous habitat inhabited by all three species and rerecorded at different distances up to 320 m. Despite standardized amplitudes and habitats, calls of the species living in simpler habitats had longer transmission ranges but were more susceptible to degradation than calls of the species living in more complex habitats. Our results suggest that narrow frequency bandwidth is an adaptation of species inhabiting complex habitats that helps their calls to degrade less at the cost of stronger attenuation and shorter transmission range. By contrast, wide frequency bandwidth extends the range but increases degradation and thus it is used only by species inhabiting structurally simpler habitats. This study shows that, in more complex habitats, the clarity of the message is preserved at the cost of range.     

Local richness and distribution of the lizard fauna in natural habitat mosaics of the Brazilian Cerrado

We investigate local lizard richness and distribution in central Brazilian Cerrado, harbouring one of the least studied herpetofaunas in the Neotropical region. Our results are based on standardized samplings at 10 localities, involving 2917 captures of 57 lizard species in 10 families. Local richness values exceeded most presented in earlier studies and varied from 13 to 28 species, with modal values between 19 and 28 species. Most of the Cerrado lizard fauna is composed of habitat‐specialists with patchy distributions in the mosaic of grasslands, savannas and forests, resulting in habitat‐structured lizard assemblages. Faunal overlap between open and forested habitats is limited, and forested and open areas may act as mutual barriers to lizard distribution. Habitat use is influenced by niche conservatism in deep lineages, with iguanians and gekkotans showing higher use of forested habitats, whereas autarchoglossans are richer and more abundant in open habitats. Contrary to trends observed in Cerrado birds and large mammals, lizard richness is significantly higher in open, interfluvial habitats that dominate the Cerrado landscape. Between‐localities variation in lizard richness seems tied to geographical distance, landscape history and phylogenetic constraints, factors operating in other well‐studied lizard faunas in open environments. Higher richness in dominant, open interfluvial habitats may be recurrent in Squamata and other small‐bodied vertebrates, posing a threat to conservation as these habitats are most vulnerable to the fast, widespread and ongoing process of habitat destruction in central Brazil.     

Long‐distance call evolution in the Felidae: effects of body weight,habitat, and phylogeny

Long‐distance calls used for mate attraction and territorial spacing are distinctive signals in the felid vocal repertoire. Their evolution is subject to natural and sexual selection, as well as various constraints. Body size is an important morphological constraint, with the scaling of the spectral characteristics of a species' vocalizations with its body size being established for several vertebrate groups. Alternatively, the structure of long‐distance calls may have been optimized for transmission in species' habitats (acoustic adaptation hypothesis). The present study assessed whether the mean dominant frequency of long‐distance calls in the Felidae (approximately 70% of all species incorporated) is influenced by the species' body size and/or conforms to the acoustic adaptation hypothesis. After controlling for phylogenetic relationships, we found a significant correlation between mean dominant frequency of a taxon's long‐distance calls and conditions for sound transmission in its habitat type (‘open/heterogeneous’ versus ‘dense’), although no significant influence of body size. Taxa living in more open habitat types have long‐distance calls with significantly lower mean dominant frequencies than those living in dense habitats. The result obtained in the present analysis is fairly robust against random removal of single or few taxa from the data, and also against the use of different branch‐length transformation models in phylogenetic regression. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 487–500.     

Environmental correlates of vocal communication of wild pygmy marmosets, Cebuella pygmaea

We quantified the acoustic characteristics of the habitats of two wild populations of pygmy marmosets, in Amazonian Ecuador to evaluate their effects on vocal signal structure. We obtained measures of ambient noise, sound attenuation and reverberation through recordings and broadcasts of exemplars of two short-range vocalizations and one long-range vocalization of the marmosets. Ambient noise levels differed among habitats. The calls of pygmy marmosets had frequencies that coincided with relatively quiet regions of the ambient noise spectra of the habitats. The three vocalization types were degraded similarly in all habitats. The two short-range signals, with a pulsatile structure, were more affected by reverberation than was the long-range, less pulsatile vocalization. This degradation could be used by the marmosets to estimate the distance of the caller animals. We obtained data on context of vocalizations from six groups of pygmy marmosets, three from each population, in both the dry and rainy seasons. The use of Trills, J calls and Long calls was related to the distance between the calling animal and the potential receivers suggesting that marmosets are using the calls in a way appropriate to the effects of habitat acoustics. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

