Walter Hödl

Bimodal signal requisite for agonistic behavior in a dart-poison frog, Epipedobates femoralis

Animal acoustic signals play seminal roles in mate attraction and regulation of male spacing, maintenance of pairbonds, localization of hosts by parasites, and feeding behavior. Among vertebrate signals, it is becoming clear that no single stereotyped signal feature reliably elicits species-specific behavior, but rather, that a suite of characters is involved. Within the largely nocturnal clade of anuran amphibians, the dart-poison frog, Epipedobates femoralis, is a diurnal species that physically and vigorously defends its calling territory against conspecific intruders. Here we report that physical attacks by a territorial male are provoked only in response to dynamic bimodal stimuli in which the acoustic playback of vocalizations is coupled with vocal sac pulsations, but not by either unimodal cues presented in isolation or static bimodal stimuli. These results suggest that integration of dynamic bimodal cues is necessary to elicit aggression in this species.

Call differences and calling site segregation in anuran species from central Amazonian floating meadows

SummaryThe acoustic behaviour of 15 sympatric and synchronically breeding species of frogs in an area of floating meadows near Manaus (Brazil) was studied for a period of 8 months. The calling positions of each species can be identified with certain physiognomic types of vegetation.Sound analyses were used to compare the mating calls. The main variables are dominant frequency, call duration and pulse repetition rate. Each of the 15 species has a distinct mating call and differs from the acoustic behaviour of each other one. Eleven species are separated in their dominant frequency ranges within their specific calling sites. Species sharing emphasised frequency ranges within identical calling sites differ greatly in at least two temporal variables.The roles of calling position, spectral, and temporal features of mating calls in species recognition and premating reproductive isolation are discussed.

MASKING INTERFERENCE AND THE EVOLUTION OF THE ACOUSTIC COMMUNICATION SYSTEM IN THE AMAZONIAN DENDROBATID FROG ALLOBATES FEMORALIS

Abstract The efficacy of communication relies on detection of species‐specific signals against the background noise. Features affecting signal detection are thus expected to evolve under selective pressures represented by masking noise. Spectral partitioning between the auditory signals of co‐occurring species has been interpreted as the outcome of the selective effects of masking interference. However, masking interference depends not only on signals frequency but on receivers range of frequency sensitivity; moreover, selection on signal frequency can be confounded by selection on body size, because these traits are often correlated. To know whether geographic variation in communication traits agrees with predictions about masking interference effects, we tested the hypothesis that variation in the male‐male communication system of the Amazonian frog, Allobates femoralis, is correlated with the occurrence of a single species calling within an overlapping frequency range, Epipedobates trivittatus. We studied frogs at eight sites, four where both species co‐occur and four where A. femoralis occurs but E. trivittatus does not. To study the sender component of the communication system of A. femoralis and to describe the use of the spectral range, we analyzed the signals spectral features of all coactive species at each site. To study the receiver component, we derived frequency‐response curves from playback experiments conducted on territorial males of A. femoralis under natural conditions. Most geographic variation in studied traits was correlated with either call frequency or with response frequency range. The occurrence of E. trivittatus significantly predicted narrower and asymmetric frequency‐response curves in A. femoralis, without concomitant differences in the call or in body size. The number of acoustically coactive species did not significantly predict variation in any of the studied traits. Our results strongly support that the receiver but not the sender component of the communication system changed due to masking interference by a single species.

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.

HOW, WHEN, AND WHERE TO PERFORM VISUAL DISPLAYS: THE CASE OF THE AMAZONIAN FROG HYLA PARVICEPS

We performed intrusion experiments and observed the course of 13 male-male agonistic interactions to gather information on the communicative role of visual signaling in the Amazonian tree frog Hyla parviceps. To obtain information on the ecological context potentially associated with visual signaling, we performed nightly censuses of calling activity and tested whether males differentially used microhabitats in relation to properties that affect both acoustic and visual communication. Among seven behaviors performed by males, two were visual displays. Foot-flagging displays and advertisement calls were used at a similar rate and at similar distances between interactants. Arm-waving displays were less common and used at a closer range than foot-flagging displays. The analysis of a dyadic transition matrix revealed that foot flagging significantly elicited foot-flagging displays by the opponent frog. Furthermore, resident males produced more arm wavings and calls than intruders, although the latter difference was not significant. We conclude that male H. parviceps respond to intruders by combining advertisement calls and visual displays, and that visual signals may serve functionally as a spacing mechanism. Comparing the properties of perches used by calling males with a random sample of available perches indicates that males prefer perches surrounded by denser and higher vegetation. Furthermore, calling activity occurred during or shortly after heavy rains and coincided with calling activity of several co-occurring species of hylid frogs, which probably decreases the locatability of calling males. We suggest that, under these conditions, the simultaneous production of auditory and visual signals may momentarily increase a senders locatability when a conspecific receiver is detected.

