No. 112: Jul-Aug 1997
Dolphins and other cetaceans have an overheating problem. For high hydrodynamic performance, their bodies must be nicely streamlined. For males, this means that their testicles must be stored internally. But dolphins are very active animals, and their muscles generate considerable heat -- too much heat for sperm to survive without some sort of special cooling system. (Recall that human males with undescended testicles may become sterile.) Since dolphins are obviously procreating, evolution must have come to the dolphins' rescue. Evolution's engineering solution installs heat exchangers in the dolphins' tails and dorsal fins. Blood heated in the vicinity of the testes is pumped through special veins in the tail and dorsal fin, where it is cooled by seawater and then returned to the dolphins' heat-sensitive innards.
Female dolphins have similar heat exchangers to cool their uteri.
The same article in Discover points out still another remarkable adaptation conferred on dolphins: They do not have to expend a lot of energy in diving to great depths. Below about 70 meters, the water pressure collapses their lungs so that they sink like rocks! Of course, returning to the surface does require some exertion.
(Zimmer, Carl; "The Dolphin Strategy," Discover, 18:72, March 1997.)
Comments. One automatically supposes that the dolphin dorsal fin is needed for stabilization when swimming -- like an airplane's rudder. But several cetacea do quite well without dorsal fins; viz., the finless dolphin found in IndoPacific waters. Could dorsal fins actually have evolved for thermal control rather than stabilization?
And how do dolphin's breathe if their lungs have collapsed? Their blood and muscles act as oxygen reservoirs -- still another evolutionary adaptation.
Speaking of evolution, as one must when discussing Nature's neat methods of solving bioengineering problems, how did random mutation and natural selection hit upon the heat-exchanger and oxygen-reservoir ideas? We know! These were more millimeter-by-millimeter climbs up Mount Improbable.
Reference. The finless dolphin and the general question of dorsal-fin evolution is the subject of Section BMA46 in our Catalog Mammals I. Details may be found at here.