Edmund Gerstein – gerstein2@aol.com
Charles E. Schmidt College of Science
Department of Psychology
777 Glades Road
Florida Atlantic University
Boca Raton, Florida 33486

Vasilis Trygonis
University of the Aegean
Lesvos Island, Greece

Steven McCulloch
Florida Atlantic University / Harbor Branch Oceanographic Institute
Fort Pierce, FL 34946

James Moir
Florida Marine Resources Council
Stuart, FL 32905

Scott Kraus
Edgerton Research Laboratory
New England Aquarium, Boston, MA 02110

Popular version of paper 4pAB13 presented at the 2014 167th ASA Meeting in Providence, RI.


North Atlantic right whales (Eubalaena glacialis) are the most endangered of the great whales inhabiting the eastern coastline of the United States and Canada. With a remnant population estimated at 450 individuals there is an urgency to understand their behavioral ecology and reduce the threats to their recovery from ship strikes and entanglement with fishing gear. North Atlantic right whales produce loud, broadband, short duration sounds referred to as gunshots. As the name depicts, these calls resemble the sound of a gun or rifle being fired (Sound file 1.)

Sound File 1. Typical gunshot sound recorded from a male in a surface active group in the southeast critical habitat (note the 191[infopopup tag=dBdefinition] peak amplitude)

The sounds have been hypothesized to function in a reproductive context, as sexual advertisement signals produced by solitary adult males to attract females and/or agonistic displays among males in surface active groups. This study provides evidence that gunshot sounds are also produced by adult females, (specifically mothers with new born calves) and examines the acoustics and behavioral contexts associated with these calls. Results from boat-based observational surveys investigating the early vocal ontogeny and behavior of right whales in the southeast critical calving habitat (off the coastal area off Southeast Georgia and Northeast Florida) are presented for a subset of mothers who produced gunshots while in close proximity to their calves and autonomous recording buoys. (Figures 1 and 2).

Figure 1. Mother and calf

Figure 2. Mother just before gunshot near recording buoy

Of 32 different isolated mother-calf pairs, gunshots were recorded from six identified females of varied ages and maternal experience. Most of the gunshots were emitted by mothers when their young (1-6 week old) calves separated from them during the calves’ curious (exploratory) approaches toward objects on the surface. The social and behavioral context of female gunshots suggest these calls were associated with maternal alarm and or communication directed toward their calves. However, they were not overtly effective as call back signals (during our observations) as the calves continued unabated on their forward paths of exploration.

The spectral and temporal characteristics of these female gunshots resemble those attributed to adult males in the Bay of Fundy (Parks et.al., 2005) and those recorded from surface active groups in the southeast critical calving habitat (Trygonis et.al., 2013). However, the (female) gunshot calls are orders of magnitude quieter (-30dB). (Figures 3, 4, & 5).

Figure 3. Gunshot from adult male in surface active group (note 191 peak power dB)

Figure 4. Gunshot from female with calf (note 158 peak power dB)

Figure 5. Gunshot from female with calf (note 147 peak power dB)

While a specific articulation site or mechanism for gunshot production is unknown, the ability to vary intensity of impulsive sounds lends support to Parks et al (2005) who suggested there is a unique anatomical trait within balaenid whale species which enabled the internal production of these impulsive gunshot sounds. The significantly lower intensity “quiet gunshots” emitted by the mothers posed minimal risk of injury to nearby calves and also resulted in relatively small detection ranges, both of which would be beneficial for mothers trying to protect their calves. Daylight observations and acoustic recording in the southeast critical calving habitat indicate that each mother and calf pair functioned as “isolated social dyads or islands” and actively avoided other whales. The mothers and calves are predominantly silent during the first six weeks of development, and exhibit only low intensity social calls as calves mature. Isolation, quiet gunshots, and the whales’ overall acoustic quiescence may function in predator avoidance and serve to minimize acoustic detection or harassment by adult males, juveniles and other conspecifics in the area.

The social and behavioral context of these gunshot calls naturally precludes any reproductive function or specific communication signaling by the female toward any individuals other than her calf. Synchronized underwater acoustic and surface video recording presented below chronicled gunshots and behavior as a mother interacted with a surface buoy in the forward path of her and the calf (Video file 1).

Video file 1. Mother approaches recording buoy, note how she does not avoid the buoy, but actively pushes it with her head and clears a path for her calf to follow. The event is accompanied with two gunshots as she interacts with the gear.

The behavior exhibited during this chance encounter offers an insight into how and when a whale (mother in this case) might interact with surface gear (fishing floats, as well as buoys), and suggests that gunshots may also be emotive indicators of heightened stress and agitation caused by anthropogenic factors, as well as naturally occurring social behavior.

References:

Parks, S. E., Hamilton, P. K., Kraus, S. D., and Tyack, P. L. (2005). “The gunshot sound produced by male North Atlantic right whales (Eubalaena glacialis) and its potential function in reproductive advertisement,” Mar. Mamm. Sci. 21, 458–475.

Trygonis V., Gerstein E.. Moir, J., & McCulloch, S. (2013). “Vocalization characteristics of North Atlantic right whale surface active groups in the calving habitat, southeastern United States,” J.Acoust. Soc. Am. 134, 4513-4531.