Just when we thought we couldn’t admire the naked mole rat any more, we learn that parenthood extends their lifespan.
In other animals, procreation is good for the species but bad for the individual, at least physiologically. This latest wrinkle in the ways naked mole rats break the rules was reported Wednesday by scientists from Germany’s Leibniz Institute on Aging, in BMC Biology.
It is physiologically illogical for breeding naked mole rats (Heterocephalus glaber) to live longer than ones who don’t breed. So how do they achieve this?
Let us start with some mole rat basics. For one thing, they are native to parts of Africa and the Levant, including Israel. Each subterranean colony has a single breeding female, who has one consort. Beneath them, as it were, are up to 300 workers who do not breed but spend their lives foraging for roots to eat, digging with their teeth, and being unpleasant to the neighbors, it seems.
“There is quite a lot of intracolony aggression. If the queen dies, there is intense aggression between high-ranking females,” remarks co-author Dr. Martin Bens of the Leibniz Institute.
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The nonbreeding mole rat seems ramped down: Worker females do not ovulate; the males have smaller testes than the active ones and produce less sperm. Under the right circumstances, though, they can ramp up to reproduce, Bens says.
One would expect the workers – not hassled by courtship, sex or rearing progeny – to outlive the harried royal couple, who typically have hundreds of offspring before they flop over and die.
Not so. “Although the breeding pair carries the metabolic cost of reproduction and, in the queen’s case, lactation, they live longer than nonbreeders and remain fertile throughout their lives,” the team reports.
That flouts the conventional wisdom that mammals either invest resources in a long life or in reproduction, Bens explains.
To investigate, the team studied the naked mole rats’ genetics.
A roundup of naked mole rat fun facts includes extraordinary resilience to cancer – though, like sharks, they are not entirely cancer-free. The reason people thought they were is that around 2,000 mole rat necropsies found no cases of invasive cancer. But in 2016, Bens explains, cancer was reported in the little rodents.
Also, naked mole rats live strictly underground and don’t see well: they can scuttle along their tunnels either forward (degenerated eyes first) or backward (tush first) with apparent equal facility. Being mammals, they have fur – like elephants do: not much, and not obvious, but it’s there. They look rather like penises with feet.
Finally, naked mole rats also have bizarrely long life spans for their diminutive size – around 30 years in captivity, and scientists have reported that the naked mole rat does not age. That sounds preposterous.
"They mean that the animals look healthy and behave normally until very old age, and then 'suddenly' die," Bens explains. “So the progressive deterioration that can be observed, for instance in humans, does not manifest to such a high degree in naked mole rats. Queens usually give birth till their last days of life."
So, yes, the clock ticks for the naked mole rat, too – but when they become breeders, their aging rate slows, Bens and the team discovered.
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All mole rats have mole rat DNA, but it is expressed differently in different individuals.
The researchers analyzed the transcriptomes – meaning, all the proteins produced from the DNA of breeding and nonbreeding naked mole rats. They also compared the results with a benchmark of breeding and nonbreeding guinea pigs.
Breeding and nonbreeding naked mole rats had different transcriptomes; the breeders also differed from guinea pigs.
The difference lay in the expression of various genes related to aging, the team explains. Hundreds of genes are known to be involved in life span, Bens points out.
For instance, breeders had higher expression of a gene involved in muscle regeneration, which could help them fight age-related muscle loss, in turn extending life span.
Another surprise lay in the expression of male/female differences. Nonbreeding naked mole rats are sexually monomorphic: we observe few differences in body size, body mass or external genitalia – looking between their legs isn’t helpful at sexing them. “Their external genitalia look very similar,” Bens tells Haaretz. Males and females also evince few behavioral differences.
What the team found, astonishingly, is that gene transcription in male/female naked mole rats was not significantly different among nonbreeders. But everything changed if they did develop into breeders.
Take worker mole rats and give them a love interest, and they morph into breeders – at which point they certainly do evince sexual dimorphism. Genes associated with sex steroid hormone production are activated in both sexes.
“We also find differences in genes associated with mitochondria [the ‘power producer’ of the cells], immune system and fat metabolism. All of the processes are involved in aging,” says Bens.
Bens is hopeful that more investigation of the naked mole rat transcriptome data can shed light on sexual maturation in humans. We could use more information: puberty seems to be coming on earlier, among Western girls for instance, with implications for public health. The reasons for the early onset are not thoroughly understood, though environmental elements, such as synthetic chemicals emulating hormones, have been fingered as possible culprits.
This is of more than academic interest. “Variations in puberty onset have implications for the risk for diseases such as breast cancer or cardiovascular diseases,” Bens points out. says. “Our data may help identify targets to mitigate these variations.”