A Mind of her own
Mothers matter most: Women and parental investment
A. Sexual, not natural, selection
2. Sexual selection: ‘the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction’
3. sex diff. from sexual rather than natural selection
4. genes that increase # of children reared will be passed on
a. traits helping such genes to be passed on are diff. in men and women
B. Anisogamy: the start of parental inequity
1. advantages of parthenogenetic species
a. no wasting time finding mate
b. no exposure to predators during copulation
c. no need to impress opposite sex
d. no sacrificing 50% genetic legacy
2. advantages of sexual reproduction
a. unique individuals
b. creates less competition
c. increase odds someone will survive changing environment due to unique immune system
d. genetic repair
3. Why have an egg and sperm?
a. small gemetes best for sperm because they’re cheep, light and mobile
b. large gamete best for egg because they must carry nourishment for zygote to survive
c. medium gametes have neither advantage
d. once anisogomy begins, size difference only gets larger
4. Parental investment
a. Robert Trivers (1972) – parental investment is ‘ any investment by a parent in an individual offspring that increase offspring’s chance of surviving at the cost of the parents ability to invest in other offspring’
b. r-K selected distinction evident between men and women
i. men = r-selected (interest in quantity of offspring)
ii. women = K-selected (quality over quantity of offspring)
c. minimum biological costs of reproduction greater in women, thus women view offspring symbolic of large investment – greater parental investment
d. men provide less parental investment because desertion leads to further reproductive prospects
5. Why such long period of parental care?
a. 9 months is evolutionary compromised
b. later than 9 months, head too big to fit through pelvic bone
c. earlier than 9 months, to immature to survive
d. But, babies still born too early in sense of maturity – complete dependent on parental care
C. Men and the attraction of polygyny
1. ideal condition for reproductive success is access to many fertile females
2. evidence that polygny is supported by evolution derives from examining sex differences
a. size/strength diff
b. delayed puberty to first enter male-male contests
c. males die earlier
d. testes size
e. strong sex drive
3. Although men are often monogamous, it appears evolution fitted them to seek many sexual opportunities
Why Men invest at all
I. Need for parental care of the father
a) How important is parental care to the survival of the infant?
b) It is more beneficial, in terms of reproductive success, to invest?
1. Looking at evolutionary past, we find a correlation between father abandonment and child mortality.
II. Paternal certainty
a) The greater the doubt, the less willing is the male to invest
b) Internal fertilization and concealed ovulation make it harder for the male to make certain of paternity.
1. mate guarding has evolved as a means to lessen this uncertainly
III. Behavior of females
a) Rewards of monogamy
1. Females should encourage monogamy by practicing fidelity.
b) Cost of polygyny
2. Females should make it difficult for men to take part in a polygynous mating system. They can do this by being less willing to have casual sex and by patronizing women who do so.
Women as choosy investors
a) Females must ensure that her body has a reasonable chance of sustaining the pregnancy
b) Loss of menstruation can be caused by
1. food shortages
2. high stress
c) Babies are more likely to be born during seasons when the climate is moderate and food supplies are plentiful.
II. Quality of male mate
a) concealed ovulation
1. encourages monogamy
2. creates uncertainty about who the father is
b) choose healthy and attractive males
c) “sperm wars”
1. Only the most successful sperm with the strongest army will win and therefore impregnate.
III. Women give more parental investment
a) Only a very small percentage of fertilized eggs make it through the first trimester
1. Low success of implanting
1. If an infant is not viable than a woman is better off abandoning it as early as possible.
c) Abortion: terminating the pregnancy
1. Mother’s age: voluntary abortion is high among young women ages 20 of less
2. Material, personal and interpersonal resources
3. Commitment of the father: the best predictor of abortion
4. Women’s own psychological stability
d) Infanticide: a last resort for a woman who believes that the investment in the current infant is not worth continuing
a) deformity at birth
b) absence of male support
c) economic hardship
2. Post-partum depression: a psychological adaptation that reduces attachment and may thus open the way to infanticide.
a) associated with inadequate emotional and material support from the father, family, and friends.
