Etolog{A, Vol. 5, 1997 Behavioral Ecology of Mating and Colony Founding in the Grass-Cutting Ant, Acromyrmex Balzani (Hymenopte
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Etolog{a, Vol. 5, 1997 Behavioral ecology of mating and colony founding in the grass-cutting ant, Acromyrmex balzani (Hymenoptera: Formicidae) Harold G. Fowler* Reproductive behavior of Acromyrmex balzani ABSTRACT. Behavioral ecology of mating and colony founding in the grass-cutting ant, Acromyrmex balzani (Hymenoptera: Formicidae). - The grass-cutting ant Acromyrmex balzani conducts nuptial flights at dusk, often during light rain. The daily timing of flights is invariant, but annual occurrence is dependent upon meterological conditions. Males form aggregations over shrubs or other prominent objects, into which females fly to be mated. These aggregation sites are used perennially. Sexual com petition among males is intense, with an average of 4 males simultaneously competing foraccess to each female in a mating ball. Multiple matings of females are common. The male genetial capsule is detat ched and retained in the female genetalia upon mating, and femalesremove these prior to flying to mate again. Females found colonies independently and are not claustral. KEY WORDS: Acromyrmex balzani; behaviour; mating; sexual competition; haplometrosis; colony founding; sex-ratio Introduction formation of leks in the male-aggregation syndrome is also common (Holldobler & Wilson, 1990). Boomsma and Leusink ( 1981) demonstrated that, in Mating behaviour in ants is still poorly spite of the diel rhythmicity of nuptial flights, cli documented. Holldobler & Wilson (1990) recognize matic conditions, especially rain, are major determi two mating strategies: the female-callingsyndrome, nants of which day they occur. and the male-aggregation syndrome. The first syn In the Neotropics, the leaf-cutting ants drome is found primarily in species with wingless (Atta and Acromyrmex spp.) are considered major females, while the second is found with alate fema agrictural pests (Mariconi, 1970), but we still know les. In the later case, timing of nuptial flights is lar very little about their basic biology. Some species, gely invarient (Holldobler & Wilson, 1990). The such as Atta texana (Buckley) swarm before dawn * Departamento de Ecologia, Instituto de Biociencias, Universidade Estatual Paulista (UNESP), 13506-900 Rio Claro, Sao Paulo, Brazil. 3 Harold G. Fowler (Moser, 1967), others, such as Atta sexdens (L.) males per lek. The !ck sites for each year were map swarm during the day (Mariconi, 1970). However, ped to verify their reuse in the following year. species of Atta form mating leks at altitudes of at Behavioral observations were performed least 300 m which must be studied with helicopters on mating activity and colony foundation beha (Amante, 1972). For species of Acromynnex, we viours. These, as well as experimental manipula know relatively little about their mating behaviour tions, will be discussed in detail in the pertinent sec (Kusnezov, 1959, 1962). Low leks have been repor tions. Meterological information was taken from a ted for A. striatus (Roger) (Fowler, 1988) and A. personal weather station mounted in the same area. versicolor (Pergande) (Rissing et al., 1986), both The number of marked nests engaging in nuptial members of the subgenus Moellerius (Fowler, flights on a given date were also recorded. 1988). Additionally, attine ants tend to be polyan drous (Kerr, 1961; Fowler et al., 1986). In this paper, Results I present data on the reproductive behaviour of anot her member of this subgenus, A. balzani Emery. Males tended to form leks over small Specifically the formation of leks, diel and yearly shrubs or other vegetation. Lek sizes, determined by timing, and the mating behaviour and colony foun sweep net samples were not large (x=33.3, 1 s.d. dation by newly mated females is discussed using 12.92, N=43). Mating swarms occured at dusk, data collected over a 5-year period. often during a light rain (Table 1). In these shrubs, femaleswere not present or hidden, but flewdirectly Materials and methods fromgrass blades near the nest entrance. Studies were conducted from 1983 through 198g, in Rio Claro, state of Sao Paulo, Brazil, in 0.2 Table 1. The die! and seasonal timing of mating flights in ha region of an abandoned pasture (campo sujo) in Acromyrmex balzani in the interior of the state of Sao Paulo, which the foraging behaviour of A. balzani was Brazil. Also shown is the estimated sex-ratio, determined by under intense study. During the period of nuptial sweep-sampling either nests or mating leks, the estimated number flights (September through November), colonies of females available per ha, as well as the number and frequency of reuse of lek sites used. were frequently observed during the day to detect pre-swarming behaviours. When observed, these Hour of Date of Numberof Reuse Sex Estimated were then