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Oecologia (2002) 132:60–67 DOI 10.1007/s00442-002-0960-2 POPULATION ECOLOGY Robert A. Johnson Semi-claustral colony founding in the seed-harvester ant Pogonomyrmex californicus: a comparative analysis of colony founding strategies Received: 4 February 2002 / Accepted: 25 April 2002 / Published online: 25 May 2002 © Springer-Verlag 2002 Abstract The evolution of queens that rear their first strategies to survive this stage. The two primary modes brood solely using body reserves, i.e. fully claustral, of nest founding are dependent colony founding, in is viewed as a major advance for higher ants because which a queen starts a colony with a retinue of workers, it eliminated the need for queens to leave the nest to and independent colony founding, in which a queen forage. In an apparently unusual secondary modification, starts a nest without workers. Both founding strategies the seed-harvester ant Pogonomyrmex californicus dis- display several variants (Hölldobler and Wilson 1977). plays obligate queen foraging, i.e. queens must forage For example, queens that found nests independently can to garner the resources necessary to survive and success- be fully claustral, in which case the queen remains fully rear their first brood. I examined the potential bene- sealed in the nest chamber and uses only body reserves fits of queen foraging by comparing ecological and phys- to support herself and raise her first brood. Alternatively, iological traits between P. californicus and several con- semi-claustral queens leave the nest to forage. Fully geners in which the queen can rear brood using only claustral nest founding is common in morphologically body reserves. The primary advantage of foraging ap- “advanced” subfamilies of ants (Myrmicinae and For- pears to lie in providing the queens of P. californicus micinae) and is the derived state, whereas the ancestral with the energy to raise significantly more brood than state of semi-claustral nest founding is common in mor- possible by congeners that use only body reserves; the phologically “primitive” subfamilies of ants (Ponerinae workers reared in the first brood were also heavier in and Myrmeciinae; Peeters 1997). mass than that predicted by their head width. Other The evolution of fully claustral queens is viewed as correlates of queen foraging in P. californicus relative to a major advance for ants because it eliminated the need tested congeners included a significantly lower total fat for queens to leave the nest to forage, where they are content for alate queens, a small queen body size, and exposed to predation and other sources of mortality a low queen to worker body mass ratio. Queens also (Haskins and Haskins 1950, 1955; Hölldobler and forage in several other well-studied species of Pogo- Wilson 1977; Rissing et al. 1989; Peeters 1997). Conse- nomyrmex, suggesting the possibility that queen foraging quently, the observation that some ant species in the sub- may be more common than previously thought in higher families Myrmicinae and Formicinae have returned to ants. semi-claustral nest founding, when most of their conge- ners are fully claustral, requires explanation. Information Keywords Brood production · Fat content · Nest on semi-claustral nest founding in higher ants is limited founding · Pogonomyrmex californicus · Queen body size to scattered behavioral observations. The only physio- logical data indicate that queens of Messor andrei lack storage proteins (Brown 1999), which comprise a neces- Introduction sary energy source for fully claustral queens to raise their first brood (Wheeler and Buck 1995, 1996). Nest founding is the most vulnerable stage in the life of It has been suggested that queen foraging is associated an ant colony (Hölldobler and Wilson 1990; Herbers with the small size of queens relative to the workers 1993; Johnson 1998), and ants have evolved numerous (Haskins and Haskins 1950; Stille 1996; Peeters 1997). Correlative evidence for this hypothesis occurs within R.A. Johnson (✉) ponerines, which display a gradation of colony founding Department of Biology, Arizona State University, Tempe, AZ 85287–1501, USA methods that range from dependence on queen foraging e-mail: [email protected] to ability of the queen to successfully rear her first work- Tel.: +1-480-8972473, Fax: +1-480-9652519 ers using only body reserves. One of the few species in 61 the latter group is Pachycondyla (= Brachyponera) lutea, farms were observed for several days to determine if the queens which is exceptional among ponerines because of the un- laid eggs prior to obtaining food. Kentucky blue grass (KBG) seeds were provided ad libitum to a subset of queens after 12 days usually large size of queens relative to workers (the maxi- in 1999 and after 10 days in 2000; other queens remained unfed. mum known caste dimorphism among ponerines; Haskins Thereafter, I recorded queen status and stage of brood develop- and Haskins 1950, 