Dispersal strategies in the highly polygynous ant Crematogaster (Orthocrema) pygmaea Forel (Formicidae: Myrmicinae)

Abstract

In ants, dispersal strategies and morphology of female sexuals are generally linked to the mode of colony founding. In species using long-range dispersal tactics, queen/worker dimorphism is generally high and young queens are able to initiate new colonies by themselves, using their metabolic reserves. By contrast, in species using short-range dispersal strategies, queen/worker dimorphism is generally low and, due to their limited metabolic reserves, queens have lost the capacity to raise their brood alone and to found their colony independently. Moreover, polygyny is also often associated with short-range dispersal strategies, although the relationship between the number of queens and the dispersal strategy in ants is not clear-cut. Here, dispersal strategies were investigated in C. pygmaea, a highly polygynous and polydomous ant species from northeastern Brazil. Field observations and laboratory experiments show that this ant exhibits a suite of traits that are more commonly associated with long-range dispersal and independent colony foundation: functional wings in both males and females, high queen/worker dimorphism, strong weight loss in mature queens, nuptial flights and, in the lab, ability of young queens to found new colonies in haplometrotic conditions. On the other hand, this species shows a high degree of polygyny with a strong seasonal component, and, at least under laboratory conditions, mature queens seem able to develop propagules if they are accompanied by at least 10 workers. These features strongly suggest that (1) some of the gynes do not engage in a long-range dispersal but become new queens in their mother colony and (2) that budding events are possible in this species. We therefore speculate that C. pygmaea has a dual dispersal strategy probably related to environmental conditions: some gynes engage in long-range dispersal followed by independent colony foundation at the beginning of rainy season, while others mate in the parental colony and are re-adopted leading to high polygyny. During the rainy season, budding events can lead to colony extension and increased polydomy. Polydomy is commonly thought to improve resource discovery and exploitation through decentralized foraging behavior, a significant advantage during the rainy season when food ressources (mainly floral/extrafloral nectaries and hemipteran honeydew) are more abundant and when colony needs for food supplies are highest.

Fig 1

Morphological differences between a gyne (unmated winged queen) (A,C) and a mature queen (B,D). The gaster is yellowish in gynes while it is dark in mature queens.

Fig 2. Frequency distribution of the queen thorax volume in Crematogaster pygmaea queens (n = 32) from five colonies.

Fig 3. Frequency distribution of fresh body weight in Crematogaster pygmaea queens (n = 279) from two colonies (Col-4, Col-5).

Fig 4. Mean number (± SD) of adult workers in Crematogaster pygmaea haplometrotic foundations (n = 8) 1 to 16 weeks after gynes fecundation.

Fig 5. Box plot comparing the effect of initial number of workers on the survival rate of queens.

Fig 6. Box plot comparing the effect of initial number of queens on the survival rate of queens (including groups without workers).

Fig 7. Box plot comparing the effect of time on the survival rate of queens (including groups without workers).

Fig 8. Effect of the number of workers per queen on the brood production.

Model: ExpDec2; Equation: y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0; y0 = 52.89 ± 12.00; A1 = -24.18; t1 = 26.80; A2 = -23.20; t2 = 26.78; Reduced Chi-Square = 1148.78; R-Square (COD) = 0.199; Adj. R-Square = 0.166.