Page 47 - Summer2017

oceans finding and devouring dense patches of prey. Howev- er, as we will see, for a social animal to range this far requires a long-range communication system.
Echolocation
The toothed whales have evolved a different formula for eco- logical success, one that depends directly on acoustic behav- ior. Many large marine predators such as fish and squid have well-developed eyes and depend on vision for detecting and capturing prey. Vision works well near the surface during the daytime, but much of the ocean is dark and vision can be limited there. Some deep-diving toothed whale species base their remarkable success on using echolocation to detect and capture prey in the dark depths of the ocean. The key to their success is they can use their lungs to make powerful sounds, then use their sophisticated mammalian auditory system to detect echoes from their prey.
Two groups of mammals have evolved sophisticated abilities to locate prey by making sounds and listening for echoes. A brilliant Italian experimentalist named Lazzaro Spallan- zani discovered in the eighteenth century that bats need to be able to hear but not to see to orient while flying in the dark (Galambos, 1942). As a biologist, I would hope that such a discovery would lead biologists to have discovered sonar, but actually we did not understand animal sonar until after human engineers developed sonars early in the twenti- eth century to detect obstacles such as icebergs and military targets such as submarines (an excellent website on ocean acoustics is http://www.dosits.org; their page on this topic is http://www.dosits.org/people/history/early1900/; you can explore underwater sounds at http://www.dosits.org/galleries/ audio/interactive/).
Echolocation was first discovered in bats a few decades after engineers developed sonars (Griffin, 1958) and in toothed whales decades later (Au, 2015). Bats and toothed whales force air from their lungs past phonic lips to generate high- frequency click sounds whose energy is directed forward to- ward targets. Echolocation has allowed both of these groups to specialize in hunting prey in conditions where darkness hampers visual hunters, such as at night or in the dark depths of the sea. Even during the day, light does not penetrate far in most seawater, so echolocators can often detect their prey at much greater ranges using sound than is possible using vision.
The sperm whale is perhaps the most specialized echoloca- tor. Sperm whales devote about a third of the volume of their large body (up to 16 m in length) to the spermaceti organ,
Figure 1. Illustration of the “bent horn” model of sound production in the sperm whale proposed by Møhl et al. (2003). The initial sound of a sperm whale click is generated by the passage of air from the right nares (Rn) through the phonic lips (also known as the monkey lips; Mo). Most of this sound energy passes backward through the spermaceti organ (So), reflects off the frontal sac (Fr), and forms a forward-directed beam as it passes through the junk (Ju). B, brain; Bl, blow hole; Di, distal air sac; Ln, left naris; Ma, mandible; MT, muscle/tendon layer; Ro, rostrum. From Madsen et al. (2002).
which lies above the skull and is filled with a specialized wax. Biologists have argued that the spermaceti organ functions as a battering ram or to regulate buoyancy, but most mod- ern biologists have reached a consensus that this large organ evolved to generate the loudest sonar signals of any animal (Møhl et al., 2003).
Figure 1 illustrates the sound production anatomy of the sperm whale. When sperm whale clicks are measured in the beam, they are as loud as the most intense naval sonars deployed on warships. This powerful sonar is estimated to allow sperm whales to detect a squid as far away as 300 m or more (Madsen et al., 2007). The ability of deep-diving toothed whales to use sonar to detect, select, and capture prey is a prime factor for their evolutionary success, sup- porting prewhaling populations of more than a million sperm whales (Whitehead, 2002) and supporting a radiation of many species of deep-diving toothed whales.
When a dolphin or toothed whale is echolocating on prey, the best sonar targets are features such as dense bone or a gas-filled cavity that have densities very different from those of seawater. When a sonar signal hits a hard target, it does not reflect very efficiently if the wavelength of the sonar sig- nal is larger than the circumference of the target. A dense bone in a prey fish for a dolphin might be about 2 cm in circumference. The speed of sound is about 1,500 m/s, so a wavelength of 2 cm (or 0.02 m) would correspond to a sonar frequency of 1,500/0.02 = 75 kHz. Thus, if the dolphin sonar needed to resolve objects about 2 centimeters in size in order
Summer 2017 | Acoustics Today | 45