The Development of Echolocation in Bottlenose Dolphins (Tursiops truncatus)

Abstract

This study aimed to expand on previous efforts to evaluate the ontogeny of echolocation in Atlantic bottlenose dolphins (Tursiops truncatus). Data consisted of echolocation recordings and concurrent behavioral observations taken from one calf in 2000 and from five additional dolphin calves and their mothers in 2002 housed at the U.S. Naval facility in San Diego, CA. A total of 361 echolocation click train samples from calves were recorded weekly over the first 6 months of the calves’ lives. The earliest calf echolocation train was recorded at 22 days postpartum and the number of echolocation attempts from calves increased steadily with age. Calf echolocation trains increased in duration and the number of clicks per train with age while train density (clicks/sec) and interclick interval values remained more consistent. Calves swimming independent of their mothers produced more click trains, especially when multiple calves were present in the social grouping. When considering these results in the context of possible maturation of a calf’s melon physiology, it seems very likely that the first two months of life are critical for the development of echolocation and related behaviors. While the first click train recorded in this sample was approximately 3 weeks of age by two different calves, it is possible that dolphin calves may innately produce functional sonar clicks immediately after birth, which were not captured in the current study. Future research will need to investigate this possibility using more controlled conditions and a better understanding of the anatomy and physiology of the sonar system of neonates as well as the possible role of the mother in echolocation development.

Acoustic Localization Method Applied to the Analysis of Dolphin Calf Acoustical Exploratory Behavior Within its Social Group

“…When dolphins were vocalizing from the inside pools, they were no longer visible and this did not allow identification of the emitter. Third, because of the dolphins' interest in the device, sometimes our recordings showed a general cacophony of clicks in which click trains mostly overlapped, making it impossible to determine the initial start and end time of click trains and, thus, compare the click train duration with results of previous studies (Favaro et al, 2013;Harder et al, 2016).…”

“…We selected sequences in which the calf was present in the video, a click train was emitted, and our customized program localized the emitter of the vocalization. For these sequences, we used the pulse train analysis function of Avisoft-SASLab Pro version 5.2.07 (Raymond Specht; Berlin, Germany) to measure click rate, mean duration of click, and mean ICI for comparative purposes with previous studies (Favaro et al, 2013;Harder et al, 2016;Lindhard, 1988;Reiss, 1988). A hysteresis of 10 dB and a start/end threshold of -2 dB were the parameters used to analyze all the click sequences.…”

“…The study of the ontogenesis of acoustical exploratory behavior in dolphins presents several difficulties (e.g., regular, long term observations of individuals almost impossible; birth rate quite low, Cornell et al, 1987; the difficulty of determining which dolphin is emitting a click train), and, as a result, only six studies, conducted on dolphins under human care, have taken on this challenge (Carder & Ridgway, 1983;Favaro et al, 2013;Harder et al, 2016;Linhard, 1988;Manoukian et al, 2002;Reiss, 1988). In this study, we presented the BaBeL system and a geometrical localization methodology that allowed analysis of a dolphin's click trains, which occurred in varied circumstances, with the presence of other dolphins around and regardless of its position in relation to the recording device.…”

“…First, following and recording the vocalizations of dolphins from early age in the wild are almost impossible and thus are only feasible with studies in captivity, where the birth of a calf occurs on average every 28 months per female (Cornell et al, 1987). As a consequence, to our knowledge, there are only six studies on the development of dolphins' echolocation, and all were carried out in dolphinaria (Carder & Ridgway, 1983;Favaro, Gnone, & Pessani, 2013;Harder et al, 2016;Linhard, 1988;Manoukian, Azzali, Farchi, & Tizzi, 2002;Reiss, 1988). Second, all these studies focusing on the development of echolocation have faced the difficulty of determining which dolphin emitted a click train and, thus, used several proxy indicators: the production of bubble streams (Reiss, 1988;Favaro et al, 2013), the intensity of the signal and the position of the calves with respect to the hydrophone (Lindhard, 1988), the presence of head scanning behaviors at the same time as click recordings (Favaro et al, 2013), the distraction of mothers in activities with trainers, and the proximity, orientation, and relative position of calves (Harder et al, 2016).…”

“…As a consequence, to our knowledge, there are only six studies on the development of dolphins' echolocation, and all were carried out in dolphinaria (Carder & Ridgway, 1983;Favaro, Gnone, & Pessani, 2013;Harder et al, 2016;Linhard, 1988;Manoukian, Azzali, Farchi, & Tizzi, 2002;Reiss, 1988). Second, all these studies focusing on the development of echolocation have faced the difficulty of determining which dolphin emitted a click train and, thus, used several proxy indicators: the production of bubble streams (Reiss, 1988;Favaro et al, 2013), the intensity of the signal and the position of the calves with respect to the hydrophone (Lindhard, 1988), the presence of head scanning behaviors at the same time as click recordings (Favaro et al, 2013), the distraction of mothers in activities with trainers, and the proximity, orientation, and relative position of calves (Harder et al, 2016). All these indicators are subject to uncertainty and only allow accurate analysis of a calf's click trains during specific behavioral circumstances (for instance, mothers distracted by trainers [Harder et al, 2016] or calves clicking with their blowhole outside the water [Favaro et al, 2013]).…”