Psych 311 - Psychophysical Development

OUTLINES

Chapter 9

Transitions: Weaning and Puberty

Rev. 1/23/03

 

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I.                Weaning

A.            Except for a small number of precocial species such as the guinea pig obtaining mothers milk is indispensable to the survival of the infant mammal

B.            Beyond providing the sole source of nourishment, milk and nursing a contribute in other ways

1.                Infants vulnerability to infection is reduced

2.                Odors associated with nursing have been implicated in psychosexual development as expressed in a male’s later selection of a mate

C.                Suckling and Independent Feeding

1.                Similarities

a.                Enable nutrients to be brought from without to within

b.                Regulated by changes in internal state

1)                Neonates like adults have the ability to regulate their intake in response to internal changes associated with prior ingestion

c.                Attachment (nipple) occurs even when it does not procure milk

1)                Milk is available only when secreted into the mammary ducts in response to the release of the pituitary hormone oxytocin

2)                The regulation of mil intake is not achieved by relinquishing the nipple has been verified experimentally by Blass, Hall, and Teicher (1979b)

d.                The ability to respond to nutritional status by adjusting intake is present from an early age

1)                It does not mean that the neonate is in the possession of a fully mature food regulatory mechanism

e.                Both can serve as reinforcement for acquisition of an instrumental response

1)                Kenny and Blass (1977) removed 6-11-16 and 20-day-old pups from their mothers and 24 hours later placed them in a Y maze

a)                Results were clear all subjects regardless of age, came to reliable choose the arm that led to the undiscovered nipples

f.                Sucking itself must be positive because it can reinforce an operant response without yielding milk

2.  Differences

a.                Nursing and Independent feeding

1)                regulated by internal states

2)                both reinforcing

3)                require different motor behaviors

4)                role of olfactory stimuli

a)                odor is not necessary with regard to independent feeding, it plays a crucial role in suckling

b.                Teicher and Blass

1)                observed that 4 to 5 day old rats do not attach to nipples that have been washed

2)                if cleansed nipples coated with a distillate of the nipple wash or with pups’ saliva, nipple attachment is reinstated

3)                study replicated by Blass, Teicher, Cramer, Bruno (1977) and was extended to pups aged 2-11 days old

a)                suckling appears to be elicited by a substance found in pups’ saliva

b)                the substance must be volatile and an olfactory cue

c)                other data shows that anosmia induced by irrigating the nasal passage with zinc sulfate also disrupts suckling

c.                Internal sucking response

1)                performed in the absence of their saliva on the nipples

2)                Teicher and Blass (1977)

a)                both mothers’ saliva and amniotic fluid reinstate nipple attachment when applied to washed nipple

b)                saliva from virgin female and mother’s urine-no effect

c)                mother apparently applies a substance to her nipples that evokes the initial suckling response

d.                Internal controls

1)                infant depends on stimuli arising from distension of the stomach

a)                number of factors that modify independent feeding are without effect in the nursing animal

b)                blood glucose

(1)                decrease causes adult animals to consume large amounts of food

(2)                declines in the young animal are without influence

2)                Lytle, Moorcraft, and Campbell (1971)

a)                reduced blood glucose in rats by administering insulin

b)                insulin increased food intake of 25 day old rats and older but not of 15 and 20 day olds

c)                reported that amphetamine (produces anorexia in adult) was ineffective prior to day 15 of age

d)                cholecystokinin has been ineffective in 5 and 10 day olds

3)                Blass, Beardsley, and Hall

a)                milk was delivered directly into pup’s mouth while it was sucking on nipple of a nonlactating animal

b)                cholecystokinin didn’t cause a cessation of milk intake

D.  Transition

1.                Is nurturing the youthful form of independent feeding?

a.                if nursing is prevented – deficits in independent feeding behavior would occur

b.                independent feeding would not emerge until a sufficient amount of nursing has transpired

c.                independent feeding becomes evident late in the preweaning period

d.                elimination of nursing behavior should affect independent feeding

