**2.3 Estrous cycle length**

Estrous cycles were monitored by daily vaginal lavage. Lavages were analyzed according to criteria described elsewhere (Freeman et al, 1994). In addition, receptive behavior (hopping and darting, ear wiggling and lordosis posture) was monitored daily. To this end, a naive male WU rat was introduced briefly in the female's home cage around ZT 11, 1 hr before dark onset to confirm a proestrous lavage typing.

Cycle length was defined by the last two monitored cycles before sampling. Most females displayed regular 4-day estrous cycles (70%). Cycle lengthening was observed in 8.5 mo old females: regular 4-day cycles decreased from 70% to 45%, while both 5-day cycles (from 10% to 24%) and acyclicity (from 0% to 10%) increased.
