Sumit (SD): Thank you, Ms. Christine Cook and Dr. Marissa Ramsier, for agreeing to answer a few questions about your recent experience evaluating the gorilla Kumbuka’s hearing and other issues related to primate hearing. Our publication is read primarily by audiologists and others associated with hearing health care. Your expertise and experience will be of great interest to our readers. I am also pleased to report that this conversation about Kumbuka’s evaluation will be accompanied by a web feature about primate hearing in general. How about we start with brief introductions? Please tell us a little bit about who you are. 

Christine (CC): I am a pediatric audiologist and supervisor of audiology at Nemours Children’s Specialty Care in Jacksonville, Florida; I did my undergraduate and graduate studies at Arizona State University. I have been with Nemours for almost 17 years, but have been an audiologist for over 25 years. My interests include early identification of pediatric hearing loss and amplification. 

Marissa (MR): I am a biological anthropologist at Humboldt State University in Northern California. I earned my PhD in anthropology at the University of California, Santa Cruz. One of my specialties is sensory ecology, specifically acoustic communication in primates. My colleagues and I have collected data on the hearing sensitivity of more than 30 primate species utilizing the minimally invasive auditory brainstem response method.

SD: A specific question for Professor Ramsier: how did you become interested in evolutionary sensory biology and then primate hearing?

MR: As a graduate student in anthropology at the University of California, Santa Cruz, I enrolled in a graduate seminar titled, The Evolution of Human Sensory Systems, by Nathaniel Dominy (now at Dartmouth College), who was to become my collaborator in this line of research. I became interested in the topic when I realized how very little we know about primate auditory sensitivity despite decades of extensive documentation of primate vocalizations—in fact, discussion of how well, if at all, various sounds are received by various primate species is basically absent in most of the literature on primate acoustic communication. I was hooked! I still remember the day I walked into Dominy’s office and proposed the topic as the focus on my doctoral research. “It will be a long and challenging road,” he said, “but an interesting one.” He was right. 

SD: Let us switch gears a little bit and talk about Kumbuka and her hearing evaluation. Let’s start with a specific question—how did the two of you get pulled into the project? Walk us through the planning process. Any special considerations? Who else was on the team? How did you arrive at the final plan on what would be done, by whom, etc.? 

CC: In early 2016, the Jacksonville Zoo and Gardens contacted Christine’s department at Nemours and asked if one of their gorillas could be tested in any way due to their suspicion of her having significant hearing loss. Christine contacted the medical director to see if it would be possible for her to do this. He approved. Since Kumbuka would need to be sedated for the hearing testing, we would have to wait until her routine exam scheduled for 2017. In late 2016, the zoo contacted Marissa, having heard about her recent involvement with a similar procedure to test the hearing sensitivity of an orangutan at the Indianapolis Zoo. From there, we determined a date for the procedures that worked for all parties. 

Initially Christine thought about doing OAEs, although wondered if an ABR would work with the human equipment. Unfortunately, there wasn’t much information about testing hearing in gorillas upon which to base a protocol. At the same time, Marissa contemplated the potential success of her system for gorillas (her system is designed to work with various mammals). In theory, it would work fine with gorillas despite their considerable head size, but she had not yet had the opportunity to develop a protocol and setting based on a normal-hearing gorilla. After a few emails back and forth, we decided that we would compare results with both Christine’s clinical system and Marissa’s nonhuman primate system. Christine also spoke with Nemours’ neuro-otologist, Dr. Drew Horlbeck, and we thought it would be beneficial to have him check Kumbuka’s ears to be sure we weren’t dealing with any cerumen impaction, as Marissa had encountered this in other nonhuman primates. 

The game plan for the day of the procedure was to have Dr. Horlbeck check and clean out the ears, then for Christine to do tympanometry and DPOAEs (and possibly TEOAEs if DPs were present), and then for both Christine and Marissa to run at least a click ABR and compare results. We were limited as to how much time we would have, as Kumbuka was also having other procedures with her routine exam and cardiology. As was suspected, all results strongly suggested that Kumbuka had significant hearing loss in both ears. 

A gorilla being hand fed at the Jacksonville zoo.SD: Let me back up a little bit. What does a gorilla’s hearing range and sensitivity look like?

MR: Good question, and one we wish we could answer more fully. We do know that gorillas are able to detect the range of frequencies present in human speech, as evidenced by their interactions with keepers and researchers and by examining vocalizations of gorillas themselves. One could hypothesize, based on the aforementioned evidence as well as size and phylogeny, that gorillas likely can hear similarly to humans, with perhaps slightly better sensitivity to infrasound. However, there is no direct evidence as to the limits of their hearing range nor the frequencies they are most adept at detecting. To our knowledge, there are no existing comparative data on the auditory sensitivities of normal-hearing gorillas, but this is something we are actively working to resolve. We hope that working with Kumbuka will mark the beginning of this endeavor. 

