A quarter of active-duty male soldiers in the US Army have low testosterone, according to a study of nearly 500 personnel. This has implications for the recent that all US service members aged 30 and older must undergo annual testosterone screening and be offered voluntary replacement therapy if they have low levels.

鈥淔or soldiers with genuine, symptomatic testosterone deficiency, appropriate treatment can improve symptoms and important aspects of physical health, including sexual function, body composition, muscle mass and bone health,鈥� says at the University of Melbourne, who wasn鈥檛 involved in the research. 鈥淗owever, there is currently insufficient evidence that [it] will improve military readiness, operational performance or fighting capacity.鈥�

at Pennington Biomedical Research Center in Louisiana and his colleagues analysed data from the , which measured testosterone levels and other health markers in active-duty personnel at five military centres across the US between 2024 and 2026.

Among the 479 male personnel tested, 26 per cent had testosterone levels below 300 nanograms per decilitre, classified as low by the . Based on this, the researchers estimate that 100,691 male soldiers in the US Army could have low testosterone readings.

Older male personnel were more likely to have low testosterone, with 33 per cent of those aged 30 and older being classified in this group. Female personnel weren鈥檛 included in the analysis because their testosterone levels were often below the limits of detection.

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When the testosterone-boosting initiative in July, Pete Hegseth, the US defence secretary, noted that testosterone levels naturally decline with age and said the decision to offer replacement therapy to soldiers with low levels was 鈥渁bout restoring and optimising your natural capabilities, protecting your longevity, ensuring you have the biological foundation required to sustain the fight鈥�. There are no ramifications for choosing not to have the therapy, he said, adding that personnel under the age of 30 will also be able to opt for testosterone screening.

But a single testosterone test isn鈥檛 a reliable measure of deficiency, says Katz. 鈥淭estosterone levels can fluctuate considerably, even within the same individual, and may be influenced by numerous factors, including the time of day blood is collected, recent food intake, inadequate sleep, obesity, acute illness and excessive physical or psychological stress,鈥� he says.

Even if testosterone levels are consistently low in blood tests, that doesn鈥檛 necessarily mean someone has a deficiency, says Katz. The American Urological Association that a clinical diagnosis be made only if an individual also has related symptoms, such as reduced energy, motivation, libido and erectile function.

Low testosterone levels may protect women from kidney injury

Men get acute kidney injury 鈥� where kidney performance suddenly declines 鈥� at a higher rate than women. An experiment with mice suggests that this could be related to having higher levels of testosterone

鈥淭estosterone should be prescribed to treat a medical condition, not simply to achieve a higher number on a blood test or as a general strategy to enhance the performance of otherwise healthy soldiers,鈥� says Katz. 鈥淚 would favour a more targeted approach [to diagnosing and treating testosterone deficiency] based on symptoms, clinical findings and relevant risk factors rather than automatically screening every man over the age of 30.鈥�

Some of the side effects of testosterone replacement therapy include acne, testicle shrinkage and reduced sperm production and fertility. It also increases the risk of (blood clots in the lungs) in older men.

at the University of Queensland in Australia says there is a risk that high testosterone levels will be conflated with being a good soldier. 鈥淭estosterone is already increasingly being used as an oversimplified proxy for masculinity and virility,鈥� he says.

WHAT鈥橲 the point of old men? We women have had our postmenopausal existence justified, in evolutionary terms, by the grandmother hypothesis. But the same cannot be said for men. The mystery is why they live long past their physical prime, with muscles turning to flab and potency waning.

In How Men Age, we have an answer. Biological anthropologist Richard Bribiescas covers some interesting uncharted territory. This is not a mere description of ageing. Instead, by considering male ageing in the light of natural selection, it aims to answer big questions including why men鈥檚 lifespans are shorter than women鈥檚, why baldness, prostate disease and erectile dysfunction are so prevalent, and how humans as a species have benefited from men鈥檚 tendency to run to fat.

From an evolutionary perspective, nothing matters more than sex. And as far as men are concerned, nothing influences sexual power more than testosterone. It increases libido, promotes muscle growth and encourages risk-taking behaviour 鈥� all of which help attract a mate. But testosterone peaks in early adulthood, so that men are past their physical prime by the age of 30. It鈥檚 tempting to see it as all downhill from there. But with wit and insight, Bribiescas shows convincingly that鈥檚 not the case.

He points out that testosterone has a dark side 鈥� it can increase a man鈥檚 metabolic rate and suppress the immune system. In other words, there鈥檚 a trade-off, or as Bribiescas puts it: 鈥渕acho makes you sick鈥�. High levels of the hormone early in life help explain why men don鈥檛 live as long as women and why they are prone to prostate cancer later on. So waning testosterone can be seen as a positive development. It may make older men less physically competitive against younger ones, but men can produce offspring throughout their lives and, argues Bribiescas, as they age they develop new reproductive strategies to achieve this.

