Biological Rhythms - Infradian

What are endogenous pacemakers?

Internal body clocks that keep biological processes to time

What are exogenous zeitgebers?

External cues that entrain internal body clocks, meaning altering body clocks to match the environment

What is an infradian rhythm?

A biological process that repeats less frequently than once every 24 hours (eg weeks/months)

What is the main example of an infradian rhythm?

The menstrual cycle

The menstrual cycle

A 28 day cycle with a 6 day period of fertility, however cycles can be shorter or longer ranging between 24 and 35 days. Biological aspects of this cycle include releasing the egg (ovulation), thickening of the womb lining and losing the womb during menstruation. These biological changes are regulated by hormones oestrogen (pre ovulation) and progesterone (post ovulation). The levels of these hormones act as endogenous pacemakers keeping the biological processes to time, acting as an internal body clock for the menstrual cycle

Seasonal affective disorder (SAD)

An infradian rhythm that takes a year to complete. Emotional mood lowers in winter and improves in summer. It is suspected that this is due to the lack of bright sunlight in winter

AO3 - McClintock (1971)

Studied 135 women who lived in the same dormitory at university, recording period onset and duration. Results showed women appeared to synchronise their cycles with their friendship groups. Other researchers have demonstrated similar findings when women cohabit (Russell 1980)

AO3 - Stern and McClintock (1998)

Investigated if synchronisation was due to women sharing pheromones. 20 women were given pads to wipe on their top lips each day. The pads had been taken from the armpits of 9 donor women at varying stages of their menstrual cycle. The researchers found that the women would either shorten or extend their menstrual cycle to match the donor, depending on when in the donor woman’s menstrual cycle the pad had been collected. Suggesting synchronisation due to the presence of pheromones, acting as an EZ, an external cue for the timing of the infradian rhythm

AO3 - Replications of McClintock and Russell

Some replications have had positive results, there have been a number showing no synchronisation, including Trevathan (1993) showing cohabiting lesbian couples did not synchronise, even though there were optimal conditions for possible synchronisation

AO3 - General limitations

• Pheromones as chemical messengers are not accepted widely as a process that happens in humans

• As menstrual cycles can vary in length, women can appear to synchronise just due to their variability in their cycles, something not taken into consideration in all of the studies showing positive results

• Or maladaptive due to women wanting to be fertilised by the male with the best genes, and needing to compete with other women for him if they synchronised

AO3 - Evolutionary theories are conflicting

Suggesting synchronisation is advantageous, a way to stop one male dominating a group and having all the children, being unable to impregnate all the women at once due to synchronisation, leading to genetic diversity in small human groups