A scientific paper was recently published which looked at how shifting patterns of daily light and darkness affect pregnancy in mice. The authors were interested in this question because studies have suggested that humans who experience such patterns, such as shift workers or women who travel repeatedly across time zones, have reduced fertility. In their study, pregnant mice were divided into three groups. All groups had 12 hours of light followed by 12 hours of darkness. The control group had the same pattern throughout the 21 days of pregnancy. The other two groups had shifting patterns. In one, the 12 hours of light started six hours earlier every five days (phase advanced group); in the other, six hours later (phase delayed group). In the control group, 90% of the mice had successful pregnancies and deliveries; in the phase advanced group, 22%; in the phase delayed group, 50%.
Circadian rhythms is the general term for biological activities that have a 24-hour cycle, like sleeping and waking, or like hormones whose amounts vary during the course of a day. There are many circadian rhythms in humans, animals, and plants.
They are internal, determined by the physiology of the animal or plant. However, they are also entrained (synchronized with) environmental events like the amount of light at night vs. during the day. This entrainment means the rhythms match what is going on in the environment and also can adjust to environmental change. In the pregnant mouse experiment, the light shifts were so large they disrupted the internal circadian timekeeper, which had cascading effects on mouse physiology and success in maintaining a pregnancy.
There are also many physiological rhythms that mesh with environmental patterns on longer or shorter time scales, for reproduction as well as many other aspects of biology. Zucker (1988), for example, found an annual rhythm to whether the amounts of a hormone called luteinizing hormone (LH) had a feedback relationship (that is, interaction) with the amounts of estrogen in ground squirrels. Typically in a mammal, LH increases estrogen production, and then when estrogen levels reach a high point the LH surges which initiates ovulation. For ground squirrels, who only become pregnant during January to March instead of having a regularly repeating cycle throughout the year, this relationship between hormones only exists during the breeding season. If the ovaries of females are surgically removed (so that their bodies don’t make estrogen), LH levels still go up to initiate the breeding season at the correct time of year; that is, levels of LH appear to be controlled by some environmental factor.
There are also social influences on the biology of reproduction in animals. Nelson (1999) summarized some of them: If four or more female mice were housed together in a cage, their cycles occurred less frequently. If they were then exposed to a male, they ovulated at the same time. In a study of albino mice, if a strange male was introduced into the cage of pregnant females, the females spontaneously aborted about 25% of the time. If the male who impregnated the female was re-introduced into the cage, there were no miscarriages. Female rats that were handled daily by researchers reached puberty at an earlier age than did rats who were not, and mice housed alone reached puberty sooner than mice housed with other females.
What does this mean for humans? There are not necessarily direct correspondences between animal and human research. Sometimes human physiology is simply differerent; sometimes, exactly the same. In addition, humans may have many influences where animals have fewer, so big, determining effects in animals may be mere suggestions in humans, one factor among many. On the other hand, the circadian research I discussed above was suggested by the possibility that shift workers and frequent travelers have fertility problems. Many social influences on human menstruation — synchronized menstrual cycles among college roommates, effects of stress — have been reported.
The possibility that intrigues me is this: We are individuals, but we are also intrinsically part of larger environments. Reproductive biology is about our inner organization of hormones, brain chemicals, goals and interests, but it is also about the viability and value of conception in specific social groups and physical environments. Our physiology is inside our skins, internal to us, but is also related to maintaining a state of balance with our physical and social environments.
Nelson, R. (1999). An introduction to behavioral endocrinology. Sunderland, MA: Sinauer.
Summa, K., Vitaterna,M., & Turek,T. (2012). Environmental perturbation of the circadian clock disrupts pregnancy in the mouse” PLoS One 7(5): e37668. doi:10.1371/journal.pone.0037668
Zucker, I. (1988). Neuroendocrine substrates of circannual rhythms. In D. Kupfer, T. Monk, & J. Barchas (Eds.), Biological rhythms and mental disorders (pp. 219-252). New York: Guilford.