There’s one scent to rule them all – and we now know what it is. A series of experiments has shown that a single molecule released by the queen of naked mole rat colonies prevents all the other females in a colony from breeding.
“It’s a super-contraceptive, if you’re a mole rat,” says Gary Lewin at the Max Delbrück Center in Berlin.
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Naked mole rats (Heterocephalus glaber) have a social structure like that of bees and ants, with colonies made up of soldiers and workers, and a single queen ruling each colony. Only the queen can breed, but how she maintains her long reign – Lewin’s team’s oldest queen is 39 – hasn’t been clear.
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“The theory was that the queen is larger and more aggressive than the other animals, exerting her dominance through pushing and shoving,” says Lewin. “But we never found that very satisfying as an explanation.”
So team member Mohammed Khallaf, also at the Max Delbrück Center, proposed identifying the mole rat bouquet – the molecules in the air around them that create their scent. Comparing the scents of hundreds of animals revealed that only the queens produce a molecule called isopropyl myristate.
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“It’s made in the reproductive organs, basically the vagina of the reproductive female,” says Lewin.
When the team sprayed isopropyl myristate daily into cages containing male and female pairs, none of the females became pregnant. Without it, almost all the females became pregnant.
Next, the team removed a queen from a colony and applied isopropyl myristate daily. There were no fights for succession and no females started breeding during the three months this was done. “We produced peacefulness,” says Lewin. “That’s probably the most dramatic experiment.”
When the team stopped applying isopropyl myristate, the high-ranking females started fighting within a week. After around three weeks one became pregnant: the new queen.
The team also showed that exposure to isopropyl myristate changes the levels of the hormones progesterone and prolactin. But they haven’t found out exactly how the molecule is detected and leads to these changes – that’s the next project, says Lewin.
The evidence for isopropyl myristate influencing reproduction is compelling, says Markus Zöttl at Linnaeus University in Sweden. “I think it’s an impressive and important study. And convincing.”
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Chris Faulkes at Queen Mary University of London is also convinced. “But the paper raises many questions, like any interesting research,” says Faulkes. These include how animals detect it, and how behavioural interactions and queen dominance interact with the scent, he says.
There is something special about isopropyl myristate, says Lewin. Isopropyl myristate is volatile, meaning it can evaporate into the air, but it’s not highly volatile, so any traces left by the queen take time to evaporate and the scent persists for at least a day.
It’s known that a queen will patrol every part of her colony, which in the wild might extend underground for 3 kilometres. “We think the reason she does that is to deposit this molecule around the colony,” says Lewin. “To make sure that every member of her colony is exposed to her scent.”
Other experiments by the team suggest the animals can consciously detect the smell. For instance, highly ranked females with a chance of becoming queen try to avoid places where isopropyl myristate is present, whereas lower-ranked animals aren’t bothered.
The team also tested a number of other species of mole rat. They didn’t find isopropyl myristate in any solitary species but they did find it in a few species whose social structure is more like that of naked mole rats. “But I would be cautious about assuming that the same pathway has a comparable function across social mole rats without direct experimental evidence,” says Zöttl.
Isopropyl myristate is also widely used in cosmetics. It is described as odourless but Lewin says some women at his lab thought they could smell something when exposed to it. A 2008 study also reported that it is released from the nipples or areolas of women during pregnancy and after childbirth.
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