2.                Hall (1975)

a.                nursing is not an essential prelude to independent feeding

b.                cannula placed in stomach of 12-48 hour old rats- an artificial diet administered

c.                infants kept isolated in Styrofoam cups floating in warm water

d.                artificially reared rats experienced no difficulty in identifying or ingesting oral food

1)                growth rates were similar

2)                nursing is nonessential for the emergence of autonomous feeding

3.                Miller, Caul, Mirsky

a.                examined three 9 year old monkeys that had been totally isolated first year of life

b.                consumed 30% more food per meal over 6 month feeding period

4.                Hall (1979)

a.                observed that when isolated incubator reared infant rats are given direct oral infusions of a liquid diet they respond like they are eating independently

5.                Other factors

a.                anorexic amphetamine- has no influence on nursing prior to day 15 but suppresses independent feeding

b.                norepinephrine- elicits milk intake in sated 10 day olds feeding away from the dam (Epstein, 1984)

c.                nursing and feeding appear to be distinct behaviors

1)                feeding occurs when the infant is separated from the mother

2)                autonomous feeding is not exhibited

3)                leads to what mechanisms suppress nursing thereby permitting independent feeding

6.                Mechanisms

a.                Mother-infant interaction

1)                Nursing and the transition to independent ingestion are in large part the result of a synchronous relationship between mother and infants

2)                During early portion of lactation period, the availability of milk is dependent on the mother’s receipt of stimuli association with suckling

a)                Such stimuli reflexively cause the pituitary hormone prolactin to be released

(1)                This promotes milk secretion

(2)                Removal of those stimuli also diminishes the ability of subsequent suckling-related stimuli to release the hormone

3)                Suckling stimulation serves two functions

a)                Immediate release of prolactin

b)                Maintenance of the responsiveness of the prolactin-releasing mechanism

4)                As young mature and being to spend time away from mother, the availability of suckling-related stimuli declines

5)                Maternal behavior is synchronized with the changing physical characteristics of the young

a)                Suckling ceases partly because the mother treats older pups differently from younger ones spending less time nursing them

b)                Data suggests that nursing female normally contributes to the weaning process by altering her responsiveness to older pups

b.                Serotonergic System

1)                Prior to about 13 days of age bother food-deprived and non-deprived rat pups attach to a nipple, whereas subsequent to day 13 only deprived young reliability attach

2)                Experiments performed by Nock, Williams and Hall (1978) involved the administration of two drugs, methysergide, and quipizine

a)                Methysergide antagonizes the action of serontonin by blocking serotonin receptors in the brain

b)                Quipizine increase brain serotonergic activity by directly stimulating serotonergic receptors

c)                Only 25% of 20-day-old rats injected with a line attached to the nipples of an anesthetized dam, 100% of methysergide-injected subjects did so

d)                Quipizine did not inhibit suckling but significantly reduces it

e)                Methysergide also antagonizes the effect of quipizine

3)                Findings of Nock et al. (1978) confirmed Williams, Rosenblatt and Hall (1979)

a)                Reported that the ability of methysergide to induce suckling behavior is age-dependent

(1)                It is effective in rat pups as old as 39 days an age at which food deprivation normally does not induce such behavior

(2)                Ineffective in10-day old rats and older then 40 days

b)                Point out that the effect reported by Nock et al. are not caused by other extraneous actions of methysergida and quipizine but, rather, they are specific to alterations of the serotonergic system

4)                Stoloff and Supinski (1984)

a)                Found that regardless of age, young rats administered methysergide chose that goal containing the accessible nipples more often than controls

5)                Modifications of the serontonergic system by pharmacological manipulations supports the idea that idea that the serontonergic system plays a role in the weaning process

a)                Placed serontonin-specific neutrotoxin 5, 7-dihydroxytryptamine (5,7-DHT) directly into third ventricle of 3 day rat brain

(1)                Produces a permanent depletion of brain serotonin by destroying serotonin-producing neurons

b)                5, 7 DHT treated deprived animals exhibited significantly more nipple attachment than controls up until day 37 of age

c)                Concluded that serontonin, although involved in the weaning process, does not play an essential role

(1)                This conclusion is unwarranted because 5, 7-DHT treatment of 3-day-olds doe not deplete serontonin but rather, causes an average reduction relative to control levels of only 50%as assessed. At 7 weeks of age

7.                Food Selection               

a.                Infant rats posses two different models of ingestive behavior.

