Auditory Perception and Social Communication in Greater Spear-Nosed Bats

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Hanging bats
     Phyllostomus hastatus

    Greater Spear-Nosed Bats (Phyllostomus hastatus)

    While most research in the lab centers around echolocation, this research focuses on perception as it relates to social communication. Greater spear-nosed bats (Phyllostomus hastatus) are large, omnivorous microchiropterans found in Central and South America. These bats live in stable social groups of females with low levels of relatedness (McCraken & Bradbury 1981; McCraken 1987). Given the stable social structure and high degree of intraspecific interactions that characterize this species, it is not surprising that they use several types of social calls as well as ultrasonic echolocation calls.

     Phyllostomus hastatus

    One social call, the "screech" call, is used to coordinate group foraging. This low frequency (5-20 kHz) call is group specific and vocally learned. Although common in birds, this is one of the few cases of vocal learning in mammals. A second social call in this species is the "isolation call" that is emitted by pups. These calls, with a frequency range of 15 to 100 kHz, function in parent-offspring recognition and contain sufficient information to be individual signatures.

    Vocalizations


    Spectrogram of high frequency (30-80 kHz), low amplitude echolocation calls.

    Loud, low frequency (5-20 kHz) screech calls are used to coordinate group foraging. This call is group specific and vocally learned. This is one of the few examples of vocal learning in mammals.

    Isolation calls are emitted by infants and are used in parent-offspring recognition. These calls have a frequency range of 15 to 100 kHz and contain sufficient information to be individual signatures. In P. hastatus there are 4 types of calls (3 shown) composed of 3 different syllables.

    Average power spectra of screech (light dashed line), isolation (solid line) and echolocating calls (thick dashed line). Average of the lowest absolute thresholds (solid line) and masked thresholds at 25 dB Hz-1 (dashed line) and 35 dB Hz-1 (grey circles) for all four bats.

    Auditory Sensitivity and Communication

    We investigated the relationship between high-frequency echolocation call perception and low-frequency social call perception by determining auditory thresholds to pure tones from 2.5 to 100 kHz. Since these bats use low frequency social calls and demonstrate vocal learning, high auditory sensitivity is expected in that frequency region. However, the only audiogram available for this species ranged from 10 to 100 kHz and showed low auditory sensitivity below 20 kHz. Thresholds were determined for four P. hastatus using a go/no-go procedure.

    Sensitivity was highest over the frequency range of intraspecific communication rather than echolocation (10-20 kHz). Highest sensitivity was at 15 kHz, which does not match foraging calls, but does match the strongest energy component in isolation calls. These findings suggest that isolation calls may be an important component of the vocal repertoire in this species.

    Non-volant pups emit isolation calls when they fall from roosts in cave ceiling depressions (Wilkinson & Bohn unpub.). Calls attract females who carry young back to the roost (Wilkinson & Bohn unpub.). Unless retrieved by an adult, death is likely either through starvation, predation, or attacks from other females (Wilkinson & Bohn unpub.). P. hastatus have low reproductive rates (one pup per year) and high infant mortality (McCraken & Bradbury 1981, Stern & Kunz 1998). Recognizing pups may be an important component of fitness in this species. Consequently, the perceptual abilities of receivers may be under strong selection for detection and discrimination of these calls.

     Phyllostomus hastatus babies

    Future Research

    Future research on P. hastatus will focus on perception of isolation calls and parental care in P. hastatus. A collaborative project with Dr. Gerald Wilkinson, it will combine perceptual experiments in the lab with behavioral research in the field. For the perceptual component we will use the four previously trained bats in discrimination experiments with pup isolation calls as stimuli. First we will determine whether females can discriminate between isolation calls and if group-level similarity or pup age affect discrimination ability. Second we will explore how females recognize isolation calls that change rapidly during ontogeny. The results of these psychoacoustical experiments will be compared with data from experiments performed with a wild population of P. hastatus in Trinidad. We will examine retrieval error rates, interactions (both agonistic and possible cooperative acts) between females around fallen pups, and female decision rules when responding to isolation calls. Although previous studies using allozymes have indicated low relatedness among female group members, we will developing microsatellites in order to determine more accurate relatedness measures among individuals.

    This material is based upon work supported by the National Science Foundation under Grant No. 0308642. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.