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June 03, 2013

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“Old boys’ network” seen in monkeys

July 16, 2012
Courtesy of UCLA
and World Science staff

If you’re a male born to a fa­ther who’s a strong and en­dur­ing com­mun­ity lead­er, you’re more likely to be­come a lead­er your­self, due to the so­cial ad­van­tages ac­cru­ing from your dad’s po­si­tion. And even if your old man is­n’t a lead­er, oth­er men in your com­mun­ity may be more likely to take you un­der their wing than your sis­ters, lav­ish­ing at­ten­tion on you and show­ing you the ropes.

New research ex­plores the so­cial dy­nam­ics of cap­u­chins, who form co­op­er­a­tive groups with an al­pha ma­le, sev­er­al sub­or­di­nate males and ma­ny fema­les. The al­pha males rise to their po­si­tion — and de­fend it — by fight­ing off oth­er males with the help of al­lies. (Image cour­tesy U.S. Fish & Wild­life Svc.)


Sound like the de­scrip­tion of an old boys’ net­work?

May­be so, but it’s al­so the so­cial struc­ture that pre­vails among white-faced cap­u­chin mon­keys, those cute lit­tle New World pri­ma­tes as­so­ci­at­ed in pop­u­lar cul­ture with or­gan grinders, says Uni­vers­ity of Cal­i­for­nia Los An­ge­les an­thro­po­l­o­gist Su­san Per­ry. Her new work of­fers a glimpse in­to how our male an­ces­tors may have jock­eyed for pow­er and passed it on to their male off­spring.

“Off­spring, es­pe­cially male off­spring, raised in a group in which their fa­ther is the al­pha [dom­i­nant] male, through­out their ju­ve­nile phase en­joy a host of ad­van­tages over less for­tu­nate mon­keys,” said Per­ry, who has stud­ied cap­u­chins for 22 years. “A sta­ble, peace­ful family en­vi­ron­ment may have been im­por­tant to the well-be­ing and fu­ture suc­cess of chil­dren among our re­mote an­ces­tors, just as it is to chil­dren to­day.”

Per­ry re­ports her find­ings in the cur­rent is­sue of the re­search jour­nal Ad­vanc­es in the Study of Be­hav­ior. 

Widely known for their clev­er­ness, dex­ter­ity and train­abil­ity, cap­u­chins en­joy a dis­tinc­tion that makes them es­pe­cially com­pel­ling to schol­ars of the ev­o­lu­tion of be­hav­ior. The cat-sized pri­ma­tes have the larg­est brains for their body size among close evo­lu­tion­ary rel­a­tives of humans, or pri­ma­tes. That makes their be­hav­ior of in­ter­est for un­der­stand­ing the ev­o­lu­tion­ary his­to­ry of their big-brained rel­a­tives — us.

“There are a lot of rea­sons to sus­pect that the same se­lec­tive forces,” or ev­o­lu­tion­ary pres­sures, “that shaped huma­ns al­so shaped cap­u­chins,” Per­ry said. This could have caused “both spe­cies to share fea­tures such as com­plex po­lit­i­cal be­hav­ior and cul­tur­ally trans­mit­ted so­cial rit­u­als.”

Since 1990, Per­ry; her hus­band, Jo­seph Man­son, al­so an an­thro­po­l­o­gist at the school; a Uni­vers­ity of Io­wa fac­ul­ty mem­ber; and 122 stu­dents, vol­un­teers and co­pi­ously trained lo­cals have spent about 79,000 hours watch­ing 444 cap­u­chins that make up 11 so­cial groups in Cos­ta Ri­ca’s Lo­mas Bar­bu­dal Bi­o­log­i­cal Re­serve. With da­ta on five genera­t­ions of these an­i­mals, which can live in­to their 50s, Per­ry’s Lo­mas Bar­bu­dal Mon­key Proj­ect is the world’s most de­tailed study of any nat­u­ral pri­mate popula­t­ion, the sci­en­tists said.

Per­ry’s lat­est find­ings ex­plore the so­cial dy­nam­ics of cap­u­chins, who form co­op­er­a­tive groups with an al­pha ma­le, sev­er­al sub­or­di­nate males and ma­ny fema­les. The av­er­age size of these groups in the Lo­mas re­serve is 19 mon­keys, Per­ry has found. The al­pha males rise to their po­si­tion — and de­fend it — by fight­ing off oth­er males with the help of al­lies.

In­i­tial­ly, females mate ex­clu­sively with the al­pha ma­le, Per­ry said. Ex­tra­or­di­nar­i­ly, sub­or­di­nate males wait to mate un­til the al­pha ma­le’s daugh­ters reach sex­u­al matur­ity, a pro­cess that takes an av­er­age of six years. The regimes of most al­pha males last about a year, but regimes at Lo­mas have lasted for as long as 18 years and for as lit­tle as a few hours.

Fol­low­ing a chang­ing of the guard, the new al­pha male im­me­di­ately kills all in­fants that have not been weaned, Per­ry has doc­u­mented. “It’s chaos,” she said. In fact, in­fanti­cide in this ma­nner is the lead­ing cause of death for in­fant cap­u­chins. With­out nurs­ing in­fants to care for, the females in the overthrown group quickly re­turn to a fer­tile state and eventually come around to mate with the new al­pha male.

