Post-mortem results of the Grays’ beaked whale stranding

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Greetings from Murdoch University’s Veterinary and Biomedical School and the Marine Mammal Health Project as we bring you some news on a recent and ongoing case.

On the 9th January, 2012, a very fresh stranded cetacean carcase was reported to the DEC by a member of the public in the early hours of the morning on Dalyellup Beach, near Bunbury. DEC staff identified it as a Gray’s Beaked Whale (Mesoplodon grayi), and tentatively categorised it as a juvenile male.

Beaked Whales (family Ziphiidae) comprise a group of 21 different species, all of them characterised by their elongated beaks and by their ability to dive to extreme depths for often extraordinary lengths of time (up to nearly 2km; 20-30min dives are common, although 85min has been recorded), making them amongst the deepest-diving marine mammals on record (for more information, see – http://australianmuseum.net.au/Grays-Beaked-Whale). This, and the fact that they are relatively solitary in their life-style compared to many other gregarious species of cetaceans, means that they are much harder to study and as such, relatively little is known about their comparative biology, anatomy and diseases. So, as this unfortunate stranding gave us the rare and valuable opportunity to examine a fresh individual; DEC staff, members of MUCRU (special thanks to Kate Sprogis and her intern students who facilitated the transfer) and the Bunbury Dolphin Discovery Centre worked collaboratively to transport it to Murdoch’s Anatomic Pathology Department for cold storage overnight and post-mortem examination early the next day.

To understand what might adversely affect an individual and cause injury and/or death, one must first consider the natural biology and comparative anatomy of the particular species. What is especially remarkable about this group of whales is their ability to deep-dive. So how do they resist the remarkable pressures at depth, as well as avoid the excessive nitrogen super-saturation of the tissues (as occurs in human decompression sickness or DCS; ‘the bends’) that can occur with inappropriately rapid ascent from depth? Whilst we don’t fully understand the full range of anatomical and physiological adaptations they have to do this (and in fact, research suggests they are not 100% immune to adverse effects, just more well-adapted and adept at avoiding and minimising them), some of the answers lie in their altered anatomy. If we compare them to ourselves, and even other cetaceans, several adaptations for deep-diving are evident:

  • They are streamlined, even compared to other cetaceans, with smaller pectoral and dorsal fins relative to body size to reduce drag (see how small the pectoral fins are in the stranding photo),
  • Like other cetaceans, they have relatively  more connective tissue supporting and holding in place their organs in their thoracic and abdominal cavities to prevent organ misplacement as these cavities are compressed under pressure at depth,
  • Like other cetaceans, they have an extensive network of blood vessels in their thorax (rete mirabile) which helps regulate blood supply and pressure when diving,
  • Like other cetaceans, they have collapsible ribs which collapse to compress the lungs at depth, which together with the fact that they do not breathe underwater (unlike human divers), means that there is no opportunity for gas exchange, therefore eliminating the risk of DCS, providing they ascend slowly (more about this later),
  • Their peripheral airways have reinforced walls which along with their collapsible ribs, helps facilitate lung collapse to prevent air exchange at depth,
  • They have reduced air spaces (sinuses) in the head, and to prevent them being crushed at depth, when they dive, they divert blood into a network of blood vessels within the remaining spaces so they dilate to fill the space,
  • They have remarkably thick and dense bone (roughly the same density as metal on computed tomography!) around a portion of their ears (the tympanic bullae) to resist pressure,
  • Their red blood cells have remarkably enhanced oxygen-carrying capacity and their muscle cells are able to utilise oxygen more efficiently too,
  • The deep-diving species such as Beaked Whales are estimated to be able to withstand enormous levels of nitrogen super-saturation as a normal event,
  • They are able to slow their heartbeat and metabolic rate when diving to reduce workload and conserve energy.

They also have many adaptations for enhanced echolocation capability given they are unable to see at depth in the absence of light. In fact, their sense of sound is highly developed.

A relatively common cause of injury sufficient to cause stranding of any cetacean can be blunt trauma; but what if things go wrong and they are unable to perform normal dive profiles with appropriate ascent times relative to the depth dived? Some overseas researchers have linked acoustic events (e.g. deployment of military sonar, underwater impulsive and explosive events related to construction, shipping noise, and seismic surveys for underwater exploration) to various findings in stranded cetaceans, in particular Beaked whales.

