Current Biology Vol 14 No 21R910

Quick guide

Dolphin cognitionLori Marino

What is a dolphin? When youthink of a dolphin you probablypicture a bottlenose dolphin,Tursiops truncatus, the familiar starof oceanaria. But bottlenosedolphins are only one among sixty-seven living species in the fullyaquatic mammalian order Cetaceaand the diverse suborderOdontoceti (‘toothed whales’). Inaddition to possessing adult teeth,the many types of toothed whaleare distinguished from their baleen-bearing relatives, the suborderMysticeti, by the use ofecholocation to perceive theirenvironment. One of themisconceptions about dolphinsthat held sway until studies in the1980s and 90s was that they areauditory specialists with poorvision. But we now know that —with the exception of the nearlyblind river dolphins — bottlenosedolphins and other toothed whalesare highly multimodal animals,easily integrating information abouttheir environment from audition andvision, and probably also othersenses such as touch.

Dolphins are brainiacs. Dolphinbrains are organized differentlyfrom human brains, but the highlyconvoluted brains of manydolphins and other odontocetesare also significantly larger than thehuman brain. The average humanbrain weighs about 1.3 kg, but thebottlenose dolphin brain, averaging1.7 kg, is about 25% heavier.Furthermore, the largest brain inthe animal kingdom, at a hefty7.8 kg, is possessed by yet anotherodontocete — the sperm whale.The larger brains of dolphins andwhales might be due to their largerbodies. So, a more meaningfulmeasure is to compare actual brainsize with that expected for thespecies’ body size. This metric isknown as encephalization quotient(EQ): brains with EQs larger than 1are larger than the expected size,while those with EQs less than 1

are smaller than the expected size.Modern humans have the highestEQ: about 7, so our brains areabout 7 times the size one wouldexpect for an animal of our bodysize. But many dolphins possessEQs in the 4–5 range, tantalizinglyclose to the modern human level,and significantly higher than allother animals. Like humans,dolphins are, without a doubt,brainiacs of the animal kingdom.

What is dolphin cognition? Theword ‘cognition’ comes from theLatin root ‘cognoscere’ meaning ‘tocome to know’ and refers to all themental activities associated withthinking, knowing andremembering. So dolphin cognitionrefers to how dolphins think, know,and remember. The overwhelmingmajority of experimental researchon dolphin cognition has beenbased on the adaptable, highlysocial bottlenose dolphin, with asmaller subset of studies on thekiller whale, the beluga whale and atiny proportion of other odontocetespecies. These studiesdemonstrate that the dolphin’s highEQ is matched by its sharpcognitive abilities.

Our cognitive cousins. Theeminent dolphin researcher LouisHerman coined the term ‘cognitivecousins’ to refer to the fact thatbottlenose dolphin cognitionappears to be at a level also typicalof great apes and humans. Thissimilarity may be surprising, asprimates and dolphins are only verydistantly related. But the twogroups share other characteristicsindicative of complex intelligenceand cognitive ability, especiallyhigh encephalization levels, longjuvenile periods and complicatedsocial lives.

Controlled experimental studieson bottlenose dolphin cognitionhave been carried out in thedomains of memory, conceptualprocesses, vocal and motormimicry, behavioral innovation,‘language’ understanding, mentalrepresentation and self-awareness.Not surprisingly, bottlenosedolphins have exceptional short-term and long-term memory forvisual, auditory and multimodalinformation, as well as abstractconcepts.

Bottlenose dolphins exhibit alevel of behavioral flexibility,innovation and imitative-ability thatis rare, if not unique, amongnonhuman animals. This meansthat dolphins are able to mentallyrepresent and form analogiesbetween their own body and that ofanother individual — evenimprovising when that otherindividual doesn’t look much like adolphin! These propensities ofdolphins undoubtedly contributemuch to the human passion forinteracting with them.

The bottlenose dolphin is oneof a very limited group ofnonhuman species — great apesand parrots are the others — thathave demonstrated compellingcapacities to understand a rule-based symbolic artificial‘language’. Louis Herman and hiscolleagues have shown thatdolphins are capable ofsemantics (comprehending visualand auditory symbols as ‘words’)and syntax (understanding thatchanges in word order changethe meaning of a sentence).Dolphins even understandsymbolic references to objectsthat are absent. Out of sight —not out of mind!

