As Earth turns, so does our physiological clock. Daylight keeps cascades of timekeeping molecules in sync as they perform regulatory dances throughout our bodies, sometimes partnering with sex hormones, sometimes waltzing alone, but always leading the way.

A surprising study led by the University of California, Davis has now discovered timekeeping molecules that dance to a completely different tune.

In the large black chafer beetle (Holotrichia parallela), a species of scarab beetle, sex hormones seem to take the lead and create an internal rhythm that cycles over 48 hours instead of the typical 24-hour clock.

Because this strange internal clock takes twice as long as a circadian cycle to start and finish, it is known as a 'circabidian' rhythm.

While a few animal behaviors are known to align with a circabidian cycle, including those of humans, mosquitoes and beetles, the mechanism that generates this cycle is an ongoing mystery.

"Observations of 48-hour rhythms in nature are rare," says University of California, Davis entomologist Joanna Chiu, who was not involved in the research. "This elegant study by Professor Leal and his collaborators has provided us with an in-depth description of how the circabidian rhythm of pheromone detection in this beetle is generated."

Infamous for being a severe agricultural pest in Asia, this lucky species of scarab beetle gets to enjoy a date night every other night. On these nights, females emerge from snug soil homes to scale to great heights (like up a small peanut plant) to beckon their mates with enticing pheromones, called l-isoleucine methyl ester (LIME) and l-linalool.

Males respond to this perfume like clockwork, according to cellular biologist Yinliang Wang from the Chinese Academy of Agricultural Sciences and colleagues.

First, the team identified the beetle's sex pheromone receptors, and monitored their activity through their molecular products and an electroantennogram which measures the response of the beetle's antennae. For comparison's sake, the researchers also tested the beetles' responses to a plant volatile.

While the beetle receptors responded to both LIME and l-linalool alone, the strongest signals arose when both pheromones teamed up together.

This double reaction of the antennae revealed the same bi-nightly (24 hour) pattern in both chemical and electrical signals, whereas the beetles' response to the plant volatiles did not cycle.

The results are baffling, as biological cycles usually keep in time with external environmental cues, such as light or temperature. But there are no known natural cycles that adhere to 48 hours.

"The circabidian rhythm reduces mating opportunities but minimizes predation," Wang and team explain in their paper. "However, H. parallela is not under known predation pressure."

Previous research suggests that although darkness doesn't disrupt this cycle, light still has some role to play, as removing their optic lobes knocks the beetles out of sync.

So there appears to be "some mechanism for doubling clock cycles in the circuit between the clock cells and behavioral command neurons," the team concludes.

At some point unfavorable conditions must have benefitted the beetles that restricted their emergence to every two days. But what on Earth caused their bodies to respond to such a unique tune, remains an intriguing mystery.

This research was published in Current Biology.