1)                Suckling

2)                The capability to ingest food independent of the mother

b                Weaning consists of suppression of suckling, allowing independent feeding to become the sole means by which food is ingested.

1)                Changing mother- young interaction and maturation of serotonergic system contribute to the transition from nursing to independent feeding.

2)                Young are now nutritionally on their own.

c.                Food selection information is provided in two ways.

1)                Weanlings often approach adults and begin to feed in their vicinity.

a)                This increases the probability that the weanling ingests the same food as the adults.

2)                Information provided by the mother during the lactation period.

d.                Galef and Henderson (1972)

1)                Fed 2 nursing rats solely with laboratory diet and 2 other rats were feed with preferred standard diet.

2)                The lactating rats were never fed in the maternity cage thus the young animals were never fed solid food nor did they experience the mother ingesting solid food

3)                On day 17 to 23 the young were placed in a choice apparatus that contained both laboratory foods and preferred foods.

a)                The pups that mothers were fed lab foods ate more of that food then pups whose mothers who were given the prefect diet.

e.                Sullivan et al. (1990)

1)                Both behavioral preference and olfactory bulb neural activity in rat pups in relation to maternal diet.

2)                Newborns were reared either by mothers fed a lab rat diet of by other fed a high sucrose diet (which has been known to alter normal maternal odor)

3)                Pups were placed in Y maze and were to choose between normal maternal odor and abnormal maternal odor; other pups were injected with 2-dg and placed in a cage with normal fed mother.

4)                Pups were fed after killed and the olfactory bulbs assessed for the uptake of 2 dg.

5)                Early olfactory exposure influenced both behavior and neuronal activity.

a)                Pups raised with normal odor exhibited enhanced 2-dg uptake.

II.            Puberty

A.            Consists in myriad changes that mark the transition from nonreproductive to reproductive status.

1.                A passage that not only entails maturation of the physical attributes required for successfully mating but requires the emergence of motivation to engage in reproductive behavior.

2.                The timing of puberty is a significant consideration, having implications with regards to reproductive potential or fitness.

3.                Clark, Spencer, Galef (1986)

a.                Female gerbils that reach puberty early have more litters containing more young/ litter

b.                The early maturing gerbils give birth to more females.

B                .Neuroendocrine Control of Puberty

1.                  General Agreement

a.                  At a particular point the arcuate nucleus of the hypothalamus secretes GnRH  (gonadotropin releasing hormone).

b.                  GnRH transported by ducts to anterior part of pituitary.

c.                  The pituitary in response increases secretion of 2 gonadotropins: luteinizing and follicle-stimulating hormone.

d.                  The increase in gonadal hormones effect sexual maturation in males and females.

2.                  A precise description of dev. Changes the hypothalamus must undergo to enable it to secrete GnRH is lacking

a.                  Adrenal secretions, prolactin, endogenous brain opiads implicated

b.                  Specification of mechanism that trigger process culmination in puberty remain elusive

3.                  Reiter and Grumback

a.                  Hypothalamic- pituitary gonadal system differentiates and functions in fetal life and early infancy but is suppressed throughout childhood

b.                  Reactivated in puberty

4.                  Elevated levels of gonadal hormones do what?

a.                  Modify internal organs related to reproduction

b.                  Cause appearance of secondary sex characteristics

c.                  In females, lead to first ovulation

d.                  Activate brain portions to motivate engaging in reproductive behavior

