Men’s ability to father children never goes away, even in old age. Women, on the other hand, have a finite window for procreation imposed by menopause. Doctors have long known that as women approach this limit they are at increased risk of having children with genetic disorders such as Down syndrome. The affects of men’s age on offspring, however, are just beginning to be understood.
The animal kingdom can offer some clues. Houbara bustards—North African birds that look like a cross between an ostrich and a roadrunner—suffer the usual aches and pains of old age. Males become less attractive to females, with less impressive displays and less lustrous feathers. But according to new research, the quality of their sperm also declines, and the detriments to their offspring are nearly equivalent to the impacts on chicks born to older females.
To arrive at these findings, an international team of researchers turned to long-term data collected from a group of captive birds housed at a breeding center in Morocco. For ten years, more than 1,000 bustards ranging in age from 1 to 23 years old had given up their sperm or undergone artificial insemination using that sperm. Altogether, the birds had produced nearly 59,000 eggs and more than 31,000 chicks. The researchers had access to records of whose sperm went to whom, and how the resulting eggs and chicks turned out.
Here you can see a male houbara bustard's famous "booming" display, used to attract females:
After both male and female birds passed their third to sixth birthday—the birds’ reproductive prime—their gamete performance began to slowly but steadily give away their age, the authors found. As they report today in Nature Communications, eggs produced by both older female and male bustards were significantly less likely to hatch than those of birds in their prime. Males that were 10 years past their prime carried an additional 7 percent risk of egg failure than younger fathers. The chicks that did manage to hatch didn’t escape age-related impacts, either. Those with an older parent of either gender weighed less at one month than nest mates with younger parents.
Based on what scientists know about reproductive biology and how the impacts to eggs and chicks manifested in the experiment, the researchers surmise that older males’ sperm is functionally less fit than in younger birds—it suffers from more deformities, and there’s less of it. The sperm is also genetically inferior, meaning the DNA it carries contains more mutations. This supports the results of a recent study that found that sperm from older human males carries more genetic mutations than eggs from older women. In humans, the mutation-heavy sperm carries greater risks of negative impacts on embryo development and results in lower numbers of successful births, that study found.
The bustard team adds that the situation is probably even more dire for older male birds than their results imply. Ornithologists at the breeding center only used ejaculates that contained at least 50 percent active sperm, but 65 percent of ejaculates from males 14 years or older didn't meet this criteria. Therefore, the results of the experiment are probably more favorable than what would be observed in the wild, since only the fittest seniors’ sperm was represented.
Either way, based on these findings, it seems that age-related declines in sexual prowess are indeed a true signal of an older male’s subpar genetic offerings to a potential mate. The results suggest that in both bustards and humans, postponing fatherhood until later in life can be done, but it carries certain unavoidable risks.