Oh, what an unmitigated horror puberty was. For girls, therewas the inevitable first menstruation that came right in the middle of gymclass: the bloody shorts, the panic--no one had told you you’d bleed thatmuch--and the conviction that you were about to die of either embarrassment orhemorrhage. Training bras, pubic hair, acne; there were no end of disquietingchanges.
For boys, there was the first wet dream, inevitably occurring at sleep-awaycamp--sneaking off to the woods to bury your disgusting soiled pajamas--thevisible erection with a mind of its own in the swimming trunks, the voice thatcracked whenever you spoke to someone of the opposite sex. Humiliation,agonies, pimply insecurities; you wondered if life would ever return to normal,and the answer, you knew, was no.
Most of all, there was the heartfelt prayer of every pubescent child: “Please,please, if I have to go through this, don’t let it happen to me one minutebefore or after it happens to everyone else. Please don’t let me be different.”
For the peer-pressured adolescent, the timing of puberty is everything. Indeed,for all mammals going about the business of being fit enough to survive andreproduce, the timing of puberty is pretty important. But some mammals dosomething about it. As social or environmental conditions shift--when there’s achange in the availability of food, for example, or in the number of animalscompeting for mates--many animals find that it sometimes pays to reach pubertyat an earlier age, and it’s sometimes better to delay the process.
Now there’s evidence that humans, too, manipulate the timing of puberty’sonset. In a controversial paper published last fall, a team of researchersproposed that girls reach puberty at an earlier age if they are raised in homesfilled with parental strife or where the father is absent because of divorce orabandonment.
The notion that our bodies can engage in such weighty deliberation shouldn’t besurprising. After all, we’re comfortable with the idea that some of our lives’landmark events are decided strategically. We do it constantly. For example: “Oh,let’s not. What a terrible time this would be for me to get pregnant--you’reout of work, my dissertation research is bogged down, the Nazis are marching onParis, and we’llhave to flee.” That sort of thing.
But such strategies are conscious. It’s harder to think about the timing ofpuberty as a strategy. We do not think, “The time is ripe--I’ve decided tostart ovulating,” any more than a bamboo plant decides, “The rainfall’s beenfabulous--I think I’ll start flowering,” or a deer decides, “Ah, spring is inthe air--time to grow me some antlers.” The “thinking” and “deciding” do notreally occur. Instead, a more complex evolutionary process is taking place:organisms that have the biological means to time an important life-historyevent (such as puberty or flowering or growing antlers) so that it occurs whenthe environment is best suited for it have an advantage over organisms thatcannot do so; therefore more of them will survive, more will reproduce, andmore will leave copies of their genes to future generations. And thus theiradaptive timing mechanism will become more common in their species over themillennia.
When does it best make sense to go through puberty? Well, to start with,puberty is an energetically costly experience, so it probably makes sense to gothrough it early only if you’ve been fed well and you’re healthy and can affordthe metabolic expense. It wouldn’t hurt to make sure that there is someoneattractive around who is available to mate with, and it also helps if theenvironment is conducive to the survival of any children you may have. When isit logical to defer puberty? If things are bad enough that you need your energyto survive instead of to reproduce, if it makes sense to still be taken care ofby Mom instead of becoming a mom yourself, if there’s no one around to matewith except close relatives.
Take, as an example, a female prepubescent mouse left on her own. Eventually,of course, she will reach puberty. But put an adult male into the cage with herand she will reach puberty earlier. This intriguing phenomenon, known as theVandenbergh effect, is produced by pheromones-- chemicals that cause abehavioral response in other animals of the same species--in the male’s urine.These pheromones, when detected by a sensing organ in the roof of the female’smouth, crank up her ovulatory machinery. The male’s urine does this trick evenwhen the male isn’t present--just dab a smidgen of male urine on females andthey reach puberty earlier. Moreover, the more male sexual hormones the mousehas in his bloodstream, the more effective he is at accelerating the onset ofpuberty.
Conversely, when a prepubescent female mouse is exposed to a large number ofadult females, puberty is delayed. Once again the signal is a pheromone, but inthis case it’s one found in the urine of the adult females. Block the youngfemale’s sensing organ, and the puberty delay no longer occurs.
