Individual differences among female rats in the timing of the preovulatory LH surge are predicted by lordosis reflex intensity.

In young female rats, we found individual differences in LH surge characteristics that could underlie differences in fertility throughout adulthood and aging. We modified a jugular cannulation technique, which enabled us to characterize four successive proestrous LH surges in regularly cycling female rats. During a surge, the timing and amount of LH were negatively correlated, so that late-onset surges reached a low peak, late in the day. Within a female rat, the timing of the LH surge, but not the blood level of LH, was stable from cycle to cycle. Thus, differences in onset and peak time between successive LH surges within one female were less than differences among females. Moreover, these individual differences in timing of the LH surge were positively correlated with individual differences in lordosis reflex intensity on proestrus. Females with early-onset LH surges had high average lordosis reflex intensity on proestrus. Therefore, lordosis reflex intensity is a nonintrusive behavioral biomarker for individual differences in the timing of the preovulatory LH surge. This relationship between behavior and the preovulatory LH surge may reflect a hormonal basis for variations in reproductive strategies throughout the life span.