SD: How did the suspicion arise that Kumbuka might have a hearing loss? 

CC/MR: The staff who routinely work with Kumbuka observed that her behavior was unusual compared to the other gorillas, and she was having difficulty socializing with the others. If someone or something was not in her line of sight, she often did not react or know what was going on in her surroundings. They also observed some behaviors very similar to those of humans with hearing loss. Kumbuka tended to be very vocal and louder compared to the other gorillas. She startled easily when other gorillas would approach out of her peripheral vision, sometimes prompting aggression. They also observed that Kumbuka would respond to things that created vibrations. 

SD: Was anything done to formally evaluate Kumbuka’s hearing before you arrived on the scene? 

CC: Not that we are aware of. However, zoo staff felt her symptoms were consistent with those reported by other facilities working with hearing impaired primates. They had also mentioned that some of the “hyperactive” behaviors noted by her previous zoo may have been consistent with Kumbuka needing to compensate for using senses other than hearing to explore her environment. 

SD: Given Kumbuka’s age and other known facts about her health, could you anticipate the outcome of the investigation? 

CC: Her age and health did not necessarily help us anticipate the outcome. Age-related hearing loss is documented in nonhuman primates, but gorillas are long-lived animals, and thus Kumbuka is not particularly old. We are not aware of anything in particular regarding her health that would lead to hearing loss. The observations of the staff that has been working with Kumbuka since she arrived at the Jacksonville Zoo were really the main influence for the anticipated outcome.

Both ABR and OAE responses were absent in both of Kumbuka’s ears.SD: Great. Walk us through the day and process, if you will. We have seen the many videos that are on various sites. Were there remarkable, unexpected, or funny events? 

CC/MR: From both of our perspectives, the two weeks prior to the actual procedure day were a whirlwind of events. What started out as being thrilled just to be able to work with the gorilla, turned into a chance to partner with each other and learn from our respective expertise in clinical settings and working with nonhuman primates. The public relations department at Nemours really went above and beyond to produce informative videos of the day at the zoo, as well as the days leading up to it. And with the Today Show picking up the story, it was a chance to reach a large audience to raise awareness about hearing loss and how we can determine this in humans and nonhuman primates. 

On the day of the procedure, we both arrived early and were able to check our equipment to see if the two systems would be compatible with respect to the electrodes, which they were. There were a couple hours of preparation, getting the room and staff wired up for sound and video, and also prepared for the various scheduled procedures. Both of us, as well as Dr. Horlbeck, were wearing Go Pro cameras, with hopes of getting some up-close footage. The procedure room was set and ready to go with many people on hand. We were all definitely out of our element with cameras and microphones following our every move…not something we normally encounter in the clinic or out in the field!! 

After a pre-procedure briefing by zoo veterinarians as to the order all of the procedures for the day and some precautionary dos and don’ts, Kumbuka would be arriving shortly from her enclosure, already sedated for transport. It took a team of people to carry Kumbuka in from the transport van to the procedure room and to get her up on the table. Things moved fairly quickly from there. As the anesthesia team got Kumbuka situated and stabilized on the table, it became apparent that Kumbuka would be on her side and not on her back as we are used to for ABRs both in human patients and normally with nonhuman ABRs. Since the goal was to not have Kumbuka under anesthesia for longer than two hours, the vet asked that we start our testing while they were doing other procedures and they would work around us and would keep quiet while testing was underway. We would do one side then the other when they were ready to turn her. 

Tympanometry yielded normal results, but audiologist Christine Cook had to work her way through an hair and cerumen in the ear canal.Dr. Horlbeck quickly got to work cleaning out any debris from Kumbuka’s left ear canal. We were surprised by how difficult it was to actually see into the ear canal with the otoscope, as well as with the lighted microscopic glasses Dr. Horlbeck was using. There is an incredible amount of hair in the ear, which made visualization of the tympanic membrane challenging. Some cerumen was removed and it was time for Christine to do tympanometry. Using a basic handheld tympanometer, a normal tympanogram was obtained on the first ear. Since middle ear function appeared to be normal, OAEs were next. Christine started with DPOAEs and had some difficulty with noise in the room. Marissa had noise cancelling headphones which we placed over the ear and that allowed the OAEs to run beautifully. Of course, no OAEs were detected and now it was time to move onto the ABRs. We decided to start with Christine’s human equipment first, using Marissa’s needle electrodes. Although Kumbuka was still on her side, there was enough space to reach under Kumbuka’s neck to get to the other ear and Marissa was able to place all three needle electrodes properly. We held our breath as Christine checked impedance, and all three electrodes read 3 ohms and we were good to go. Christine ran a click ABR at the equipment limits with insert earphones and saw no response. She adjusted gain a bit to see if this would change anything on the screen, but still no waveform was evident. She tried a couple tone bursts just to confirm and again saw no response. 

