Crematogaster

Longino, J. T., 2003, The Crematogaster (Hymenoptera, Formicidae, Myrmicinae) of Costa Rica., Zootaxa 151, pp. 1-150 : 4-7

Crematogaster

Natural History Overview

Crematogaster are often common ants, and they play a major ecological role in Neotropical forests. Colonies may be large, blanketing forest canopies, or small, contained within a single dead twig. Large colonies are usually polydomous, with multiple nests. Most species nest in dead wood, from narrow gauge hollow stems to large dead branches or trunks. One species, C. stollii , nests in live stems. Although major Crematogaster lineages in the Asian and African tropics are specialized plant ants, and at least one or two species are plant ants in the Amazonian region, none are known to be specialized plant ants in Costa Rica. Crematogaster bryophilia often nests under epiphyte mats. Although many species can make carton from masticated plant fibers, most use relatively small amounts to form partitions inside the nest or to restrict the opening of a nest in dead wood. Several Costa Rican species use carton more extensively. Crematogaster stollii makes carton galleries on tree trunks and branches, connecting their nests in the live branch tips. These carton galleries are indistinguishable from those of Azteca forelii , an ant species with similar nesting behavior, and both are very similar to the galleries of the arboreal termites that are so common in lowland forests. Crematogaster montezumia and C. arcuata make external carton nests that encircle small stems. These nests are plain carton, and lack epiphytes. In contrast, two Costa Rican species make carton nests that sprout epiphytes, forming ant gardens. Crematogaster longispina makes loose ant gardens on tree trunks (Kleinfeldt 1978), and C. jardinero lives in the high canopy, forming "archipelago" clusters of discrete ant gardens.

Although most species are arboreal, a few nest in the leaf litter. Species that nest in the leaf litter are usually yellow, nocturnal, and rarely encountered. One leaf litter species, C. sotobosque , is brown, forages diurnally on low vegetation, and is moderately abundant in lowland wet forest.

Most species are monogynous; a few are polygynous. Ergatogynes or intercastes have been reported for C. minutissima (Holliday 1903), C. minutissima smithi , and a species tentatively identified as C. curvispinosa (Heinze et al. 1998, Heinze et al. 1999). The Heinze et al. studies of smithi revealed that these intercastes, morphologically intermediate between workers and queens, function mainly to provide trophic eggs for the colony. They never perform foraging, maintenance, or defensive duties. They mainly lay eggs, most of which are eaten by larvae. The ergatogynes lack a spermatheca and cannot be inseminated, but their eggs are viable and produce males if left to develop. Among the Costa Rican fauna, ergatogynes are known to occur in C. bryophilia , C. curvispinosa , and C. nigropilosa . Nests are often found with only ergatogynes, workers, and brood. It is unknown whether these are colony fragments, with queenright nests elsewhere, or whole colonies founded by ergatogynes.

Two categories of queens occur among the Costa Rica fauna. In one group the propodeum is tall and narrow and drops very steeply from the scutellum (Fig. 1A), and sculpture and pilosity characters are similar to workers. I refer to these as "normal" queens. These species appear to have typical colony founding behavior, with standard nuptial flights and claustral colony founding by individual queens, and they are the most abundant species in communities. In another group the propodeum has a shallower slope and extends well beyond the scutellum (Fig. 1B), and sculpture and pilosity characters often differ greatly from workers. In particular, queens are often highly polished and shiny. Queens in this group also show varying degrees of development of falcate mandibles. Costa Rican species having these distinctive queens are acuta , arcuata , distans , evallans , jardinero , montezumia , and raptor . I refer to these as the "acuta-group." These are all very low density species, and very little is known of their colony founding behavior. The morphology is similar to other ant lineages that are known to be temporary social parasites (Forel 1928, Hölldobler and Wilson 1990). Queens of temporary social parasites insinuate themselves into nests of other species, killing or incapacitating the host queen, and use the heterospecific worker force to establish their own colony. Two anecdotal observations are consistent with temporary social parasitism as a colony founding mechanism in the acuta-group. I observed a mixed nest in which a queen of C. montezumia occurred in a small nest with workers of C. curvispinosa , and Adrienne Nicotra, a student working at La Selva Biological Station, observed a queen of C. raptor in a small queenright nest of C. carinata . These are the only such observations so far, and the colony founding behavior of acuta-group species is in need of investigation.

Most species of Crematogaster , especially those with large polydomous colonies, are aggressive and territorial. Crematogaster carinata is exceptional in having large polydomous and polygynous colonies that overlap with many other ant species. Workers are not aggressive and may even share the same nest structures with other species. Forel (1898) observed C. carinata (as C. limata parabiotica ) and Dolichoderus debilis Emery inhabiting the same nest in Colombia, and coined the term parabiosis to describe the phenomenon of mutual nest sharing. In Costa Rica, C. carinata can be found coinhabiting ant gardens with Odontomachus panamensis Forel and sharing nest space with D. debilis or D. inermis Mackay . The nesting behavior and taxonomic uncertainties in the complex are further discussed under the C. carinata species account. Crematogaster limata may also exhibit an ability to overlap non-aggressively with other species because it has been observed in close association with the large tropical ponerine ant Ectatomma tuberculatum (Olivier) (Wheeler 1986).

Crematogaster appear to be very generalized and omnivorous foragers. Individual scouts search for resources and recruit nestmates when resources are encountered. They rapidly recruit to baits of sugar or protein (e.g., tuna, dead insects). Although rarely predators of active prey, I have often seen them attacking pupae or otherwise immobilized live prey. They readily tend Homoptera, and species vary in the degree of reliance on Homoptera. Crematogaster stollii appears to rely entirely on Homoptera and perhaps cryptic plant resources. Workers are found only inside of live stems or under their carton galleries and they never forage on the surface. Their chambers in live stems are packed with Coccoidea that are feeding from the inside of the stems. This phenomenon, in which ants live and feed entirely within live plant stems, with no external patrolling by the ants and no obvious myrmecophytic adaptations on the plant's part, has evolved convergently in several ant lineages, including species of Azteca and Myrmelachista .

In Costa Rica, Crematogaster are abundant in all lowland habitats. In mangroves, C. crinosa is often a dominant species. In lowland dry or wet forest sites, a community of over 15 species may occur. These are concentrated in second growth vegetation, forest edges, and forest canopy. Relatively few species are found in wet forest understory and forest floor litter. At higher elevations the dominant Crematogaster drop out by about 500 to 1000m, depending on the openness of the habitat. A few species, such as C. bryophilia , C. moelleri , and C. sumichrasti , are montane forest specialists that are more common at mid elevations than at sea level.