5.                  Puberty not determined solely by physiological maturation

a.                  Results from interaction between maturation and environment

b                  Outcome determines if and when it will occur

C.  Day Length

1.                  Constant Illumination

a.                  Female rats exposed to this from day of birth or from day 21 have acceleration of puberty (earlier age at which vaginal membrane ruptures)

b.                  Normally takes place between days 35 and 36

c.                  Also earlier first ovulation

d.                  Ovulation and vaginal opening apparently caused by increased level of follicle-stimulating hormone

2.                  Effect of total darkness is unclear

a.                  Early Studies

1)                  Exposure to constant dark produced retardation of sexual maturation

2)                  Retardation also found for animals blinded at birth or on day 15

b.                  Later studies yielded contradictory results

1)                  Orback and Kling

a)                  No effect of blinding performed on days 6 and 10

2)                  Relkin

a)                  Darkness can accelerate development by 4 days if darkness is imposed from day of birth

3.                  Duration of Light Exposure

a.                  Lecyk (1962)

1)                  Separate groups of prepubertal males and females exposed to various durations of light

2)                  Began on Sept. 22- placed voles under gradually increasing light approximating natural change in duration occurring between 3/21 and 6/24

3)                  Another group kept under constant 16/8 hour light/dark cycle

4)                  Third group exposed to 8 hours of light until Nov. 5 when increased to 13 hours gradually by Dec. 6

5)                  Control group exposed only to natural light

6)                  Results

a)                  By early December all but controls had reached puberty

b)                  Controls exhibited no signs of sexual maturation

b.                  Lecyk (again)

1)                  Demonstrated sexual maturation can be completely inhibited by exposing 10 day old voles of either sex to gradually decreasing illumination or by exposing them to 7 hours of light a day

c.                  Thiboult

1)                  Exposed male voles to 5, 10, 15, or 20 hours of illumination a day

2)                  Animals given 15 and 20 hours had heavier testes than those exposed for 5 or 10 hours

3)                  Problem:

a)                  No control group

d.                  Similar findings for white-footed mice

1)                  Diff. seen between exposed from birth to 10 or 14 hours of light

2)                  Males exposed to short day length had significantly smaller testes than those exposed to 14 hours

3)                  Females

a)                  Short day length had smaller uteri and ovaries

(1)                  Ovaries didn’t begin to reach maturity until day 90

b)                  Long day length ovaries became mature by day 50

e.                  Johnston and Zucker with prepubertal cotton rat

1)                  Those exposed to 10 hours of light per day had smaller testes and lighter seminal vesicles than those with 14 hours per day

2)                  All females exposed to long day exhibited vaginal opening by day 50

a)                  This was found for only 50% of short day subjects

f.                  The Lamb –different

1)                  Females kept from birth on a long day routine exhibit a delay in puberty

2)                  Suggests short days should promote puberty but this is not the case

3)                  Lambs exposed to 9 hrs. per day are similar to animals maintained on long day routine

4)                  Because born in spring – experience a sequence of day lengths

a)                  Both long and short are essential

b)                  One week of long days starting at 21 weeks of age followed by continuous short days leads to onset of reproductive cycle

(1)                  Similar to spring (long) then fall (short)

g.                  Non human Primates

1)                  Wilson

a)                  Raised some spring born fem. Rhesus monkeys starting at 12 mos. Of age in outdoor enclosures under natural conditions

b)                  Others indoors in 12 hrs per day environment

c)                  Results

(1)                  Most indoor raised (more light) reached puberty sooner

(2)                  Light accelerates development in nonhuman primates

h.                  Humans

1)                  Zacharias and Wurtman

a)                  Compared age at menarche in blind and normal girls

b)                  Blindness associated with early menarche

(1)                  Especially if blindness accompanied loss of light perception

c)                  Results

(1)                  50% of non deprived reach 1st menstrual cycle by 149th month

(2)                  50% of deprived attained it at 140 months

(3)                  Unless blindness is associated with other physiological change that could affect onset of sexual maturity – the absence of light fosters maturity in human females