The vocal repertoires of the bush dog (Speothos venaticus), crab-eating fox (Cerdocyon thous), and maned wolf (Chrysocyon brachyurus)

The vocalizations of the bush dog (Speothos venaticus), crab-eating fox (Cerdocyon thous), and maned wolf (Chrysocyon brachyurus) were studied in a captive setting. The three species' repertoires were similar and included whines, long-distance calls, and growls. The whines and growls varied in amplitude, duration, and repetition rate; these variations and the frequencies of occurrence probably were influenced by both social organization and habitat characteristics. The bush dog, which lives in family groups and inhabits rain forests, had an elaborate whine-scream continuum that at close distances conveyed a wide range of moods. On the other hand, the maned wolf, which is solitary and frequents open pampas, used amplitude variations of the growl to signal hostile intentions over short, medium, and long distances.The long-distance vocalizations of the three species were the least variable in structure; the bush dog and crab-eating fox used the call to promote contact with separated pairmates or family members. Based on recordings during development, these two calls appeared to arise from the repetitive whine, a variant of the basic whine syllable. The maned wolf also had a contact-promoting howl, apparently an extended whine of high amplitude, but it was rarely recorded during the present study. More prominent was the maned wolf roar-bark, which functioned to space individuals over large distances, and appeared to be a growl of high amplitude.     

Do aggressive signals evolve towards higher reliability or lower costs of assessment?

It has been suggested that the evolution of signals must be a wasteful process for the signaller, aimed at the maximization of signal honesty. However, the reliability of communication depends not only on the costs paid by signallers but also on the costs paid by receivers during assessment, and less attention has been given to the interaction between these two types of costs during the evolution of signalling systems. A signaller and receiver may accept some level of signal dishonesty by choosing signals that are cheaper in terms of assessment but that are stabilized with less reliable mechanisms. I studied the potential trade‐off between signal reliability and the costs of signal assessment in the corncrake (Crex crex). I found that the birds prefer signals that are less costly regarding assessment rather than more reliable. Despite the fact that the fundamental frequency of calls was a strong predictor of male size, it was ignored by receivers unless they could directly compare signal variants. My data revealed a response advantage of costly signals when comparison between calls differing with fundamental frequencies is fast and straightforward, whereas cheap signalling is preferred in natural conditions. These data might improve our understanding of the influence of receivers on signal design because they support the hypothesis that fully honest signalling systems may be prone to dishonesty based on the effects of receiver costs and be replaced by signals that are cheaper in production and reception but more susceptible to cheating.     

Sometimes noise is beneficial: stream noise informs vocal communication in the little torrent frog <Emphasis Type="Italic">Amolops torrentis</Emphasis>

Many kinds of environmental noise can interfere with acoustic communication and efficient decision making in terrestrial species. Here we identified an exception to this generalization in a streamside species, the little torrent frog (Amolops torrentis) which communicates in a stream noise environment. To determine whether stream noise can act as a cue regarding the microhabitat characteristics of senders, we performed phonotaxis experiments using stimulus pairs constructed with synthetic male calls (high or low dominant frequency) and stream noise with varied signal-to-noise ratios. We found that females prefer calls with high amplitude stream noise added compared to those with low amplitude stream noise added for both high and low dominant frequency stimulus pairs; however, stream noise itself was not attractive in the absence of calls. These results show that stream noise can function as a cue that may be used by females for enhancing the attractiveness of calls. Stream noise associates closely with rocks, topographies and vegetation and may thus provide useful microhabitat information for signal receivers, thereby acting on sexual selection. These data therefore contribute to our understanding of how the perception of mate attractiveness in heterogeneous ecological environments can evolve.     

