A new paper has shown examples of cetacean skulls with fractured ear bones and hypothesises what may have caused them…
It is well known that marine mammals, particularly cetaceans, are very dependent on hearing and all evidence suggests that hearing is their primary sensory system. Adaptations to the skull and the hearing system reduce conduction of sound through the bones and help for directional hearing underwater. Of particular importance is the tympanic bulla which is a sort of capsule around the middle ear and is essential in the cetacean sound reception pathway.
A new paper has been published reporting on 11 examples of cetacean tympanic bulla that show healed fractures. The species examined included: fin, sei and minke whales, common dolphins and Baird’s beaked whales. All large whale samples were obtained from the Canadian Museum of Nature collected from Canadian whaling operations while others came from bycatch samples. The samples examined were definitely not post mortem fractures (ie: as a result of the preservation/transport of skeletons) as the authors only report on those fractures that showed remodelling and healing of fractures which had to happen while the animal was still alive. The fractured and healed tympanic bulla were extensive in some cases and would have caused hearing damage for the animals. However, due to the levels of remodelling and healing, the animals lived for a long time after the trauma occurred.
The authors hypothesise on how they could have been caused:
- Blunt trauma
- Ship strike?
- Intraspecific conflicts?
- not likely as an impact that shatters the tympanic bulla would probably cause death
- Pressure waves from high explosives causing blast injuries
- explosive harpoons during whaling?
- Seismic exploration?
- Dynamite fishing?
- Malfunction in tissue with pressure changes from deep diving?
- Foreign objects disrupting pressure regulation
- Rare for baleen whales to have middle ear parasites
- Abnormal sneeze or cough
- Loud acoustic sources
- Disease weakened bones
- Lytic lesion/fracture
This paper presents some really interesting findings and hypotheses. It certainly leads us to ask lots of questions such as: how does having a fractured ear bone effect the animals? how well can they cope with diving, feeding etc with fractured ear bones? How often does it happen? Are certain species more susceptible? We will be keeping an eye out for any further developments in this area…