Nicholas Flores Martin is a PhD student within the CDT-SIS group, currently working on developing an acoustic-based behavioural guidance system for fish. His talk dealt with the mechanics of humpback whale vocalisation and investigating the potential use of ambient sound by grey whales for navigation and foraging.
After a short introduction on humpback whales (Megaptera novaeangliae) and a brief history of the body of scientific work related to humpback whale song, Nicholas explained the complex anatomy used by humpbacks to vocalise and the concept of formants. The humpback whale sound generator involves vocal folds located in the larynx, which can be set into oscillations, possibly in both directions, by an airflow powered either by the lungs or by the laryngeal sac. The whale respiratory tract has three possible mutually exclusive configurations. At this point Nicholas pointed out that these configurations, while plausible, have never been confirmed owing to the near-impossibility of placing a camera down a humpback whale larynx or keeping it in a tank.
Humpback whale calls are characterised by vocal tract resonances called formants, originating mainly in the nasal cavity, but also in the laryngeal sac. Because the latter behaves increasingly like a low-pass filter as it inflates, formants arising from the laryngeal sac would be expected to decrease in frequency as the lung empties. A starting point to check whether this is true would therefore be to carry out linear prediction analysis on humpback whale calls.
Explaining his results, Nicholas said that no clear picture was apparent from the shifts in the formants encountered, although a greater number of formant peaks showed a decrease in frequency over the duration of a call. He concluded by suggesting that the analysis be carried out on a larger scale – such as on “phrases” or “themes” within a song, and that acoustic resonances originating from the laryngeal sac may possibly be distinguished from nasal cavity resonances via changes in their bandwidth over the duration of the call.
Nicholas then introduced the second part of his talk, on research conducted for his Master’s degree – giving a background on the grey whale (Eschrichtius robustus), a species that is also famed for its long distance migrations. In his study Nicholas wanted to look into whether grey whales might be using the wealth of acoustic information available to them as a foraging and navigational aid. He also wanted to look at whether there was any evidence of periodic choruses in kelp beds, using data originally recorded by Jennifer Wladichuk as part of her doctoral studies.
Acoustic maps of Cow Bay, Canada
While going through his methodology, Nicholas pointed out the recipe for a field-constructed sonobuoy which he used while in the field. The buoy was put together from bits and pieces found around the whale lab, which included: an empty packet of noodles, an old polystyrene float, and a short piece of rope!
Nicholas explained while grey whales likely do use ambient sound as an indirect indicator of the general direction of food it is doubtful that this is their main foraging strategy since they have been observed carrying out test dives, switching prey after finding insufficient numbers of their preferred choice, and leaving a bay after spending a certain amount of time foraging – behaviour that would not be associated with a predator that hunts mainly through passive listening. He also mentioned that based on the highly unique sound patterns found in each bay, it is more likely that these whales use passive listening as a key navigational aid.
In agreement with Wladichuk’s findings, Nicholas revealed he had found no evidence for periodic choruses in kelp beds, although certain frequencies originating from snapping shrimp were significantly more intense during the day.
Nicholas concluded with a brief for questions, during which he revealed his plans to repeat his formant analysis on a larger scale – such as on “phrases” or “themes” within a humpback whale song. Following a question by fellow Post graduate student Nikhil Mistry, he also plans to model the greatest distance that noise from a kelp bed can be heard by a whale.
Nicholas is currently a Postgraduate Research Student in Environmental Engineering at the University of Southampton, based at the Centre for Doctoral Training – Sustainable Infrastructure Systems, Southampton. He graduated in 2008 with a BSc (Hons) in Biology and Chemistry from the University of Malta from, before completing an MSc in Oceanography (MBiol and Ecol.) in 2011 at the National Oceanography Centre, Southampton.