Abstract:
The unique nomadic life-history pattern of army ants (army ant adaptive syndrome), including obligate colony fission and
strongly male-biased sex-ratios, makes army ants prone to heavily reduced effective population sizes (Ne). Excessive multiple
mating by queens (polyandry) has been suggested to compensate these negative effects by increasing genetic variance in
colonies and populations. However, the combined effects and evolutionary consequences of polyandry and army ant life
history on genetic colony and population structure have only been studied in a few selected species. Here we provide new
genetic data on paternity frequencies, colony structure and paternity skew for the five Neotropical army ants Eciton
mexicanum, E. vagans, Labidus coecus, L. praedator and Nomamyrmex esenbeckii; and compare those data among a total of
nine army ant species (including literature data). The number of effective matings per queen ranged from about 6 up to 25
in our tested species, and we show that such extreme polyandry is in two ways highly adaptive. First, given the detected
low intracolonial relatedness and population differentiation extreme polyandry may counteract inbreeding and low Ne.
Second, as indicated by a negative correlation of paternity frequency and paternity skew, queens maximize intracolonial
genotypic variance by increasingly equalizing paternity shares with higher numbers of sires. Thus, extreme polyandry is not
only an integral part of the army ant syndrome, but generally adaptive in social insects by improving genetic variance, even
at the high end spectrum of mating frequencies.
