¡Descarga Reproductive Patterns and Sexual Differentiation: Hormones and Behaviors in Mammals - Prof y más Apuntes en PDF de Psicobiología solo en Docsity!
Module 4: Theme
11a
Diversity of reproductive patterns
Mode of gamete production
Gonadal activity: development of eggs and sperm and increased sex steroid hormone secretion.
Mating: copulatory behaviour.
A) Parthenogenetic or asexual. B) Gonochoristic. C) Sequential hermaphroditic (protogyny y protandry). D) Simultaneous hermaphroditic. E) Serial hermaphroditic
Associated reproductive pattern: species that live in temperate regions where seasonal cycles are predictable and prolonged. Gonads are fully developed at the time of mating and circulating levels of sex hormones are maximal. Dissociated reproductive pattern: species that live in extreme environments where seasonal changes are predictable, but short. Gonads are small, and levels of sex steroids at the time of mating are low.
Constant reproductive pattern: species that live in harsh environments where breeding conditions are completely unpredictable. Gonads are maintained at nearly maximal development, so when breeding opportunities arise, mating can occur immediately
Development:
1. Oviparity: lay eggs, with little or no other embryonic development inside the mother. 2. Ovoviviparity: develop eggs that remain within the mother’s body until they hatch or are about to hatch. 3. Viviparity: the embryo develops inside the mother’s body. The mother then gives live birth.
Nourishment:
1. Lecitotrophy : the embryo is nourished exclusively by the egg yolk 2. Matrotrophy: the embryo is nourished by parental provisioning of extra2 0 1 0 vitelline nutrients i. Prenatal (placentotrophy) ii. Postnatal (lactation)
Origin and maintenance of sex
The twofold cost of sexual reproduction:
- Genome dilution (Williams): only half of the parental genome is transferred to offspring. - Male production (Maynard 2 0 1 0 Smith): females waste half of their resources on producing males, who in turn invest minimally in offspring.
Four hypotheses to account for the maintenance of sexual reproduction:
- Ratchet effect (Muller): Deleterious mutations can ratchet up over time in asexual species. Sexual reproduction is an alternative to rid the population from such deleterious mutations. - Lottery (Williams) : sexual reproduction provides the genetic diversity that may be adaptive in an abiotic environment which is highly unpredictable and changing. - Red Queen (Van Valen): in a biotic environment where species compete with one another (predators and preys), sexual reproduction would be more adaptive. - Co 2 0 1 0 evolution (Hamilton): of parasites and hosts.
will eventually form the accessory sex organs are dual. Male accessory sex organs require two products of embryonic testes: testosterone (T) and Müllerian inhibitory hormone (MIH).
Determination of sex in
mammals: genitalia
During development, a genital tubercle forms in front of the genital sinus. The urogenital ridges are flanked by two flaps of skin called folds. In the absence of hormonal influences, a clitoris develops from the genital tubercle, and the genital folds become the vaginal labia. In the presence of androgens, the genital tubercle develops into a penis, and the genital folds fuse into the scrotum
XX and XY cells are different prior to the secretion of gonadal hormones, and gonadal hormones affect XX and XY cells unequally.
Redefining sexual differentiation. In a twenty 2 0 1 0 first 2 0 1 0 century view of sexual differentiation of the brain, the importance of genetics and environment are incorporated along with the effects of hormones to provide a more nuanced portrayal of the types of variables that cause sex differences. The result is that some aspects of male and female brain, behaviour and physiology are unique from each other, whereas others are highly similar. Two important aspects of the redefined view are not illustrated here: sex differences are pervasive throughout the brain and not restricted to reproductively relevant neural circuits, and variability in the degree to which brain regions are masculinized or feminized in one individual results in a mosaic of relative maleness or femaleness and thereby greatly increases the variance between individuals of the same sex in a population.
Organizational/activational hypothesis
Hormones and prohormones involved in sexual differentiation of accessory sex
organs, external genitalia, and neural sex.
