Pleasure, pain, hormones, nature, nurture… There are many players that determine when or even if sex happens. In this short, 3-parts series, Susana Lima, head of the Neuroethology lab at Champalimaud Centre for the Unknown in Lisbon, tells us about her work on cracking the neural circuits that wire up sexual behaviour.
Part 1: Finding “the one”
Susana Lima has been studying the neural circuits that control sexual behaviour since she joined the Champalimaud Foundation in 2008. She is certainly not one to beat around the bush. She talks about topics which leave most of us blushing from head to toe – erection, ejaculation, menstruation – with the ease and humor of a true scientist.
“Life without sex would not only be less interesting, it would be impossible,” Susana begins with a smile, “but despite its obvious importance for the existence of the species, unlike feeding, it is not actually necessary for the survival of the individual. You may feel less happy, but you will live. In fact, sex might even be dangerous! Think about it, during sex individuals have to remove many levels of self defence. So, given this complicated balance of benefits and risks the question is – how does the brain ensure that we and other animals engage in safe and productive sexual behaviour?”
Susana’s passion for understanding the neural mechanisms that control sexual behaviour began when she was finishing her postdoctoral work at Cold Spring Harbour Laboratory in the United States. As she recalls, “At that moment, two events came together that determined both my personal and professional paths. The first was coming across a book called The Mating Mind by Geoffrey Miller, which ignited my interest in the field, and the second was meeting Zach Mainen who became my husband that same year.”
“You know the feeling when you meet someone and there is some click? When Zach and I got together what happened between us was very special and different. More than love, it was how natural it felt, how effortless. It made me wonder – what were the features that my brain was looking for and identifying, for the first time, in this person? And more generally, how does the brain know when it finds ‘the one’?”
Susana was set on finding an answer to this question and so, when she started her lab in Lisbon, she began by studying Mate Choice, which underlies that special “click” that lead individuals to prefer one partner over another.
In order to do that, together with Léa Zinck, a postdoctoral researcher in the lab at the time, Susana decided to take advantage of a naturally occurring situation involving the house mouse.
As Susana explains, “It is not commonly known, but about 10,000 years ago the house mouse was a single species. As men started migrating with agriculture, following different routes, the house mouse went with them. Some migrated to northern Africa, others to northern Europe and some to Asia. Throughout the years, the house mouse species began diverging, leading to the creation of two subspecies. These subspecies were physically separated, until they met some 3000 years ago in the middle of Europe. What is beautiful about this, is that once they met, they didn’t mix. This created a surprisingly sharp border in the middle of Europe running all the way from Denmark to Bulgaria, where the two subspecies live in close proximity, but don’t really mate.”
Why don’t they mate? What are the features that the brains of these mice are looking for and not finding at that mysterious borderline? In order to find out, Susana and Léa looked for the conditions under which these two mice subspecies would mate.
The team devised a series of experiments that included tethering males from each subspecies in separate compartments to allow the female to choose freely. They also tested the female’s preference during different periods of their reproductive cycle.
In an article published in June 2013, the pair shared their discovery. “We found that two variables determined the female’s choice – Reproductive State and Availability.”
“When the reproductive state of the female was conducive to pregnancy, they would prefer males of their own subspecies. However, if they were in the phase of their reproductive cycle where pregnancy was not possible, they did not exhibit a special preference towards males of either subspecies.”
It appeared that reproductive state was a very important part of the answer, but it still wasn’t a deal breaker for the females, availability was. “If they didn’t have the option of mating with a male of their own subspecies, they would readily mate with the other type. It showed us that females were not all that picky when it came down to it.”
But still, they do have a preference, otherwise that borderline in the middle of Europe would have been quite a bit fuzzier. Susana and Léa wondered whether this preference was something the mice were born with, or something they learned from their environment. In other words, was the preference the result of nature, or nurture?”
In a separate study, the pair tried to untangle the two factors by directly testing the effect of nurture. Specifically, they conducted a set of experiments where female pups were placed in foster families of the other subspecies and then had to choose mates as adults.
The results were surprising – half of the females who were raised in foster families prefered males of their own subspecies, while the other half preferred the “wrong” alternative. “We were puzzled by these results at first. This type of 50:50 split was not what we were expecting. But then, by looking more carefully at the behaviour, we observed it was actually very simple: they preferred whichever male they saw first! It was all a matter of chance! Since they could freely choose which side to go to, some females would randomly visit the ‘right’ male first, while some would visit the ‘wrong’ one. Then, the first male they saw became their prefered choice. These results were different for females who were raised within their own subspecies. They would visit both males, and only then decide.”
Why would that happen? What role did upbringing play in the eventual preference development of these mice?
“In nature, females don’t mate with the first male they come across. Instead, they compare males in order to get the best available option. It has been hypothesised that this capability is acquired through a learning process called sexual imprinting, which happens during early life. We would like to propose an alternative, or at least, an add on to this hypothesis: that sexual imprinting, in addition to constructing a set of features of an ideal mate, prompts females to follow favourable decision strategies such as ‘mate comparison’ by inhibiting suboptimal ones such as ‘go for the first one you see’. In mice raised in foster families, this doesn’t seem to happen, showing that finding ‘the one’, at least when it comes to mice, is a result of neither nature, nor nurture, but of an interaction between them.” She concludes.
Liad Hollender works as a Science Writer at the Science Communication Office at Champalimaud Research
Edited by: Catarina Ramos (Science Communication office)
Photo Credit: Liad Hollender