Ants of Madagascar

Genera

Crematogaster

Blaimer (2010). The then subgenus Decacrema species from Madagascar were the focus of this taxonomic revision. The subgenus has since been abandoned and the species here now are part of the Crematogaster hova group (see Blaimer 2012c).

There is a key to the Madagascar species of this group: Key to Crematogaster hova-group workers of Madagascar

In Madagascar, ants of the genus Crematogaster occur in all forest habitats and probably are the most dominant and abundant ants in the canopy, rivalled only by Camponotus, Tetraponera and Pheidole. Weaver ants (Oecophylla), which dominate arboreal habitats throughout the rest of the Palaeotropics, are absent from Madagascar; this could have presented ecological and evolutionary opportunities for other genera such as Crematogaster and led to increased species diversification (Fisher, 2003). In Madagascar, nest constructions such as carton nests are built exclusively by Crematogaster species from at least three of the ‘subgenera’ (Decacrema, Oxygyne and Crematogaster). Carton nests are made from masticated plant material that the ants then plaster together in layers (see Weissflog, 2001). Other preferred arboreal nesting habitats are dead twigs or branches in the canopy or lower vegetation, and beneath canopy moss and epiphyte mats; a few species stray from this largely arboreal lifestyle and nest on the ground, either in rotten logs or under stones. Most arboreal species seem to also forage extensively on the ground, but this aspect of life history needs further investigation. In contrast to many other tropical regions, Crematogaster is not known to be involved in any close associations with plants in Madagascar (Fisher, pers. comm.).

The subgenus Decacrema Forel. A specialised ‘plant-ant’ habit has brought the Decacrema-group the most attention of all ‘subgenera’ within Crematogaster. Species from South–East Asia assigned to this ‘subgenus’ have mutualistic associations with Macaranga trees, and these relationships have been investigated as model systems for co-evolution (e.g. Feldhaar et al., 2003; Quek et al., 2004). The Crematogaster (Decacrema)-group has been morphologically defined by having 10-segmented antennae versus 11 segments (in workers and queens, males have 12 segments) in all other ‘subgenera’ of Crematogaster. Species from Madagascar, Sub–Saharan Africa, Singapore and the Malay Archipelago have been assigned to Decacrema (Emery, 1922; Hosoishi & Ogata, 2010), for a total of 24 species and subspecies names (Bolton et al., 2007). Although the ecology of the Macaranga-inhabiting Decacrema has been well circumscribed (Feldhaar et al., 2010; Fiala et al., 1999) and their taxonomy is undergoing revision (Feldhaar et al., in prep.), taxonomic and biological information on the remaining Asian, African or Malagasy species in the group is very sparse. The cohesion of Decacrema as a group however has been challenged by preliminary data and there is compelling molecular evidence that at least the Macaranga-Decacrema and the Malagasy Decacrema do not form a clade (Blaimer, 2012c). The morphology of all but one examined African species further refutes a close relationship to the Decacrema of Madagascar (Blaimer, pers. observ.) and warrants further study.

Natural history and taxonomy of Decacrema in Madagascar. Molecular and morphological data strongly support the monophyly of the Malagasy Decacrema (Blaimer, 2012c.) and thus justify their treatment as a cohesive natural group in this revision. In Madagascar, Decacrema ants are found throughout all forest habitats within the five biomes on the island. Nesting habits are arboreal except for one species that is known to occasionally nest on the ground and is adapted to drier habitats. Many of the canopy-nesting species construct carton nests around small branches or attached to tree trunks, or alternatively nest in dead branches or twigs. It is unclear whether these nesting habits are species-specific. Malagasy Decacrema are medium to large-sized ants compared to other Crematogaster species in this region, and they often seem to be the dominant element of the Crematogaster, or even of the entire canopy ant fauna where they occur. Partly this may be mediated by the carton-nesting ability that allows for large and often polydomous colonies. Carton nesters often tend and house mealybugs in the main nest or in special carton shelters, and both twig- and carton-nesting species can further be found living in association with myrmecophilous beetles (Blaimer, pers. observ.).

Genera

Crematogaster

References