Behavioral Ecology Blog

There is an exquisitely interesting paper in this edition of The American Naturalist (Sex in the Morning or in the Evening? Females Adjust Daily Mating Patterns to the Intensity of Sexual Harassment ), providing yet another layer of complexity on the issue of sexual conflict. Basically, the authors Løvlie and TommasoPizzari studied feral populations of chickens to reach their conclusions. The story goes like this:

1. As per usual, the ideal rate of copulation is greater in males than in females.
2. This is usually thought to be as a result of the different cost:benefit ratios between the sexes. For females, costs are higher, so females limit access to eggs both by limiting # of males and # of copulations.
3. If the primary cost of mating to females involves the use of eggs, then females should be less resistant when there are no eggs to be fertilized (of the probability of fertilization is less)- especially when resistance to male advances is costly. Female resistance to male copulation attempts has been shown to be costly in many species.
4. In chickens, the probability of fertilization is greater in the evening. As a result, both male harassment and female resistance is greater in the evening than in the morning. Basically, the stakes are higher for all involved.

Here is the Abstract:

Selection on males to mate at a higher rate than females often results in male harassment of females and counteracting female responses. When the reproductive value of copulation changes over time, these mating strategies are expected to be time dependent. Here, we demonstrate that variation in the intensity of male harassment leads to drastic changes in female daily mating patterns. In feral populations of fowl Gallus gallus domesticus, male harassment is intense, particularly in the evening when insemination’s are most likely to result in fertilization. We experimentally manipulated the intensity of male harassment through similar-sized groups of different sex ratios. Male mating propensity was always higher than females’, particularly in male-biased groups and in the evening, when males were closer to and more likely to approach females. Females counteracted male harassment by escalating resistance to mating and- crucially- by shifting their daily mating pattern: in strongly female-biased groups with relaxed sexual harassment, females solicited sex in the evening, while in male-biased groups, they solicited sex in the morning, thus avoiding harassment in the evening. Together, these results indicate that intersexual conflict may occur not only over mating rates but also over when in the day to copulate.

There is an interesting point to be made here in terms of human mating behavior… Although the study is of chickens, the basically cost benefit analysis and set of evolutionary forces are identical. So here it is… Think about it…. One of the parameters that determines the probability of fertilization in humans is the amount of semen retained in the vagina. Now think about the issue of leakage when standing upright after sex versus remaining recumbent.. *ask a woman if you can’t figure this one out*. Now generally speaking, females are more likely to remain recumbent in the evening/night (during sleep) than they are in the morning… If this is the case, than the probability of fertilization is greater in the evening/night. As you might expect if these things are in fact true, that nighttime copulation is more costly for females, and more valuable for males. It would be an interesting human study to determine if in fact males pressure females for sex more in the evening, and if resistance is resultingly higher. I suspect that you might find that it is..

So the off shoot to this is that for males, if “scoring” is what your goal, you might just meet less resistance in the morning than you would in the evening, all else being equal.. In addition to this, you have the potential additional benefit of reduced risk of accidental fertilization.