Steven W. Rissing, Robert A. Johnson, and Gregory B. Pollock.
Natal nest distribution and pleometrosis in the desert leaf-cutter ant, Acromyrmex versicolor (Pergande) (Hymenoptera: Formicidae).
Psyche 93(3-4):177-186, 1986.

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NATAL NEST DISTRIBUTION AND PLEOMETROSIS IN THE DESERT LEAF-CUTTER ANT
ACROMYRMEX VERSICOLOR (PERGANDE)
(HYMENOPTERA: FORMICIDAE)
While most ant colonies are started by single queens, colony foundation by groups of queens, pleometrosis, also occurs (Wilson 1971, Holldobler and Wilson 1977). Several extensively studied, highly pleometrotic species are notably similar with respect to important aspects of colony ontogeny and population dynamics. Myrmecocystus mimicus, Solenopsis invicta and Veromessor per- gandei queens found colonies mutualistically without respect to relatedness (Bartz and Holldobler 1982, Tschinkel and Howard 1983, Pollock and Rissing 1985, Rissing and Pollock 1986). Further, while adult colonies of these species are highly territorial (Holl- dobler 1976a. 1981; Wilson et al. 1971; Went et al. 1972, Wheeler and Rissing 1975), natal colonies are clumped with brood raiding and subsequent worker defection from brood-raided colonies occur- ring (references cited above for M. mimicus and S. invicta, for V. pergandei: Rissing and Pollock, in press). Given such frequently deleterious natal colony interactions, adaptive value of habitat selection by founding queens resulting in clumping of natal nests is unclear. Natal nests of M. mimicus are generally clumped in areas devoid of adult nests (Bartz and Holldobler 1982), yet still occur near such nests (B. Holldobler, pers. comm,), and queens of S. invicta show some preference for microtopogra~hic features (Tschinkel and Howard 1983). Here we present data relating habitat selection and clumping of natal nests of the highly pleometrotic leaf- cutter ant Acromyrmex versicolor (Pergande) directly to survival of founding queens. The only other report regarding any aspect of colony initiation in this species is a description of mating flights following summer rains in the Sonoran Desert by Wheeler (1 9 17). 'Department of Zoology, Arizona State University, Tempe, AZ 85287 **School of Social Science; University of California, Irvine; Irvine, CA 92717 Manuscript received by the editor April 6, 1986.



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178 Psyche [vo~. 93
A major flight of A. versicolor occurred on 19 September 1985 on a study area in North Scottsdale, AZ 5.6 km north of Maricopa County along Pima Rd, approximately 3.2 km west of the McDow- ell Mountains. The habitat in this area is typical of the Sonoran Desert with Larrea tridentata and Franseria dumosa dominant shrubs and Olneya tesota and Cercidium microphyllum dominant trees along numerous shallow washes in the gravellsandy soil. A major storm front produced rain throughout the region the previous day; 2.6 cm of rain was recorded at the Arizona State Laboratory of Climatology located 32 km south of the study area. Habitat choice by A. versicolor queens was examined by running a transect 20 m long and 2 m wide from the base of 10 haphazardly chosen trees on the study area. Transect direction was chosen haphazardly. Distance of each starting nest from base of tree was recorded and standardized into "canopy units" by dividing by dis- tance from base of tree to outer canopy edge along each transect. This standardization was necessitated by variance in tree size and canopy extent. Distance to nearest neighboring tree was measured for 20 haphazardly chosen trees and converted to canopy units using the larger canopy extent of each pair. Number of queens per starting nest was determined by excavating 43 nests during this time. Addi- tionally, 21 starting nests were excavated on 22 September on a study area of similar habitat in South Mountain Park, Phoenix, AZ, 38.5 km southwest of the main study area. Effect of temperature in a starting nest on queen survivorship was determined by placing 18 queens (from the above excavated nests) in a large test tube plugged with cotton and containing a large ball of cotton saturated with water to prevent desiccation. Test tubes were placed in a darkened incubator at 20, 25,. . .45OC in random predetermined order; subsequently an additional tube was exposed to 42.5OC. To mimic late afternoon temperature exposures in the field, tubes remained at their given temperature for 2 hours. Queens incapable of righting themselves after 2 hours were considered dead. Likely temperatures in starting nests were determined by taking soil temperatures 5 and 10 cm below surface at the trunk base (0 canopy units), canopy edge (1 canopy unit) and in the open (> 1 canopy unit) on the main study area between 16:OO and 17:OO h on 24



