How does spermaceti make a whale float?

Question

In the biochemistry book I'm reading (Box 10-1, Lehninger Principles of Biochemistry), an example is given in whales of how spermaceti (a wax located in the whale head), when at sea level is a liquid, and allows the whale to float due to its buoyancy.

As the whale dives deeper, the wax turns into a solid (due to the colder temperatures), and hence this more dense wax is less buoyant and prevents the whale from floating back up due to the greater density of the seawater at these depths.

My limited understanding is that density effects buoyancy in so far as that the weight of the object displaces an equivalent weight of water. If the quantity of the water displaced is small enough that the surface area of the object isn't completely submerged, it can float.

Now with the whale example, considering the wax is fully enclosed by the head, how would this change in local density effect the buoyancy of the whale as a whole? The weight of the wax still remains the same (in so far as the example used in the book), the only thing that changes is its density inside the whale.

So with a constant weight, and only a variable local density enclosed by an object (the head) and limited to these conditions only (as this is all the example in the book uses), how is this possible?

Answer 1

Buoyancy is controlled by "Archimedes' principle", as described in Britannica:

... any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force, the magnitude of which is equal to the weight of the fluid displaced by the body.

Here the weight is a force, not a mass, so the force that pushes an object is related to the density and volume of the object itself. In this context, considering the spermaceti, it's the temperature of the oil that matters, because it's going to change its density, thus the buoyancy force. However, the example you are referring to is a hypothesis on the spermaceti function developed by Clarke (1978). In summarizing, he stated that sperm whales are almost neutral at the surface, so they need to control their buoyancy in order to dive. So, it's possible they change the vascularization on the head (but also the blubber!) to change the density and descend or ascend.

Still, this remains a hypothesis that has not been confirmed nor rejected. Miller et al. (2004) found data that don't support the theory proposed by Clarke, as the change in temperature and so density in the spermaceti should not be relevant for buoyancy during dives and sperm whales are not neutral at the surface; moreover, they actively swim while searching for food at the bottom, so they contrast the buoyancy force. A more studied and developed theory for the function of the spermaceti system (with the nasal apparatus) is that sound production and the change in density has a relevant role found for sound transmission.

Full references:

1. Clarke, M. R. (1978). Buoyancy control as a function of the spermaceti organ in the sperm whale. Journal of the Marine Biological Association of the United Kingdom, 58(1), 27-71;
2. Miller, P. J., Johnson, M. P., Tyack, P. L., & Terray, E. A. (2004). Swimming gaits, passive drag and buoyancy of diving sperm whales Physeter macrocephalus. Journal of Experimental Biology, 207(11), 1953-1967.

Answer 2

The buoyancy theory of the function of the spermaceti organ has essentially no empirical support. On the contrary, there is overwhelming support for the idea that the spermaceti organ, together with the associated structures named the "monkey's muzzle" and the "junk" (named so by whalers) are sound producing and focusing structures. The sound production theory was originally proposed by Norris and Harvey (1972) and has gained substantial support from experimental studies within the last decades. See for example Møhl et al (2003). In brief, the sounds are produced by air being pressed through the monkey's muzzle in the front of the nose, just below the blowhole. The sound is transmitted through the spermaceti oil in spermaceti organ, reflected by a parabolic mirror created by small air-pockets on the front of the skull and transmitted forward again through spermaceti-filled 'acoustic lenses' (known as the 'junk') below the spermaceti organ and into the water as an extremely directional and powerful sound beam. Next time you happen to see a sperm whale skull in a museum, take a look at the skull and once you see it, it is very difficult to see what else could explain the strange shape of the forehead.

The buoyancy model for the spermaceti organ on the other hand has some severe shortcomings. The theory is that the whale sucks in cold water through the blowhole when it wants to dive, hereby cooling and solidifying the spermaceti. The density increases by this cooling, making the whale sink. When the whale wants to surface, it heats the spermaceti by means of warm blood, whereby it expands and the whale rises. However, this would require a massive blood supply to the nose, inconsistent with anatomy, and the experimental data gathered by data recorders on diving sperm whales have shown no indication of water being inhaled upon diving. On the contrary, observations are entirely consistent with the airways being airfilled during the dive and involved in the sound production (Wahlberg et al. 2005). See also the paper by Cranfort et al (1996) where a plausible evolutionary explanation to the origin of the spermaceti organ is proposed.

1. Cranford, T. W., Amundin, M. and Norris, K. S. 1996. Functional morphology and homology in the odontocete nasal complex: implications for sound generation. Journal of Morphology 228:223-285.
2. Norris, K. S. and Harvey, G. W. 1972. A theory for the function of the spermaceti organ of the sperm whale (Physeter catodon L.). Pages 397-417 Animal orientation and navigation. NASA, Washington DC.
3. Møhl, B., Wahlberg, M., Madsen, P. T., Heerfordt, A. and Lund, A. 2003. The monopulsed nature of sperm whale clicks. Journal of the Acoustical Society of America 114:1143-1154.
4. Wahlberg, M., Frantzis, A., Alexiadou, P., Madsen, P. T. and Møhl, B. 2005. Click production during breathing in a sperm whale (Physeter macrocephalus) (L). Journal of the Acoustical Society of America 118:3404-3407.