Next, we quickly plugged the electrodes into Marissa’s cable and she ran her ABR next, focusing on mid-range frequencies that, based on humans and other primates, should have evoked a strong response even if (typically high-frequency) hearing loss was present. Again, no response was evident. Since the ABR equipment designed for humans had not been run on a primate, and the primate system has not typically been utilized clinically, it was reassuring to see that both systems yielded the same results. Kumbuka was repositioned and we turned our attention to the right ear. We repeated the same sequence of testing and encountered the same results for all tests completed. One unexpected event that did occur during the procedure, was that Kumbuka began to stir when they were turning her over and all non-essential people were quickly escorted out of the procedure room until the anesthesia team got her settled again. Other than that, the procedures went according to plan, again with both of our results in agreement. 

SD: So, Kumbuka did not have any appreciable ABR peaks suggesting a pretty large hearing loss. 

CC/MR: That is correct. And although there are no baseline data to confirm protocols and settings for gorillas, the complete lack of response is highly suggestive of substantial hearing loss given that the ABR method works for humans, all other nonhuman primates tested with the system used, and also has been used on species across the animal kingdom. Furthermore, prior to starting the ABR testing, the absent DPOAEs suggested there was most likely at least a moderate hearing loss in both ears. 

SD: Is there any way to know what kind of hearing loss she has?

CC: Well, since her tympanometry and otoscopic exam were normal, we can surmise that her loss is likely to be sensorineural in nature. Although we won’t be able to determine cause or if the loss is congenital, the observations from Jacksonville Zoo’s and previous zoos’ staff tend to suggest this could possible by a longstanding hearing loss for Kumbuka. 

SD: Is the zoo planning on doing anything different with Kumbuka going forward? 

CC/MR: We hope that confirming what looks like a significant hearing loss for Kumbuka will help the staff to continue to modify their behaviors and interactions by using more visual cues during training. One of their goals is to help Kumbuka assimilate better socially with the other gorillas and eventually be bred. 

SD: How does this experience with Kumbuka teach this or other zoos about primates or, for that matter, other aging animals?

CC/MR: This experience with Kumbuka is important for numerous reasons. First, it highlights the fact that hearing loss is something to which nonhuman primates and other animals are susceptible, not only as these animals age, but possibly also due to genetics, noise exposure, and pathologies. Luckily, the keepers and staff at the Jacksonville Zoo and Gardens recognized Kumbuka’s situation and were already taking appropriate steps to ensure she received the most appropriate care. The zoo staff also pointed out, and we agree, that although the results of this experience will not likely result in correcting Kumbuka’s impairment, that raising awareness may result in other facilities noticing behaviors that may indicate hearing loss, which could improve the situation for other animals. The media coverage of this event already have alerted us to the interesting fact that other facilities have previously attempted to utilize similar methods to test the hearing of gorillas, highlighting that there exists both a need and an interest in developing baseline hearing data and testing protocols for gorillas and other animals so that potential hearing loss can be fully evaluated. This is important not only for captive care, but also for beginning an exploration of potential causes for hearing loss in these and other animals, so that steps can be taken to prevent or minimize preventable cases such as those traced to noise exposure or even other medical interventions.

SD: Professor Ramsier, has this experience opened any new research questions for you that you plan to pursue?

MR: Yes. There is a small but growing number of researchers focused on furthering our understanding of hearing in nonhuman primates, not only for captive management, but for understanding how hearing and habitat acoustics may affect the survival of highly endangered primates in the wild. This experience highlights that there is a lot to be done, but also a lot of support for doing so. Although the results of the tests we ran suggest that Kumbuka has substantial hearing loss, this was my first chance to see these data in a gorilla. Establishing baseline data and best settings for working with gorillas and other animals will be an important next step. It was also fantastic to have the opportunity to work with Christine and Dr. Horlbeck. I am eager to further explore the use of otoacoustic emissions in nonhuman primates as an alternative or addition to ABR data – in addition to its use here, there are existing data that suggest it is a promising approach. It is also always a pleasure to work with facilities that take pride in excellent captive care and show a strong interest in species survival plans and conservation. 

SD: Ms. Cook, are you going to do anything differently in your practice because of this experience? 

CC: Not necessarily in my day to day practice, but I will always treasure the experience I had with testing a gorilla with the same equipment and protocols used with the children I see every day at Nemours. I look forward to partnering with Marissa again to test another gorilla in the future, if the opportunity arises. 


The views and opinions expressed in this article are those of the authors and do not necessarily represent the official policy, position, or opinion of the American Academy of Audiology.