How cricket frog females deal with a noisy world: habitat-related differences in auditory tuning

Cricket frogs (Acris crepitans) occupy a variety of acousticallydifferent habitats ranging from pine forest to open grassland.There is geographic variation in their calls and the tuningof their basilar papilla (BP) correlated with habitat. Here,we characterize the spectral content of environmental noisefrom two habitats, one a pine forest (Stengl) and one a grassland(Gill) habitat. We then used rounded exponential filter functionsbased on the mean tuning of auditory fibers in Stengl and Gillfemales to model the BP tuning characteristics of an averagefemale from the two cricket frog populations occupying thosehabitats to compare their ability to filter out environmentalnoise. Noise recordings were made at both sites from 1800 to2400 h on multiple nights throughout a breeding season (Marchthrough early August). Noise spectra were similar at both sites.Cross-correlation analyses of the sampled noise indicated thatnoise spectra were consistent throughout the night and variedlittle over the season other than during the month of May. Themodel auditory filter simulating an average Stengl female wassignificantly better than one simulating an average Gill femaleat filtering environmental noise at both sites. Previous workhad shown that cricket frog calls suffered greater attenuationand degradation in the Stengl site than the Gill site but thatthe male calls from Stengl frogs suffered less attenuation andless degradation than Gill calls during transmission throughboth habitats. These new results demonstrate that frogs fromthe more acoustically challenging Stengl habitat have enhancedboth the sender and receiver portions of their communicationsystem, evolving calls that transmit better and auditory filtersthat better eliminate noise.     

Spectral characteristics of intense mew calls in cat species of the genus <Emphasis Type="Italic">Felis</Emphasis> (Mammalia: Carnivora: Felidae)

Spectral characteristics of intense mew calls of six cat (sub)species in the genus Felis were studied in captivity: European wildcat (Felis s. silvestris), African wildcat (F. s. lybica), Asiatic steppe cat (F. s. ornata), black-footed cat (F. nigripes), jungle cat (F. chaus), and sand cat (F. margarita). The body weight of the largest (jungle cat) of the six taxa is about six times that of the smallest (black-footed cat), and they live in different habitat types ranging from open desert virtually devoid of vegetation (sand cat) to various types of rather dense forest and shrubland (European wildcat). These habitats differ considerably in the conditions for sound propagation. In this study we analyzed whether and how spectral characteristics of the intense mew calls of these cat taxa are related to their body weight/size and predominant habitat type (open vs. dense). Neither the mean fundamental frequency nor the mean dominant frequency of the intense calls of these cat (sub)species showed an inverse correlation with their respective body weights (“frequency scaling rule”). Rather, the mean dominant frequency is significantly positively correlated with body weight, being lower in the calls of the smaller taxa living in open habitat compared to those of the larger taxa living in dense habitat types. The hypothesis supported best by our data is that spectral features of intense mew calls in the Felis taxa studied have evolved to reduce attenuation when propagating through their respective habitat types. B. Tonkin-Leyhausen is retired and has no institutional affiliation any longer.     

Multimodal communication in a noisy environment: a case study of the Bornean rock frog Staurois parvus

High background noise is an impediment to signal detection and perception. We report the use of multiple solutions to improve signal perception in the acoustic and visual modality by the Bornean rock frog, Staurois parvus. We discovered that vocal communication was not impaired by continuous abiotic background noise characterised by fast-flowing water. Males modified amplitude, pitch, repetition rate and duration of notes within their advertisement call. The difference in sound pressure between advertisement calls and background noise at the call dominant frequency of 5578 Hz was 8 dB, a difference sufficient for receiver detection. In addition, males used several visual signals to communicate with conspecifics with foot flagging and foot flashing being the most common and conspicuous visual displays, followed by arm waving, upright posture, crouching, and an open-mouth display. We used acoustic playback experiments to test the efficacy-based alerting signal hypothesis of multimodal communication. In support of the alerting hypothesis, we found that acoustic signals and foot flagging are functionally linked with advertisement calling preceding foot flagging. We conclude that S. parvus has solved the problem of continuous broadband low-frequency noise by both modifying its advertisement call in multiple ways and by using numerous visual signals. This is the first example of a frog using multiple acoustic and visual solutions to communicate in an environment characterised by continuous noise.     