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19861 Rissing, Johnson, & Pollock-Acromyrmex 179 September and 4 October 1985. All queens excavated 1-2 days fol- lowing the mating flight were found 5-10 cm below the soil surface. The possible importance of relatedness in formation of queen associations was tested according to the methods of Rissing and Pollock (1986). Eight plastic "choice boxes" (30 X 15 X 8 cm, half filled with sand moistened in each corner and at the midpoints along the long sides) were established with 2 sets of 4 queens, one set collected from each of the two study sites (38.5 km apart). In 5 boxes, queens were color marked according to collection locale; different patterns of the same two colors were used to avoid providing cues for recognition. As an additional control for possible paint odor, queens in the remaining 3 boxes were not marked. Boxes were excavated 24 hrs later when queens had dug below the surface; location of each queen was noted relative to the others. Acromyrmex versicolor queens strongly prefer to start nests immediately below the outer canopy of trees; while mean inter-tree distance was 6.10 + 1.85 canopy units (= 21.37 å 5.14 m between tree bases; N = 20), the average starting nest was .87 + .53 canopy units (= 3.15 31 1.88 m; N = 1 15) from a tree base (Fig. 1). Although measurements were not taken, the same distribution of starting nests was observed at South Mountain Park. All queens examined for temperature tolerance survived exposure to temperature up to and including 40å¡ for at least 2 hr; survivorship was 0, however, at 42Så¡ and above. Temperatures above 40å¡ were found in the open between trees (>I canopy units) at soil depths normally occu- pied by newly starting colonies (wm = 42.0 (k l.I)OC, N = 5; Xiocm = 36.9 (A 1.2)OC, N = 10); temperatures this high were not found at canopy edges (= 1 canopy unit) (X5Cm = 38.2 (+ 5.5)OC, N = 5; Xiocm = 32.8 (31 3.7)OC, N = 9) or at tree bases (= 0 canopy units) (X5cm = 28.91 k 2.4), N = 2; Xiocm = 27.4 (k 0.8)OC, N = 5). Acromyrmex versicolor is highly pleometrotic; 82.5% of all queens excavated (N = 160 queens from 64 nests) were from pleometrotic associations (Table 1). Relatedness appears unimpor- tant in a queen's decision to enter a foundress association; five of the 8 "choice boxes" resulted in a single starting nest occupied by all 8 queens. The remaining 3 boxes had two starting nests each: of these 6 starting nests, 4 contained queens from both collection locales, 1



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Psyche
[Vol. 93
LLJ
9
Ill
a
ec
3
0
0
0
8
CANOPY UNITS
Figure 1. Habitat choice by Acromyrmex versicolor queens. "Canopy units" represent distance of a starting nest from the base of tree divided by distance from tree base to outer extent of canopy along that transect. contained 3 queens from one site only, and the last contained a solitary foundress.
The distinct habitat choice of A. versicolor queens (Fig. 1) seems a likely response to high soil temperatures in sunlit areas during the mid-late summer flight season of this species. Queens initiating nests in open areas will likely experience lethal temperatures even 5 cm below the soil surface. We routinely found queens at this depth 2-3 days following the mating flight. These nests, however, were located under trees where potentially lethal temperatures were never recorded. Acromyrmex versicolor queen death after 2 hours of exposure to temperatures 40-43OC is consistent with upper temper- ature tolerances of foragers of this species of 42-43OC (Gamboa 1976).
Queens of several other ant species display varying degrees of habitat choice. Queens of Lasius niger and Lasiusflavus both prefer bare, sunlit soil where they establish colonies more successfully than



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19861 Rissing, Johnson, & Pollock-Acromyrmex 181 Table 1.
Pleometrosis in Acrom yrmex versicolor.
No. queens in nest
Collection frequency
1 28
2 16
3 6
4 7
5 3
7 2
9 1
16 1
in shaded soil (Pontin 1960). Queens of the tropical leaf-cutting ant Atta cephalotes appear capable of choosing between major habitat types (mature evergreen woodlands as opposed to deciduous forest or cultivated fields) (Rockwood 1973). Similar ability to choose between major habitat types (woods versus open fields) while flying occurs in Lasius neoniger and Solenopsis molesta (Wilson and Hunt 1966).
Preference by queens for the canopy edge (as opposed to any- where under a tree) may represent a trade-off for shade while still being as warm as possible for rapid development of an initial worker force and eventual establishment of a foraging territory. This would be consistent with the high degree of pleometrosis in this species (see below) and with the "maxi-therm" hypothesis of Hamilton (1 973). Location of a starting nest under a tree canopy (especially 0. tesota whose branches frequently droop to the ground) would permit easy and safe access to vegetation for initiation and growth of the fungus garden characteristic of all leaf-cutters. Unlike most species of higher ants, queens of Acromyrmex spp., including A. versicolor, routinely forage for vegetation, especially at colony initiation (Weber 1972, Gamboa 1974). Trees, including 0. tesota, are commonly harvested by A. versicolor (Gamboa 1975).
Of the several hundred adult colonies of A. versicolor we have observed within the vicinity of Phoenix, AZ, virtually all have been located directly under adult trees and never in the open between trees. Acromyrmex, Atta and related genera are a largely tropical, New World group of ants; A. versicolor is the northernmost of 24 Acromyrmex species and certainly one of the most desert-adapted of all the leaf-cutters (Creighton 1950, Weber 1972). Habitat choice by founding queens and location of adult nests under trees may be