Woman as Heavy Investors
I. Infant Survival
a) Without a mother, the life expectancy of infants is cut short.
1. If either parent died within the first year of an infant’s life, or father died before the birth of a child, it was found that child
mortality was twice as high within the first year if the mother
died compared to the father (according to the church records of
a north-western German town between 1668-1879).
b) An infant’s access to breast milk is critical.
1. Human breast milk contains far less fat then most animals, a
human baby requires many more feedings a day as well as the night.
2. Breast milk contains anti-bodies that are essential in for an infant to fight off infections, specifically diarrhea which can be potentially very dangerous to a baby’s health.
3. Oxytocin is a chemical released in a mother’s brain during nursing to allow the milk to flow. This chemical also provides the mother with a sense of warmth and comfort, which brings the mother and infant closer.
II. Mother and Infant Relationship
a) The general development of an infant to a juvenile is clearly shown in three
1. The infant is completely dependant on the mother, mother protects infant.
2. Infant begins to eat easily digestible foods as well as interact with peers. The mother is used as a base while the youngster does minimal exploring alone.
3. Finally, the mother may have a new infant and the juvenile begins to socialize outside of its immediate group.
b) When upset, children are far more to seek comfort from their mom compared
to the father.
c) In general, strange males scare infants more than strange females. After
observing primates, infants are use to seeking protection in their mothers and fearing older male primates who were prone to infanticide in order to impregnate an existing mother with his genes.
d) Mothers spend one and a half to twice as much time holding and playing with the child compared to the father.
a) Mother’s perform routine care-taking chores about two to four times more often than fathers.
1. Even in households in which both parents work, such as much of
b) When a divorce occurs, custody is awarded to the mother in 85 percent of the cases.
I. Male and Female Roles
a) Males devote their time to finding the right mate, females devote their time to raising the offspring to maturity.
1. A female’s job is seen as much less troublesome by many even though parenting is critical to the survival of the offspring, perhaps more than finding the right mate.
b) The excitement of the pursuit of a mate by a male is perhaps why more research is devoted to mate selection over parenting, which can be seen as repetitive and boring.
c) The excessive study of male-male competition only confuses the fact that
males competed because they were much less important.
1. A loss of females would be much more hazardous to the human population than the loss of males.
From an evolutionary perspective, chapter two strives to explain why mothers matter most in the sense of parental investment and how this accounts for the some of the major differences between men and women. In relating
The genes that increase the quantity if children reared are responsible for sex differences. The traits, which promote these genes, are different in men and women, resulting in sexual dimorphisms. Such genes may be enhanced by “sex-specific hormones”, so we would only view sex differences “where they have a direct influence on sex-specific reproductive strategy.”
There are other ways to reproduce other than sexual reproduction. Parthenogentic species (not mating required) have many advantages such as, not wasting time to find a mate, not exposing themselves to predators during copulation, there’s no concern in impressing the opposite sex and no need to sacrifice 50% of their genetic parentage. However, it is the sharing of genes that makes sexual reproduction the more desirable system. Mating results in unique individuals. This creates less competition because offspring are different from other individuals, including family, so they will “occupy a variety of different environmental niches.” Sexual reproduction also increases the odds of survival in that some are bound to survive through the changing environment due to unique immune systems.
Since mating is beneficial, the question remains as to why we have and egg and a sperm. In sexual reproduction gametes that contain half of one’s genetic information that can pair with another to form the zygote, is essential. However, variability and mutations occurred and some gametes were giving more or less then half of there chromosomal information. Thus, those gametes giving less produced more, and those giving more produced less. Small gametes became good for sperm because they are cheep, light and mobile. Large gametes became good for eggs because they are adequate to carry the nourishment for the zygote. Medium gametes had neither advantage and once anisogamy (size difference between gamete) began, the difference only became larger.