“A ma­raud­ing male does­n’t know how long he’s go­ing to be in the al­pha po­si­tion, so he really needs to hur­ry up and start pro­duc­ing ba­bies if he is go­ing to have any hope of pro­tect­ing them long enough to see them in­to adult­hood,” she said.

As de­struc­tive as a chang­ing of the guard is for young cap­u­chins, a sta­ble home life is equally as ben­e­fi­cial, Per­ry re­ports. Re­sults from Lo­mas in­di­cate that 63 per­cent of males who grow up in groups that are de­mo­graph­ic­ally sta­ble be­cause of a long-en­dur­ing al­pha male be­come al­pha males when they leave the pro­verbial nest, Per­ry re­ports. By con­trast, only 11 per­cent of males from groups that ex­pe­ri­enced an al­pha male turno­ver in the first five years of their lives go on to be­come al­pha ma­les.

“S­ince al­pha males are re­spon­si­ble for sir­ing the ma­jor­ity of off­spring, gain­ing the lead­ership spot is very im­por­tant for re­pro­duc­tive suc­cess,” Per­ry said. “Most like­ly, none of their genes will be passed to the next genera­t­ion if they do not be­come an al­pha ma­le.”

The ad­van­tages that cap­u­chins get from be­ing reared in sta­ble groups may help ac­count for their suc­cess lat­er in life, Per­ry said. “If an al­pha male ma­nages to en­dure as the head of a group, more in­fants will sur­vive,” she said. “That means there will be more play part­ners for all of the kids who grow up in the group. They will have bet­ter op­por­tun­i­ties to de­vel­op their so­cial skills.”

Hon­ing so­cial skills is im­por­tant for cap­u­chins be­cause, as with huma­ns, long-term al­pha males re­main in pow­er not so much be­cause of their fight­ing abil­ity but be­cause of their abil­ity to so­cially ma­ni­pu­late oth­ers and ma­nage their al­lies, she said. The bet­ter out­comes for cap­u­chins from sta­ble groups may al­so have some­thing to do with be­ing spared the stress of a takeo­ver dur­ing the early phase of their de­vel­opment, when a lot of brain and body sys­tems are be­ing de­vel­oped, she added.

Com­pared with their peers from un­sta­ble back­grounds, males who spent their first five years of life in a group with a sta­ble al­pha male seem to en­joy anoth­er im­por­tant ad­van­tage: They strike out on their own — or in pairs or small groups — much lat­er in life. On av­er­age, the males from sta­ble back­grounds spent close to two years long­er in the groups in which they were raised than their peers from un­sta­ble back­grounds, Per­ry found.

“These males have a bet­ter chance of sur­viv­ing to migra­t­ion age and they have more choices of those to mi­grate with, so they’re much bet­ter pre­pared for adult life,” she said.

Their ad­van­tages al­low them to sire more prog­e­ny, which over time has a tre­men­dous im­pact. “Males from sta­ble back­grounds stand a bet­ter chance of be­com­ing among the few males that are ex­tra­or­di­narily suc­cessful in repro­duc­ing,” Per­ry ex­plained.

Males reared in groups un­der the lead­ership of en­dur­ing al­pha males are not the only ben­e­fi­ci­aries of the old boys’ net­work, Per­ry has found. In a study of male and female in­fants in the first year of life at Lo­mas, all male in­fants en­joyed con­si­der­able ad­van­tages over their sis­ters, she re­ports. Adult males showed a three-fold pref­er­ence for groom­ing male in­fants — a key mark­er of af­filia­t­ion — over female in­fants.

“A­dult males are only in­ter­ested in the male ba­bies, not the female ba­bies, where­as the female cap­u­chins are in­ter­ested in ba­bies of both sex­es,” she said.

In de­vel­oping rela­t­ion­ships with the male in­fants and young ma­les, sub­or­di­nate males take as ac­tive a role as the al­pha ma­les, Per­ry has found. “Sub­or­di­nate males are very rarely the fa­thers of the young ma­les, but they play with them a lot,” she said. “They’re really good baby-sitters.” But more than child’s play is at work in these ex­changes, she added. “The adult males are es­tab­lish­ing rela­t­ion­ships with young males who can lat­er be­come use­ful al­lies in at­tempts to ac­quire and de­fend breed­ing po­si­tions.”