Strong male/male competition allows for nonchoosy females: high levels of polygynandry in a territorial frog with paternal care

Our knowledge about genetic mating systems and the underlying causes for and consequences of variation in reproductive success has substantially improved in recent years. When linked to longitudinal population studies, cross‐generational pedigrees across wild populations can help answer a wide suite of questions in ecology and evolutionary biology. We used microsatellite markers and exhaustive sampling of two successive adult generations to obtain population‐wide estimates of individual reproductive output of males and females in a natural population of the Neotropical frog Allobates femoralis (Aromobatidae), a pan‐Amazonian species that features prolonged iteroparous breeding, male territoriality and male parental care. Parentage analysis revealed a polygynandrous mating system in which high proportions of males (35.5%) and females (56.0%) produced progeny that survived until adulthood. Despite contrasting reproductive strategies, successfully reproducing males and females had similar numbers of mating partners that sired the adult progeny (both sexes: median 2; range 1–6); the numbers of their offspring that reached adulthood were also similar (both sexes: median 2; range 1–8). Measures of reproductive skew indicate selection on males only for their opportunity to breed. Reproductive success was significantly higher in territorial than in nonterritorial males, but unrelated to territory size in males or to body size in both sexes. We hypothesize that female polyandry in this species has evolved because of enhanced offspring survival when paternal care is allocated to multiple partners.

Growth, morphometrics, size at maturity, sexual dimorphism and condition index of Austropotamobius torrentium Schrank

Growth, morphometric relationships and the size at maturity of the Stone crayfish Austropotamobius torrentium, a highly endangered European crayfish species, were investigated, in order to provide information for future protection management. Analyses of the weight–length relationship of A. torrentium revealed that males gained weight faster than females. This difference is due to an increased growth of chelae of males larger than 50 mm, which is assumed to be the size of sexual maturity of males. Female crayfish reached sexual maturity at 59 – 65 mm total length. The sex ratio in the studied sections of the brook Biberbach in Lower Austria was balanced. The estimated annual instantaneous growth rates ranged from 0 to 1.84. The growth rate declined rapidly in both sexes with increasing crayfish size. The crayfish condition showed the tendency to decline from May to August. The average crayfish condition was significantly higher in the section with superior current heterogeneity and percentage of potential shelter stones.

Multimodal signaling in the Small Torrent Frog (Micrixalus saxicola) in a complex acoustic environment

Many animals use multimodal (both visual and acoustic) components in courtship signals. The acoustic communication of anuran amphibians can be masked by the presence of environmental background noise, and multimodal displays may enhance receiver detection in complex acoustic environments. In the present study, we measured sound pressure levels of concurrently calling males of the Small Torrent Frog (Micrixalus saxicola) and used acoustic playbacks and an inflatable balloon mimicking a vocal sac to investigate male responses to controlled unimodal (acoustic) and multimodal (acoustic and visual) dynamic stimuli in the frogs’ natural habitat. Our results suggest that abiotic noise of the stream does not constrain signal detection, but males are faced with acoustic interference and masking from conspecific chorus noise. Multimodal stimuli elicited greater response from males and triggered significantly more visual signal responses than unimodal stimuli. We suggest that the vocal sac acts as a visual cue and improves detection and discrimination of acoustic signals by making them more salient to receivers amidst complex biotic background noise.

The annual number of breeding adults and the effective population size of syntopic newts (Triturus cristatus, T. marmoratus)

Pond‐breeding amphibians are deme‐structured organisms with a population genetic structure particularly susceptible to demographic threats. We estimated the effective number of breeding adults (Nb) and the effective population size (Ne) of the European urodele amphibians Triturus cristatus (the crested newt) and T. marmoratus (the marbled newt), using temporal shifts in microsatellite allele frequencies. Eight microsatellite loci isolated from a T. cristatus library were used, five of which proved polymorphic in T. marmoratus, albeit with high frequencies of null alleles at two loci. Three ponds in western France were sampled, situated 4–10 kilometres apart and inhabited by both species. Parent–offspring cohort comparisons were used to measure Nb; samples collected at time intervals of nine or 12 years, respectively, were used to measure Ne. The adult population census size (N) was determined by mark–recapture techniques. With one exception, genetic distances (FST) between temporal samples were lower than among populations. Nb ranged between 10.6 and 101.8 individuals, Ne ranged between 9.6 and 13.4 individuals. For the pond where both parameters were available, Nb/N (overall range: 0.10–0.19) was marginally larger than Ne/N (overall range: 0.09–0.16), which is reflected in the temporal stability of N. In line with the observed differences in reproductive life‐histories between the species, Nb/N ratios for newts were about one order of magnitude higher than for the anuran amphibian Bufo bufo. Despite of the colonization of the study area by T. cristatus only some decades ago, no significant genetic bottleneck could be detected. Our findings give rise to concerns about the long‐term demographic viability of amphibian populations in situations typical for European landscapes.

The rôle of call frequency and the auditory papillae in phonotactic behavior in male Dart-poison frogs Epipedobates femoralis (Dendrobatidae)

Territorial males of the pan-Amazonian Dart-poison frog, Epipedobates femoralis, are known to present stereotypic phonotactic responses to the playback of conspecific and synthetic calls. Fixed site attachment and a long calling period within an environment of little temperature change render this terrestrial and diurnal pan-Amazonian frog a rewarding species for field bioacoustics. In experiments at the field station Arataï, French Guiana, we tested whether the prominent frequency modulation of the advertisement-call notes is critical for eliciting phonotactic responses. Substitution of the natural upward sweep by either a pure tone within the species frequency range or a reverse sweep did not alter the males’ phonotactic behavior. Playbacks with artificial advertisement calls embedded in high levels of either low-pass or high-pass masking noise designed to saturate nerve fibers from either the amphibian papilla or basilar papilla showed that male phonotactic behavior in this species is subserved by activation of the basilar papilla of the inner ear.