''Geodorcus sororum'' is a large flightless species of

stag beetle Stag beetles are a family of about 1,200 species of beetles in the family Lucanidae, currently classified in four subfamilies.Smith, A.B.T. (2006). A review of the family-group names for the superfamily Scarabaeoidea (Coleoptera) with corrections ...

in the family

Lucanidae Stag beetles are a family of about 1,200 species of beetles in the family Lucanidae, currently classified in four subfamilies.Smith, A.B.T. (2006). A review of the family-group names for the superfamily Scarabaeoidea (Coleoptera) with corrections ...

. It was discovered in 1973 by Mr. A. Wright on an expedition to Middle Sister Island/Te Awanui, one of The Sisters Islands/Rangitatahi which are part of the

Chatham Islands The Chatham Islands ( ) ( Moriori: ''Rēkohu'', 'Misty Sun'; mi, Wharekauri) are an archipelago in the Pacific Ocean about east of New Zealand's South Island. They are administered as part of New Zealand. The archipelago consists of about t ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...

. This holotype specimen is held in the New Zealand Arthropod Collection. It was first described by

Beverley Holloway Beverley Anne Holloway (born October 1931) is a New Zealand entomologist. Holloway is a preeminent lucanid systematist and was awarded the New Zealand Commemoration Medal in 1990 for services to New Zealand as a scientist. She has also been ...

in 2007. The name ''sororum'' is translated from Latin to mean "belonging to the sisters".

Description and taxonomy

This beetle ranges in length (including mandibles) from 18.0 to 28.5 mm (males) and from 19.4 to 24 mm (females). It has a glossy exoskeleton which ranges in colour from reddish-black to black. Like ''

Geodorcus servandus ''Geodorcus servandus'' is a large flightless species of stag beetle in the family Lucanidae. It was discovered by P.R. Kettle in December 1960 and this holotype specimen is held in the New Zealand Arthropod Collection. It was first described b ...

'', its

elytra An elytron (; ; , ) is a modified, hardened forewing of beetles (Coleoptera), though a few of the true bugs ( Hemiptera) such as the family Schizopteridae are extremely similar; in true bugs, the forewings are called hemelytra (sometimes altern ...

are not distinctly ribbed. This is a feature that distinguishes ''G. sororum'' from ''G. capito'' which is also found in the

. They demonstrate

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

with an overall size difference and obvious differences in mandible shape and size. The two species of ''Geodorcus'' beetle on the Chatham Islands share a common ancestor. Mitochondrial DNA analysis shows that ''G. capito'' and ''G. sororum'' are more closely related than either is to the other ''Geodorcus'' species found on mainland New Zealand. Individuals of ''G. sororum'' are very closely related to each other and have low genetic diversity.

Distribution

This species has only been observed on Middle Sister Island, which is located 19 km north-west of Chatham Island. Collections of the beetle have been made from near sea level to 40 m. This island is a breeding ground for large seabirds including

northern royal albatross The northern royal albatross or toroa, (''Diomedea sanfordi''), is a large seabird in the albatross family. It was split from the closely related southern royal albatross as recently as 1998, though not all scientists support that conclusion and ...

and

Buller's albatross Buller's albatross (''Thalassarche bulleri'') or Buller's mollymawk, is a small mollymawk in the albatross family. It breeds on islands around New Zealand, and feeds in the seas off Australia and the South Pacific. Taxonomy Mollymawks are a ...

Habitat

Middle Sister Island has sparse vegetation consisting mainly of coastal herbs and ice plant. Other ''Geodorcus'' species are associated with rotten logs, but this habitat is absent from the treeless Sisters Islands, which are two pyramidical rocks about 100 feet high, covered with scanty bushes, and frequented by countless numbers of sea-birds.

Diet

''Geodorcus'' beetle larvae tend to feed on decaying wood or the fungi involved in decomposition. ''Geodorcus sororum'' larvae must consume an alternative, as the plant communities on Middle Sister Island consist only of small shrubs and herbs. The adult beetles must also consume an alternative food source to the other members of this genus, considering the absence of woody vegetation on this small island. The presence of a humus-rich soil should allow this species to complete its life cycle.

Conservation

Like all ''Geodorcus'' species, ''G. sororum'' is absolutely protected under Schedule 7 of The 1953 Wildlife Act, making it an offense to collect or harm a specimen. It has a conservation status as nationally critical due to its small population on Middle Sister Island and presence in only one location. The Sisters Islands are currently rodent free. They are visited by birdwatchers and the introduction of rodents would be catastrophic for this species.

References

{{Taxonbar, from=Q21265846 Endemic fauna of New Zealand Beetles of New Zealand Fauna of the Chatham Islands Lucaninae Beetles described in 2007 Endemic insects of New Zealand