Wild Female Olive Baboons Adapt their Grunt Vocalizations to Environmental Conditions

Sound propagates differently and visibility varies according to the habitat type. Animals should therefore adapt the acoustic structure and the usage of their vocal signals to the environment. In the present study, we examined the influence of the habitat on the vocal behaviour of wild olive baboons ( Papio hamadryas anubis ) in two populations: one living in Gashaka-Gumti National Park, Nigeria, and the other in Budongo Forest, Uganda. We investigated whether female baboons modified the acoustic structure of their grunts and their rate of grunting when they wandered between closed and open habitat types. As an adaptation to the environmental conditions, baboons might utter calls with a longer duration, a lower fundamental frequency and/or energy concentrated in lower frequencies in a closed habitat like forest than in an open habitat. Baboons should also grunt more frequently in the closed habitat. Analyses showed that in both populations grunts uttered in forest were significantly longer than in open habitat. Additionally, baboons from Uganda showed a significantly higher grunt rate in forest than in open habitat. These results revealed a certain degree of plasticity in vocal production and call usage with regard to the habitat type. However, results in Nigeria suggested that, besides habitat structure, other proximate factors like the context of calling and the proximity between group members could also have an influence on the actual communication patterns.     

Changes in the Frequency Structure of a Mating Call Decrease Its Attractiveness to Females in the Cricket Frog Acris crepitans blanchardi

In many species, females often prefer male signals that are more complex than in nature or beyond the range of calls naturally produced by conspecific males in spectral, temporal and amplitude features. In this study we examined both the ability of females to recognize signals outside the normal range of spectral frequency variation seen in male advertisement calls, and the influence of increasing call complexity by adding spectral components to enhance the attractiveness of a male advertisement call in the cricket frog Acris crepitans blanchardi, while keeping its amplitude constant. We used two different natural male call groups and created the following synthetic call groups: with a dominant frequency at 3500 Hz, i.e. at the normal dominant frequency with a frequency band within the sensitivity range of the inner ear basilar papilla; with a dominant frequency at 700 Hz, i.e. outside the normal range of variation and with a frequency band outside the sensitivity range of the basilar papilla but within the range of the amphibian papilla; with two dominant frequencies, one at 700 Hz and another at 3500 Hz, stimulating the basilar and amphibian papilla simultaneously. In double choice experiments we tested all combinations of the three call groups, and we tested the 3500 Hz call groups against the same natural call groups. Additionally, we tested the 700 Hz call groups against white noise to see whether these signals are meaningful in mate choice. Females preferred 3500 Hz call groups over all other call groups. The synthetic call group was as attractive to females as the same natural call group. The 700 Hz call group was not meaningful in mate choice. The combined (700 Hz + 3500 Hz) call group was significantly less attractive to females than the 3500 Hz call group. Thus, making a call more spectrally complex without increasing its overall amplitude decreases its attractiveness to cricket frog females.     

The Acoustic Properties of Low Intensity Vocalizations Match Hearing Sensitivity in the Webbed-Toed Gecko,Gekko subpalmatus

The design of acoustic signals and hearing sensitivity in socially communicating species would normally be expected to closely match in order to minimize signal degradation and attenuation during signal propagation. Nevertheless, other factors such as sensory biases as well as morphological and physiological constraints may affect strict correspondence between signal features and hearing sensitivity. Thus study of the relationships between sender and receiver characteristics in species utilizing acoustic communication can provide information about how acoustic communication systems evolve. The genus Gekko includes species emitting high-amplitude vocalizations for long-range communication (loud callers) as well as species producing only low-amplitude vocalizations when in close contact with conspecifics (quiet callers) which have rarely been investigated. In order to investigate relationships between auditory physiology and the frequency characteristics of acoustic signals in a quiet caller, Gekko subpalmatus we measured the subjects’ vocal signal characteristics as well as auditory brainstem responses (ABRs) to assess auditory sensitivity. The results show that G. subpalmatus males emit low amplitude calls when encountering females, ranging in dominant frequency from 2.47 to 4.17 kHz with an average at 3.35 kHz. The auditory range with highest sensitivity closely matches the dominant frequency of the vocalizations. This correspondence is consistent with the notion that quiet and loud calling species are under similar selection pressures for matching auditory sensitivity with spectral characteristics of vocalizations.     

