Sperm whales: Divers of the deep
Physeter macrocephalus, commonly known as the sperm whale is currently the largest toothed whale as well as largest toothed predator we know of today. They are the only known living member of their genus and one out of three species in the sperm whale family, including pygmy and dwarf sperm whales. The sperm whale is a pelagic marine mammal with a worldwide distribution migrating seasonally for feeding and breeding. Mature adults can grow to approximately 16.5 metres but have been know to reach 25 metres in length. Throughout history these gentle giants have been famous for one special trait they possess, the ability to dive to extreme depths.
Over the years, sperm whale diving behaviours have been extensively researched. With this knowledge we now know they can physically dive for hours at a time to depths of 1,200 metres but regularly dive for 45 minutes at a time to depths of between 600-900 metres with 9 minute surface intervals. The max recorded depth of 2,035 metres has placed these creatures second only to the Cuvier’s beaked whale in their diving capabilities.
Pressure’s of the deep
Diving to such depths as the sperm whale is no easy task, many Limiting factors arise for any organism that dives to these deep depths for example, pressure change. As depth increases pressure increases in the form of atmosphere’s, for every ten metres the pressure increases by one atmosphere. For example, 20 metres would equal 3 atmospheres worth of pressure, each level of atmospheric pressure equates to approximately 14.7 pounds per square inch. Due to this change in pressure the average sperm whale diving to approximately 900 metres would experience a pressure change of around 90 times stronger than that of surface atmospheric pressure levels. Times 90 by 14.7 and you get 1323 pounds per square inch of pressure. If humans attempted depths of this range, we would be crushed under the enormous weight. However, sperm whales can achieve this due to their flexibility. The rib cage of a sperm whale is capable of collapsing due to being bound by cartilage. Therefore, rather than crushing its rib cage every time they dove to such extreme depths it would collapse leaving the sperm whale unharmed when it returns to the surface.
Taking a deep breath
Sperm whales being air breathing mammals must be able to store a large amount of oxygen for long periods of time. The lungs of a sperm whale are capable of collapsing when deep diving as too allow the creature to dive deeper, this is because the inflated lungs would act as a buoyancy aid. Oxygen is stored and transported through the body via red blood cells in haemoglobin. In whales 60% of their blood is haemoglobin, this is double our haemoglobin concentration. This allows them to store twice as much oxygen as we can. Whales also have a higher percentage of blood taking up approximately 10-20% of their overall body volume allowing for even more oxygen storage. Oxygen is also stored in muscle tissue through myoglobin, whale myoglobin levels can be up to 30% higher than terrestrial mammals. This is due to marine mammal myoglobin being positively charged, as this allows marine mammals such as whales to contain a larger quantity of myoglobin in the muscle tissue without the particles sticking together as they repel each other resulting in higher oxygen stores. Myoglobin is distributed through the muscles of the whale and can hold approximately 35% of the oxygen stores. This allows oxygen stores to last and constantly feed the brain with oxygen whilst submerged.
The video below from YouTube describes how marine mammals supply themselves with oxygen
Sperm whales can do several things to help conserve oxygen stores on long dives. One major aspect being the conscious control of their heart rate, greatly reducing their cardiac output halving its heart rate. This in turn reduces blood flow to non-essential organs and some muscles. Certain blood pathways will also be shunted and or blocked to certain areas of the body unimportant to the animal during its dive periods, this can include digestive areas such as the stomach all while keeping a steady blood flow to the brain. Unfortunately, this can be harmful to the animal during prolonged cases, therefore rest periods are very common and important.
Feeding in the dark deep
Beyond 200 metres light barely penetrates the ocean. Under the right circumstances light can penetrate up to 1000 metres although this is very rare, because of this the sperm whale has adapted to hunt in these dark conditions allowing for full exploitation of this expansive resource. Echolocation is an adaptation sperm whales have developed allowing them to find their prey and locate any obstacles. A series of clicks are emitted using the spermaceti organ located in the head, here air is blown across structures like our own vocal cords lying within the head. These structures generate sound waves which researchers believe are focused by the oil-filled spermaceti organ. Acting like an acoustic lens this organ alters its shape focusing the sound on different points, acting like an auditory imaging system for the whale. When this sound reaches an obstacle or animal (potential prey) it bounces off and returns to the whale. As water has a consistent pressure the sound will travel at the same speed, therefore the whale can calculate how far the sound wave travelled when it is bounced back. This provides the whale an accurate measurement of how far away another animal or obstacle is. As whales has two ears they can also confirm which direction the sound is coming from pinpointing the exact location of potential prey, making sperm whales masters of deep sea hunting.
The mixture of developments in echolocation and specialised anatomy have made sperm whales brilliant deep-sea hunters. With these attributes they are capable of taking full advantage of deep ocean environments, providing them with an extensive food source rarely available to other organisms. This combined with the lack of competitors have allowed the sperm whale to become a formidable deep-sea predator. Although we may know a lot on the diving habits of sperm whales and other deep-sea diving air breathers much is still unknown to us. With the improvement in technology we may be able to learn more about them in the future.