Finally, one of the most intriguingdomains of research on dolphincognition is that of self-awareness.Self-awareness is the ability tocomprehend and think aboutoneself in the physical and mentalrealms. Although relativelyunstudied, the few experiments inthis realm show that dolphins havecapabilities that are exceedinglyrare in the animal kingdom.Bottlenose dolphins have beenshown to be aware of their ownbehaviors and body parts and theirown levels of subjective uncertaintyduring a difficult memory test. In

Figure 1. A bottlenose dolphin examininga mark on his head in a mirror.

2001, my colleague Diana Reissand I reported conclusive evidencethat — along with only great apesand humans — bottlenose dolphinsare capable of recognizingthemselves in a mirror and using amirror to investigate their ownbodies (Figure 1). This intriguingline of research awaits much moreextensive exploration.

The dolphin mystique.Throughout the ages, an enduringmystique has developed arounddolphins. Even today somepeople continue to imputedolphins with mystical abilitiessuch as extra-sensory perceptionand, in alternative medicinecircles, special healing powers.An entire industry in ‘dolphin-assisted therapy’ has beenfounded on the idea that dolphinshave the capacity to healillnesses through the use of theirsonar or by touch. There is noscientific support for any of theseoutlandish claims. But ourappreciation of dolphins does notneed to depend on their havingsupernatural powers. What weknow about dolphin cognitionfrom scientific research isimmensely more exciting than anymyths could be. Scientificresearch continues to add to ourfactual understanding of dolphinsas highly intelligent, complex andcommunicative animals that arevery different but intriguinglysimilar to ourselves.

Where do I find out more?Herman, L.M. (2002). Exploring the

cognitive world of the bottlenosedolphin. In: The Cognitive Animal, M.Bekoff, C. Allen, and G.M.Burghardt, ed. (Cambridge, Mass.:MIT Press), pp 275–283.

Marino, L. (2002). Convergence ofcomplex cognitive abilities incetaceans and primates. BrainBehav. Evol. 59, 21–32.

Reiss, D., and Marino, L. (2001). Mirrorself-recognition in the bottlenosedolphin: A case of cognitiveconvergence. Proc. Natl. Acad. Sci.USA 98, 5937–5942.

Schusterman, R.J., Thomas, J.A., andWood, F.G. (1986). DolphinCognition and Behavior: AComparative Approach. (Hillsdale,N.J.: Lawrence Erlbaum Assoc).

Neuroscience and Behavioral BiologyProgram, Emory University, Atlanta,Georgia 30322, USA. E-mail: lmarino@emory.edu

MagazineR911

While trees can often seemdwarfed in significance by theirsurrounding human artefacts, inmore rural areas they canacquire much more localprominence and, because of alack of development, some canbe of significant historical,cultural or ecological interest.

A new book, published lastmonth, highlights many trees ofoutstanding significance inBritain ranging from the originalBramley apple tree in a privatevillage garden inNottinghamshire, the chestnuttree planted by David Razzio asa mark of his love for MaryQueen of Scots to the apple treethat is thought to haveprompted Isaac Newton’s workon gravity.

But the book has anotherpurpose: to back the TreeCouncil’s campaign for thepreservation and maintenanceof the country’s ‘greenmonuments’ — the name theygive to the trees linked withsignificant events in history. Atpresent the protection of valuedtrees is divided between

different governmentdepartments but all too oftenthey have no protection at all.The fate of rural trees largelylies with landowners whoincreasingly may not have theresources to maintain heritagetrees. “Astonishingly, manysuch trees are unprotected bylaw and could be felledtomorrow without contraveningany regulations,” says PaulineBuchanan Black, Director-General of the Tree Council.

The Tree Council’s greenmonument campaign thereforeaims to raise funds to helplandowners to identify heritagetrees and also to providefencing and signage to protectthem. They also wish to be ableto provide the best informationfor landowners on maintainingthem.

As Britain is one of theEuropean countries with thelowest level of tree cover,protection of some keyspecimens linked to thecountry’s cultural and scientifichistory can only help furtherwoodland interests.

Branch support

Fruitful insights: a falling apple from this tree is credited with triggering IsaacNewton’s s thinking about gravity. (Photo: The Heritage Trees of Britain andNorthern Ireland, published by Constable and Robinson, London.)