4.                Mechanism of Action

a.                Lighting Conditions affect time of puberty

1)                Melatonin is released by the pineal gland during period of darkness.

a)                Foster, Olster and Yellon (1985) reported that lambs exposed to 9 hours of light a day display a greater duration of melatonin than subjects exposed to 15 hours of light a day

(1)                This ability endows melatonin with the potential to serve as a time-keeping hormone in the sexually immature lamb

b)                Yellon and Foster (1986) denervation of the pineal (eliminates melatonin release) prevents a puberty-inducing photoperiod from bringing on ovulation in the lamb

D.            Uterine Position

1.                It appears to modulate timing of puberty

a.                Female mice in between 2 males in utero have an average age of vaginal opening at 37.3 days.  A female between no males is 34.2 days and a female adjacent to one male is 35.7 days.

b.                This is similar in rats and gerbils.  Females located adjacent to 1 or 2 males were less likely to reach sexual maturity early than females in contact with no males. 

E.             Odor

1.                Pheromones, of the same species, influence aspects of reproduction

a.                They can signal sexual readiness, provide the basis for recognizing previous mating partners and regulate the ovarian cyclicity.

2.                Advancement of Puberty

a.                Olfactory Stimuli

1)                Vandenburg (1969)

a)                Exposed groups of 6 21-day-old sexually immature female mice to either adult males or odors from adult males

(1)                Found that males odor accelerates puberty of sexually immature females specifically if the odor was contained in the urine.

2)                Lombardi, Vandenburg and Whitsett (1976)

a).                The castrated male failed to accelerate puberty when placed in a cage with an immature female.  This suggests that androgen is involved in the formation of the pheromones. 

3)                Bronson and Desjardins (1974)

a)                Determined levels of hormones as immature females were exposed to an adult male or urine from a male

(1)                Within 1 to 3 hours females exhibited an increase in luteinizing hormone (LH) secretion followed by a 15 to 20 fold increase in levels of estrogen.

(2)                On day 3 levels fall and ovulation occurs later that day.

b.                Isolating and Identifying a pheromone Vandenbergh, Whitsett and Lombardi (1975)

1)                Reported puberty accelerating pheromone in male is associated with protein component of urine

c.                Vandenbergh, Finlayson, Dobrogosz, Dills, and Kost (1976)

1)                Used column chromatography found a fraction exhibiting pheromonal activity

a)                Does NOT contain androgenic material, but puberty acceleration is androgen dependent

b)                The involvement of androgen in the genesis of the pheromone must be producing a non androgenic substance

d.                Drickamer and Hoover (1979)

1)                Urine from pregnant/lactating mice advances the onset of puberty as determined by the first estrous cycle

2)                Minimum of 3 days of exposure to be effective

3)                Chemicals in urine accelerate sexual maturity in female

a)                Pheromones communicate that the environment is favorable for reproduction

b)                To the young female’s advantage to reach puberty early

c)                Female reproductive system more modifiable than male

e.                Maruniak, Coquelin and Bronson (1978)

1)                Pre-pubertal male mice secrete LH in response to the urine of adult females

2)                Hormonal response decreases after repeated exposure

3)                The influence of adult female on the sexual maturation of the male is unknown

3.                Delay of Puberty

a.                Drickamer (1974)

1)                Puberty occurs later in group housed mice than singly housed

2)                Housed 21 day old female mice singly or groups

a)                Onset of vaginal opening and 1st estrus delayed in grouped animals

b)                Magnitude of delay related to size of group

c)                Group housed mice weigh more than singly housed mice

3)                Singly housed females exposed only to group housed females’ urine

a)                Found virtually identical to those group housed mice

(1)                Delay in vaginal opening and first estrus and increased body weight

(2)                Puberty delaying properties contained in the urine of both adult and pre-pubertal grouped females

b)                Determined puberty delaying pheromone appears only in the urine when group housed females interact physically