Testosterone [T] is a prohormone for an estrogen (estradiol, [E]) and for an androgen (dihydrotestosterone [DHT]). E is the actual hormone that masculinizes some species’ brains and DHT is the actual hormone that masculinizes some species’ external genitalia. T is the actual hormone which is involved in the masculinization of many species’ sexually dimorphic characteristics, including many that develop around puberty.
In primates in general, and in humans in particular, oestrogen receptors do not seem to play any significant role in the masculinization of their central nervous system. So, the aromatization of T into E is not involved in primate brains’ masculinization.
Hormones relevant in behavioural endocrinology
Hormones are chemicals, secreted by one cell group [endocrine and exocrine glands], that travel through the bloodstream to act on targets (within and outside the body, respectively.
Men tend to have a longer 4th digit relative to 2nd digit than women meaning that men have lower 2D: 4D rations than women
2D:4D ratio: low ratios tend to be associated with men and high ratios with women; nevertheless, there is a substantial overlap between both sexes’ distribution of percentages of ratios and percentages of individuals of each sex
- Releasing hormones: a class of hormones produced by the hypothalamus, that transverse the hypothalamus 2 0 1 0 pituitary portal system to control the pituitary’s release of tropic hormones. - Tropic hormones: a class of anterior pituitary hormones that affect the secretion of other endocrine glands (e.g., the gonads, the adrenal glands, etc.).
The hypothalamus-Pituitary-Gonadal axis: structures and hormones involved
The hypothalamus-Pituitary-adrenals axis: structures and hormones involved
Male sexual behaviour
Coolidge effect: after one bout of copulation the animals will not mate again for a period of time – this is called the refractory period. A Coolidge effect is claimed to be operating if this refractory period is greatly reduced when the male is exposed to novel females.
Environmental effects on female reproductive cycles: Whitten, Bruce and
Vandenberg effects
Hormonal correlates of parental behaviour in mammals
Copulation may or may not lead to fertilization; if it does, it is internal anyway and it becomes the starting point for a reproductive project, which in turn may or may not reach full term.
Gestation (or pregnancy) is also internal, it takes place within the female’s womb. There are a number of hormones that support pregnancy. In humans (and other primates), levels of estrogen (E) and progesterone (P) are high right up to the end of gestation, then they drop precipitously immediately postpartum. In contrast, levels of adrenocorticotropic hormone (ACTH) and cortisol increase throughout gestation.
In some species, stimulation of uterus/vagina during parturition is responsible for an elevation of oxytocin (OT) and endorphins. Both are important for the establishment and maintenance of mother 2 0 1 0 offspring bonds postnatally
After parturition, prolactin (PRL) levels increase and are responsible for the development of mammary glands, which are involved in lactation. The newborns’ survival in mammals is closely linked to their mothers’ capacity to lactate and so feed their offspring postnatally.
Suckling by the infant, and the stimulation of the mother’s nipple, is responsible for keeping high levels of anterior pituitary PRL and posterior pituitary OT.
Suckling effects
Suckling has two effects:
**1. Stimulates OT synthesis (milk synthesis and maternal behaviour)
- Inhibits DA synthesis and by so doing PRL levels are then increased**
Testosterone, prolactin, cortisol, vasopressin, pairbonding, fathering, and child
care
There is a relationship between T levels and relationship status (i.e., paired versus unpaired). Men involved in committed relationships had lower testosterone than those unpaired, and only in samples collected late in the day (six hours after waking).
Men who had not been in a relationship in the past 3 years, men who had been in a relationship in the past 3 years but were currently single, and men who have been in a relationship for 1– months, 7–12 months, or longer than 12 months. The early stages of such relationships may constitute a period of courtship associated with high T levels, followed by a deepening commitment, associated with lower T levels.
These results provide longitudinal confirmation that T exhibits a bidirectional relationship with reproductive strategy in human males.
- Single nonfathers with higher T at baseline were more likely to be partnered fathers 4.5 y later.
- After becoming partnered fathers, these men experienced dramatic reductions in both waking and evening T, which were substantially greater than the age 2 0 1 0 related declines observed in single nonfathers.
- (^) The finding that caregiving fathers had lower T than fathers who did not invest in care supports the hypothesis that father 2 0 1 0 child interaction likely contributes to suppressed paternal T among fathers.