1955; Hölldobler and Wilson 1990; ment every 2–3 days until all queens had produced workers or Peeters 1997). died; this was for 54 days in 1999 and 64 days in 2000. I com- pared survival curves of fed and unfed queens in each year using The primary disadvantage for fully claustral queens the Kaplan-Meier survival analysis log-rank test for censored data seems to be that a fixed amount of energy is available to (PROC SURVIVAL of SPSS; SPSS 1990); the comparison ran raise the first brood. This requires that the queen allocate from the day on which seeds were first provided through to the her energy to maximize colony success, either by pro- last day of observation. ducing many small workers or few large workers. Theo- The quantitative comparison of survival and brood production for single queens used a one-factor (two food levels) laboratory ry predicts that queens should maximize the initial num- experiment; each treatment contained 85 bottles for a total of ber of workers produced because some minimum num- 170 bottles. The experiment used 8-ounce glass bottles that con- ber is needed to perform the gamut of colony tasks that tained 250 ml soil and 35 ml water. A plastic petri dish containing include raising the second brood. Additional brood allow several holes covered each bottle. The bottles were randomly posi- tioned in a darkened room maintained at 25–30°C. KBG seeds these tasks to be performed more efficiently through were provided ad libitum to one-half of the queens beginning on division of labor rather than sequentially by the same in- day 7, and 10–15 ml water were added to all bottles every 10 days dividual (Oster and Wilson 1978). As predicted, fully beginning on day 20. Bottles were emptied after 53 days to deter- claustral ant queens invariably produce many small mine queen status and number of brood (larvae, pupae, and work- ers). This interval was about 10 days after observing the first workers (these small workers from the first brood are worker, thus allowing time for most queens to successfully rear called minim workers or minims). Empirical data also workers, but not so long as to deplete queen energy reserves (see demonstrate the adaptive advantage of producing many Johnson 1998). Survival of fed and unfed queens was compared small workers because the number of workers (not their by a Chi-square test, and number of brood was compared by a size) is the primary factor affecting production of the t-test. All of the above experiments were set up so that the initial wet mass of fed and unfed queens was similar (t-test, P >0.50), second brood (Porter and Tschinkel 1986). thus eliminating effects due to queen mass. This study was motivated by observations that queens of the seed-harvester ant Pogonomyrmex californicus (subfamily Myrmicinae) forage in the field and laboratory. Comparative data with other species of Pogonomyrmex I assessed the possible benefits of queen foraging by quantifying parameters related to nest founding for P. ca- Comparative data for fully claustral congeners provide insight into the evolution of foraging by queens of P. californicus. Ecological lifornicus and several congeners. Two hypotheses were and physiological variables that were measured included time to tested: first, foraging releases queens from the size versus emergence of the first minim worker, number of brood (larvae, number tradeoff relative to producing their first brood; pupae, and workers) produced by each queen, dry mass and head second, queen foraging is correlated with body size con- width of minim workers, dry mass of queens and workers from straints. I also assessed: (1) if foraging is necessary for mature colonies, the ratio of queen to worker mass, and fat content of alate queens; sample size for each variable is given in Table 1. queens of P. californicus to survive and successfully rear Congeners that were examined included P. barbatus and P. rugosus their first minim workers, and (2) if there are additional from the P. barbatus complex, P. occidentalis and P. salinus from ecological and physiological correlates of queen foraging the P. occidentalis complex, and P. maricopa from the P. californ- in comparison with fully claustral congeners. To this end, icus complex; the latter species is closely related to P. californicus (Cole 1968; Taber 1990). I measured several ecological and physiological parame- Data related to colony establishment were obtained for all spe- ters across several species of Pogonomyrmex. cies by placing single queens in glass ant farms filled with moist- ened soil (see above). Nests were placed in a darkened incubator at 30°C, and water was added as necessary to maintain moist nest con- Materials and methods ditions. Queens of P. californicus were provided KBG seeds ad libitum; queens of the other species remained unfed to verify the as- Queen survival and brood production in P. californicus sumption that they were fully claustral.

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