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February 19, 2013

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Extra spatial abilities in males may be hormonal “side effect”

Feb. 19, 2013
Courtesy of the University of Illinois
and World Science staff

The slight su­pe­ri­or­ity of spa­tial abil­i­ties in av­er­age males com­pared to av­er­age fe­males is prob­ably just a “side ef­fect” of men’s high­er tes­tos­ter­one lev­els, a new study pro­poses.

Some ev­o­lu­tion­ary psy­chol­o­gists have ar­gued that ma­les’ slight, but sig­nif­i­cant, su­pe­ri­or­ity in spa­tial naviga­t­ion—a phe­nom­e­non shown re­peat­edly in many spe­cies, in­clud­ing hu­mans—is probably “adap­tive.” That means that over the course of ev­o­lu­tion the trait gave males an ad­van­tage that helped them have more chil­dren than their peers. As a re­sult the genes con­fer­ring the ad­van­tage, like most genes con­fer­ring a re­pro­duc­tive ad­van­tage, would spread fur­ther in the popula­t­ion.

But a new anal­y­sis pub­lished in The Quar­terly Re­view of Bi­ol­o­gy finds no ev­i­dence for this adap­tive­ness hy­poth­e­sis as re­gards male spa­tial abil­i­ties. 

The re­search­ers, led by Uni­vers­ity of Il­li­nois psy­chol­o­gist Jus­tin Rhodes, looked at 35 stud­ies that in­clud­ed da­ta about the ter­ri­to­rial ranges and spa­tial abil­i­ties of 11 spe­cies of an­i­mals, in­clud­ing peo­ple.

Rhodes and col­leagues found that in eight of these, males showed mod­er­ately high­er spa­tial skills, re­gard­less of the size of their ter­ri­to­ries or the ex­tent to which males ranged far­ther than fe­ma­les. The find­ings back up an often-o­verlooked hy­poth­e­sis, Rhodes said: the male ad­van­tage in spa­tial naviga­t­ion may just be a “side ef­fect” of the male sex hor­mone, tes­tos­ter­one. Pre­vi­ous stud­ies have shown that wom­en who take tes­tos­ter­one tend to see an im­prove­ment in their spa­tial naviga­t­ion skills, he not­ed.

Rhodes and col­leagues ob­ject in gen­er­al to what they call “crea­t­ion sto­ries” that seek to ex­plain sex­u­al phe­nom­e­na like the female or­gasm, rape or men­o­pause in terms of adap­tive­ness. Some ev­o­lu­tion­ary psy­chol­o­gists de­scribe rape, for ex­am­ple, as an al­ter­nate mat­ing strat­e­gy for males who oth­er­wise are re­pro­duc­tively un­suc­cess­ful. Oth­ers say men­o­pause evolved in wom­en to en­hance the sur­viv­al of their genes by in­creas­ing the time spent nur­tur­ing their grandchil­dren. Some of these ideas ap­peal to com­mon sense, Rhodes said, but they’re “gen­er­ally are not testable.”

Re­search­ers tend to overlook the fact that many phys­i­cal and be­hav­ior­al traits arise thanks to ran­dom events, or are simply side ef­fects of oth­er changes that of­fer real ev­o­lu­tion­ary ad­van­tages, he added.

“For ex­am­ple, wom­en have nip­ples be­cause it’s an adapta­t­ion; it pro­motes the sur­viv­al of their off­spring,” Rhodes said. “Men get it be­cause it does­n’t harm them. So if we see some­thing that’s ad­van­tageous for one sex, the oth­er sex will get it be­cause it’s in­her­it­ing the same genes—un­less it’s bad for that sex.”

Sim­i­lar­ly, sci­en­tists who claim that the dif­fer­ent spa­tial skills in men and wom­en are adaptive must ex­plain why wom­en failed to in­her­it the su­pe­ri­or spa­tial skills of their naviga­t­ionally en­hanced fa­thers, Rhodes said.

