What drives the probability of being born male or female?

A new study published today by Marwell Wildlife (UK) and University of Alcala (Spain) has shown that the genetic quality of fathers plays a significant role in determining the sex of their offspring.

It has long been known that the condition of mammalian mothers, often determined by the quality of their environment, can influence the sex of their offspring. The role of fathers in determining sex of offspring has been either discounted or unexplored until recently. 

The study, published in the journal Proceedings of the Royal Society B, used captive breeding records from the international studbook for a critically endangered species of antelope, the Eastern bongo. 

The paper “Drivers of sex ratio bias in the Eastern bongo: lower inbreeding increases the probability of being born male” was co-authored by Aurelio Malo from the University of Alcala and Tania Gilbert and Philip Riordan from Marwell Wildlife, a conservation charity that owns and operates Marwell Zoo in Hampshire. 

Dr Aurelio Malo, Ramón and Cajal Researcher at University of Alcalá, said: “In short, two things influence the sex of the offspring; one is how inbred the father is and the second is how inbred the offspring is.” 

Fathers with higher levels of inbreeding themselves were more likely to sire males than females. The mothers’ level of inbreeding only influenced the number of offspring, but not their sex. Furthermore, offspring that are less inbred have a higher probability of being born male. 

Dr Tania Gilbert, Conservation Biologist at Marwell Wildlife, said: “This opens up a new avenue of research to predict sex ratio allocation in an applied conservation context.” 

Animal populations are becoming smaller in size and increasingly fragmented, according to findings from recent UN assessments of nature, produced by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). 

Dr Philip Riordan, Head of Conservation Biology at Marwell Wildlife, who has also worked on the IPBES assessments, highlighted the conservation implications of these findings: “The potential impact of this research for isolated populations of wild mammals, where inbreeding risk can increase, could be to further reduce their chances of survival, ultimately exacerbating an already perilous state.” 

The paper “Drivers of sex ratio bias in the Eastern bongo: lower inbreeding increases the probability of being born male” is published in Proceedings of the Royal Society B.