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Dolphin braininess due to social life, studies suggest

May 30, 2007
Special to World Science  

Dol­phins and their close kin are widely thought to rank among our plan­et’s most in­tel­li­gent crea­tures. They al­so have some of the larg­est brains rel­a­tive to their body size.

 New research offers a pos­si­ble ans­wer for why. Two just-pub­lished stud­ies con­c­lude that dol­phins’ highly de­vel­oped brains like­ly evolved as a re­sult of their so­cially com­p­lex life­styles. 

Dol­phins form so­cial bonds in a variety of ways, of­ten by gen­tly rub­bing each oth­er and by play­ing. (Im­age cour­te­sy U.S. In­te­ri­or Dept.)

The findings echo a pop­u­lar the­o­ry of hu­man in­tel­li­gence: the “so­cial brain” pro­pos­al, which emerged in the 1980s. It claims our large brains evolved to meet the cog­ni­tive de­mands of liv­ing in com­plex so­cial groups.

The dol­phin brain prob­ab­ly owes its size to “life­style pat­terns,” such as the fact that these mam­mals are “so­cial­ly com­p­lex and high­ly com­mu­nica­tive pred­a­tors,” wrote psy­chol­o­gist Lori Ma­ri­no of Em­o­ry Un­ivers­ity in At­lan­ta, Ga., in a pa­per de­tail­ing one stu­dy. It ap­pears in the May 21 ad­vance on­line is­sue of the re­search jour­nal The Ana­tom­i­cal Rec­ord.

Oth­er proposed ex­plana­t­ions for dolphins’ volu­mi­nous brains—that they’re ad­ap­ta­t­ions for their long-ago ev­o­lu­tion­ary tran­si­tion from land to wa­ter, or to han­dle their un­usu­al so­nar naviga­t­ion sys­tem—are less plau­si­ble, Ma­ri­no claimed.

Her work fo­cused mainly on dol­phins, por­poises and toothed whales, which to­geth­er com­prise a line­age known as Odon­to­cetes.

Just how in­tel­li­gent they are is de­bat­ed. Past stud­ies have found re­mark­a­ble feats in dol­phins in par­tic­u­lar: tool use, var­ied and im­ag­i­na­tive games, rec­og­niz­ing them­selves in mir­rors, “nam­ing” them­selves, even trick­ing their hu­man train­ers in­to feed­ing them ex­tra. But some ex­perts say dol­phin clev­er­ness is over­rat­ed. A con­tro­ver­sial study in the May 2006 is­sue of the jour­nal Bi­o­log­i­cal Re­views of the Cam­b­ridge Phil­o­so­ph­i­cal So­ci­e­ty ar­gued that the large brains are merely an ad­ap­ta­t­ion to cold-wa­ter life.

Brain size is only loosely re­lat­ed to in­tel­li­gence, but much re­search fo­cus­es on brain size, as it’s much more clearly meas­ur­a­ble than in­tel­li­gence. Many odon­to­cetes have brains that are big­ger rel­a­tive to their bod­ies than any oth­er mam­mals ex­cept hu­mans, Ma­ri­no not­ed. 

Dol­phins, por­poises and whales of all types—col­lec­tive­ly known as ce­ta­cean­s—de­scend from a line­age of hoofed an­i­mals known as even-toed un­gu­lates, which in­clude cat­tle, rein­deer, camels, pigs, goats, and sheep. In one of evo­lu­tion­ary his­tory’s most re­mark­able tran­si­tions, ce­ta­ceans moved back in­to the seas from whence their re­mote, fishy an­ces­tors had emerged. They fully re-adapted to aquat­ic life by an es­t­i­mated 40 mil­lion years ago.

But pa­le­on­to­logical ev­i­dence shows the major brain en­large­ments didn’t oc­cur un­til at least five mil­lion years lat­er, be­ly­ing the idea of a di­rect link be­tween the two events, she wrote. Her study did­n’t di­rectly ad­dress the cold-wa­ter the­o­ry, pro­posed by Da­vid Man­ger at the Un­ivers­ity of the Wit­wa­ters­rand, South Af­ri­ca. Man­ger had ar­gued that dol­phin brains contain a pre­pon­der­ance of fatty gli­al cells, which pro­duce heat; skep­tics noted that such cells also add con­nect­ivity to the brain, and that they abound in hu­man brains as well.

Ma­ri­no did, how­ev­er, toss cold wa­ter at anoth­er pro­pos­al—that large ce­ta­cean brains are a func­tion of their com­plex naviga­t­ion sys­tem, called ech­oloca­t­ion. This in­volves gaug­ing an ob­jec­t’s loca­t­ion by meas­ur­ing how long it takes for an ech­o to re­turn from it.

