Bioacoustics of Streblognathus

Ecology and behavior. Relatively little is known about most aspects of Streblognathus ecology. They occur in arid thorn scrub and grasslands in extreme southeastern Africa, and apparently are specialist predators of tenebrionid beetles ( Brown, 2000 ; Robertson, 2002 ). Workers stridulate when disturbed ( Ware, 1994 ) and are able to differentiate nestmates from non-nestmates (Schlüns et al. , 1996). In contrast, a fair bit is known about the reproductive and social behavior of the genus. Colonies are small, with usually around 100 workers or fewer (mean = 35 for S. aethiopicus ; mean = 95 for S. peetersi ; Ware et al. , 1990 ; Peeters, 1993). The queen caste is entirely absent, with reproduction instead being performed by a single mated gamergate worker. This gamergate, or “alpha” worker, is morphologically indistinguishable from the other workers but differs from them in its ovarian development ( Ware et al. , 1990 ), hormone levels ( Brent et al. , 2006 ), neurochemistry ( Cuvillier-Hot & Lenoir, 2006 ), cuticular hydrocarbons ( Cuvillier-Hot et al. , 2005 ), and relative proportions of mandibular gland secretions ( Jones et al. , 1998 ). Within a colony, workers are behaviorally differentiated into foragers, nest workers and the sole reproductive gamergate ( Ware et al. , 1990 ). A dominance hierarchy exists among the workers in a colony, with high-ranking workers subordinate to the alpha but dominant over the low-ranking individuals. Gamergates inhibit reproduction by subordinate workers through chemical signaling ( Cuvillier-Hot et al. , 2004b ). The reproductive division of labor within the colony is further maintained by the low-ranking workers, who identify the alpha worker and aggressively prevent sub-alpha workers from ascending to dominance unless the alpha senesces. In such instances, high-ranking workers aggressively compete until a single alpha ascends to dominance; this is usually the previous “beta,” or second ranked, worker ( Cuvillier-Hot et al. , 2004a , 2004b ). Phylogenetic and taxonomic considerations. Streblognathus was erected by Mayr (1862) to house the species Ponera aethiopica F. Smith. All subsequent authors have maintained distinct generic status for this taxon. Robertson (2002) revised the genus and divided S. aethiopicus into two species. We continue to treat Streblognathus as a distinct genus on both morphological and molecular grounds. Schmidt's (2013) molecular phylogeny of the Ponerinae places Streblognathus solidly within the Odontomachus group, though its sister group is unresolved. Carpenter (1930) , in describing the fossil ponerine genus Archiponera , argued for a close relationship between Streblognathus and Dinoponera , even going so far as to suggest that they be considered a supergenus. He apparently based this hypothesized relationship on the relatively medial placement of the eyes, the presence of paired teeth on the anterior margin of the clypeus, and presumably their large size; to this list of similarities could be added the loss of a queen caste. This purported relationship has been repeated in much of the subsequent literature on Streblognathus and Dinoponera ( e.g., Haskins, 1970 ; Haskins & Zahl, 1971 ), but is clearly false. The similarities between Streblognathus and Dinoponera are apparently the result of convergence, as morphological and molecular evidence otherwise argue against a sister relationship between these genera. Archiponera , which Carpenter believed was close to both Dinoponera and Streblognathus , has uncertain affinities but is unlikely to actually represent an ancestor or sister group to either of these extant genera (see further discussion under Dinoponera ). The true sister group of Streblognathus is still unresolved.