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182 Psyche [vo~. 93
an important behavioral adaptation permitting range extension into the Sonoran Desert. Acromyrmex versicolor queens clump around the essential resource of favorable nest sites (tree shade with ready access to lower canopy leaves). The mating system of this species may also permit "tracking" of this resource. Unlike some desert species that have massive mating swarms (e.g. Pogonomyrmex spp.: Chapman 1957; Nagel and Rettenmeyer 1973; Holldobler 1 976b, c; Davidson 1982), A. versicolor mates in small, localized groups at or near the ground in open areas between trees (Wheeler 1917; R. A. Johnson, pers. obs.). This behavior mimics closely the mating behavior of M. mimicus (M. Cazier, pers. comm.) and likely V. pergandei (Pollock and Rissing 1985), pleometrotic species with clumped natal nests. Whether such localized mating aggregations have led to a highly female biased sex ratio, as appears to have occurred in V. pergandei (Pollock and Rissing 1985), is currently unknown for these other pleometrotic species. Some other ant species with clumped, natal nests engage in inter- nest brood raiding in the process of establishing natal territories (references cited above). This may select for pleometrosis (Rissing and Pollock, in press) which generally results in more rapid produc- tion of a larger initial worker force (Waloff 1957; Stumper 1962; Markin et al. 1972; Taki 1976; Mintzer 1979; Bartz and Holldobler 1982, Tschinkel and Howard 1983, Rissing and Pollock, in press). Colonies of these species are also territorial as adults. Brood raiding also seems likely in Atta texana, another pleometrotic desert leaf- cutter (Mintzer and Vinson 1985), which "merges" young colonies in the laboratory and field (Echols 1966). Adult colonies of Acromyr- mex versicolor are territorial (Gamboa 1974). We suggest such terri- toriality, when coupled with natal nest clumping through habitat choice, makes brood raiding and associated forms of natal nest competition likely for this species as well. A final similarity between A. versicolor and other pleometrotic ants with clumped natal nests discussed here is the apparent forma- tion of foundress associations without respect to relatedness. Queens collected from distant locales readily associate. Similar observations exist for M. mimicus (Bartz and Holldobler 1982), A. texana (Mintzer and Vinson 1985) and V. pergandei (Rissing and Pollock 1986); electrophoretic evidence indicates S. invicta queens also associate randomly (Ross and Fletcher 1985). This differs



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19861 Kissing, Johnson, & Pollock-Acromyrmex 183 dramatically from the close relatedness of cofoundresses in primi- tively eusocial wasps (Pfennig et al. 1983 and included references). Given the normally claustral method of colony foundation in ants, relatedness to potential cofoundresses should be unimportant in this essentially mutualistic process (Rissing and Pollock 1986). Colony foundation in leaf-cutters (including A. versicolor), however, is not claustral; foundresses forage (references cited above). This presents an opportunity to extend and examine the dynamics of the mutual- istic process of colony foundation by unrelated females. Work in this area is currently planned.
Queens of the desert leaf-cutter ant, Acromyrmex versicolor exhibit distinct habitat choice during colony foundation; almost all natal nests are located directly under the canopy edge of large trees. Soil temperatures in these sites are conducive to queen survivorship during the first several days of colony initiation while those in open areas between trees are high enough to result in queen death. This habitat choice results in clumping of many natal nests under indi- vidual trees implying strong natal colony competition. Indeed, as with several other ant species exhibiting such competition, starting colonies are frequently pleometrotic; 82.5% of all queens excavated were from such multiple foundress associations. As with other pleometrotic ant species, mating aggregations of A. versicolor are small and localized, and relatedness appears unimportant in a queen's decision to enter a foundress association. G. E. Walsberg provided advice regarding queen temperature tol- erance tests.
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