Oct 21

The birds, the bees, and the rats

For many years, American society has pushed the idea that men control sex. But if pop culture has taught us anything, it’s that women enjoy being in charge, too. When scientists began to research sexual behaviors in animals, they assumed that during sex, males were the initiators and females played a more passive role. As research advances, scientists are finding that’s not the case. Much how in Khia’s famous song “My Neck, My Back,” she sings explicitly about dictating when and how she wants to have sex, female rats want to control their sexual encounters in similar ways. Female rats both solicit sex and want to control the pace. And female rats only find sex rewarding when they’re the ones who get to call the shots.

By studying sexual behavior in rats, we might be able to learn about something about humans.

Like humans, female rats aren’t fertile all the time. In humans, we’re familiar with the menstrual cycle, in which ovulation (the release of an egg from the ovary) occurs about once a month. Female rats have a similar cycle, but they ovulate multiple eggs every four to five days. From the standpoint of evolution, it makes the most sense for sex to happen when the chance of pregnancy is the highest – when eggs are present in the fallopian tube and can be fertilized by sperm. To maximize these odds, ovulation is precisely timed so that when the eggs are released, the rats are awake to have sex.

Female rats will only solicit sex during this period of fertility. To attract a male rat, a female will wiggle her ears, hop and dart around the cage, and present herself to the male in a position that allows him to mount her. These mating behaviors are dependent on the hormones estrogen and progesterone, which are only high during the fertile part of the rat reproductive cycle. When estrogen and progesterone levels are low, females won’t display any of these mating behaviors and will avoid males altogether. Males’ interest in sex isn’t as variable, and they engage in mating and sex behaviors consistently. But if given the choice, female rats will dictate when they have sex, and will only do so when they are likely to become pregnant.

Scientists have also shown that if they can, female rats will control the pace of sex. But do they like sex in this case? It’s hard to determine whether rats really “like” things because we can’t ask them directly. Instead, scientists study reward. Animals find something rewarding if they like it, they actively seek more of it, and they can learn to associate it with something previously neutral. Scientists often measure reward using a test called conditioned place preference. In this test, they can couple stimuli like drugs, food, or sex with a certain physical location. Over time, animals will learn that the stimulus is only given in one place and associate the two. Scientists infer that if the animal spends more time in the place where the stimulus is given (i.e. develops a place preference), the animal must find that stimulus rewarding.

When male and female rats are placed in the same cage, the male will initiate sex with the female regardless of whether she is fertile or not. If this interaction occurs in a conditioned place preference test, females only develop a very weak preference for the location with the male, indicating that they don’t find the interaction that rewarding. But mating conditions in the lab aren’t what rats would experience in their natural environment. Unlike in lab settings, in the wild, female rats have places to hide from males. And in the lab, when female rats are given a place to hide, their reaction to sex changes. That’s because female rats prefer to control the frequency and timing of sex, a behavior referred to as paced mating. When they are given a place to hide from the male rat, female rats can choose to “take breaks” during sex, and in this case, they find sex rewarding.

“Scientist doing experiments with rat in a laboratory” (Editor’s note: this photo gravely misrepresents how scientists work with animals, we just thought it was funny.)

Dr. Katie Yoest recently conducted a study where she had female rats choose between getting access to a male rat or access to a tasty snack. She found that infertile female rats chose the treat more often than a male rat, but fertile female rats preferred the male rat over the treat. Dr. Yoest said that she went into her study thinking that the important part of the pacing behavior was going over to see the male. But she saw that when the females moved back away from the male, they remained focused on him and not other things in the cage. She explained, “That was something that we thought was kind of surprising… it does seem like this pacing, sort of ‘time out’ period… does seem to be more of an active state than we originally thought it was.”

Many chemicals in the brain control whether rodents and humans find things rewarding, and in the case of paced mating, reward is controlled by dopamine and opioids. Although we sometimes think of dopamine and opioids in the context of drugs of abuse, dopamine and opioids are produced by the brain during natural events too. Although the details are still being teased out, scientists think that dopamine is involved in the motivation to engage in paced mating behavior in the first place, whereas opioids are more involved with the rewarding feeling after mating. When we disable dopamine or opioid signaling, female rats don’t develop a place preference for paced mating. This means that the motivational and rewarding aspects of paced mating depend on signaling by dopamine and opioids and their receptors.

There’s still a lot we don’t know about female sexual behavior in animals and humans, but current research seeks to better understand mating dynamics and the biology that underlies them.  Given the prevalence of biases about female sexuality, it’s important to remember that the more open-minded we are, the more we can learn. But at least for now, one thing is clear: female rats only find sex rewarding when the timing is right.

By Liza Brusman