Sperm whales-Why such a big head?

by Matthew Norton

Sperm whales are large toothed whales and are known for diving into the ocean depths to feed on squid, forming close social groups and featuring in the famous novel Moby Dick. However, its most distinctive characteristic is its enlarged forehead, which contains a reservoir of spermaceti, an oily mixture of fats and waxes, around an upper ‘spermaceti organ’ and lower ‘junk’ compartment. Various theories have been proposed to explain their function, especially the spermaceti organ. These include a role in controlling air movement within the whale’s body, for using their head as a ‘battering ram’ and to control their buoyancy at depth.

1200px-Sperm_whale_head_anatomy_(transverse_+_sagittal)
The spermaceti organ and junk compartment make much of the enlarged forehead and both are covered in spermaceti, an oily mixture of fats and waxes

The most prevailing theory is that both the spermaceti organ and junk compartment are involved in echolocation, to explain how requires some context. Echolocation in toothed whales involves sending out sounds, ‘clicks’, which when they hit an object in the water causes the sound to be bounced back as an echo. These echoes cause vibrations in a fatty mass, in the case of sperm whales this is the junk compartment, which can then be interpreted to locate the object, which could be their prey.

For sperm whales there are added challenges that come with hunting by echolocation in the deep sea, such as the total darkness and the sparse distribution of life. Squid, the primary prey of sperm whales, also give off weak echoes which makes it difficult to get a precise location with the usual echo locating clicks. Sperm whales overcome this by using a different type of sound called ‘creaks’, a rapid series of clicks, to home in on their targets.

Sperm_whale_and_squid
Sperm whales prey on deep sea squid, using echolocation to home in on them.

Producing the echolocating creaks is where the spermaceti organ comes in. Around the spermaceti organ there are air sacs which are believed to reflect sound waves as they are bounced around the organ. This splits the sound wave into multiple pulses with each pulse travelling a different path within the organ depending on how far away the air sac, from which the pulse is reflected, is from the source of the sound wave. Therefore each pulse is released into the water at different time intervals, separated by milliseconds, creating the series of sounds that make up the creaks. Meanwhile the junk compartment is primary receiver of the returning sound waves in the water.

The enlarged head may seem an extreme feature for the sperm whale to evolved, just so they can produce this extra type of sound. But as previously mentioned, the deep sea is an extreme environment for any species to try and make a living, and this is even more true for sperm whales who must make regular migrations to and from the ocean surface in search of both air and food.

From a human perspective

Sperm whales were prime targets for the whaling industry, especially in the 19th and mid-20th century. As in other targeted whale species, their blubber was a valuable commodity, but the real prize was the spermaceti oil, which was an effective lubricant and burnt bright in oil lamps.

Fortunately, whaling is nowhere near as widespread as it used to be thanks to the 1972 United States Endangered Species Act and 1987 International Whaling Commission moratorium. Some small scale sperm whale hunting continues in Japan and Indonesia, but global populations overall have been given a much needed chance to recover. In the 21st century they are doing just that with numbers estimated to be in the hundreds of thousands of sperm whales, compared to the pre-whaling figure of 1.1 million. Recovering is the key word as they are still classed as vulnerable by the IUCN and endangered under the United States Endangered Species Act. These figures and assessments are based on data by various research methods and it is interesting to see how such methods have evolved over the years.

Spermaceti
The spermaceti oil was very profitable during whaling times.

Historic whaling practices may be responsible for the decimation of global population in the first place, but their records provides us with a useful baseline from which to work out how sperm whale populations have changed during and post-whaling. However, this whaling data is recorded as the rate at which sperm whales are caught, which is not influenced by just how many whales are in the water. For example technological advancements, which make whaling vessels more effective at catching their targets, can be a contributing factor that needs to be accounted for.

Whaling records can also give us a good idea of where to start looking for sperm whales using alternative research methods. One such method involves regular whale spotting trips along particular stretches of ocean, looking out for sperm whales as they surface and recording how many they see. Photo-identification techniques may also be used to keep track of individual whales, using unique marks and patterns on their tail. Over time these markings may change and risk a previously recorded whale from being confused for a new individual, but if the surveys are done often enough these changes may be noticed and accounted for.

Another common method is to use sound recording equipment to estimate how many sperm whales there are from their clicks, creaks and other vocal signals. The multi-pulse structure of their creaks also makes it easier to distinguish sperm whales from other species, and the different types of vocalisations they produce allow us to study their behaviour. There has been some suggestion that variations in the size of the spermaceti organ, produced by variation in head size, can help us to identify individuals from their creaks, but I think the method needs to be refined and tested.

The data from all these techniques give us only a glimpse into how sperm whale populations are recovering and all the figures that are reported are educated guesses based on this limited information. Nonetheless the evidence overall points to the recovery of sperm whale populations and it is the balance of evidence, while keeping in mind its limitations, is essentially how, although some will say this is an oversimplification, scientific study works.

Sperm_whale_and_Bottlenose_whale
Thanks for reading

Sources

Wikipedia. 2018a. Sperm whale. https://en.wikipedia.org/wiki/Sperm_whale. Last accessed 09/07/2018

Mohl. 2001. Sound transmission in the nose of the sperm whale Physeter catodon. A post mortem study

Mohl et al. 2002. Sound transmission in the spermaceti complex of a recently expired sperm whale calf

Clarke. 1978a. Structure and proportions of the spermaceti organ in the sperm whale

Clarke. 1978b. Buoyancy control as a function of the spermaceti organ in the sperm whale

Melville. 1851. Moby Dick. Evergreen Ed (2017). ISBN: 978-1-84749-644-7

Mohl et al. 2003. The monopulsed nature of sperm whale clicks

Miller et al. 2004. Sperm whale behaviour indicates the use of echolocation click buzzes ‘creaks’ in prey capture

Madsen et al. 2002. Male sperm whale (Physeter macrocephalus) acoustics in a high-latitude habitat: implications for echolocation and communication

Norris and Harvey. 1972. A theory for the function of the spermaceti organ of the sperm whale (Physeter catodon L.). NASA. Tech. Rep. Doc ID: 19720017437

Flewellen and Morris. 1978. Sound velocity measurements on samples from the spermaceti organ of the sperm whale (Physeter catodon)

Whitehead. 2002. Estimates of the current global population size and historical trajectory for sperm whales

Wikipedia. 2018b. Sperm oil. https://en.wikipedia.org/wiki/Sperm_oil. Last accessed 09/07/2018

Taylor et al. 2008. The IUCN Red List of Threatened Species 2008: e.T41755A10554884. http://www.iucnredlist.org/details/41755/0. Last accessed 09/07/2018

FAO. 2018. Physeter catodon (Linnaeus, 1758). http://www.fao.org/fishery/species/2736/en. Last accessed 09/07/2018

Perry et al. 1999. The great whales: History and status of six species listed as endangered under the US Endangered Species Act of 1973

Dufault and Whitehead. 1995. An assessment of changes with time in the marking patterns used for photoidentification of individual sperm whales, Physeter macrocephalus

Gero et al. 2008. Heterogeneous social associations within a sperm whale, Physeter macrocephalus, unit reflect pairwise relatedness

Barlow and Taylor. Estimates of sperm whale abundance in the northeastern temperate Pacific from a combined acoustic and visual survey

Rendell and Whitehead. 2003. Vocal clans in sperm whales (Physeter macrocephalus)

Image sources

Kurzon. 2013. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D. https://commons.wikimedia.org/w/index.php?curid=26884229

All other images are public domain and do not require attribution

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