The four Odontolabis sommeri lowei beetle morphs, (L-R) Alpha, Beta, Boltcutter, Gamma

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The stag beetles with bolt-cutter jaws

Adult males of some stag beetle species have been found to exhibit more jaw- and head-size variations than previously thought.

A fourth form has been identified in at least two species of Odontolabis stag beetle. This number is currently unparalleled in any other stag beetle genus.

Male stag beetles usually have two types of mouthparts, known as mandibles. Each may offer the beetle different advantages in finding and competing for mates.

There are also a few species that have three different mouthpart variations. These are known as trimorphic species.

But new research shows that in some species of Odontolabis, found in southeast Asia, males have one of four mandible forms, making them tetramorphic.

Keita Matsumoto, a curatorial assistant at the Museum and author of the study, explains just how unique these tetramorphic beetles are.

More mandibles, more complexity

Keita explains, 'Two morphs are standard for stag beetles with males that vary in size. 

The three Odontolabis cuvera morphs (L-R Alpha, Beta, Gamma)

Odontolabis cuvera is a trimorphic species, with (L-R) Alpha, Beta and Gamma mandible variations

'But in this genus, Odontolabis, it's quite complex. It's got dimorphic species, trimorphic, and then there is also tetramorphic, which is a world first.

Keita's study introduced the fourth variation, named 'Boltcutter'. This newly identified morph is seen in the species Odontolabis brookeana and Odontolabis sommeri lowei - both found across the islands of Borneo, Sumatra and Java.

Why do stag beetles have different mandible shapes?

As well as studying head size, Keita examined the shape of the mouthparts.

One possible reason for the differences in stag beetle mouthparts is that they could each offer advantages and disadvantages when it comes to finding a mate.  

However, no official studies of differing behaviour have been published to date. 

Alphas (or major morphs) are the largest of the four types, with the longest and most elaborate mandibles. The size and shape of the mandibles may make them ideal for wrestling, allowing the beetle to pick up rivals.

Betas could also be suited to a life in the wrestling ring, although their head-size and mandibles are distinctly smaller and less elaborate.

Gammas are the smallest of the four morphs and so would be unlikely to stand a chance against an Alpha or Beta in a fight - but there is hope for them.

Odontolabis sommeri lowei boltcutter

A boltcutter morph of O. sommeri lowei

Keita speculates, 'The minor males may display a sneaking tactic. They approach the females, mate and then run off.

'Minor males have short mandibles. They weigh less, so their mobility is increased and might be higher than the Alpha morph. They can travel further and increase the rate of encounters with females.'

As for the new Boltcutter morph, its behaviour is not currently known. This study was completed using mounted Museum specimens, so their behaviour in their natural environment hasn't yet been studied - although their robust pincers seem adapted for cutting, perhaps to directly injure rivals.

Keita says, 'It would be quite interesting to see what kind of tactic they use. Are they more useful for fighting or gripping? Are there any downsides to having these big, flashy mandibles?'

Why are Museum collections important?

The new research was based on the Museum's Coleoptera collection, centring on the Odontolabis specimens mainly from the previously private collection of Hugues E Bomans.

During his life, Bomans published 113 papers on stag beetles, describing eight new genera and more than 200 new species. The collection, which proved vital to the study, was acquired by the Museum and holds an astonishing 25,000 specimens.

Of the research, Keita says, 'I looked through the collection, took photographs, collected data, then analysed the data to find patterns.

'It has highlighted the importance of accumulating specimens over several generations, maintaining the collection to world standard and allowing scientists to conduct their research.'