Effect of yohimbine on the reproductive behavior of the male Nile crocodile Crocodylus niloticus.

The effect of yohimbine, an alpha 2-adrenergic blocker, on the reproductive activity of 12-year-old male, captive Nile crocodiles was tested on 18 animals equally divided into 3 ponds. In each pond, there were 6 males and 30 females. In the first group, six of six males received yohimbine capsules of 30 mg, twice a day, for 1 week. In the second group, two males of a group of six were administered a similar dose of yohimbine. Another group of six males in a separate pond were used as a control group. The yohimbine treatment had a significant (p less than 0.05) effect on the frequency of headslap display (the main behavioral pattern of reproductive activity) in all treated animals but did not result in a significant increase in copulation frequency. In the two treated groups, the reproductive period was prolonged by 3 weeks, 8-11 weeks. We found a significant increase in the fertility percentage of eggs laid by females in the male-treated groups (30.5% in control group females, 34.1% in the partly treated, and 39.0% in the pond where all six males were treated), even though no increase in mating frequencies was noted. This might be due to more successful copulations. During the treatment period, the daily activity peak, when the male's headslap performance was the most frequent, was shifted from 5:00-6:00 p.m. in the control to 4:00-5:00 p.m. in the treated group. This effect caused females to enter the water earlier than in the control group.