Gamete size was only the beginning of the larger commitment women had to make to parentage. Robert Trivers (1972) claimed that the main “reproductive difference between the sexes” is parental investment, defined as, “any investment by a parent in an individual offspring that increases the offspring’s chance of surviving at the cost of the parents ability to invest in other offspring.” Women only produce about twenty eggs/month compared to 300 million sperm/ejaculation. If the woman becomes pregnant, her bodies will be occupied for nine months. By the time the baby arrives the female views the baby as symbolic of a large emotional and physical investment, while the male doesn’t have quit the same connection. Males often desert females because desertion may lead to more opportunities with fertile females.
Although nine months seems to give males too much time to desert a pregnant female, it is a “compromised gestation period.” If the baby is born any earlier it will be too immature to survive, and any later than nine months the baby’s head becomes too large to fit through pelvic bones. Yet, infants are still born too early in the sense that they are completely dependant in parental care.
Men are attracted to polygny and sometimes desert a female because the ideal condition for reproductive success is access to multiple fertile females. Evidence that suggests that evolution fitted men to seek multiple partners is found when comparing sex differences. The size/strength differences highlight that men competed for females and the idea of delayed puberty so that men first experience male-male competition all suggests that men have been trying to access multiple females since the roots of evolution. On average, men die earlier. This may be due to the fact that they are r-selected and care more about passing on their genes than they care about survival. Males also have a stronger sex drive, suggesting that evolution fitted them to copulate with many females daily to enhance their reproductive success.
Why Men invest at all
Given the male reproductive rate, it would seem advantageous for males to be polygynous. Yet in some circumstances male are better off investing in their mate and her offspring. For some species of birds, without the joint care of two adults, the offspring would die. But in the majority of mammals, this is not the case. One parent can raise the offspring successfully.
Many important factors come into play as to whether a male should invest or not. One factor is if and by how much parental care improves the likelihood of infant survival. The male must weight the benefits in terms of reproductive success; the benefits of promiscuous mating and offspring abandonment against the possible detriments of having none of these offspring survive. In this day and age, child mortality is very low, but evolutionary past shows, across culture, an association between child mortality and father absence.
Another important factor in parental investment is paternal certainty. The less certain the male is, the less willing is the male to invest. Investing is someone else’s offspring is extremely detrimental to one’s own reproductive success. In humans, both internal fertilization and concealed ovulation have made it harder to recognize paternity. Males cannot be certain that it is his sperm that has fertilized the egg. In addition, in species such as humans where female can raise the offspring alone, there is further temptation for infidelity. Because of this, mate guarding has evolved. A male must guard his female for a substantial period of time in order to ensure that she has not and does not mate with anybody else.
The third factor in parental investment is the behavior of females. A woman’s reluctance to have casual sex makes it difficult for men to pursue polygny while at the same time reassures prospective long-term mates that she will be faithful in a monogamous relationship.
Women as choosy investors
Copulation for women is substantially more of an investment than it is for men. Because of this, women are quality not quantity specialists. Due to their reproductive rate and small reproductive window, women must be choosy in deciding when and with whom to copulate, and also whether or not to invest in the child once conception has occurred. First, she must make certain that her body has a reasonable chance of sustaining the pregnancy. Suppression or even loss of menstruation is very common in environments of poor resources and high stress. These factors are relevant to the woman’s current and past circumstances. Women who suffer from chronic stress will find her body adapting to the environment and she will still ovulate. On the other hand woman who are not adapt to high stress will stop ovulating. Environments that may make one woman stop ovulating, may seem like a walk in the park for another.