Blast events suffered within a certain radius can cause concussive damage with changes ranging in severity depending on proximity and the blast force sustained (e.g. fracture of the auditory ossicles, rupture of the ear drum, haemorrhage into the ear(s) and the cranial soft tissues, cochlear and saccular damage amongst others). Additionally, less severe blast and noise events can cause pain, hearing loss (ranging from temporary to permanent depending on severity), tinnitus and loss of balance; which may result in an inability to echolocate, navigate and perform normal dive profiles adequate for sufficient decompression and which may cause an animal to strand.

Several researchers also postulate that some noise, even if it does not directly harm the animal, may startle it sufficient to cause it to alter its behaviour such that inappropriately rapid ascension times from depth may occur (i.e. abnormal dive profiles), resulting in changes similar to those seen in DCS in humans and a controversial, poorly understood syndrome in cetaceans referred to as ‘Gas Bubble Disease’ (GBD). This is difficult to prove as it cannot be directly observed. Still others hypothesise that gas bubble formation may occur as a physical effect of sonar on nitrogen super-saturated tissues; again, difficult to prove. However, GBD remains controversial and is largely impossible to prove; some overseas researchers use gas analysis of gas from lesions (currently not available to us) to support the diagnosis – in reality though, the cause and the progression of changes that result in disease is not yet clear in the literature.

So, in examining our case, we wanted to (a) document any signs of trauma that could be attributable to blunt trauma and/or blast trauma, (b) document any changes that could be consistent with acoustic trauma, (c) document any changes consistent with GBD, (d) document any parasitic or infectious disease, (e) examine the gut to try and characterise what made up its diet, (f) confirm the sex/age class (juvenile male), and (g) document the morphometric measurements to contribute to comparative anatomy studies.

To aid our visualisation of internal structures, following external examination we removed the head and neck as one and sent this to Dr Zoé Lenard, Specialist Veterinary Radiologist (Veterinary Imaging Centre, Perth Veterinary Specialists; see – http://www.perthvetspecialists.com.au/index.php?option=com_content&view=article&id=69&Itemid=87.) for a Computed Tomography (CT) scan (the size of the body meant it would not fit in the machine, necessitating removal of the head). No overt abnormalities were found on CT; however the lack of structural abnormalities does not rule out the occurrence of a functional disturbance resulting in hearing/echolocation/balance dysfunction. To our knowledge, this is the first CT scan recorded for this species, and we managed to document some fascinating comparative anatomy. We have since been liaising with Asst Prof/Senior Scientist Dr Darlene Ketten about the case to both maximise and further our understanding (Woods Hole Oceanographic Institute/WHOI and Harvard Medical School; see – http://csi.whoi.edu/darlene-r-ketten-ph-d). Dr Ketten is a world-renowned expert on marine mammal hearing and acoustic impacts and WHOI are at the forefront of research into these and other marine mammal issues. You can access a free-access gallery of CT images from WHOI here – http://csi.whoi.edu/gallery.

On full post-mortem examination we did not find any overt evidence of blunt trauma, however there were some interesting preliminary findings which need to be investigated further (investigation is ongoing). Unfortunately, given the estimated time since death, many changes may merely be due to post-mortem decomposition (which can in many cases significantly hamper a pathologist’s ability to definitively diagnose disease/cause of death in many cases), and it is hoped pending microscopic tissue examination (and tissue culture for the presence of any significant bacteria) will help determine whether or not this is the case.

So, whilst our preliminary findings are inconclusive (and the scientific jury on GBD is still out!), we await further results. At the very least it examination has been important in ruling out overt blunt trauma as a cause of death in this individual. Despite the lack so far of an overt link to physical or acoustic trauma in this case, and even though for various reasons we may ultimately never know the definitive cause of death in this particular individual; this case has still given us pause to consider the fact that with increasing human usage of the ocean comes increased anthropogenic ocean noise. This noise may potentially have an adverse effect on cetaceans, given they are almost solely dependent on their sense of sound as their principle sense. This should be taken into account whenever making decisions with management implications for cetaceans.

 

Dr Nahiid Stephens and Dr Carly Holyoake, Marine Mammal Health Project.

Collaborators – Asst Prof Dr Darlene Ketten and Dr Zoé Lenard.

 

Lars Bejder PhD
Lars Bejder PhD
Professor Lars Bejder PhD is the Research Leader of the Murdoch University Cetacean Research Unit.
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