Measurement and cycle variability: reexamining the case for ovarian-cycle synchrony in primates

Critical reviews of menstrual synchrony studies, over the last 15 years, strongly suggest that synchrony may be a methodological artifact. The primary focus of this paper is on reexamining the evidence for synchrony in the two other primate species: chimpanzees (Pan troglodytes) and golden lion tamarins (Leontopithecus rosalia). It is demonstrated, with computer simulation, that cycle variability can yield misleading measurements of synchrony when it is assumed that the mean absolute difference between points in cycles (e.g. menses onsets, onset of anogenital swelling, peak estrogen levels) is 14 the mean cycle length. This together with inappropriate statistical analyses can lead to the spurious detection of synchrony. In addition, computer simulations indicate that the null hypothesis (i.e. that females do not synchronize their cycles) should not have been rejected in either of these studies. These errors suggest that synchrony in non-human primates is also a methodological artifact. I conclude by suggesting that: (i) from the perspective of female mate choice, synchrony is likely a state to be avoided; and (ii) we must be especially vigilant of studies reporting the discovery of mechanisms of synchrony.