PARENTAL BEHAVIOR

What is parental behavior?

Parental behavior evolved to supplement physiological mechanisms of reproduction. In other words, parental behavior is thought to have evolved when fertilized eggs did not develop, hatch and result in the survival of the young.

Species that exhibit parental care produce fewer young

Young develop more slowly, but are more likely to survive to reproductive age with parental care.

Modes of reproduction in which there is parental care:

viviparity - live bearing

oviparity - egg laying

ovoviviparity- eggs retained in reproductive tract (guppy) or pouch (marsupial frog)

Among the mammals and birds almost all species exhibit parental care. The exceptions are species of bird such as the cowbird, and the cookoo that are parasite egg layers.

In mammals, parental behavior varies with the mode of reproduction

the monotremes (platypus) are oviporous but after incubating the eggs the female exhibits parental behavior and also lactates and nurses the young. The marsupials give birth within the time period of the reproductive cycle (and hormones are not different from the estrous cycel). Hormonal basis of parental behavior is most highly evolved in the true mammals.

In birds, parental behavior is hormonal dependent and is initiated with the onset of brooding behavior and continues through the period when the young are being fed with crop milk.

Patterns of parental behavior are related to the developmental status of the young at birth.

There are of course other factors. For example the nutritional composition of the milk can influence the amount of time a species spends nursing. In rabbits and hares, the milk has a very high nutritional content and mothers nurse only once a day for a few minutes. Rats nurse their young at 2-3 hour intervals. Hooded seal pups nurse almost continuously (there of course is also an influence of the amount of energy needed by the pup to survive in its particular habitat and environment).

How developmental factors affect parental behavior - Types of Young

ALTRICIAL YOUNG

(immature at birth) -> cannot see or hear, unable to locomote, cannot thermoregulate

PRECOCIAL YOUNG

(young well developed) -> have vision and hearing, can locomote themselves, thermoregulate alone, need mother for food (nursing)

SEMI-PRECOCIAL YOUNG

(young intermediate in development) -> eyes and ears open, cannot locomote.

 

PARENTAL BEHAVIOR PATTERNS

Nesting pattern - build nest before eggs are laid or before birth of young -> altricial young

---> Birds, monotremes, mammals with altricial young

The "Pouch" is a variation on the nest pattern

-> kangaroos, wallabies, opossum, altricial young are nursed in mother's pouch

Leading-Following Pattern (precocial young)

Mother leads young who follow

-may nest for a few days

- typical of parents who must continuously forage for nutrition

-> mammals with precocial young; chickens, waterfowl (imprinting of young); sea mammals (whales and porpoises).

Clinging-Carrying Pattern (semi-altricial young)

young cannot keep up with mother so cling to her

mother also place young in temporary nests (cribs)

-> semi-altricial young (primates)

-> The exact pattern varied with the foraging pattern and social organization of the group (grazing/availability of food, does male contribute, etc)

 

THE HORMONES OF PREGNANCY (In the Rat)

1. Copulation - induced neuroendocrine reflex resulting in progesterone release from the corpus luteum and implantation/placenta formation

2. LH & Prolactin from Anterior pituitary maintain pregnancy for 10-12 days.

3. Placenta maintains pregnancy with its own versions of LH and Prolactin

4. Pituitary terminates pregnancy: prolactin and prostaglandin F2alpha induce estradiol release from the ovary. Oxytocin induces uterine contractions. Relaxin relaxes smooth muscle.

MAMMARY GLAND DEVELOPMENT - exemplifies hormone-behavior relations in parental behavior and behavior plays an important role in the physiological changes that occur.

Phase One: Hormonal Control

-estrogen stimulates duct development

-progesterone stimulates proliferation of the secretary cells in the aveoli of the mammary gland

-prolactin stimulates synthesis of milk by secretory cells.

Phase Two: Behavioral Activation

-pregnant females must lick nipples and genitals to get 100% mammary gland development

-experiments with collared females vs brush stimulation show that when there is no licking milk production is reduced.

