3. Sex: Evolutionary, Hormonal, and Neural Basis Flashcards
Reproductive behavior can be divided into 4 stages
Sexual attraction, appetitive behavior, copulation, postcopulatory behavior
Sexual attraction
Brings males and females together, which may be synchronized with:
- Physiological readiness to reproduce
Females of many species won’t engage in sexual activity unless ovulating
- Learned associations, such as appearances or displays
In some species, mates males and females will form pair bonds
Appetitive behavior
Establishes, maintains, or promotes sexual interaction
- If a female is willing to copulate, she is said to be sexually receptive and exhibits lordosis (also referred to as estrus or heat)
- A proceptive female rat may approach males, engage in hopping and darting, perform ear-wiggling, or produce ultrasonic vocalizations
- Male appetitive behaviors include staying near the female, sniffing, nest-building, etc.
Appetitive phase lasts longer than the copulatory phase
Copulation
Involves one or more intromissions in which the male penis is inserted in the female vagina
Following stimulation, the male ejaculates sperm-bearing semen into the female
After copulation, a refractory phase follows
Length varies across species
The Coolidge Effect
The Coolidge Effect
Refers to the faster resumption of mating behavior with a different partner
Postcopulatory behavior
Varies across species
In a copulatory lock, occurring in dogs and some mice, the penis swells temporarily and cannot be withdrawn from the female
What are the 2 purposes of gonads?
To produce steroid hormones and gametes
Gonadal steroids activate sexual behavior
Circulating gonadal steroid hormones are required for males and females of many species to engage in sexual activity
Gonadectomy (removing gonads) eliminates sexual behavior
Estrous cycle
In rats, females ovulate, or release eggs, every 4 or 5 days
This is called the estrous cycle
What is the estrous cycle controlled by?
Hormones, via the hypothalamic-pituitary-gonadal axis (HPG axis)
Anterior pituitary
What determines the estrous cycle’s length?
The rate-limiting step is the time it takes to develop the egg
Testes
Produce and secrete testosterone, which is an androgen
Male HPG axis
GnRH from hypothalamus causes an increase in LH (from anterior pituitary), which regulates testosterone production in the Leydig cells of the testes
Not pulses, but steady and constant
Production of testosterone and sperm remain fairly constant once they begin at puberty, unlike the cyclical nature of the female cycle
Male castration
A castrated male loses interest in mating because testosterone is no longer produced, behavior is restored with testosterone treatment
Level of T does not seem to matter, in that having higher levels does not further increase sexual behavior except in seasonal breeders
Female HPG axis
The hypothalamus kicks everything off by releasing gonadotropin releasing-hormone in a pulsatile fashion
GnRH stimulates anterior pituitary to release follicle-stimulating hormone and luteinizing hormone
FSH acts on the ovary to cause development of the follicle; LH increases release of estradiol from the ovary
Estradiol acts to increase further LH secretion (positive feedback), resulting in a surge of LH that causes ovulation
Progesterone production then increases, inhibiting the hypothalamus from releasing more GnRH (negative feedback)
If the egg isn’t fertilized, progesterone levels drop, and the cycle starts over again
Female hormonal contraceptive
Oral contraceptives contain synthetic progesterone that inhibits release of GnRH
The lack of GnRH prevents release of FSH and LH, so no egg is released by the ovary
Active ingredient in the pill
Sometimes estrogens are added to the pill, but progesterone is the active ingredient
Female sexual behavior: Receptivity
The estradiol and progesterone produced during the ovulatory cycle are also important for female sexual behavior, increasing her receptivity and motivation for sex during behavioral estrus
The hormones that lead to ovulation are the same ones that increase her receptivity so she responds appropriately
A female rat who is not in estrus
will fight a male who makes advances
A female rat who is in estrus
will exhibit lordosis (arched back) when a male mounts her, allowing for copulation to occur (vaginal canal accessible)
The point of having the same hormone do these different things is:
- It’s more efficient because she’s mating when she has the highest chance of becoming pregnant
- Focus energy and motivation
- Energetically efficient
Avoid diseases and predators
_____ is crucial to the lordosis response through steroid actions
the ventromedial hypothalamus
Estrogen’s central action
Increases dendritic branching and connections of neurons in the VMH and periaqueductal gray (PAG)
estrogen’s action in the peripheral nervous system
Estrogens act in the sensory fields to increase sensitivity so female will have lordosis posture
Estrogens also stimulate production of progesterone receptors
Lordosis
The VMH sends projections to the PAG and the midbrain, which projects to the medullary reticular formation in the hindbrain
This, in turn, projects to the spinal cord via reticulospinal tract
The neural circuitry of the brain regulates reproductive behavior: males
In male rats, the medial preoptic area coordinates male copulatory behavior
The mPOA sends axons to the ventral midbrain and then to the basal ganglia to coordinate mounting
Axons project through brain stem nuclei to the spinal cord
Pheromones
Chemical signals that communicate info between animals of the same species to help coordinate their reproductive activities
vomeronasal organ
important for activating male response to pheromonal signals
VNO info is sent to accessory olfactory bulb, which then projects to the medial amygdala and then, in turn, to the mPOA
the MPOA
Integrates hormonal and sensory info, such as pheromones, and coordinate the motor patterns of copulation
Hormones in animals
Hormones can signal readiness within an animal or provide info between animals
Ex: pheromones in urine of mice and goldfish
Human sexual behavior
Humans and other primates are capable of engaging in sexual behavior at any point in their cycles
There have been some reported effects of hormones on sexual motivation
Some studies show a peak in sexual motivation around time of ovulation
A low dose of T can stimulate interest and activity in men
Goal of reproduction
Pass along your genes to the next generation
Altricial animals
Born in an undeveloped state and depend on parental care, as human infants do
Precocial animals
Born in a more developed state and are able to survive with relatively little maternal care
Rat mothers show 4 behaviors
Nest building
Retrieving pups
Licking and grooming
Nursing
Female rats who haven’t had their own litters will not act maternally
Rat mothers during pregnancy
During pregnancy, hormones change her brain to display these maternal behaviors, and the smell of pups is no longer aversive
Licking and grooming
Licking and grooming is a way to program offspring’s behavior when they’re adults → epigenetics (changing gene expression to change protein outcomes)
Low licking
Methyl groups are added → stress hormone receptor is reduced → reduced opportunity for negative feedback → less stress hormone receptors means more stress hormone is produced → high-stress levels, high anxiety, they end up being low licking and grooming
High licking
No methyl groups added → high expression of stress hormone receptor