Vocal variation in <Emphasis Type="Italic">Chiroxiphia boliviana</Emphasis> (Aves; Pipridae) along an Andean elevational gradient

Bird vocalizations are likely shaped both by natural and sexual selection. Here we test the sensory-drive hypothesis, which states that communication signals diverge as a direct adaptation to the signaling environment and can evolve to minimize degradation and maximize transmission. We examined the effects of elevation and other habitat variables on variation in vocalizations of Chiroxiphia boliviana (Aves, Pipridae) along an elevational gradient (1300–2500 m) in cloud and humid montane forests in the Andes of Bolivia. We also conducted sound transmission experiments to determine if reverberation and attenuation changed along the gradient. Reverberation increased at higher elevations, and attenuation decreased at higher elevations and increased for higher frequencies. We recorded vocalizations from ~?50 individuals throughout the elevational gradient and examined variation in duration and bandwidth of short calls (used as contact calls between males), 2 display calls (advertisement for females) and 2 types of male–male duets (including interval times between males). Duration of short calls, display 1 and duet 1 increased with elevation. Bandwidth of short calls increased at mid-elevation categories and decreased at high elevations, whereas bandwidth of display 1 and duet 1 decreased with elevation. We also directly related the transmission properties to vocalizations and found that bandwidth of short calls decreased with reverberation and attenuation, bandwidth of display 2 decreased with reverberation, and duration of duet 1 both increased and decreased with attenuation (at 3 and 4 kHz, respectively). This study suggests that vocalizations by C. boliviana may be adapted to the habitat transmission properties along the elevational gradient; and perhaps that increasing song length and concentrating energy within a narrow bandwidth may lead to an increase in amplitude and improvement in transmission. Overall, our results support the sensory-drive hypothesis and suggest that this form of selection is likely common along tropical elevational gradients.     

Variation in Plant Substrates and its Consequences for Insect Vibrational Communication

Many insects and other arthropods communicate using plant‐borne vibrational signals. Vibration transmission along plant stems imposes a frequency filter on signals, and may cause signal degradation from reflected waves. Furthermore, different plant species and plant parts can differ in their transmission properties. This variability in the communication channel may constrain the reliability of signals, with important consequences for the evolution of vibrational communication systems, as well as for researchers studying signal variation at an individual, population, or species level. In this study we estimate the magnitude of substrate‐related variation in the mate advertisement signals of a treehopper (Hemiptera: Membracidae: Umbonia crassicornis). We used laser vibrometry to record the signals produced by 25 adult males on two different plant species, one host and one non‐host. We recorded male signals on two plants per species; within each plant, signals were recorded simultaneously at two distances. We measured three spectral characteristics (dominant frequency, relative amplitude of the signals’ high and low frequency components, frequency at the end of the signal) and two temporal characteristics (signal duration and click repetition rate). Spectral characteristics were influenced by the distance at which the signal was recorded, and this influence varied among plant species and individuals. Temporal characteristics were less influenced, although signal length was influenced by distance, an effect that varied among individual plants. Overall, the magnitude of the effects was small. Furthermore, there was significant within‐individual repeatability of almost all signal traits across different plant substrates. Signal characteristics were thus reliably associated with individuals, even when they signaled on different plants.     

Essay on Contemporary Issues in Ethology: Variation among Mammalian Alarm Call Systems and the Problem of Meaning in Animal Signals

Understanding the information conveyed by animal signals requires studies of both production and perception. It is important to determine the relationship between signal morphology and the circumstances of production, the way signaller behavior varies with motivational state and the role of context in mediating responses to signals. Alarm calls are well-suited to research of this type because they are widespread in birds and mammals and typically evoke unambiguous responses. We review studies of alarm calling in primates and ground-dwelling sciurid rodents, concentrating especially on whether these signal systems may be viewed as ‘functionally referential’, that is, as conveying sufficient information about an event for receivers to select appropriate responses. Comparisons of the physical, behavioral and habitat characteristics of these species suggest that incompatibility of the escape responses required to avoid different classes of predators may have been an important factor in the evolution of functionally referential alarm calls.