“The only way you will get a sex dif­fer­ence in an adaptive trait is where a trait is good for one sex and bad for the oth­er,” he said. “But how is naviga­t­ion bad for wom­en? This is a flaw in the log­ic.”

“When peo­ple hear ar­gu­ments made or sto­ries told, par­tic­u­larly about hu­man be­hav­iors be­ing prod­ucts of adapta­t­ion, I think they should ask the ques­tion: ‘Where is the ev­i­dence?’” Rhodes said. The re­search team al­so in­clud­ed a phi­los­o­pher from the Uni­vers­ity of Wis­con­sin at Mad­i­son and a sci­ent­ist from the Uni­vers­ity of Cal­i­for­nia at Riv­er­side.

The 11 animal species reviewed in the study were cut­tle­fish, deer mice, hors­es, hu­mans, lab­o­r­a­to­ry mice, mead­ow voles, pine voles, prai­rie voles, rats, rhe­sus macaques and talas­tuco­tucos, a type of bur­row­ing ro­dent. 

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The slight superiority of spatial abilities in average males compared to average females is probably just a “side effect” of men’s higher testosterone levels, a new study proposes. Some evolutionary psychologists have argued that the males’ slight, but significant, superiority in spatial navigation—a phenomenon shown repeatedly in many species, including humans—is probably “adaptive.” That means that over the course of evolution the trait gave males an advantage that helped them have more children than their peers. As a result the genes conferring the advantage, like most genes conferring a reproductive advantage, would spread further in the population. But a new analysis published in The Quarterly Review of Biology finds no evidence for this adaptiveness hypothesis as regards male spatial abilities. The researchers, led by University of Illinois psychologist Justin Rhodes, looked at 35 studies that included data about the territorial ranges and spatial abilities of 11 species of animals: cuttlefish, deer mice, horses, humans, laboratory mice, meadow voles, pine voles, prairie voles, rats, rhesus macaques and talastuco-tucos (a type of burrowing rodent). Rhodes and colleagues found that in eight out of 11 species, males showed moderately higher spatial skills, regardless of the size of their territories or the extent to which males ranged farther than females. The findings back up an often-overlooked hypothesis, Rhodes said: the male advantage in spatial navigation may just be a “side effect” of the male sex hormone, testosterone. Previous studies have shown that women who take testosterone tend to see an improvement in their spatial navigation skills, he noted. Rhodes and colleagues object in general to what they call “creation stories” that seek to explain sexual phenomena like the female orgasm, rape or menopause in terms of adaptiveness. Some evolutionary psychologists describe rape, for example, as an alternate mating strategy for males who otherwise are reproductively unsuccessful. Others say menopause evolved in women to enhance the survival of their genes by increasing the time spent nurturing their grandchildren. Some of these ideas appeal to common sense, Rhodes said, but they’re “generally are not testable.” Researchers tend to overlook the fact that many physical and behavioral traits arise thanks to random events, or are simply side effects of other changes that offer real evolutionary advantages, he added. “For example, women have nipples because it’s an adaptation; it promotes the survival of their offspring,” Rhodes said. “Men get it because it doesn’t harm them. So if we see something that’s advantageous for one sex, the other sex will get it because it’s inheriting the same genes—unless it’s bad for that sex.” Similarly, scientists who claim that the different spatial skills in men and women are adaptive must explain why women failed to inherit the superior spatial skills of their navigationally enhanced fathers, Rhodes said. “The only way you will get a sex difference (in an adaptive trait) is where a trait is good for one sex and bad for the other,” he said. “But how is navigation bad for women? This is a flaw in the logic.” “When people hear arguments made or stories told, particularly about human behaviors being products of adaptation, I think they should ask the question: ‘Where is the evidence?’” Rhodes said. The research team also included a philosopher from the University of Wisconsin at Madison and a scientist from the University of California at Riverside.