Formica tibetana, Schultz & Seifert, 2025

Schultz, Roland & Seifert, Bernhard, 2025, Formica sentschuensis Ruzsky 1915 and Formica tibetana n. sp. - two unique and little known Formica species from Tibet (Hymenoptera: Formicidae), Soil Organisms (Northwood, London, England) 97 (1), pp. 85-95 : 91-94

Formica tibetana n. sp.

Etymology: named according to its geographical range.

Type material: Holotype worker labelled „CHI: 34.5406°N, 102.4179°E, Luqy- 8.4 km SW, 3296 m, Viehweide, LqA R2 , 2011-08-17, Chi 2011 178“ and „ Holotype Formica tibetana Schultz & Seifert “; two mounted paratype workers on a separate pin plus 8 paratype workers in ethanol with the same collecting data labels; all type specimens from the same nest sample; depository SMN Görlitz.

Material examined. Numeric phenotypical data were taken in 36 nest samples with 83 workers, in one gyne from a nest sample and in one isolated gyne caught walking on ground. Additional 9 nest samples were classified by sim- ple visual inspection. They originated from the Chinese provinces Quinghai, Gansu and Sichuan. For details see supplementary information SI1 .

Geographic range. Known from the provinces Quinghai, Gansu and Sichuan in an area delimited by 31.5°N, 35.7°N, 99.8°E and 103.9°E. The altitudinal range extends from 2663 to 3999 m a.s.l. Compared to F. sentschuensis it has a more northern distribution ( Fig. 1 map) and a lower altitudinal range .

Diagnosis: --Worker (Tab. 1, Figs. 5-7 ): Rather small size, CS 1127 ± 131 µm. Head more elongated than in sentschuensis but scape only slightly longer (CL/CW 1.125 ± 0.037, SL/CS 1.029 ± 0.021). Distance between posterior ocellae moderately large (OceD/CS 0.194 ± 0.010). Eye small (EYE/CS 0.267 ± 0.010). Distance between metathoracal spiracles, slightly smaller than in sentschuensis (MtSt/CS 0.124 ± 0.014), distance between propodeal spiracles and between metathoracal spiracle and caudoventral metapleuron larger than in sentschuensis (MtPpSt/CS 0.348 ± 0.011, MtMtP 0.621 ± 0.012). Setae on whole body slightly shorter than in sentschuensis (GHL/CS 11.25 ± 1.63%). Head, mesosoma, petiole and gaster with more numerous setae than in sentschuensis (nGen 14.3 ±3.2, nCH 39.9 ±6.6, nGu 26.5 ±6.6, nMn 25.0 ± 6.4, nPrMe 40.9 ± 9.1, nPe 12.5 ± 3.0). Extensor sides of femora and tibiae with a much larger number of setae than in sentschuensis (nHFex 34.1 ± 6.7, nHT 32.0 ± 5.8). Dorsal plane of scape with much more numerous erect and semierect setae than in sentschuensis (nSc 23.4 ± 5.2). Distance of pubescence on gaster tergites larger than in sentschuensis (sqP- DG 8.99 ± 1.27). Distance of microripples on gaster tergites as in sentschuensis (RipD 7.06 ± 0.50). Microsculpture of vertex and frontal triangle similar to sentschuensis . Clypeus with a sharp median keel stretching over 75% of its length in the largest and over 95% in the smallest workers. The excavation of anteromedian clypeus is conspicuous in frontodorsal view ( Fig. 7) but only suggested in full face view. Vertex usually dark to blackish brown. Genae, clypeus, mesosoma, legs and scapes usually light reddish brown. Gaster dark reddish brown to blackish brown. Specimens with darker overall pigmentation do occur.

-- Gyne: The two available gynes have the following data CS 1566 , 1724 µm; CL /CW 0.979, 0.955; SL/ CS 0.881, 0.899; ML / CS 1.691, 1.679; MW / CS 1.050, 0.961; PEW / CS 0.520, 0.511; ML / CS 1.691, 1.679; MW / CS 1.050, 0.961; GHL/ CS 15.49 %, 17.40%; sqPDG 11.47, 9.57; RipD 8.1, 9.5; nSc 26.0, 14.5; nGen 14.5, 7.5; nGu 30.0, 17.0; nPrMe 41.0, 36.0; nPe 15.5, 10.5; nHFex 30.0, 33.0; nHT 30.5, 20.0. Both show the diagnostic excavation of anteromedian clypeal margin and their seta counts correspond to those of workers .

Taxonomic comments. Formica tibetana n.sp. is clearly separable from F. sentschuensis on worker individual level alone by the much higher setae numbers on dorsum of scape and extensor profile of hind femur. The maximum nSC and nHFex are 3.5 and 16.5 in F. sentschuensis whereas the minima of these characters are 12.2 and 21.0 in F. tibetana n.sp. A principal component analysis considering all 20 characters given in Tab. 1 provides two clearly separated clusters ( Fig. 8 PCA). Furthermore, there are no mixed nest samples known which excludes that F. sentschuensis and F. tibetana n.sp. could represent an intraspecific polymorphism. There is no indication that one of the ten taxa of Formica described by Chang & He (2002) from China could be a senior synonym of F. tibetana n.sp.

