Toothed whales Animal echolocation

diagram illustrating sound generation, propagation , reception in toothed whale. outgoing sounds red , incoming ones green

biosonar valuable toothed whales (suborder odontoceti), including dolphins, porpoises, river dolphins, killer whales , sperm whales, because live in underwater habitat has favourable acoustic characteristics , vision extremely limited in range due absorption or turbidity.

cetacean evolution consisted of 3 main radiations. throughout middle , late eocene periods (49-31.5 million years ago), archaeocetes, primitive toothed cetacea arose terrestrial mammals creation of aquatic adaptations, known archaic cetacea. these primitive aquatic mammals did not possess ability echolocate, although did have adapted underwater hearing. morphology of acoustically isolated ear bones in basilosaurid archaeocetes indicates order had directional hearing underwater @ low mid frequencies late middle eocene. however, extinction of archaeocete @ onset of oligocene, 2 new lineages in oligocene period (31.5-28 million years ago) comprised second radiation. these mysticetes (baleen whales) , odontocetes can dated middle oligocene in new zealand. based on past phylogenies, has been found evolution of odontocetes monophyletic, suggesting echolocation evolved once 36 34 million years ago. dispersal rates routes of odontocetes included transoceanic travel new adaptive zones. third radiation occurred later in neogene, when present dolphins , relatives evolved common species in modern sea.

the evolution of echolocation attributed several theories. there 2 proposed drives hypotheses of cetacean radiation, 1 biotic , other abiotic in nature. first, adaptive radiation, result of rapid divergence new adaptive zones. results in diverse, ecologically different clades incomparable. clade neocete (crown cetacean) has been characterized evolution archaeocetes , dispersion across world s oceans, , estuaries , rivers. these ecological opportunities result of abundant dietary resources low competition hunting. hypothesis of lineage diversification, however, can unconvincing due lack of support rapid speciation in cetacean history. second, more abiotic drive better supported. physical restructuring of oceans has played role in echolocation radiation. result of global climate change @ eocene-oligocene boundary; greenhouse icehouse world. tectonic openings created emergence of southern ocean free flowing antarctic circumpolar current. these events allowed selection regime characterized ability locate , capture prey in turbid river waters, or allow odontocetes invade , feed @ depths below photic zone. further studies have found echolocation below photic zone have been predation adaptation diel migrating cephalopods. since advent, there has been adaptive radiation in family delphinidae (dolphins) in echolocation has become extremely derived.

two proteins have been found play major role in toothed whale echolocation. prestin, motor protein of outer hair cells of inner ear of mammalian cochlea, has association between number of nonsynonymous substitutions , hearing sensitivity. has undergone 2 clear episodes of accelerated protein evolution in cetaceans: on ancestral branch of odontocetes , on branch leading delphinioidae. first episode of acceleration connected odontocete divergence, when echolocation first developed, , second occurs increase in echolocation frequency seen in delphinioidae family. cldn14, member of tight junction proteins form barriers between inner ear cells, shows same evolutionary pattern prestin. 2 events of protein evolution, prestin , cldn14, occurred @ same times tectonic opening of drake passage (34-31 ma) , antarctic ice growth @ middle miocene climate transition (14 ma), divergence of odontocetes , mysticetes occurring former, , speciation of delphinioidae latter. there strong connection between these proteins, ocean restructuring events, , echolocation evolution.

one specific type of echolocation, narrow-band high frequency (nbhf) clicks, evolved @ least 4 times in groups of odontocetes, including pygmy sperm whale (kogiidae) , porpoise (phocoenidae) families, pontoporia blainvillei, genus cephalorhynchus, , part of genus lagenorhynchus. these high frequency clicks evolved adaptation of predator avoidance, inhabit areas have many killer whales , signals inaudible killer whales due absence of energy below 100 khz.

another reason variation in echolocation habitat. sonar systems limiting factor deciding whether returning echo detected echo-to-noise ratio (enr). enr given emitted source level (sl) plus target strength, minus two-way transmission loss (absorption , spreading) , received noise. animals adapt either maximize range under noise-limited conditions (increase source level) or reduce noise clutter in shallow and/or littered habitat (decrease source level). in cluttered habitats, such coastal areas, prey ranges smaller, , species commerson s dolphin (cephalorhynchus commersonii ) have lowered source levels better suit environment.

toothed whales emit focused beam of high-frequency clicks in direction head pointing. sounds generated passing air bony nares through phonic lips. these sounds reflected dense concave bone of cranium , air sac @ base. focused beam modulated large fatty organ known melon . acts acoustic lens because composed of lipids of differing densities. toothed whales use clicks in series, or click train, echolocation, while sperm whale may produce clicks individually. toothed whale whistles not appear used in echolocation. different rates of click production in click train give rise familiar barks, squeals , growls of bottlenose dolphin. click train repetition rate on 600 per second called burst pulse. in bottlenose dolphins, auditory brain response resolves individual clicks 600 per second, yields graded response higher repetition rates.

it has been suggested smaller toothed whales may have tooth arrangement suited aid in echolocation. placement of teeth in jaw of bottlenose dolphin, example, not symmetrical when seen vertical plane, , asymmetry possibly aid in dolphin sensing if echoes biosonar coming 1 side or other. however, idea lacks experimental support.

echoes received using complex fatty structures around lower jaw primary reception path, transmitted middle ear via continuous fat body. lateral sound may received though fatty lobes surrounding ears similar density water. researchers believe when approach object of interest, protect against louder echo quietening emitted sound. in bats known happen, here hearing sensitivity reduced close target.