Big horned rhinoceros beetles are healthiest
The size of a male rhinoceros beetle's horn is a genuine indicator of its health, according to researchers.
The horns vary in size from small bumps to two-thirds of the insect's body length and are used in fights.
Investigating the variation, US scientists found cells in the horn are more sensitive to "nutrition signals" than cells in other parts of the body.
They suggest their findings could explain the evolution of super-sized body parts in the natural world.
Battling beetle facts
- Rhinoceros beetles are some of the world's largest insects and the longest species of beetle. Dynastes hercules can measure up to 17cm long.
- They are members of the scarab family and number more than 300 different species.
- Living in tropical areas across the Western Hemisphere, males fight rivals on a variety of terrain including sugar cane, tunnels in the soil, emerging branches and bamboo shoots.
The study, led by Dr Douglas Emlen from the University of Montana, US, is published in the journal Science.
Although scientists have long assumed that exaggerated body parts accurately represent the ability of a male to survive and reproduce, the link has not been proven.
To understand the relationship, Dr Emlen and colleagues compared the beetle's horn with other body parts including the wings and genitalia.
They found that the horn's cells were much more sensitive to "nutrition signals": fluctuations in insulin due to diet quality and resistance to illness.
This discovery explained the differences in horn sizes between high and low quality males but it also offered an explanation of how the horns grow to such impressive sizes.
Dr Emlen explained that these insulin pathways are also known to regulate tissue growth and body size. Therefore if a body part contains cells that are more sensitive to these signals it will grow to reflect the health of the beetle.
"If the horn cells are super sensitive to these nutrition signals, then the same mechanism that makes horns huge in high-quality, good condition males will also make horns especially tiny in low condition, poor-quality males," said Dr Emlen.
Referred to as "weapons", the beetles use their forked horns to fight off competition from other other males to "win" access to females for mating.
Many other species exhibit similarly oversized features to advertise their health and help either warn off rivals or woo partners, in a process known as sexual selection.
In the past, researchers have debated whether it is possible for such body parts to "lie": duping females or foes with misleading signals.
But Dr Emlen and colleagues found that the horns were honest.
"Our surprise finding that the same change in mechanism that appears to make an ornament or weapon especially big also makes them hyper-variable and super-sensitive to nutrition, means that these structures are likely to be intrinsically reliable as signals," he explained.
He also suggested that the team's findings could apply to other animals including those sporting "ornamental" features to attract attention such as long-tailed birds of paradise.
End Quote Dr Douglas Emlen University of Montana, US
These structures are likely to be intrinsically reliable as signals”
"Historically, when bigger versions of traits cropped up, they would have been conspicuous and receivers, choosy females or rival males, that noticed them and responded to them would have fared well, since the trait is an unfakeable signal of male quality," said Dr Emlen.
"This would have favoured receivers who paid attention to these traits, and individuals with bigger and bigger traits.
"The bigger they got, the more reliable they would become, enhancing this process and helping drive the evolution of bigger and bigger size."
Dr Emlen pointed to the evolution of insulin-sensitive cells in exaggerated body parts as the key driver in one of the natural world's most recognisable patterns.
"The critical properties that have long been known to apply to nature's most extravagant structures, the showy ornaments and weapons of sexual selection, all fall out as a logical consequence of this one simple evolutionary change," he told BBC Nature.