Biology. Since we have never observed nests containing workers of Serviformica species, Formica tibetana n.sp. is undoubtedly not dulotic and colony foundation is obviously independent. The latter becomes clear from nest density data collected during the PaDeMoS project on 19 study plots on grasslands in NE Tibet. Only four Formica species were found in these investigations: Formica picea candida Smith 1878 , Formica tibetana n.sp., Formica (Coptoformica) brunneonitida Dlussky 1964 and Formica (Coptoformica) manchu Wheeler 1929 . The mean nest densities on these test plots were 0.42 nests / 100 m ² in the temporarily socially parasitic Coptoformica species, 8.32 nests / 100 m ² in Formica tibetana n.sp. and 27.38 nests / 100 m ² in Formica picea candida . Coptoformica was present on 6, Formica picea candida on 15 and Formica tibetana n.sp. on 6 study plots. The latter two species are apparently highly competitive with strong mutual exclusion showing syntopic occurrence on only two study plots. The realized niche overlap OR as defined by Seifert (2017, p. 25-26) is only 1.9%. These data exclude socially parasitic colony foundation of Formica tibetana n.sp. in F. picea candida . For comparison: OR is 44.9% between the temporarily socially parasitic Coptoformica species and Formica picea candida that serves as host for colony foundation in these two Coptoformica species (Seifert & Schultz 2021). It appears unlikely that F. tibetana n.sp. may serve as host of Coptoformica because the realized niche overlap OR is only 0.6%. More simply expressed: syntopic occurrence of Coptoformica and F. tibetana n.sp. was observed only on a single study plot, but here the established host F. picea candida was also present.

The second argument against independent colony foundation is gyne morphology. The large gaster size of one single dealate gyne observed walking on ground for colony foundation corresponds to the situation in independently founding Serviformica gynes and not to the small gaster of the dependently founding Formica rufa group gynes. We have no numeric comparison data on gaster size but we have data on CS, CL/CW, SL/CS, PEW/CS and ML/CS of 327 gynes of 12 species of the F. rufa group and of 152 gynes of 14 Serviformica species. A principal component analysis run with these data places the two Formica tibetana n.sp. gynes widely distant from the F. rufa group cluster and closer to Serviformica and the diagram also indicates that the character syndrome of F. tibetana n.sp. is outstanding ( Fig. 9 ).

The habitats in Sichuan are mainly in the highly montane zone in light conifer stands (6 findings), grassland with few conifers (2) and a rubble field with shrubs (1). Habitats in the eastern part of the Tibetan Plateau (Quinghai and Ganzu) are mainly mesophilic grasslands or pastures (24)), light conifer forest (4), wet grassland (3), stony flood plains (2) and grassland with shrubs (1). Formica tibetana n.sp. has been observed to construct nests in the soil, beneath stones, or as soil mounds within areas with dense vegetation. In optimal habitats these ants can reach nest densities ranging from 15 to 36 nests per 100 m 2. In a manner similar to other ant species, such as some Myrmica , Lasius , and Formica , this species constructs earth mounds, referred to as solariums, within dense and tall grassland ( Fig. 10). The primary purpose of these mounds is to protrude from the shading vegetation to harness solar energy efficiently. This is particularly important for rapid warming in spring allowing a faster brood development. Observations conducted at the base of a north-facing cattle pasture near the site Sazin Gömpa in Qinghai revealed that Formica tibetana n.sp. constructs mounds with southward-facing solar collector surfaces. At the latitude of the Sazin Gömpa site (35.5°n) and during the months of February and March, an inclination of 14° results in 122% of the solar heat input compared to a horizontal plane whereas the advantage in the period April to June is only 102%.

The hypothesized sequence of the development of the writing-desk shape of Formica tibetana n.sp. mounds at Sazin Gömpa commences with Potentilla fruticosa forming rings around soil nests. These dwarf shrubs are not consumed by grazing cattle hence providing some kind of protective ring against trampling. Subsequently, Formica tibetana n.sp. brings up mineral soil to construct a solarium. After a while the Potentilla dies off, and the hard Kobresia turf begins to spread over the mound. This process is accompanied by the proliferation of crustose lichens, which further contributes to the mechanical stabilization of surface. The shape of the distinctive desk-like mounds is maintained more or less when the nest population of F. tibetana n.sp. eventually dies.

A second pathway to form desk-like mounds is their establishment in areas with sparse grazing and dense vegetation. In such environments, Formica tibetana n.sp. has been observed to construct soil mounds from the beginning. When grazing increases, the tall grasses gradually disappear while the mounds become covered with Kobresia pygmaea and crustose lichens ( Fig. 11 ). The ant population of these mounds dies out after a while and the mounds remain as relicts of a time of less grazing and higher vegetation. Aged mounds are frequently completely overgrown by Kobresia and lichens, resulting in the formation of a very hard and felty root-mat of approximately 8-10 cm thickness. These old mounds have diameters of 35 to 90 cm and heights of 30 to 50 cm. These very stable mounds are obviously persisting for a long time – with and without ant populations. Many of these aged mounds are occupied by pikas ( Ochotona ), which cut holes at the mound bases. Formica tibetana n.sp. nests restricted to the southern mound side in close proximity to the ground indicate a resettlement of abandoned mounds.

Figures

Figure 1. Geographic distribution of Formica tibetana n.sp. (pink dots) and of Formica sentschuensis Ruzsky, 1915 (black dots, black rhomb type locality) in the Chinese provinces Quinghai, Ganzu and Sichuan. The size of dots is proportional to the number of samples found in a site.

Figure 5. Head of the holotype of Formica tibetana n.sp. in full face view.

Figure 8. Principal component analysis of worker individuals considering the 21 morphological characters given in Tab.1. Squares – Formica tibetana n.sp., dots – Formica sentschuensis; specimens from the holotype samples filled in red.

Figure 9. Principal component of gyne individuals of 12 species of Palaearctic Formica rufa group species with dependent nest foundation (black squares), of 14 independently founding Palaearctic Serviformica species and of Formica tibetana n.sp. (black dots) considering head size CS and the shape variables CL/CW,SL/CS,PEW/CS and ML/CS.

Figure 11. Development of Formica tibetana n.sp. nest mound.