Origins of "Myrmicaria"

by
Diane W. Davidson

email: davidson@biology.utah.edu

     Borneo is such a fantastic place for ants! There are 'exploding' ants in the Camponotus saundersi group, Echinopla - the "porcupines" of ants, and ants in the bihamata group of Polyrhachis, with their wonderfully large, curved spines, perhaps adaptations against consumption by avian predators. (This is one of the few formicines that forages diurnally and conspicuously, in large aggregations, in the treetops.) Then there are the Myrmicaria, a diverse lot as presently defined. Here I will expose the meagerness of my knowledge of Myrmicaria by publicizing (but not publishing) a speculation as to their origins. If this conjecture is confirmed, the genus Myrmicaria may or may not represent a monophyletic group.
      This speculation arises out of work that my grad student, Steve Cook, and I did in Brunei during summer of 2002. Near the research station at Kuala Belalong, we discovered 4 "species" that are currently assignable to Myrmicaria, but which fell into two quite distinct categories. Two morphologically very similar species with yellow head and thorax and brown gaster nested beneath leaves in the low arboreal zone. One (M. melanogaster) sent its workers to the treetops diurnally and the other (possibly M. lutea), nocturnally. Workers of these species (in the arachnoides group) look a great deal like the minor castes of Pheidole.
      The remaining two, as yet unidentified, "species" (M. brunnea, and, perhaps, nr. brunnea) were darker colored, nested terrestrially, and foraged solitarily. Whereas the smaller of these "species" just canvassed leaves in search of whatever (including plant exudates), the other tended cicadelids in the arboreal zone. Both of the species looked rather like the major castes of Pheidole, and also like terrestrial Myrmicaria I had witnessed in southern India in 2001. We began to think that the two pairs of species might have arisen independently, perhaps from different ancestors in the genus Pheidole, or possibly from a single ancestor, through the loss of one or the other worker caste.
      Supporting this conjecture are data on head shapes, on load capacities and uptake rates for liquid foods, and on foraging modes. First, in proportion to both body lengths and head lengths, the terrestrial nesters had broader heads than did the arboreal species. Second, as measured volumetrically, the arboreal species loaded liquids slowly and in small total amounts. In contrast, the terrestrial species loaded somewhat more quickly and in remarkably greater amounts in relation to their body sizes (5.8 and 7.2 mm, vs. 7.3 and 6.8 in the arboreal species). Because Pheidole soldiers often function as a storage caste, they would be expected to have greater capacities for storing liquid foods. Third, while the arboreal species foraged in aggregate (i.e., in columns), the terrestrial species foraged individually. Finally, Steve's initial look at the proventriculi of these species has them all looking pretty similar and much like those of Pheidole. In sum, these ants appear to be very like Pheidole; moreover, characteristics of the arboreal species resemble those of Pheidole workers, and those of terrestrial species are similar to traits in Pheidole soldiers.
      In summary, the hypothesis is that the two types of Myrmicaria evolved from either the same Pheidole species or from different ancestors in that genus. At first glance, the latter scenario seems more parsimonious. However, since all of these ants smell of the terpenes typical of Myrmicaria, it could just be that they evolved from a single ancestor via losses of the opposite castes (disruptive selection maybe?). Whether or not Myrmicaria is monophyletic would depend on whether there was a single vs. multiple ancestors. Bearing on this hypothesis are Roy Snelling's observations of Myrmicaria in Africa, where he has frequently found colonies with large-bodied workers nesting within a few meters of those with small-bodied workers, and not interacting aggressively with them. We saw just this with our small and large-bodied M. brunnea "species", and we've even seen workers of the two types sharing foraging columns to sugar drops we provided. However, we have also found the large-bodied "species" nesting on their own. Roy suggests the possibility of a developmental switch from small to large workers as colonies grow. If this is so, then evolutionary loss of the developmental switch could have resulted in taxa like our arboreal species, i.e., with complete loss of the major class.
      Regarding this wildly speculative hypothesis, I would appreciate any feedback, morphological or genetic, positive or negative, and especially from experts on the two genera involved here. Whether you agree or disagree that my argument is plausible, the matter of the origins of Myrmicaria remains a fascinating enigma, and perhaps this note will stimulate some future work on the question