Dolphin braininess due to social life, studies suggest

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.)


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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." 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.


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 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.


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. The study did­n't di­rectly ad­dress
the cold-wa­ter the­o­ry, it 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.

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 is that dol­phin brain ar­eas de­vot­ed to sound
pro­cess­ing-though rather large-are too lim­it­ed in size to ac­count for
the over­all size in­crease. 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," This could have been
driv­en by fac­tors paral­lel­ing those that pushed the evo­lu­tion of
oth­er "so­cially com­plex mam­mals."


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.



Their sit­u­a­tion drives a "need to de­vel­op so­cial strate­gies
in­volv­ing the rec­og­ni­tion of a large num­ber of in­di­vid­u­als and
their re­la­tion­ships with oth­ers . "All three 'peaks' of large brain size
ev­o­lu­tion in mam­mals," -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.