Why dive so deep?

Until recently the deepest diving mammals ever recorded were the elephant seals, Mirounga angustirostris and M. leonina. They have the ability to dive to depths of 1500 metres for times close to 2 hours (Stewart & DeLong 1990). However, in 2014, Schorr et al. publish the “first long-term behavioural records from Cuvier’s beaked whales” (Schorr et al. 2014).

Fig 3. Cuvier’s beaked whale holds the record for the deepest and longest foraging dive of any marine mammal, 9,816ft. Roughly, that’s the same distance taken if we stacked the Empire State Building on top of itself 8 times! Image copyright of https://birdingbytrain.wordpress.com/

Like most cetaceans, beaked whales spend only 10% of their at the surface making them difficult to research. By fitting satellite-linked tags (D-tags; Fig 4) to the animals, they were able to record the dives of Cuvier’s beaked whales, Ziphius cavirostris, in the Santa Cruz basin, for 3 month periods between 2010 and 2012. During this time the deepest dive recorded reached 2992 metres and the longest lasted 137.5 minutes. The deepest recorded dive of any mammal. To visualize this depth imagine if we took the Empire State Building and stacked it on top of itself 8 times (Fig 3). 

The D-tag is attached to the whale using 4 suction cups that will release after time.
Figure 4. The D-tag is attached to the whale using 4 suction cups that will release after time.

So why are beaked whales so routinely diving to extreme depths? Much like other deep diving mammals, such as Mirounga spp. and the sperm whale (Physeter macrocephalus), beaked whales dive to hunt for prey, as well avoiding being hunted themselves. Predators such as killer whales (Orcinus orca), present a threat to beaked whales in upper waters (Baird et al. 1988). They are born large, assumingly so that they can dive deep sooner to escape from predators (Madsen et al. 2014). From studies of the stomach contents from dead, beached subjects the diet of beaked whales has been determined to be mainly made up deep-water squid residing in depths of around 1200 metres (Blanco & Raga 2000, Madsen et al. 2014).

Fig 5. Echolocation. The passing of ‘trains’ of clicks forming sound waves that are emitted and reflected back if they hit a prey item.
Fig 5. Echolocation. The passing of ‘trains’ of clicks forming sound waves that are emitted and reflected back if they hit a prey item.

Although their eyes are adapted to see low wavelengths of blue light (Hozel 2002) in the darkness of the deep sea beaked whales can’t rely on their vision to locate prey. Like many odontocetes (toothed whales), beaked whales use echolocation to locate their prey (Figure 5). They do this by sending out ‘clicks’ produced by their phonic lips, located in the nasal passage. This click is then focussed through a space comprised of a mass of tissue in the head, referred to as the ‘melon’. In beaked whales the melon is often bulbous and large, this is indicative of squid eating species (Dunn et al. 2012). The ‘clicks’ are sent out into the water. If they hit a prey item the sound is reflected back and channeled into a fat filled cavity under the jaw bone. Where the whale can judge how far away and in what direction the prey is. Sound waves that return quickly inform the whale that the prey is close (Hozel 2002). Blainville’s beaked whale are among the most understood toothed whales to hunt by echolocation. When these whales were tagged with sound monitoring devices they recorded no only the transmitted sound but the reflected sound returning, giving unique insight into this hunting technique and knowledge that could be modelled on other odontocetes. We now know that these whales emit approximately 3500 clicks to detect their prey, having a foraging success rate of around 25 prey per dive (Figure 6)(Johnson et al. 2006, Madsen et al. 2013).

Fig 5. A dive profile of a Blainville’s beaked whale. During deep foraging dives the animal produces echolocation clicks (yellow line). The red marks indicate buzzes, where the animal has found food.  Image copyright of Current Biology.
Fig 6. A dive profile of a Blainville’s beaked whale. During deep foraging dives the animal produces echolocation clicks (yellow line). The red marks indicate buzzes, where the animal has found food.
Image copyright of Current Biology.

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A look at extreme mammal divers