For a start, although they may lack raw strength, their experience and guile often make them better providers than their younger counterparts. Bribiescas has done fieldwork with the Ache people of Paraguay, and points to research showing that men鈥檚 hunting success peaks in their 40s, long after their testosterone levels peak.

What鈥檚 more, older men tend to become more nurturing. As testosterone decreases, a man鈥檚 girth increases, and the metabolic changes associated with growing adiposity promote care of offspring. Bribiescas calls this the 鈥減udgy dad hypothesis鈥�, and argues that it has implications for the evolution of our species as a whole.

Humans live far longer than other primates. For longevity to evolve, natural selection must favour long-lived individuals. Older women cannot reproduce, so they are out of the running. But if, throughout human history, pudgy older men have been fathering children, then they will have passed on genes associated with longevity to both daughters and sons.

Old men, therefore, could be the reason we all live so long. It would appear there is some point to them after all.

This article appeared in print under the headline 鈥淕olden oldies鈥�

This lion king (pictured above) is a queen. And she鈥檚 not the only one.

Five lionesses in Botswana have grown a mane and are showing male-like behaviours. One is even roaring and mounting other females.

Male lions are distinguished by their mane, which they use to attract females, and they roar to protect their territory or call upon members of their pride. Females lack a mane and are not as vocal.

But sometimes lionesses grow a mane and even behave a bit like males. However, until now, reports of such maned lionesses have been extremely rare and largely anecdotal. We knew they existed, but little about how they behave.

Now, Geoffrey D. Gilfillan at the University of Sussex in Falmer, UK, and colleagues have reported five lionesses sporting a mane at the Moremi Game Reserve in Botswana鈥檚 Okavango delta.

Gilfillan started studying these lionesses back in March 2014, and for the next two years he focused on recording the behaviour of one of them, called SaF05. She had an underdeveloped mane and was larger than most females.

鈥淲hile SaF05 is mostly female in her behaviour 鈥� staying with the pride, mating males 鈥� she also has some male behaviours, such as increased scent-marking and roaring, as well as mounting other females,鈥� says Gilfillan.

鈥淎lthough females do roar and scent-mark like males, they usually do so less frequently,鈥� he says. 鈥淪aF05, however, was much more male-like in her behaviour, regularly scent-marking and roaring.鈥�

A likely explanation is an increased level of testosterone as these lionesses mature, says , president and chief conservation officer at the global wild cat conservation organisation Panthera.

In lions, testosterone directly affects the development of manes. Castrated males, for example, lose their ability to produce testosterone and promptly lose their mane, too.

In 2011, a at the National Zoological Gardens of South Africa developed a mane. Tests revealed high levels of testosterone due to a problem in her ovaries, and once they were removed she reverted to a typical lioness.

The idea that testosterone is implicated in the Botswana lionesses is also backed by observations of their reproductive success, says at Virginia Tech in Blacksburg.

鈥淲hile some of the maned lionesses were observed mating, none of them became pregnant, suggesting they are infertile, a known consequence of high levels of androgens such as testosterone in females,鈥� she says. 鈥淭he behavioural changes suggest this is likely the case.鈥�

Hunter suspects this explanation applies to the animals studied by Gilfillan and his colleagues. 鈥淕iven all five known maned females come from the Okavango region, there must be a genetic component in this population underlying the phenomenon,鈥� he says.

鈥淚 don鈥檛 think this is anything to be concerned about,鈥� says Hunter. 鈥淎lthough the females are apparently infertile, they otherwise appear to live long, healthy lives. And from a conservation perspective, there is nothing to suggest the pattern is increasing or will ever be anything more than a rare, local phenomenon.鈥�

No one seems to be studying the exact genetic and hormonal causes of this phenomenon at the moment. 鈥淚 guess there are just one or a few genes altered,鈥� says at Imperial College London, who had a student briefly work on the possible causes. 鈥淚 believe some masculinised genes have been documented in domesticated cats 鈥� it would be good to look into this, especially given that the cat genome is available as reference.鈥�

Could the masculinised females in fact be a boon to the pride when it comes to competing with other prides? It鈥檚 possible, it seems.

Gilfillan says he once saw SaF05 bring down a zebra. 鈥淎 neighbouring pride stole the zebra from SaF05, but in return SaF05 killed two of their cubs.鈥�

Cub-killing behaviour is rare in females but common in males.