So, you have finally found the partner of your dreams - but no matter how hard you try, no children have come along. Could science offer new answers to mate choice and infertility?
- Text Marianne Mustonen
- Photos Jaakko Heiskanen, Niko Jouhkimainen and Jukka Kekäläinen
For several years, researchers at the University of Eastern Finland have been studying human reproduction and mate choice through the lens of evolutionary biology.
“This continues to be an important topic of research since it offers new information on factors such as how we choose our reproductive partners. We are attempting to discover new mechanisms that lead to failures to reproduce,” says Professor Jukka Kekäläinen from the Department of Environmental and Biological Sciences.
The starting point for Kekäläinen’s research group was to see whether they could improve the diagnostics of the reasons for infertility by studying the communication mechanisms between gametes.
The current definition describes infertility as a disease of the reproductive system, presumably arising from pathological factors related to the man or the woman. However, determining which partner is the cause for the infertility is eventually impossible in up to 30–40% of studied couples.
In studies, it has been tested if biochemical communication between gametes might function as a mechanism of mate choice, improving the probability of fertilisation in couples who are compatible in regard to gametes.
However, Kekäläinen says that often it is not a question of one of the partners having poor quality of gametes. More likely, it is possible that some immunological factors make the gametes of certain couples less compatible than of others.
“Just because your gametes might not be a match, does not mean that the couple would also be otherwise incompatible, for example in terms of personality,” he points out.
Only the most genetically compatible sperm might be able to fertilise the egg
The research undertaken by Kekäläinen’s group has shown that there may also be genetic compatibility differences between reproductive partners.
“Sperm are under sexual selection in the female reproductive system. This challenges prior understanding, according to which it would be completely random which sperm cell fertilises the egg,” says Kekäläinen.
The results of the study suggest that the differences between the human leucocyte antigen (HLA) genes of males and females influence the sperm’s ability to fertilise the egg. Some previous studies have claimed that women may prefer the body odour of men who have different HLA genes to them when choosing a reproductive partner, but the results of the studies are rather conflicting.
“We also do not know whether men with different HLA genes have a better chance of fertilising the egg,” he points out.
In other words, the fertilising ability of the sperm might be strongly dependent on the genetic compatibility of the couple.
“It seems that the encounter of gametes is a very selective event where particularly the genes of our immunological system play a role.”
Definition of infertility should be broadened
Infertility treatments provide options for people who are struggling to have children, but they are not a cure-all. Many women fail to get pregnant despite several attempts, or it takes multiple rounds of treatment. Many couples never discover the exact reason behind their infertility.
For this reason, diagnosing infertility is very difficult. Kekäläinen thinks that the definition of infertility should be broadened.
Sexual selection influences the likelihood of fertilisation.
Jukka Kekäläinen
Professor
In natural fertilisation, the female reproductive system only allows a very carefully selected sperm population to proceed to the vicinity of an unfertilised egg. This phenomenon is known as the cryptic female choice, and it usually favours the sperm of a genetically compatible partner.
“The cryptic female choice is a common phenomenon across the animal kingdom – as well as in humans, as recent research suggests,” says Kekäläinen.
“The genetic compatibility of the couple's gametes is likely based on a complex, network of male and female genes.”
Partner’s good odour does not guarantee fertilisation
The sense of smell has a big role in mate choice – the rows of perfumes in stores indicate as much.
Is it possible that if you think your partner smells good, their gametes are more compatible with your gametes?
This was the research question in the latest study at the University of Eastern Finland.
“The human leucocyte antigen genes that are essential for the functioning of the immune system are known to mediate mate choice at both the individual and gamete levels. Previous studies have shown that HLA genes clearly influence odour-based mating preferences meaning that our mating decisions are affected by the HLA genes we and our potential partners carry,” says Kekäläinen.
However, it remains unclear whether mate choice based on HLA genes operates in the same way both before and after intercourse, i.e. at the individual and gamete levels.
The study analysed the HLA compatibility of male-female couples, taking into account both classical and non-classical HLA genes. The results showed that women preferred the odour of men with HLA genes similar to their own, but sperm motility was higher in the presence of the sperm-activating chemical factors (follicular fluid) of HLA-dissimilar women.
“This suggests that the men who women consider the most attractive based on their odour might not be the best partners in terms of fertilisation,” says Kekäläinen.
In other words, mate choice processes at the individual and gamete levels may be conflicted. Mate choice at the gamete level may play a crucial and definitive role since it may prevent the egg from becoming fertilised when the partners are genetically incompatible.
Studying the compatibility of gametes more extensively on a genome-wide scale is the next step for Kekäläinen’s research group.
“We are looking for new genes that could act as mediators of mate choice at the individual or gamete level.”
This story is part of UEF Insight, the University of Eastern Finland’s online magazine.
The theme of our 2/2026 issue, sex and gender, examines sexuality, sex and gender from a variety of perspectives, including health, reproductive biology and human interaction.
