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The evolution of animal personalities

May 30, 2007
Courtesy Santa Fe Institute
and World Science staff

An­i­mals dif­fer strik­ingly in char­ac­ter and tem­per­a­ment, bi­ol­o­gists say, but just re­cently has it be­come clear that per­son­al­i­ties are a wide­spread phe­nom­e­non in the an­i­mal king­dom.

A bird in which per­son­al­i­ty dif­fer­ences have been stud­ied, the Car­o­li­na Chick­a­dee. Au­thors of a re­port in the April Jour­nal of Gen­er­al Psy­chol­o­gy found that in­di­vid­u­al chick­adees dis­play "strong be­hav­ior­al con­sist­en­cy de­spite... ma­jor change[s] in so­cial context," a sign of in­di­vid­u­al per­son­al­i­ties. (Im­age cour­te­sy U.S. Na­tion­al In­sti­tutes of Health)


Per­son­al­ity dif­ferences have been do­c­u­mented in more than 60 spe­cies in­clud­ing pri­ma­tes, ro­dents, birds, fish, in­sects and squid, said re­search­ers who pre­sented a study on the sub­ject in the May 31 is­sue of the re­search jour­nal Na­ture.

In the work, Max Wolf of the Un­ivers­ity of Gro­nin­gen, The Ne­ther­lands, and col­leagues of­fered an ev­o­lu­tion­ary ex­plana­t­ion for the or­i­gin of an­i­mal per­son­al­i­ties, de­fined as con­sist­ent be­hav­ior over time and in dif­ferent situa­t­ions.

The is­sue is poorly un­der­stood, ac­cord­ing to the group. 

Under ev­o­lu­tion­ary the­ory, a popula­t­ion’s gene pool changes as more suc­cess­ful in­di­vid­u­als re­pro­duce more, and thus spread their genes—while poorly adapted in­di­vid­u­als’ genes tend to die out. These con­si­d­er­a­tions raise three ques­tions, ac­cord­ing to Wolf and col­leagues:
  • Why do dif­ferent per­son­al­ity types ex­ist with­in one popula­t­ion when, at first sight, one would ex­pect one type to be more suc­cess­ful than an­oth­er?

  • Why are in­di­vid­u­als not more flex­i­ble con­sid­er­ing that per­son­al­ity rig­id­ity some­times leads to seem­ingly in­ef­fi­cient be­hav­ior? 

  • Why do we find the same types of traits cor­re­lat­ed with each oth­er in very dif­ferent kinds of an­i­mals?

The au­thors be­gan with two ob­serva­t­ions. First, per­son­al­ity varia­t­ions of­ten in­volve bas­ic dif­ferences in overall will­ing­ness to take risks. Sec­ond, in­di­vid­u­als are of­ten con­fronted with a trade-off be­tween cur­rent and fu­ture re­pro­duc­tion: the more one in­vests in cur­rent re­pro­duc­tion, the less re­sources re­main for fu­ture op­por­tun­i­ties, and vi­ce versa. 

This led the au­thors to ar­gue that in many cases a sim­ple un­der­ly­ing prin­ci­ple shapes per­son­al­i­ties: the more an in­di­vid­ual stands to lose in terms of fu­ture re­pro­duc­tion, the more cau­tious it’s likely to be, over time and all kinds of situa­t­ions.

Us­ing a math­e­mat­i­cal mod­el, the re­search­ers found that this fun­da­men­tal trade-off can give rise to popula­t­ions where some in­di­vid­u­als put more em­pha­sis on fu­ture re­pro­duc­tion than oth­ers. Those who in­vest in fu­ture re­pro­duc­tion evolve to be con­sist­ently risk-averse in dif­ferent con­texts, such as en­coun­ters with preda­tors and ag­gres­sive in­ter­ac­tions. Those who fo­cus on cur­rent re­pro­duc­tive suc­cess are more risk-prone, ac­cord­ing to Wolf, who is cur­rently at the Santa Fe In­sti­tute in New Mex­ico.


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Animals differ strikingly in character and temperament. Yet just recently has it become clear that personalities are a widespread phenomenon in the animal kingdom, biologists say. Personality differences have been described in more than 60 species including primates, rodents, birds, fish, insects and mollusks, said researchers who presented a study on the subject in the May 31 issue of the research journal Nature. In the work, Max Wolf of the University of Groeningen, Germany, and colleagues offered an evolutionary explanation for the origin of animal personalities, defined as consistent behavior over time and in different situations. The issue is poorly understood, according to the group. Evolution is the principle that a population’s gene pool changes as more successful individuals reproduce more, and thus spread their genes through the group—while poorly adapted individuals’ genes tend to die out. This raises three questions, according to Wolf and colleagues: Why do different personality types exist within one population when, at first sight, one would expect one type to be more successful than another? Why are individuals not more flexible considering that personality rigidity sometimes leads to seemingly inefficient behavior? Why do we find the same types of traits correlated with each other in very different kinds of animals? The authors began with two observations. First, personality variations often involve basic differences in overall willingness to take risks. Second, individuals are often confronted with a trade-off between current and future reproduction: the more one invests in current reproduction, the less resources remain for future opportunities, and vice versa. This led the authors to argue that in many cases a simple underlying principle shapes personalities: the more an individual stands to lose in terms of future reproduction, the more cautious it’s likely to be, over time and all kinds of situations. Using a mathematical model, the researchers found that this fundamental trade-off can give rise to populations where some individuals put more emphasis on future reproduction than others. Those who invest in future reproduction evolve to be consistently risk-averse in different contexts, such as encounters with predators and aggressive interactions. Individuals who focus on current reproductive success are more risk-prone.