(1)                When separated by wire mesh urine fails to delay puberty

(2)                3-7 days of pheromone exposure during 1st week post-weaning is required to produce delay

(3)                Urine taken directly from the bladder contains pheromone whether group/singly housed

(a)                Absence of pheromone in singly housed females is due to a mechanism between the bladder and urethra

(b)                Group housing prevents urethra from deactivating the pheromone

(c)                Ovaries are NOT involved in pheromone production

(d)                Adrenal gland plays essential role

4)                Inhibition of female sexual maturation found in other animal species as well

a).                California Vole, Gerbil, Pig, and Marmosat

b.                Lawton and Whitsett (1979)

1)                Exposed weanling male deer to soiled bedding removed from a cage containing either an adult male or to urine collected from and adult male

a)                After 5 weeks of exposure both groups had lighter seminal vesicles than controls

c.                Rissman, Sheffield, Kratzman, Firtune, and Johnston (1984)

1)                California Vole affected by olfactory stimuli

a)                Puberty can be delayed if exposed to bedding from their mother’s cage in contrast to an unrelated male

b)                Exposure to father’s bedding produces intermediate effect on androgen levels and no effect on seminal vesicles

(1)                By delaying puberty the young male may build up body mass and fat supplies

(2)                Increase chances of reproduction

F.                Population Density

1.                Population size is reduced by inhibition of reproduction due to diminished fertility, increased prenatal & postnatal mortality, disruption of maternal behaviors, and inhibition of sexual maturation.

a.                Two techniques used to examine endogenous population controlling mechanisms.

1)                One is observation of naturally occurring populations of various densities.

2)                The other consists of examining artificially created populations of differing densities.

2.                Natural Populations

a.                Southwick (1953)

1)                Captured (virtually) entire populations of mice inhabiting 40 different corn stacks.

2)                Density was found by dividing the # of post weaning mice (weighing min of 7.6 g) by the cubic meter capacity of the stack.

3)                Grouped by 4 density classes

a)                Low, medium, high, and very high.

4)                When ovaries were examined they showed that females judged by body weight being post-pubertal were capable of reproducing varied inversely as a function of density.

a)                98.5%, 93.6%, 87.9%, and 83.7% (respectively) were fecund.

b)                A reliable difference in fecundity was not found for males.

c)                Also obtained from voles, lemmings, and rabbits.

b.                Although data collected from these studies point to an association between density and sexual maturation, a problem in the comparison of natural populations makes interpretation of the findings difficult.

1)                Populations with the greatest densities are probably older than less dense populations

2)                Differences in maturity of a population might be related to differences in population structure and social behaviors.

a)                Might influence puberty

b)                Timing in puberty is causally related to density, age of population, or a combination of the two.

c)                Alleviated by constructing artificial populations.

3.                Artificial Populations

a.                Crowcroft and Rowe (1957) founded four colonies of wild house mice.

1)                1 adult male and 2 adult females

2)                Pen was 6 sq. ft. containing nesting boxes, and unlimited food and water.

3)                Animals examined and counted every two weeks.

4)                At the 2nd 5-month period, most females did not attain vaginal opening.

b.                Terrman (1965) reported similar findings

1)                Most females produced no litters, which suggested that many failed to reach sexual maturity.

2)                Density-dependent inhibition of puberty has been reported by several experimenters.

3)                Data pertaining to males is unclear, experimenters have found conflicting findings.

4.                Mechanisms

a.                Christian (1963)

1)                Found that behavioral factors associated with groupings, especially heightened levels of aggression, and are the sources of stress.

2)                Stress in turn activates the pituitary-adrenal axis.

3)                ACTH administered can readily inhibit sexual maturation in female mice.

a)                Ovaries of mice treated with the hormone daily for 12 weeks showed no sign of ovulating.

b)                Because ACTH is effective in both intact and adrenalectonized animals, ACTH produces the effect directly rather than through the release of corticosterone.

c)                ACTH has little effect on male reproduction.

b.                Massey and Vandenbergh (1980)

1)                Found that delayed puberty in dense populations involves pheromones.

