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•parę capnctry ta che system Bcth ktads o# eapionacłons sufler bon (be psoWun that they iptere a tssbnmid prłnr i; 4e of raniidlnmy dusry. sMdk is that cudsUn »the outeome <uf the blance bttsero oosa and benefat Because the cost od mata-

that ensures nśhcleat energy to pro-vlde for feta! brata deseSopment. No such aflometric argument can eter taapiy that you han to endie a large brata or a large body. The large brata or brata past Is a cost that anbnaJ* muv factor tato their caicułacton* when consfcłerlng whether or not a large xidyora bratał* a terrible soiutlon to a partttuiar ecołogłcal problem. Słdfls to morę energy-rkłi or mcee easfiy procesaed dkts may be rssr nclal precurtor* of signifteant łn-crtases in brata or brata part ta*.² Tkds woukł espłata why frugfrores han łarger brata* dian fołteores do and why hormnlds harve łarger brain* than great apes do.

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ta body da tkań do foiharous pri-■*W Th» ha* been taaerpreted as wr^Ayrr^t thar fiugiens haw arłcher flht drnn fahwu do and das han spasie anrp to dhtrt tato detal psstk Large brata* are chus seen as

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ItartMdeafly uuflheiy	that large brains
taB ewiue otareiy because they can.
Large brata* nfli eeol	taesdy taben the
•deata factor in fh	eir fawor is stdfl'
dtst 10 escscosse tfae	steep cost grach
eta. Dtadopsusula	wrratats arr un
doubtad^y bnponant.	, but rashsr than
betag causa! daefr r	oie is that of a
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the MBgeOc apuena do noc add up: Pmecocfai anmadi do not hame bógher mecabobc raoes tfaon do dtrt-doi ma—ndt deapfte do fact that tbey barn iwommI brała dn that ant. on amagr Mu aa large. We Iłimfae do not need to coodder «ś-*^fc"^r*r^Jr^{<l 1}" ameoefordewelopenental bjfodieKł any fcrdtr ta da Gonćezt dttaatide.

trafna on brata i noc tcfl us why *ed as they dkL They may tdl us that if you mnt to ewbe a large brata, tben you mutr evcfve a large body ta order to carry theenergedccosaof dotagsooratBet

Because the cost of malntammg a large brain (s so great rt is rrtrinsica&y unfikely that large brains will evołve merely because they can. Large brains wiB evołve onły when the setection factor in their favor ts sufficient to overcome the steep cost gradient

Ttós leases us wtdi Juot tao ctasses of hypodmo. de eooloped and de soda! At kast three werdons of dn former can be łdeodfled. nfciefa I nfli term tfae dktay mental mapa, and ntrwlw loragtag hypodmę* in es-sence. theae arpe. respeedtafty. that prlmate *pede» sHfl need łarger brains d (9 tbey are frughorous because irultt are mon ephemerd and paschy ta ilu S iMuCwdliei rtiei h nu erc ind hence resjuire mon memory to ftnd;

09 tbey haM łarger ranga because of the greater memory rapdremena of large seta menral map*; or 6nj their dkt mpdres them to esiract resonroe* tram a matrta ta wbich they are ero-bedded fe.g.„ they must runów findt ptdp fnom a cane. sclnadase gum Iow kom a tree. eztract terautes kom a termitariutn. ar bunt specles that are crypticorbełiaee eva»»Mly).

For oiwtou* reason*. I used tbe per-centageoffhdtintbedtetasanappfo-prłate indez for tfae dktary hypothesis.

I used the stze of the rangę area and the length of the dayJoumey a* akema-the tadlces for the mental mapptag hypothesis. though 1 present die data onfy for the fint of these here. The fint łndeecorresponds to the caseśn wbich antraałs have tobeable to mantpułate Information abouf the łocation* of r sources reiatńe to themsehes in a FurHdean space (an eatampk amid be the nut-cracking acdelths of the Tai chfmpanzees¹*): the seoond corrc-sponds to the posstaflhy that the cór*' stratat Bes in the needs of some aspea of merdał nayigation. The eatraethe foraging hypothesk* is less easy to char-actertze in quandtathe term* because there is no objedbe measure ci the degree to wbich dka vary in their eattracthencjs. Howewr. Gibson* pro-vided a cłassification of primate spe-des tato four categories of diet that diflier in their degree of estraedtaneas. We can test thb hypothesis by asktag whether there h a consistent sariation in brain stze among these four categories. with the apedes having the morę estractlue dka hasing łarger brata* than those wkh the less eatraethe dkts.

FinaDy. we need an index of social compkaity. In my ortginal anałyses. I used *octal group słze as a staipłe measure of sochd compkacity. Akhough at beat racher crude, this measure nonethdeui captures one aspea of the