checked at 30 min intervals during the day Year swarm swarm1 lek-sites frequency2 Ratio] females/ha to follow swarming. Upon swarming, mating flights were follo 1983 19:25 12 Oc 16 0.767n 630 wed intensely. Once leks were established, these 1984 19:32 29 Sept 12 0.833 0.623s 800 were observed continuously and frequent collections 1985 19:28 18 Oct 13 0.846 0.669s 429 made to determine the sex ratios of the population 1986 19:44 23 Oct 18 0.667 0.733s 593 19:33 and the number of founding queens available per ha. 1987 14 Oct 9 1.000 0.688n 683 These were determined by following either swar 1988 19:30 19 Oct 11 0.727 0.820n 543 ming in individual nests and counting the number of I all matingflights occurred during light rain. males per females, or by following swarms and 2 frequencycalculated by thenumber of samesites usedin consecutiveyears/total counting the number of femalesentering each during numberof sites usedin thesecond year. the swarming interval. At least one sweep-net sample 3 Jnvestnent in males = estimated numberof males/estimatednumber of males and of each lek was taken to estimate the number of females;n rn,onest and s lekare estimates. 4 Etologia, Vol. 5, 1997 Frequency observed ting in the shrub would ball the female before she could fly, and approximately 50% of the observed 35.---���������������� females were copulated again before flight (42 of 87). Males with their genetial capsules removed, 0 3 would fall to the ground and were rapidly preyed upon by other ants, in particular Pheidole oxyops 25 (Forel) and Solenopsis saevissima (Fr. Smith). After termination of the mating flight, iso 2 0 ............. lated females disperse by walking over the ground. They rapidly break their wings ( I min.) and then eit her begin excavations, or find shelter under vegeta 1 5 tion and begin excavations the next morning. Most (89%) of these mated females, however, produce a 1 0 nest entrance and small chamber during the period immediately followingmating. Excavated chambers were 30 cm or less in depth. No evidence of multi 5 ple queens (pleometrosis) was foundin either obser vations following mating, or in nest excavations. 2 4 5 >6 Incipient nest entrances were not plugged, and queens were seen foraging near the nest during Number of males per female short periods of the day. Fig. 1. The frequency of the number of males observed tempting copulations with female A. balzani. The observed distribution Discussion does not vary significantly froma zero-truncated Poisson distri bution (X' 4 = 5.92, P > 0.05). Ridley (1988) found a striking tendency for female fecundity of polyandrous insects to increase with mating success, unlike in monoga Upon entering the lek, the female would mous insects. However, the detachment of male then descend to the ground or vegetation over which genetalia upon termination of mating and its reten the lek occurred with an engaged male. Often, at tion in the female genetalia may serve to reduce the these times, a mating ball of near-by males would frequency of multiple-matings by females, as form (Fig. 1), with frequencydistributions not diffe occurs in other Hymenoptera (Thornhill and Alcock ring from the Poisson. The couple would engage 1983). Matings are limited to the temporal window gcnetalia in spite of the mating ball, with each copu available, and thus the efforts of females to rid lation lasting an average of 18 min (s.d., 4.6 min, themselves of the male gcnetalia limit time availa N=28). Upon termination, the gcnetial capsule of ble for other matings. Therefore, we are still uncer the male was detached, and the female removed the tin if Ridley's (1988) patterns for polyandrous genetial capsule with her mandibles before re-ente insects are valid for A. balzani. Although we have ring the lek. To remove the male's genetial capsule, no data on sperm numbers or the sperm storage the female spent 9 min (s.d., 6.8 min, N= 11 ), and capacity of the spermatheca of the female, it is then would have to climb the shrub over which the likely that females do not mate with more than two !ck had formed to take flight again. Often male res- to three males (Fowler et al., 1996). The strongly 5 Harold G. Fowler male-biased sex ratio we found suggests that com Acknowledgments petition may be intense among males. As Arnold & Duvall (1994) suggest, males should experience a Partialsupport forthis research was provided stronger sexual selection gradient, even in lcks, by the Conselho Nacional de Pesquisas (CNPq) of used here in the broad form (Carraza, 1992), than Brazil, through grants 300171/88-9 and 500614/91-1, females under polyandrous mating systems. and the International Foundation for Science through The strongly biased male sex ratio found a grant to Flavio Schlittler. I thank Hayden Fowler for in field colonies, as found in other species of his assistance during these studies, and MariaThereza Acromyrmex (Fowler et al., 1986) may indicate Fink and Marcelo N.