Cave and Subterranean Ants

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The following is based on Roncin & Deharveng (2003).


Assessing the troglobitic status of a taxon is not easy. However, troglomorphy (Christiansen, 1962), or better troglobiomorphy (Boutin, in press), i.e. the set of morphological traits characteristics of cave organisms, is always a strong indication of troglobitism {while the opposite is not always true). Troglobiomorphy in arthropods is defined by four morphological traits: loss of wings in winged arthropod groups, reduction of eyes, reduction of tegumentary pigment and elongation of appendages (Vandel, 1964; Christiansen, 1965; Culver, 1982; Marques and Gnaspini, 2001). All known specimens of Leptogenys khammouanensis are workers, which are always apterous in ants. We do not know therefore the state of the first character in this but the last three traits are present.

Ants in Caves

Though frequently cited from caves, ants have not provided so far any unambiguously troglobitic species. Most records (Wilson, 1962; Tinaut and Lopez, 2001) concern in fact accidental occurrences, generally not far from cave entrance (Decu et al., 1998). Most species supposed to be strictly cavernicolous have been later found also outside caves. Even the rare Hypoponera ragusai considered by Tinaut (2001) to be limited to caves in Europe has been collected outside caves in France (Bernard, 1968) and in the Mediterranean islands of Lampedusa and Linosa (Mei, 1992, 1995).

This rarity of ants in caves concerns essentially temperate regions. Though limited, available evidence suggests that Formicidae might be much more frequent in tropical caves. More than sixty species have already been collected from the dark part of various caves of Southeast Asia (Roncin et al., 2001, and unpublished data). Most of these species, well-known outside, are represented by isolated specimens in cave collections. But several are regular guano inhabitants, like Hypoponera confinis in the Farm caves of Myanmar (Annandale et al., 1913) and an unidentified species in the Mulu caves of Sarawak (Chapman, 1982). In fact, ants were present in most guano caves recently sampled in Southeast Asia, with Hypoponera the dominant genus (Roncin et al., 2001 ). They were sometimes found very far from cave entrance, living in loose colonies, foraging in or around the guano piles, where they are believed to prey on a rich fauna of micro- and meso-arthropods. None of these regular guano species are troglobiomorphic, and all were also collected outside caves. Among the hundreds of caves prospected so far in all regions of Southeast Asia (see Juberthie and Decu, 2001 for an overview), troglobiomorphic species were found only in Laos with L. khammouanensis.

Having analysed what was known about cave ants, Wilson (1962) hypothesized that social insects “never become truly troglobitic” because “they are unable to maintain sufficiently large cave demes” (implicitly because of food scarcity). The well-known link between troglobiomorphy and oligotrophic habitats (Deharveng and Bedos, 2000) certainly explains the extreme difficulty for ants to establish long-term colonies and to adapt to cave environment. This view is challenged today by the discovery of L. khammouanensis with its clear troglobiomorphic morphology. The unusually large underground voids of Laos may have given the opportunity for such an evolution to take place, by providing large food reservoirs on the long-term. To confirm this exciting hypothesis, it remains to document the peculiarities of the biology and social life of the new species (Tinaut and Lopez, 2001).

Troglobiomorphic characters in Leptogenys - The genus Leptogenys has already been found in caves. Leptogenys jeanneli Santschi, 1914 was described from a cave in Tanzania, and Leptogenys diminuta (Smith, 1857) was collected in the Batu Caves of peninsular Malaysia (Wilson, 1962; McClure, 1965). However, none of these species is cave-restricted, and none exhibits the combination of troglobiomorphic traits of L. khammouanensis.

A group of Leptogenys processionalis sensu Taylor (1969) (=fallax-group of Andersen, 2000), which comprises three Australian species, Leptogenys fallax (Mayr, 1876), Leptogenys tricosa Taylor, 1969 and Leptogenys fortior Forel 1900, and the Oriental species Leptogenys myops (Emery, 1887), Leptogenys crassicornis Emery, 1895 and Leptogenys processionalis (Jerdon, 1851 ). Species of this group possess also light color and reduced eyes varying from a single (L. tricosa) to about fifteen facets, but antennae are short, and size is small. Such a morphology appears as a classical adaptation to endogeous, not to cave life. At least several African species in the guineensis- and the nitida-group also possess small eyes, and are yellow brown to dark brown. However their head and appendages are so not elongated.

Leptogenys khammouanensis was encountered far from the entrance in two big caves, Tham Nam Non (22 km long, the longest cave of continental Southeast Asia, Mouret, 2001) and Tham The (2.2 km long, Brouquisse, 1999). In spite of being 25 km apart and in different hydrogeological systems, both caves do belong to the same, uninterrupted, huge limestone unit. In Tham Nam Non, L. khammouanensis was collected at about 4.5 km of the entrance. These giant caves of the Khammouan karst host a rich troglobitic fauna, only recently discovered: microphthalmic crabs (Erebusa calobates, Yeo and Ng), various blind terrestrial lsopods, Araneids and Millipeds (Polydesmidae, Glomeridae), Campodeids, springtails, blind Nocticolid cockroaches, blind or microphthalmic Diestrammena sp. crickets (Besson et al., 2001 ). As numerous species of Leptogenys are woodlices-hunters (for a full list of references see: Holldobler and Wilson, 1990; Dejean 1997), the terrestrial isopods frequent in these caves could constitute a potential diet of L. khammouanensis, but this has to be confirmed.

In its slender habitus (very elongate head, mesosoma, petiole, antennae and legs), Leptogenys khammouanensis is similar to Leptogenys ergatogyna from Africa and to Leptogenys assamensis from Assam. But both species differ from L. khammouanensis by their black color and larger eyes, and cannot be considered as troglobiomorphic. No species of Leptogenys therefore approaches L. khammouanensis in its combination of troglobiomorphic characters.

Only one species of Formicidae associated with caves, Aphaenogaster cardenai, could be associated to subterranean habitats (Decu et al., 1998, Tinaut and Lopez, 2001). This rare Spanish species has always been collected in cryptic habitats: under big rocks, galleries of rodents and caves. However in this latter habitat no nests have ever been found, which led these authors to postulate that A. cardenai is more probably an inhabitant of the MSS (“Milieu Souterrain Superficial” of Juberthie et al. 1980, “superficial underground compartment” in Humphreys, 2000), than a strictly cave dwelling species.

A. cardenai is related to Aphaenogaster splendida and Aphaenogaster ovaticeps. All have reduced eyes, slender body, elongate appendages and often pale color, compared to other species of the genus. Bernard (1968: 136) stressed the peculiar morphology of the species then known and noted that they have “un facies aphaenopsien de cavernicole" (“an Aphaenops-like morphology of cave species”). He supposed they inhabit hypogean habitats, probably deep cracks that they rarely leave, hence their rarity in collection. However a few captures from surface habitats (Forel, 1911; Wheeler and Mann, 1916) could indicate nocturnal activity more than hypogean life.

A true cave ant?

Two Candidates

What is known about Aphaenogaster gamagumayaa strongly supports this being a troglobiotic ant. See the biology section of the species page for details.

Leptogenys khammouanensis is the second example in Formicidae with the Aphaenogaster of the splendida-group, and the first Ponerinae, which combines hypogean life with microphthalmy, light color, and elongated appendages, i.e. the typical adaptive characters of cave inhabiting arthropods. None of the splendida-group species of Aphaenogaster is cave-restricted. Conversely, L. khammouanensis was observed only in deep parts of caves and in rather significant numbers, bringing an exciting question to the fore: is L. khammouanensis the first truly troglobitic ant?