Richard Collins: Alpha males show lead in the pencil 

In some species, older alpha males produce more offspring and are not considered 'past it'

Rhesus macaques. It was thought males of high rank would have more offspring than lower ranked males. Not so.

In his book, Darwin’s Island, geneticist Steve Jones claims that ‘Turlough O’Donnell, who died in 1423, had 18 sons and 59 grandsons’. "A fifth of the men of northwest Ireland share a more or less identical version of the Y chromosome, which means that they all trace ancestry from the same male". 

A 12-point stag, commanding his harem, would be  Turlough’s equivalent among red deer. Likewise, the largest elephant seal bull rules the roost in a ‘rookery’. However, does being an alpha male, with virtually unimpeded access to compliant females, necessarily guarantee increased paternity? It has always been thought that it does, but a research paper, just published, challenges that assumption.

In 1938, about 30 rhesus macaques were transported from India to Cayo Santiago, a small island off Puerto Rico. Their descendants still live there. Visitors are barred from the ‘isla de los monos’, to minimise human impact on its primate inhabitants.

"There have been a number of studies that explore female reproductive senescence in humans and other primates", lead author Kirsta Milch told ScienceDaily, "but comparatively little work on male reproductive senescence". 

Addressing this imbalance, she and colleagues monitored the mating activities of 21 adult males on Monkey Island throughout a year. The babies born during the period were genotyped, revealing the identities of their parents.

In many primate communities, one male is dominant for a period, during which he fathers most of the troop’s offspring. With advancing age, however, he has increased difficulty maintaining his status. Eventually, he is challenged and deposed by a younger buck.

It was expected, therefore, that the high-ranking macaques on Monkey Island would father more offspring than those lower down in the pecking order. 

However, the research showed that this was not the case. The life expectancy of the males there is 14.4 years, the maximum lifespan being 29. "Older males had significantly lower reproductive output, particularly those aged 18 to 21 years" the study revealed. Alpha-male monkeys produced "very few, or no, offspring".

"Maids, when you’re young, never wed an old man" the song warns, but female macaques don’t shun elderly suitors in favour of younger ones. Old males had equal, or higher, access to receptive partners. Nor had they ‘lost their falorum’ or ‘their dingdorum’; they were observed copulating more often than younger studs. "Ageing rhesus macaque males do not have erectile issues", the researchers said.

The causes of reproductive senescence on Monkey Island, must therefore be ‘post-copulatory’. Perhaps the sperm of older studs is of inferior quality, or the offspring of elderly fathers die in infancy.

Zoologists tend to correlate copulations they observe with subsequent paternity; but "our results indicate that behavioural observations of mating behaviour are not reliable indicators of male reproductive output and should not be used", the researchers say.

"Infertility is a worldwide clinical problem for human health and affects 8% to 12% of couples" and "there has been a significant increase in parental age in recent years", say the authors. Although the monkeys are only distant cousins of ours, the findings of this research might have implications for us.

Krista Milich et al. Age negatively impacts reproduction in high-ranking male rhesus macaques on Cayo Santiago, Puerto Rico. Scientific Reports. August 2020.