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Analysis of Acoustic Structure and Function in Ultrasounds of Neonatal Microtus

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The purpose of this study was to determine conditions under which neonatal Microtus produced ultrasounds, to analyze the structure of these ultrasounds, and to investigate the probable function of ultrasounds in communication between the microtine parents and neonates. Five species of Microtus, M. pennsylvanicus, M. montanus, M. californicus, M. longicaudus, and M. ochrogaster, were trapped and maintained in the laboratory. In a first study a total of 2I neonates were subjected to I2 different conditions for 30 sec to determine in a qualitative manner whether the neonates emitted few or many ultrasounds. Results of this study showed that microtine neonates produced ultrasounds under stressful conditions in which an important factor eliciting ultrasounds was cold stress. Ultrasounds were given by pups in the field when their nests were opened. In a second study, neonates from four different litters of each species were used as subjects for analyzing the structure of ultrasounds. Two of the litters were sampled every other day beginning at day one and ending at day ten. The other two litters were sampled only once. In two species, M. californicus and M. ochrogaster, recordings were continued until pups reached I8 days of age. The investigator stimulated pups to produce ultrasounds by suspending them from their tails. Six parameters of the ultrasounds, namely the ultrasonic set, pulse duration, interpulse duration, frequency pattern, frequency of the fundamental, and bandwidth of the fundamental, were analyzed. Results of study 2 showed that all species gave ultrasounds of a similar pattern. The pattern consisted of ultrasounds of similar length and relative intensity repeated at short regular intervals. Typically, from two to ten calls constituted each ultrasonic set. Following each ultrasonic set, a pause occurred before another ultrasonic set began. In all five species, four variations in the ultrasonic calls were detected. These four types of ultrasounds were named the plain whistle, the whistle chirp, the wavering whistle, and the hoarse whistle. Certain structural features of the ultrasounds changed significantly with age. The interpulse duration decreased systematically with age. The bandwidth of the fundamental increased regularly with age in three species. The frequency of the fundamental decreased inconsistently, but in all species the frequency was significantly higher at day one than at day ten. Changes in the pulse duration were not significant. Four species, M. pennsylvanicus, M. montanus, M. californicus, and M. longicaudus, produced ultrasounds which were remarkably alike in pulse duration, interpulse duration, fundamental frequency, and frequency pattern. In contrast, ultrasounds produced by M. ochrogaster were structurally different from calls given by the four similar species. Structural differences in ultrasounds produced by M. ochrogaster support morphological data that M. ochrogaster is not closely related phylogenetically to the other four species. As microtine pups passed ten days of age, the structural properties of ultrasounds began to deteriorate and increasingly fewer ultrasounds were given. The relative intensity, pulse duration, and interpulse duration became irregular and structural breaks occurred frequently in the fundamental. Adult microtines produced few or no ultrasounds when they were subjected to the same conditions which caused neonates to emit ultrasounds. Structural features of ultrasounds produced by neonatal Microtus suggest that neonates giving these calls would be difficult to localize by predators. Therefore the probable adaptiveness of ultrasounds is that they function in short range communication between the mother and pups without revealing the location of the pups to predators. As pups mature and are able to escape from predators, it may no longer be necessary for their survival that they communicate by sounds which are difficult to localize. This may in part explain the disappearance of ultrasounds as pups grow older since there is no reason to retain an acoustic signal which no longer has survival value. Study 3 tested the communicative importance of ultrasounds produced by neonatal Microtus. The test subjects were all parents and included nine pairs of M. montanus, six pairs of M. californicus, and five pairs of M. ochrogaster. Responses of parents were observed in a Y-maze, and all tests were conducted in a dimly illuminated room. Parents were subjected six times to three test conditions, namely no stimulus, olfactory stimulus (non-vocalizing pup), and a pup producing ultrasounds. Parents could give no responses, incorrect responses, or correct responses to the olfactory and ultrasonic stimuli. Visual cues were eliminated by partitions between the non-vocalizing or vocalizing pups and the parent. This experiment tested the importance of ultrasonic as opposed to olfactory cues in initiating retrieval in microtine parents. Parents showed significantly more correct responses to the pups giving ultrasounds than to the non-vocalizing pups. Adults rarely gave an incorrect response to the pups producing ultrasouds. In contrast adults responded incorrectly and correctly about the same number of times to the olfactory stimulus. Results of the experiment provided evidence that ultrasounds function in communication between the microtine parents and neonates.

Affiliations: 1: Department of Biology, University of Colorado, Boulder, Colorado, U.S.A.

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