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If you’re a male born to a father who’s a strong and enduring community leader, you’re far more likely to become a leader yourself, due to the social advantages accruing from your dad’s position. And even if your old man isn’t a leader, other men in your community are more likely to take you under their wing than your sisters, lavishing attention on you and showing you the ropes. Sound like the basic description of an old boys’ network? Maybe so, but it’s also the social structure that prevails among white-faced capuchin monkeys, those cute little New World primates associated in popular culture with organ grinders, said University of California Los Angeles anthropologist Susan Perry. Her new work offers a glimpse into how our male ancestors may have jockeyed for power and passed it on to their male offspring. “Offspring, especially male offspring, raised in a group in which their father is the alpha male, throughout their juvenile phase enjoy a host of advantages over less fortunate monkeys,” said Perry, who has studied capuchins for 22 years. “A stable, peaceful family environment may have been important to the well-being and future success of children among our remote ancestors, just as it is to children today.” Perry reports her findings in the current issue of the research journal Advances in the Study of Behavior. Widely known for their cleverness, dexterity and trainability, capuchins enjoy a distinction that makes them especially compelling to scholars of the evolution of behavior. The cat-sized primates have the largest brains for their body size among non-human primates, making their behavior of interest for understanding the evolutionary history of their big-brained relatives — us. “There are a lot of reasons to suspect that the same selective forces,” or evolutionary pressures, “that shaped humans also shaped capuchins,” Perry said. This could have caused “both species to share features such as complex political behavior and culturally transmitted social rituals.” Since 1990, Perry; her husband, Joseph Manson, also an anthropologist at the school; a University of Iowa faculty member; and 122 students, volunteers and copiously trained locals have spent about 79,000 hours watching 444 capuchins that make up 11 social groups in Costa Rica’s Lomas Barbudal Biological Reserve. With data on five generations of these animals, which can live into their 50s, Perry’s Lomas Barbudal Monkey Project is the world’s most detailed study of any natural primate population, the scientists said. Perry’s latest findings explore the social dynamics of capuchins, who form cooperative groups with an alpha male, several subordinate males and many females. The average size of these groups in the Lomas reserve is 19 monkeys, Perry has found. The alpha males rise to their position — and defend it — by fighting off other males with the help of allies. Initially, females mate exclusively with the alpha male, Perry said. Extraordinarily, subordinate males wait to mate until the alpha male’s daughters reach sexual maturity, a process that takes an average of six years. The regimes of most alpha males last about a year, but regimes at Lomas have lasted for as long as 18 years and for as little as a fraction of a day. Following a changing of the guard, the new alpha male immediately kills all infants that have not been weaned, Perry has documented. “It’s chaos,” she said. In fact, infanticide in this manner is the leading cause of death for infant capuchins. Without nursing infants to care for, the females in the overthrown group quickly return to a fertile state and eventually come around to mate with the new alpha male. “A marauding male doesn’t know how long he’s going to be in the alpha position, so he really needs to hurry up and start producing babies if he is going to have any hope of protecting them long enough to see them into adulthood,” she said. As destructive as a changing of the guard is for young capuchins, a stable home life is equally as beneficial, Perry reports. Results from Lomas indicate that 63 percent of males who grow up in groups that are demographically stable because of a long-enduring alpha male become alpha males when they leave the proverbial nest, Perry reports. By contrast, only 11 percent of males from groups that experienced an alpha male turnover in the first five years of their lives go on to become alpha males. “Since alpha males are responsible for siring the majority of offspring, gaining the leadership spot is very important for reproductive success,” Perry said. “Most likely, none of their genes will be passed to the next generation if they do not become an alpha male.” The advantages that capuchins get from being reared in stable groups may help account for their success later in life, Perry said. “If an alpha male manages to endure as the head of a group, more infants will survive,” she said. “That means there will be more play partners for all of the kids who grow up in the group. They will have better opportunities to develop their social skills.” Honing social skills is important for capuchins because, as with humans, long-term alpha males remain in power not so much because of their fighting ability but because of their ability to socially manipulate others and manage their allies, she said. The better outcomes for capuchins from stable groups may also have something to do with being spared the stress of a takeover during the early phase of their development, when a lot of brain and body systems are being developed, she added. Compared with their peers from unstable backgrounds, males who spent their first five years of life in a group with a stable alpha male seem to enjoy another important advantage: They strike out on their own — or in pairs or small groups — much later in life. On average, the males from stable backgrounds spent close to two years longer in the groups in which they were raised than their peers from unstable backgrounds, Perry found. “These males have a better chance of surviving to migration age and they have more choices of those to migrate with, so they’re much better prepared for adult life,” she said. Their advantages allow them to sire more progeny, which over time has a tremendous impact. “Males from stable backgrounds stand a better chance of becoming among the few males that are extraordinarily successful in reproducing,” Perry explained. Males reared in groups under the leadership of enduring alpha males are not the only beneficiaries of the old boys’ network, Perry has found. In a study of male and female infants in the first year of life at Lomas, all male infants enjoyed considerable advantages over their sisters, she reports. Adult males showed a three-fold preference for grooming male infants — a key marker of affiliation — over female infants. “Adult males are only interested in the male babies, not the female babies, whereas the female capuchins are interested in babies of both sexes,” she said. In developing relationships with the male infants and young males, subordinate males take as active a role as the alpha males, Perry has found. “Subordinate males are very rarely the fathers of the young males, but they play with them a lot,” she said. “They’re really good baby-sitters.” But more than child’s play is at work in these exchanges, she added. “The adult males are establishing relationships with young males who can later become useful allies in attempts to acquire and defend breeding positions.”