Phase Three: During Nursing

-Prolactin stimulates milk synthesis

- Oxytocin stimulates milk ejection - very powerful effect.

The mother rat exhibits maternal behavior for 3-4 weeks after parturition.

- hormones of pregnancy and parturition initiate maternal behaviors

- after behaviors are established all behaviors except lactation are maintained without hormones

- behavior of young maintain hormone-independent maternal behaviors and lactation.

There are four components of maternal behavior in the rat:

1) nest building

2) pup licking

3) nursing

4) pup retrieval (search out pup in response to ultrasound, carry back pup)

Prepartum onset of maternal behavior associated with:

1) prepartum decline in progesterone and increase in estrogen

2) uterine contractions (oxytocin)

3) vaginal-cervical stimulation

Also : maternal aggression elicited by estrogen then is maintained during lactation by presence of the pups. Declines as the pups are weaned.

How is maternal behavior initiated hormonally?

1) Estrogen in POA (bilateral) activates maternal behavior

antiestrogen in POA (bilateral) 2 days before parturition prevents the onset of maternal behavior postpartum

2) The longer a female has been pregnant the more rapidly she becomes maternal and the lower the dose of estrogen required to activate maternal behavior

-> priming during pregnancy prepares for hormonal activation of maternal behavior

Estrogen in POA during 2nd 1/2 of pregnancy

-> estrogen receptors increase between GD 1 -> 13 and then are maintained until parturition (day 22)

-> similar changes in ER do not occur in other areas of the hypothalamus during this time (not a general phenomena, no sexual behavior induced)

Rise in estrogen at termination of pregnancy and withdrawal from progesterone necessary for onset of maternal behavior

-> prolactin also increases and is necessary for maternal behavior

-> oxytocin in estrogen primed females influences maternal behavior (prevents maternal cannibalization)

lesions of oxytocin cell bodies in nucleus produces deficits in pup retrieval, nest building, licking, etc

 

MATERNAL BEHAVIOR IN THE SHEEP

Young are precocial (leading-following pattern)

In a herd the mother and lamb must identify each other. The mother learns to recognize her lamb by its vocalizations, odor and appearance. For this, the mother-infant bond is essential. The ewe rejects all other lambs and will not permit them to suckle. Maternal behavior is established during parturition (narrow window of about 45 min). Hormones are necessary for maternal care of young (licking) and for maternal imprinting.

1. Estrogen priming is essential for maternal behavior in the sheep. Progesterone and prolactin are not important for maternal behavior in the ewe.

2. Oxytocin stimulates uterine contractions resulting in parturition. During parturition cervical-vaginal stimulation and the release of oxytocin are thought to be essential for the establishment of matrnal behavior. Endorphins also released during parturition act synergistically with oxytocin to stimulate maternal behavior. The ewe approaches the lamb, bleats, permits suckling, and sniffing, etc.

- oxytocin also alters the olfactory system so that amniotic fluid is attractive to the ewe (ewe licks off the lamb) -> this behavior is important for subsequent identification of the lamb.

-> oxytocin, therefore, sets up behavior and stimuli for formation of mother-infant bond in sheep.

***This is a central effect of oxytocin in the sheep. CSF concentration of oxytocin increase during this time. Oxytocin administered to the CSF of estrogen-primed females will induce maternal behavior. Once bond is established, hormones are not needed to maintain the behavior.

The formation of the mother-infant bond is dependent on the olfactory signals from the lamb and the amniotic fluid. Oxytocin changes the properties of the amniotic fluid, so that it is attractive to the ewe.

SUMMARY

1. The hormones of pregnancy and parturition are the hormones that promote maternal behavior

2. Exact maternal behaviors are species-specific and depende on the developmental status of young at birth

3. In general

-> estrogen acts in POA to prime matrnal behavior systems

-> activation of maternal behavior can be induced by estrogen, prolacxtin, and/or oxytocin (depending on the species)

-> Once maternal behavior is established it can be hormone-independent