In addition to the environment, a woman should be picky about which man she chooses to copulate with. Two ways that her body can do this is by concealing ovulation and by favoring healthy and attractive males. The purpose of concealed ovulation has been argued. Some argue that it is to create uncertainty about who the father of the child might be. This way the woman can fool each man into believing they are the father and so encourage them to supply recourses and defense. This will make for a more likelihood of infant survival. Others argue that the purpose is to encourage monogamy. With concealed ovulation, the male must go through a period of courtship in order to ensure that she has not and is not copulating with other males. In addition, concealed ovulation cuts down the amount of aggression between males and against females. The author has come to the conclusion that concealed ovulation first came about in order to confuse paternity and prevent infanticide. Once it had evolved, the species in which it occurred switched to a monogamous mating system as to ensure the paternal investment of one male.
Once copulation has taken place, a women’s body still remains choosy. Whether or not conception occurs depends on the quality of the sperm and her own feelings about the male. Only about one million of every 600 million sperm are capable of fertilizing an egg. The job of the other 600 million is to serve as blockers from any subsequent sperm and as killers of any foreign sperm that is already present. This sets in motion a “sperm war” where only the most successful sperm win. Even still, the women have means of controlling whether conception occurs. Only a very small percentage of fertilized eggs make it through the first trimester. In addition, even after pregnancy is established, woman still exercise choice about continuing with it (abortion?) and even about continuing to care for the child after its birth (infanticide?). Abortion and infanticide occur for many reasons, and as awful as it may sound, it is sometimes in the mothers’ best interest to do so. This has been named the
Women as Heavy Investors and Pursuing Half the Point
Most infants, regardless of species, are much more heavily dependant on the mother than the father. When mother and infant are separated there are instant effects. An infant’s stress levels increase dramatically causing their immune system to weaken. Thus, there is a strong correlation of separation of mother and child and early infant mortality. This was observed in 1986 by Jane Goodall. Goodall observed a young primate sit by the corpse of his dead mother for weeks and eventually died himself from stress and hunger related illnesses. Repeated studies in
The dependence of child on mother is based on the need for nourishment, protection, as well as general affection. Lactation provides benefits for the mother as well. Breast-feeding suppresses ovulation, which allows for certain spacing to occur between infants. This suppression seems to be the cause of the slow population growth of the early human race. The doubling rate of current generations is much quicker due to the development of artificial methods to nourish newborn infants, thus allowing ovulation. The mother provides essential disease fighting anti-bodies and fat for the baby through lactation. In many species the mother protects the infant from harm from predators as well as other males in the group who are often guilty of infanticide. In humans, infants also require this protection from the many dangerous elements in the environment. Also, lactation causes the release of oxytocin in the mother brains, which seems to also reach the infant. Oxytocin causes a sense of bonding and comfort for both mother and infant, which increase the attachment and love for one another. Oxytocin is also released during orgasm.
The general interaction between mother and infant can be observed in three stages represented in chimpanzees. In the first stage the infant is completely dependant on the mother for protection and food. Little motor skills are possessed. In the second stage, the infant begins to develop the ability to communicate with others. Solid food is also consumed at this point. The mother is now used as a base for the child to return to after short excursions of exploration and interaction with peers.
Mothers seem to be much more familiar with their infants. Within 48 hours of birth, a mother can recognize her infant’s distinct cry. In addition, the mother can distinguish between a cry of pain and a cry of hunger.
Infants perceive males very different than females. This seems to be rooted in evolution because of the high rate of infanticide committed by animal males in order to impregnate the mother with their own genes. Thus, causing an instinctual fear in infants of strange males.
In general, mothers also seem to do almost twice of the care-taking activities as well as engage in much more activities with the child. Even in regions such as
The reproductive process requires much more investment from females than males. Males compete for females in order to spread their genes with little repercussions to their actions. Because of the high investment by females, they essentially call the shots and decide whom to copulate with. Thus a few males and many females can still effectively produce offspring compared to vice-versa of few females and many males.
Lastly, there seems to be an apparent neglect to any studies about parenting. The author, Anne Campbell seems to attribute this to the more exciting idea of mating over parenting. However,