Researchers have speculated that these signals might play some role inregulating population density. For example, a large number of adult femalesprobably indicate a dense population. The puberty-delaying pheromones serve asa brake on fecundity: if the population density is high enough, there will soonbe food shortages, and why should a starving female waste energy on ovulatingand getting pregnant when the odds of her carrying through a pregnancy arepretty small?
Such speculation is clearly supported by a study conducted by zoologistsAdrianne Massey at the North Carolina Biotechnology Centerand John Vandenbergh (of Vandenbergh effect fame) at North Carolina State University. The tworesearchers examined mouse populations in the acre or so of grass found in eachloop of a highway cloverleaf out in the hills of North Carolina. Patches like theseconstitute “biogeographic islands,” closed mini-ecosystems where there are fewmice immigrants (the ones who try to switch to a different cloverleaf patchusually get squashed by the cars on the road in between). Massey andVandenbergh studied the fluctuation of mouse populations in each patch. Whenthe population of a particular patch became large enough, the females began tomake the puberty-delaying pheromone. And when the population dipped, thefemales stopped making this pheromone. Males indiscriminately made the puberty-accelerating odorant all the time, so when population density dropped and thedelaying signals from the females were absent, puberty was accelerated. Thebodies of prepubescent females were able to monitor the environment around themand choose wisely as to when to leap into the reproductive business.
Male mammals often show a similar savvy. Some antelopes, for example, such asgazelles and impalas, grow up in social groups in which a number of females andtheir offspring live with a single breeding male. Other mature males liveeither as wandering loners or in all-male bands, butting heads and honing theirfighting skills for the moment when they stage a coup d’Žtat against thebreeding male.
For a male growing up with his mother and the rest of the breeding group,puberty carries a stiff price. When an adolescent male begins to sproutpubescent “gender badges” such as horns, the breeding male perceives theyoungster as a sexual rival and begins harassing him, driving him out of thegroup. The apron string is cut rather abruptly, with consequences that are nottrivial--when males go out on their own, their risk of being eaten by predatorssoars. (It’s been theorized that the danger of a young male’s showing signs ofmaturity is what led females to sprout horns also: the female headgear could bean evolutionary dodge developed by protective mothers to draw attention awayfrom their growing male offspring.)
What is a prepubescent male antelope to do? By reaching puberty too early, heis subject to the harassment of the dominant male when he may not yet be readyto survive the rigors of being driven from hearth and home. But by delayingpuberty too long, he forgoes reproductive potential.
Clearly he needs to make a carefully considered decision--and apparently he does.Richard Estes of Harvard’s Museum of Comparative Zoologyhas examined the timing of puberty in antelopes and found that if the outsideworld looks pretty challenging and dangerous--a lot of other antelopes aroundto fight over food, not much food out there anyway, and a lot of lurkingpredators--the males delay the onset of puberty.
Similar strategizing occurs in primates. If you have ever watched orangutans ina zoo, you may have noted that the males seem to come in two varieties. One isshort-haired, limber, and agile. The other is much heavier and lumbering, withodd, fatty cheek flanges, thick long hair, and a large muscular throat pouch.Primatologists studying orangutans in the wild always assumed that the moregracile form was simply an adolescent, the latter being the adult version.However, in zoo populations, some males retain the gracile form well intoadulthood, even a decade later than normal. Invariably these are males livingnear an older, more socially dominant male. Some sort of signal given off bythe dominant male makes the body of the subordinate delay the development ofthe adult secondary sexual features that give the heavier form its appearance.Take away the dominant male, and the eternally youthful subordinate rapidlydevelops all the mature traits.
None of this strategizing has yet been shown to occur in male humans, butpsychologist Jay Belsky and anthropologist Patricia Draper of Penn State andpsychologist Laurence Steinberg of Temple University have found something verymuch like it in females. Their argument is actually the flip side of thedelaying tactic used by antelopes and orangutans. In a nutshell, their premiseis: When times are tough and life is unstable, it makes sense to hurry up,reach puberty, and start reproducing sooner, or it may be too late and you’lldie childless.
