Animal Echolocation - Toothed Whales

Toothed Whales

Biosonar is valuable to Toothed whales (suborder odontoceti), including dolphins, porpoises, river dolphins, killer whales and sperm whales, because they live in an underwater habitat that has favourable acoustic characteristics and where vision is extremely limited in range due to absorption or turbidity.

Cetacean evolution consisted of three main radiations. Throughout the middle and late Eocene periods (49-31.5 million years ago), archaeocetes, primitive toothed Cetacea that arose from terrestrial mammals with the creation of aquatic adaptations, were the only known archaic Cetacea. These primitive aquatic mammals did not possess the ability to echolocate, although they did have slightly adapted underwater hearing. The morphology of acoustically isolated ear bones in basilosaurid archaeocetes indicates that this order had directional hearing underwater at low to mid frequencies by the late middle Eocene. However, with the extinction of archaeocete at the onset of the Oligocene, two new lineages in the early Oligocene period (31.5-28 million years ago) compromised a second radiation. These early mysticete (baleen whales) and odontocete can be dated back to the middle Oligocene in New Zealand. Based on past phylogenies, it has been found that the evolution of odontocetes is monophyletic, suggesting that echolocation evolved only once 36 to 34 million years ago. Dispersal rates routes of early odontocetes included transoceanic travel to new adaptive zones. The third radiation occurred later in the Neogene, when present dolphins and their relatives evolved to be the most common species in the modern sea.

The evolution of echolocation could be attributed several theories. There are two proposed drives for the hypotheses of cetacean radiation, one biotic and the other abiotic in nature. The first, adaptive radiation, is the result of a rapid divergence into new adaptive zones. This results in diverse, ecologically different clades that are incomparable. Clade Neocete (crown cetacean) has been characterized by an evolution from archaeocetes and a dispersion across the world's oceans, and even estuarites and rivers. These ecological opportunities were the result of abundant dietary resources with low competition for hunting. This hypothesis of lineage diversification, however, can be unconvincing due to a lack of support for rapid speciation early in cetacean history. A second, more abiotic drive is more supported. Physical restructuring of the oceans has played a role in echolocation radiation. This was a result of global climate change at the Eocene-Oligocene boundary; from a greenhouse to an icehouse world. Tectonic openings created the emergence of the Southern ocean with a free flowing Antarctic Circumpolar current (Fordyce 1980, 2003; Lindberg & Pyenson 2007; Steeman et al. 2009). These events allowed for a selection regime characterized by the ability to locate and capture prey in turbid river waters, or allow odontocetes to invade and feed at depths below the photic zone. Further studies have found that echolocation below the photic zone could have been a predation adaptation to diel migrating cephalopods. Since its advent, there has been adaptive radiation especially in the Delphinidae family (dolphins) in which echolocation has become extremely derived.

One specific type of echolocation, narrow-band high frequency (NBHF) clicks, evolved at least four times in groups of odontocetes, including pygmy sperm whale (Kogiidae) and porpoise (Phocoenidae) families, Pontoporia blainvillei, the genus Cephalorhynchus, and part of the genus Lagenorhynchus. These high frequency clicks likely evolved as adaptation of predator avoidance, as they inhabit areas that have many killer whales and the signals are inaudible to killer whales due to the absence of energy below 100 kHz. Another reason for variation in echolocation frequencies is habitat. Shallow waters, where many of these species live, tend to have more debris; a more directional transmission reduces clutter in reception.

Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips. These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities. Most toothed whales use clicks in a series, or click train, for echolocation, while the sperm whale may produce clicks individually. Toothed whale whistles do not appear to be used in echolocation. Different rates of click production in a click train give rise to the familiar barks, squeals and growls of the bottlenose dolphin. A click train with a repetition rate over 600 per second is called a burst pulse. In bottlenose dolphins, the auditory brain response resolves individual clicks up to 600 per second, but yields a graded response for higher repetition rates.

It has been suggested that some smaller toothed whales may have their tooth arrangement suited to aid in echolocation. The placement of teeth in the jaw of a bottlenose dolphin, as an example, are not symmetrical when seen from a vertical plane, and this asymmetry could possibly be an aid in the dolphin sensing if echoes from its biosonar are coming from one side or the other. However, this idea lacks experimental support.

Echoes are received using complex fatty structures around the lower jaw as the primary reception path, from where they are transmitted to the middle ear via a continuous fat body (Ketten 1992,2000). Lateral sound may be received though fatty lobes surrounding the ears with a similar density to water. Some researchers believe that when they approach the object of interest, they protect themselves against the louder echo by quietening the emitted sound. In bats this is known to happen, but here the hearing sensitivity is also reduced close to a target.

Before the echolocation abilities of "porpoises" were officially discovered, Jacques Yves Cousteau suggested that they might exist. In his first book, The Silent World (1953, pp. 206–207), he reported that his research vessel, the Élie Monier, was heading to the Straits of Gibraltar and noticed a group of porpoises following them. Cousteau changed course a few degrees off the optimal course to the center of the strait, and the porpoises followed for a few minutes, then diverged toward mid-channel again. It was obvious that they knew where the optimal course lay, even if the humans didn't. Cousteau concluded that the cetaceans had something like sonar, which was a relatively new feature on submarines.