2)                Established freely growing populations of wild mice on 2 “highway islands”.

a)                One population eventually became dense.

b)                The other remained sparse.

3)                Investigators discovered that only urine collected from females from the dense population (when that pop. was at max. density) had puberty delaying properties in immature females.

4)                Urine of females from the sparsely populated island had no effect.

G.                Nutrition

1.                Puberty and causal relationship to growth

a.                Kennedy and Mitra (1963)

1)                Reared infant rats in either small (3pups) or large (18-20 pups) litters.

2)                Procedures furnished following 3 groups of weanlings differentiated by body weight:

a)                Optimally grown (>40g)

b)                Retarded (between 20 and 25g)

c)                Very retarded (<15g)

3)                After weaning day 21- the experimental groups had unlimited access to food

4)                The others rat pups reared in normal size litters of 7 to 9 pups until weaning were underfed after weaning.

5)                They were then placed on restricted diet/feeding schedule that virtually stopped growth for 10 days.

6)                Procedure was repeated until they reached 75g-approx. day 65.

7)                Animals raised in large litters and then given unlimited access to food reached a weight of 75g by day 25.

2.                Vaginal opening, first estrus, and first mating.

a.                Apparent that puberty was delayed in animals restricted in their pre-weaning or post-weaning growth rates relative to optimally grown animals.

b.                Post-weaning growth restriction delayed the onset of the first estrus and first mating more than pre-weaning restriction

3.                Restricted rate of growth and the onset of puberty

a.                Lambs (Fitzgerald, 1984);

1)                Fed a diet that retards growth exhibit significant delay in onset of puberty and reduction in plasma levels of LH.  (Surges of LH are a critical factor in the initiation of puberty.)

b.                Rats (Dyer et. al. 1985)

1)                Data from adult rats suggest that restricted feeding activates an opioid-sensitive neural pathway that directly inhibits LH secretion.

4.                Sexual Maturation-influenced by the content of the diet.

a.                Kirtley and Maher (1979)

1)                Fed on group of weanling rats a high fat diet and another group a low fat diet.

2)                Both diets of equal caloric value.

3)                Average day high fat group displayed vaginal opening was 32.8 days vs. 39.7 days for that of the low fat group.

4)                Average day for first estrus of the high fat group was 33.2 days vs. 40.8 days for that of the low fat group

5)                Time of vaginal opening and first estrus, both groups exhibited similar indexes of obesity (body weight/nasodnal length x 100)

6)                -Delay in sexual maturation for pre-pubertal male and female rats when fed a diet low in protein (Glass et. al. 1979 and Ramaley, 1981)

H.                Stress

1.                Life Experience and Reproductive behavior

a.                Belsky, Steinberg and Draper (1991)

1)                Believed that humans “vary their mating and childrearing behavior in accordance with the contextual conditions in which they develop, so as to maximize their reproductive success.”

a)                81 girls for whom puberty commenced at 12 or earlier, 49% had grown up in homes with absent fathers.

b)                Only 24% of girls for whom did not exhibit puberty until 14 years + were from homes with absent fathers.

b.                Steinberg (1988)

1)                Girls who had experienced relatively high levels of familial conflict displayed a greater degree of sexual maturation over the course of a year than did controls.

c.                Belsky et al. (1991)

1)                Suggests that stress causes the individual to be more biologically reactive to environmental factors that have the potential to accelerate puberty—factors could include pheromones.

d.                Other Explanations

1)                The relation between stress and early sexual behavior may be indirect and not causal

2)                Perhaps families experiencing conflict off the children less parental supervision and guidance than families in which conflicts are at a minimum—more children of the former may be expected to experiment with sex.

I.                Conclusion

1.                Research with nonhuman subjects has revealed that environmental factors help regulate the onset of puberty—little is know about how the environment affects human puberty.

2.                Unlikely that early-maturing humans produce more offspring than those maturing later—the timing of puberty does influence social development though.