This thinking bears the mark of a certain style of ecology. One of the basicdichotomies made by ecologists is between “stable” species and “opportunistic”species. Stable species live in unchanging, predictable environments. They livelong lives in large populations; they are large- bodied; and they lavish a lotof parental attention on relatively few offspring. They opt for quality versusquantity when it comes to reproduction; years later members of these speciesare still doing the kid’s laundry and paying tuition for chirping lessons.Compared with closely related species that don’t have as many of these traits,stable species tend to reach puberty later. For them there is no rush: they canafford to grow big and healthy before starting to invest all that energy in afew pregnancies. Think of elephants, with their 2-year pregnancies and 65-yearlife spans. Or ancient redwood trees. Or humans, a classic stable species.
In contrast are the opportunistic species, which live in ecosystems that areunstable and unpredictable--with long periods of severe weather, foodshortages, and environments that are favorable only now and then. Animals thatlive near bodies of water that dry up seasonally or plants that grow best insoil just after a fire are opportunistic. They’re gamblers, living boom-or-bustexistences. The population will be tiny for long periods. Then, when conditionssuddenly become right, there’s tremendous pressure to take advantage of themfast, and wham--everyone starts mating or pollinating. Quantity over quality,reproduce as much as possible, don’t bother taking care of the kids, just floodthe market with offspring while the going is good. Opportunistic species tendto be small- bodied and short-lived, their population size fluctuates wildly,and once conditions are favorable, they reach puberty rapidly. Think of rabbitsreproducing like rabbits and invading some new region, or of some kind of scrubweed that pops up overnight in the disturbed soil on the edge of a constructionsite.
About a decade ago, Draper and her colleague Henry Harpending theorized thatwhen girls are abandoned by their fathers during childhood, they grow up tobecome women who behave more like an opportunistic species. The girls havelearned that males make unreliable parents, so they don’t bother searching fora reliable mate--they have sex at an earlier age, have more sexual partners,more kids, and are more likely to have rocky marriages.
When Draper and Harpending proposed their theory, it was already well knownthat women who grow up in unstable homes are more likely to behave thisway--the novel thing about Draper and Harpending’s work was that they tried toexplain this pattern from an ecological-evolutionary perspective rather than apsychological or socioeconomic one. Now Draper, Belsky, and Steinberg havebuilt on this approach by making the controversial suggestion that in unstablesettings it’s not only the behavior of such girls that becomes moreopportunistic, it’s their bodies as well, and so the girls reach pubertyearlier.
A handful of studies supports this idea. In two studies led by Belsky and oneby Steinberg, girls who reported more strained relations with their parents,and girls raised in homes with persistent family conflict reached pubertyearlier than girls from more contented families. In a detailed 1990 study,Michele Surbey of Mount Allison Universityin Canada(who at the time proposed a theory quite close to that of Belsky andcolleagues) found that girls raised with the father absent because of divorceor abandonment also had earlier puberty. An Australian research group had foundthe same thing in 1972. Among all the various studies the effect was prettysmall--on average, puberty was accelerated by five months.
The various authors wrestle a bit with potential physiological mechanisms forthis effect (evolutionary theory is concerned with why puberty might beaccelerated; physiological mechanisms, the nuts and bolts of hormones andnervous system, explain how puberty might be accelerated). Surbey, citinganimal pheromone studies, hypothesizes that biological fathers might release apuberty-delaying chemical, perhaps an airborne one sensed by the nose--take thefather and his pheromones away and puberty comes earlier. The problem here ishow to explain Belsky’s results, in which the presumably pheromonally-richfather sticks around and fights constantly with the mother yet puberty is stillaccelerated in the girls.
But regardless of what the mechanism might be, the mere idea that socialinstability leads to earlier puberty has upset a lot of people. Someinvestigators who have tried to duplicate these studies haven’t gotten the sameresults. Other critics note that measures of “parental strife” or “strain” ingirls’ relations with parents rely largely on the girls’ own reports, so there’splenty of room for bias and inconsistency.
The biggest problem with the findings and the underlying theory is that theyare exactly the opposite of much of what is known about the effect of stress onpuberty. A lot of the research suggests that “when times are tough and life isunstable, hold off on reaching puberty--it’s a bad time to get pregnant and thekid won’t survive, so don’t waste energy ovulating.” And we’re not just talkingabout antelopes here. Girls who experience the stress of sustained physicalexercise (for example, serious ballet dancers) reach puberty later thanaverage, as do girls with anorexia nervosa.
