Posted by: Rob Viens | July 15, 2012

Pilots of the Sea

On Sunday the 15th the Beagle was speeding southward with a good wind (at least until evening):

“From noon of yesterday to the same time to day we had run 160 knots & all congratulated ourselves on soon doubling Cape St Mary’s.— On the contrary however we experienced the true uncertainty of a sailors life.— By the evening it blew a gale right in our teeth.— Top-gallant masts were sent on deck.— & with close reefed main top-sail, trysails & fore sails, we beat up against a heavy sea.” (July 15)

Darwin was feeling well enough today to actually appreciate a large number of cetaceans that were traveling for a time with the Beagle. He writes:

“In the morning I was much interested by watching a large herd of Grampuses, which followed the ship for some time.— They were about 15 feet in length, & generally rose together, cutting & splashing the water with great violence.— In the distance some whales were seen blowing.— All these have been the black whale.— The Spermaceti is the sort which the Southern Whalers pursue.” (July 15)

As noted in an earlier post (see Look, a Grampus! from April Fool’s Day), the term “grampus” has been used to refer to a lot of different animals. It is believed to be derived from either Latin (grandis piscis) or French (grand poisson) for “great fish”, and is almost always used for a large, dark-colored dolphin. Some of the cetaceans that have been called a “grampus” include orcas, Risso’s dolphins and pilot whales. All of these animals have wide ranges which include the southern coast of Brazil. But since orcas are typically larger than 15 feet, and Risso’s dolphins are typically more of a grey color, I’m going to hypothesize that Darwin was describing pilot whales. I’ll at least use that as an excuse to concentrate on pilot whales in my post today (I’ll come back to Risso’s dolphins another day).

Dolphins belong to the family Delphinidae, and although they are commonly referred to as “whales”, both orcas and pilot whales are actually large dolphins in this family.

Long-finned pilot whales in the North Atlantic (from Wikipedia Commons)

pilot whales

There are two species of pilot whales – long-finned pilot whales (Globicephala melas) and short-finned pilot whales (Globicephala macrohynchus). (In reality, although one species typically has longer fins, that characteristic is not always the case for individual pilot whales.  So the two species are actually difficult to tell apart from one another.) As you can see from the image below (from Wikipedia Commons) the two species overlap in the region of southern Brazil, so it could have been either one that Darwin saw.
pilot whales range
(blue=short-finned, green=long-finned)

Pilot whales are very social animals and live their entire life as part of a pod. (In fact, their name comes from the fact that they were always observed in pods that had an apparent leader – a pilot.) The pods, which are typically 10-30 individuals, are biologically related to one another and led by a matriarch.  Both male and female offspring stay with the pod, and even older females who can no longer reproduce, continue to lactate and take care of offspring. Periodically multiple pods will join together.  These gatherings may be partly social, but seem to primarily be for mating.  Males do not mate with females in the same pod, and during these congregations, the males will often mate with a female from another pod before returning home to mommy (yes – they never really leave home). The offspring of that pairing will stay with the female’s pod.

With strong social ties, it is not uncommon to find complex communication, and pilot whales are no exception.  Check out a recording of pilot whales here on Wikipedia Commons.

It is also believed that these strong social ties are the reason for “mass beachings” of pilot whale – where the entire pod may follow an individual (or try to come to the aid of an individual) who has become stranded on shore. These “mass strandings” rarely end well.

Pilot Whale (from Wikipedia Commons)

pilot whales

One last cool fact:  Pilot whales are one of a handful of species that experience “unihemispheric slow-wave sleep” (USWS), which basically means that they have the ability to have one half of their brain go into deep sleep while the other half stays awake.  So effectively there is always half of their brain that is awake and alert.  This may be an adaptation to having to breath air while living in the water. Whatever the cause, being able to keep half your brain awake would be pretty cool – I wish I could do that on some of these late nights!

Though before you get too excited about this “higher level of consciousness” I have to point out that domestic chickens can do this, too. Sort of makes it a little less desirable…(RJV)

PS – Did I mention that pilot whales prefer to eat squid. They were probably following their food source when then encountered this strange, stubbly naturalist who watched them with great interest so many years ago. If they had their own journals, what stories they could tell…

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