Contradicting this notion, Marino wrote, is that dol­phin brain ar­eas de­vot­ed to sound pro­cess­ing—though rather large—are too lim­it­ed in size to ac­count for the over­all size in­crease. Al­so, she con­ti­nued, land an­i­mals such as bats use ech­oloca­t­ion with­out hav­ing par­tic­u­larly de­vel­oped brains. “The neu­ro­ana­tom­i­cal ev­i­dence sug­gests that the large ce­ta­cean brain sup­ports a com­plex gen­er­al in­tel­li­gence,” Ma­ri­no wrote. This, she added, could have been driv­en by fac­tors paral­lel­ing those that pushed the evo­lu­tion of oth­er “so­cially com­plex mam­mals.”

Rich­ard Con­nor, a bi­ol­o­gist at the Uni­ver­si­ty of Mas­sa­chu­setts at Dart­mouth, Mass., reached some si­m­i­lar con­clu­sions in a pa­per pub­lished in the April 29 is­sue of the re­search jour­nal Phil­o­soph­i­cal Trans­ac­tions of the Roy­al So­ci­e­ty of Lon­don B. He stud­ied bot­tle­nose dol­phins in Shark Bay, Aus­tral­ia, which live in what he de­s­cribed as a large, un­bounded so­ci­e­ty with oft-shift­ing af­fil­i­a­tions and al­liances. 

Their sit­u­a­tion drives a “need to de­vel­op so­cial strate­gies in­volv­ing the rec­og­ni­tion of a large num­ber of in­di­vid­u­als and their re­la­tion­ships with oth­ers,” he wrote. “All three ‘peaks’ of large brain size ev­o­lu­tion in mam­mals,” he added—in odon­to­cetes, hu­mans and ele­phants—evolved from si­m­i­lar so­cial en­vi­ron­ments. These in­volved “ex­treme mu­tu­al de­pend­ence based on ex­ter­nal threats,” from mem­bers of other spe­cies or of their own.

* * *

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Home page image: Bottlenose dolphins (Courtesy U.S. Marine Mammal Commission)

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Dolphins and their close kin are widely considered some the of our planet’s most intelligent animals. They also have some of the largest brains relative to their body size. A new study has concluded that dolphins’ highly developed brains probably evolved as a result of their socially complex lifestyles. Other explanations—that the large brains are adaptations for their long-ago evolutionary transition from land to water, or to handle their unusual sonar navigation system—are less plausible, the research concluded. Dolphin brain complexity seems to be “a function of lifestyle patterns,” such as the fact that these mammals are “socially complex and highly communicative predators,” wrote psychologist Lori Marino of Emory University in Atlanta in a paper detailing the study. The work appears in the May 21 advance online issue of the research journal The Anatomical Record. The proposal echoes a popular theory of human intelligence: the “social brain” proposal, which emerged in the 1980s and claims our large brains evolved to meet the cognitive demands of living in complex social groups. Marino’s study focused mainly on dolphins, porpoises and the toothed whales, which together comprise a lineage known as Odontocetes. Just how intelligent they are is debated. Past studies have found remarkable feats in dolphins in particular: tool use, varied and imaginative games, recognizing themselves in mirrors, “naming” each other, even tricking their human trainers into feeding them extra. But some experts say their cleverness is overrated. A controversial study in the May 2006 issue of the journal Biological Reviews of the Cambridge Philosophical Society argued that the large brains are merely an adaptation to living in cold water. Brain size is only loosely related to intelligence, but much research focuses on brain size as it’s much more clearly measurable than intelligence. Many odontocetes have brains that are bigger relative to their bodies than any other animals except humans, Marino noted. Dolphins, porpoises and whales of all types—collectively known as the cetaceans—descend from a lineage of hoofed animals known as even-toed ungulates, which include cattle, reindeer, camels, pigs, goats, and sheep. Cetaceans moved into the sea and became fully aquatic by 40 million years ago, according to Marino. But paleontological evidence shows the brain enlargement took place millions of years later, belying the idea of any direct link between the two events, she wrote. Her study didn’t directly address the cold-water theory, proposed by David Manger at the University of the Witwatersrand, South Africa. Marino did, however, toss cold water on another proposal—that large cetacean brains are a function of their complex navigation system, called echolocation, which involves gauging an object’s location by measuring how long it takes for an echo to return from it. Weighing against this idea is the fact that the dolphin brain areas devoted to sound processing, though large, are limited in size, Marino wrote. Also, land animals such as bats use echolocation without having particularly developed brains. “The neuroanatomical evidence suggests that the large cetacean brain supports a complex general intelligence,” Marino wrote, “perhaps driven by factors convergent with other socially complex mammals.”