Additional evidence that extreme physical stress suppresses reproductivephysiology instead of stimulating it comes from studies of women who havealready reached puberty. Consistently, physical stresses such as weight loss,heavy exercise, or illness delay or even block ovulation. Moreover, it’s beenfound that a variety of psychological stresses, rather than physical ones, canalso trigger the cessation of ovulation. This latter finding is particularlyrelevant, given that the girls growing up in unstable home environments wereprobably being psychologically, but not physically, stressed.
The authors of the new theory try to reconcile these differences. If Surbey isright that the girls reach puberty earlier because some puberty-delayingpheromone from their fathers is missing, then this is no longer a contradictorystory about stress and maturation but instead a story about pheromones andmaturation. Her data also suggest this might not be a broad theory about “whenlife is unreliable, reach puberty earlier,” but merely about “when the primarymale around you is unreliable, reach puberty earlier.” Surbey found that havinga mother rather than a father absent didn’t accelerate puberty, yet growing upwithout a mother should be just as much of a lesson to a child that life isstressful.
Belsky, for his part, contends that there could be a difference between theeffects of extreme, potentially life-threatening stress and milder forms ofstress, such that a starving anorexic might delay puberty, while a stressed-outyet relatively healthy child of a broken home might reach it sooner.
One other important factor may explain the effect. The age of puberty onset ispartially inherited--mothers who reach puberty early tend to have daughters whodo so as well. Perhaps early sexual maturation is somehow related to havingunstable marital relationships later. In that scenario, girls in unstable homesreach puberty earlier because they inherit that biological trait from theirmother, not because the home environment is unstable. Surbey did find that themothers of her early- maturing girls had indeed reached puberty earlythemselves. When the mother’s age of puberty onset was taken into account, theoverall effect of home environment decreased.
The ideas of Belsky, Surbey, and their colleagues may or may not turn out to bevalid; clearly much more research is needed before we’ll know, and the issuewill in all likelihood depend on finely wrought details of scientific argument.So why have their various notions been the subject of heated discussion notjust in academic circles but in newspapers around the country?
Because lurking behind the scientific arguments is a big public policy conflictover what to do about teenage pregnancy. The pregnancy rate for teens in the United States isastonishingly high for a Western country. It’s twice that of Canada or England, for example. Teen motherscome disproportionately from poor inner-city populations rife with unstable,broken families. Children of such mothers are more likely to have health andlearning problems, and thus to underachieve and place a burden on socialservices for years to come.
Belsky thinks the theory he’s helped formulate has something to say about thismess. In his view, the unstable early environment of many an inner-city teenagegirl produces earlier puberty and the whole range of attendant opportunisticbehavior that ends in an increased risk of pregnancy. “These teenagers aren’tnecessarily carrying out bad behavior,” he says. “Their bodies are justresponding to forces that have emerged over the course of evolution.”
Belsky and Surbey’s critics, however, worry that the theory suggests that onecan blame biology instead of society for the problem, and blaming biologyimplies that we can’t do much about teen pregnancy. Yet for a bloatedly wealthycountry, they note, we expend very little effort to give a massive underclassany hope for jobs or education. Our mass media are permeated with a sexual hardsell that few adolescents have the means to respond to maturely. Our governmentmakes war on sex education and access to contraceptives and makes itincreasingly difficult for the young and the poor to have abortions. Are thesethe reasons that we have such an appallingly high rate of teen pregnanciesamong our poor? Or is it because early family conflict commands such girls tofollow a biologically determined program of early pregnancy?
Belsky doesn’t think the explanations are mutually exclusive. He does notbelieve that his theory is about how biology can cause earlier pregnancy, butabout how a biological drive can make a young person from an unstable home morelikely to respond to various societal pressures. This is what he calls a “nature-basedtheory of nurture.” If true, it predicts that the effects on teen pregnancyrates of growing up in an unstable home would be stronger among poor kids thanamong middle-class kids.
And if he is right--if early stress and resulting biological changes do notform a direct link to more teen pregnancies but instead are one of the manyfactors influencing how teenagers respond to the seductive world aroundthem--then perhaps the policy implications of his view are not so worrisome.Much of behavioral biology works through this sort of biology-environmentinteraction. These results are not an excuse for society to say, in effect, “Theregoes that darn biology again--nothing we can do about it,” and wash its handsof the mess. The findings are, if anything, a scientific rationale for whysociety has the responsibility to try to solve these social ills.