Monday, February 6, 2012

Dolphins on Helium

When you or I breathe helium at a birthday party, we get fun, squeaky munchkin voices. That's pretty fun, but what would happen if you gave helium to a dolphin? Cetacean scientists have done just that, and it wasn't because they'd been playing too much beer-pong .
What possible reason (other than the fact it's totally hilarious) could marine mammal scientists have to give a dolphin helium? For the answer, we first have to know a little bit about dolphin behavior.

In the wild, dolphins can dive hundreds of feet deep. At those kinds of depths, gasses get compressed and become more dense. The compression of gasses is one reason why human divers rarely go that deep. As the air is compressed, the molecules of the air get closer together, and the air takes up less space. Dolphins have actually evolved the ability to let their lungs collapse as the air compresses, which has a side benefit of making them less bouyant and making diving easier!(1) The picture at right (from Moore et al(2), 2011) shows a cat-scan of a dolphins lung as it compresses under pressure.
How does helium come into all of this? Helium affects our voices because it is lighter than air. At depth, the air in a dolphin's lungs is heavier than normal air - pretty much the opposite of the effect of helium. So we could expect the effect of deep diving on a dolphin's 'voice' to be the opposite of breathing helium on a human's voice. The opposite of breathing Helium for a human is breathing Sulfer Hexaflouride, a gas which is heavier than air. Do NOT try this at home.

Dolphin communication underwater should theoretically be effected somewhat like it affects the mythbusters guy - they should sound like scary Dolphin Terminators. Let's see what actually happened...

The two whistles are a little bit different(3), but neither of these sounds anything like Barry White. What gives? The reason dolphins are not effected by compressed air the way we think they should be lies in the difference between the way humans and dolphins make sounds. In real life (as opposed to on Flipper, whose voice was done by a monkey), dolphins communicate mainly through high-pitched whistles. Dolphin whistles are produced by changing the air pressure around an organ inside of the dolphin’s head. (Most toothed whales actually have two of these organs, but animals in the sperm whale family do not). This organ looks a little bit like a pair of lips, and functions a little bit like them too(4), as we’ll see in a minute.
To produce a whistle, the dolphin changes the air pressure around the phonic lips, something like you or I would blow air through our lips to make a “raspberry” sound. As you tighten the muscles in your lips while blowing air through them, you change how fast your lips flap, changing the pitch of the sound. In addition, every video in existence on lip buzzing includes a creepy-looking mustache.

Dolphins are doing this same sort of thing, but inside their heads!
Human voices, unlike dolphin 'voices,' are effected by the way sound bounces around inside our throat and mouth. Helium increases the speed of sound inside your mouth, which changes the way the sound waves bounce around in there. When the sound actually gets out of your mouth, you sound really funny. This video by the Naked Scientists explains it really well:
The sounds dolphins make don't bounce around in an air space before the leave the dolphin's head, so they sound exactly the same with and without helium.
And that, my friends, is why dolphins aren't actually much fun at a party.

1 Skrovan, R.C., Williams, T.M., Berry, P.S., Moore, P.W., and Davis, R.W. 1999. The diving physiology of bottlenose dolphins (Tursiops truncatus) II. Biomechanics and changes in buoyancy at depth. Journal of Experimental Biology 202: 2749-2761.
2 Moore, M.J., Hammar, T., Arruda, J., Cramer, S., Dennison, S., Montie, E., and Fahlman, A. 2011. Hyperbaric computer tomographic measurement of lung compression in seals and dolphins. Journal of Experimental Biology 214: 2390-2397.
3 Madsen P.T., Jensen F.H., Carder D. and Ridgway, S. 2011. Dolphin whistles: a functional misnomer revealed by heliox breathing".Biology Letters, doi:10.1098/rsbl.2011.0701
4 Cranford, T.W., Elsberry, W.R., VanBonn, W.G., Jeffress, J.A., Chaplin, M.S., Blackwood, D.J., Carder, D.A., Kamolnick, T., Todd, M.A., and Ridgeway, S.H. 2011. Observation and analysis of sonar signal generation in the bottlenose dolphin (Turciops truncatus): Evidence for two sonal sources. Journal of Experimental Biology 407: 81-96.
This post was inspired by Paul Nachtigall's Seminar and:
Jensen F.H., Perez J. M., Johnson M., Aguilar Soto N. and Madsen P.T.(2011) , "Calling under pressure: short-finned pilot whales make social calls during deep foraging dives".Proceedings of the Royal Society B. doi: 10.1098/rspb.2010.2604.

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