Seismic Communication in Elephants

The other week, following an afternoon SUMC-league softball game, Shortstop/Outfielder/Chair of my Thesis Committee/Professor Tony Ricci casually mentioned that a Stanford professor affiliated with Otolaryngology studies elephants. This was news to me. I’m wondering if it might be news to you.

Folks, I’m a big fan of elephants. A love of large, charismatic megafauna was one of the primary reasons why I spent most of my youth (elementary school through middle of my sophomore year in college) certain that I was going to become a large, exotic animal veterinarian. So please, understand that I was moderately thrilled to learn that I am two degrees of separation away from a scientist who studies seismic communication in elephants: Dr. Caitlin E. O’Connell-Rodwell.

Dr. O’Connell-Rodwell recently spoke about her research at TEDxStanford. As you can see, I have embedded the video of her talk. Below, I describe some of what is known about seismic communication in elephants, as gleaned from a 2007 review article authored by Dr. O’Connell-Rodwell. I have not included all the fascinating details from that review; I highly recommend interested persons to give it a look. Finally, I’ve included at the bottom a video highlighting the elephant research site in the Etosha National Park in Namibia, which is fully solar powered.

What is seismic communication?

Imagine a rabbit. Now, imagine that rabbit spotting a predator, using a foot to thump the ground, releasing a percussive alarm call. Many animals, both large and small use seismic cues for prey/predator detection, habitat sensing and communication. Elephants, for example, respond to the seismic component of an alarm call by bunching into tight groups, younger individuals shielded by older adults.

These cues can be generated through percussion, as with the rabbit. Think also of the movement of a herd of elephants (or the stomp of a single elephant), their large masses producing powerful low frequency seismic signals. These cues are effective over long distances. The jump of a human weighing 165 lbs (~75 kg) produces a seismic signal detectable at a distance of 1.1 km. The stomp of an elephant weighing 5997 lbs (~2720 kg), is modeled to travel ~32 km.

In addition, as in the case of the response to an alarm call mentioned above, coupling low-frequency acoustic waves with the ground can generate seismic cues. The low-frequency vocalizations of both African and Asian elephants travel through the ground over long distances; this is especially useful in the jungle, where dense vegetation rapidly degrades auditory information, but will not affect seismic cues. When auditory information is not degraded by habitat, an elephant’s call, traveling both through the air and through the ground, will be detected twice. Seismic signals travel at different speeds than airborne signals; this differential time-of-arrival depends on distance from the source, and can therefore be used to provide information about how far the receiver is from the source of the call. Dr. O'Connell-Rodwell and her colleagues observed that elephants actively facilitate localization of seismic signals. In response to a seismic signal, elephants will align themselves perpendicular to the source, placing their ears the greatest distance apart from the source, presumably serving to maximize the interaural time differences (ITD) used for localization of low-frequency sounds.

How does seismic communication work?

Elephants are thought to detect seismic signals through their large feet, which are well suited for detecting vibrations. Two possibly pathways for such detection are 1) conduction of vibrations from the ground, through the feet, legs, shoulders and into the middle ear, and 2) somatosensory detection, via mechanoreceptors (Pacinian corpuscles) located in the foot. Due to behavioral observations, a strong hypothesis is that the elephant’s foot is the initial interface. As of 2007, behavioral experiments to directly test the sensitivity of an elephant’s foot to vibrations were ongoing; a captive elephant was trained to perform at vibrotactile threshold study.

Whatever the exact mechanism of detection, it is clear that elephants can detect subtle differences between difference seismic cues. Research by Dr. O’Connell-Rodwell has demonstrated that elephants can distinguish between the seismic playbacks of alarm calls made by familiar and non-familiar individuals. The underlying mechanism for this ability is thought to involve discrimination of high-resolution frequency differences between seismic signals, with these differences due to variations in the vocalizations of each individual elephant.

 The research site, methods

Dr. O'Connell-Rodwell observes elephants at a research site located at a watering hole at the Etosha National Park, in Namibia. To test a herd's response to seismic cues, electronic shakers that convert sounds into vibrations are buried in the ground around the watering hole. Previously recorded elephant vocalizations of known context are played back to the group. Examples of the types of stimuli used for playback include: familiar alarm calls, as emitted by individuals from the group while lions were hunting them; unfamiliar alarm calls, as emitted by different family groups living in Kenya. (from: O'Connell-Rodwell et al. 2007)

A final note: the entire time I was editing this post, Carole King was singing in my head.

Here you all are:

Sources and Additional Articles

O'Connell-Rodwell (2007). Keeping an "Ear" to the Ground: Seismic Communication in Elephants. Physiology 22:287-294. Link

O'Connell-Rodwell et al. (2007). Wild African elephants (Loxodonta africana) discriminate between familiar and unfamiliar conspecific seismic alarm calls. J Acoust Soc Am 122(2): 823-830. Link

Stanford News article from June 1, 2005, profiling Dr. O'Connell-Rodwell. Link

Inside Stanford Medicine article from Oct 2, 2012, highlighting recent work by Dr. O’Connell-Rodwell. Link

Webpage of the O’Connell-Rodwell lab. Link

Posts by Dr. Caitlin O’Connell-Rodwell for the NYTimes Scientist at Work blog. Link

TEDxStanford: Caitlin O'Connell-Rodwell. Link