Today’s post draws on topics covered in the previous 2 posts, so it may be useful to browse those posts before reading this one.
Epigenetic inheritance of maternal care behavior in rats
Yesterday I told you about how the quality of maternal care result in epigenetic changes in glucocorticoid receptor expression in the hippocampus. The result of this epigenetic modification is that the offspring of neglectful or depressed mothers have stronger endocrinological and behavioral responses to stress than the offspring of mothers who were more nurturing.
Glucocorticoid receptor genes aren’t the only part of the genome experiencing epigenetic modifications in response to maternal care behaviors. The amount of care a mother directs towards her female offspring results in epigenetic changes to a specific type of estrogen receptor in the medial preoptic area (MPOA) of the brain as well (Champagne et al. 2003). This part of the brain is important in the expression of maternal care behaviors.
The result is that female rat mothers give their offspring the same quality of care that they themselves received as pups. Crumby mothers yield more crumby mothers and nurturing mothers yield more nurturing mothers.
It is, however, possible to get high-care mothers to decrease the amount of care that they provide to their offspring by subjecting them to stressful situations during gestation (Smith et al. 2004; Champagne and Meaney 2006). After being subjected to stress during this critical period, mothers who were once nurturing will decrease the amount of care that they provide to all subsequent litters. This finding highlights the importance of monitoring a mother’s health and stress-level while pregnant.
On the other hand, you can also get low-care mothers to increase care under certain conditions. One study found that removing pups from the litter and handling them before putting them back caused low-care mothers to spend more time caring for the pups (Francis et al. 1999). In these cases, the female offspring who were well cared for by a usually negligent mother went on to exhibit lots of maternal care behaviors towards their own pups as adults. This example highlights the important point that epigenetics alone does not dictate behavior, but instead that genetics, epigenetics and the environment are all important and interacting forces.
Epigenetic effects of maternal care on reproductive behaviors in adult female offspring in rats
Maternal care behaviors by rat mothers also have epigenetic effects on the mating tactics adopted by females as adults (see Cameron et al. 2005 for a review). The amount of maternal care behavior a female pup receives results in epigenetic changes to the same type of estrogen receptor referred to earlier, but in the ventromedial nucleus of the hypothalamus instead of in the MPOA. All you really need to extract from that jargon is that epigenetic changes are happening in a part of the brain that controls sexual behaviors.
The female offspring of rat mothers who were neglectful are far more promiscuous than females from high maternal care mothers. The neglected females engage in more behaviors intended to get a male’s attention and are more receptive to a male who is trying to mount her. Females who received more care are not only less attention seeking and promiscuous, but are also more aggressive to ward off interested male suitors.
Importantly, these behavioral differences translate directly into differences in reproductive output. In one study, 80% of the females from neglectful mothers became pregnant after spending time with a male rat while only 50% of the offspring from high-care mothers became pregnant under the same scenario. Interestingly, females from low care mothers also reach puberty earlier.
To summarize, we’ve discovered that the female offspring from mothers who provided little care reach puberty earlier, put more energy towards attracting males, are more willing to mate, are more likely to get pregnant after mating and themselves provide minimal care to their offspring. In essence, they’re producing lots of kids quickly and aren’t providing much care for them. This is analogous to what is called an “r-strategy” in evolutionary biology. Organisms adapting this strategy focus on producing lots of kids, even if they are of low quality, because the environment is harsh (or at least it has been harsh for them) and quantity is favored over quality.
On the other hand, females from mothers who provided lots of care reach puberty later, are less receptive to male advances (in fact they’re often downright aggressive towards males) and provide more care for their offspring. Organisms that focus on producing few offspring of high quality are said to be adapting a “K-strategy”.
We all have plenty of anecdotal evidence about females who were neglected as children seeking sexual attention from males at an early age. Perhaps future research will find epigenetic links to this behavior, but this area has yet to be explored.
It will also be interestomg to see if the future brings epigenetic treatments aimed at altering estrogen receptor expression in the MPOA in mothers with a history of neglecting their children. As rat studies have shown that female offspring have an epigenetically-programmed tendency to treat their offspring in a way similar to how they were treated, perhaps proper medical treatment could break family legacies of child-abuse in particular families. At the moment though, we’re unable to exert directed changes to the epigenome (for example, we can’t change epigenetic programming in one part of the brain without effecting it in another part).
I’m now off to go research the link between the environment, epigenetics and obesity, so you’ll have that to look forward to in tomorrow’s post!