Vol.:(0123456789) 1 3 Insectes Sociaux https://doi.org/10.1007/s00040-018-00683-8 RESEARCH ARTICLE Making the right choice: how Crematogaster scutellaris queens choose to co-found in relation to nest availability A. Masoni 1  · F. Frizzi 1  · S. Turillazzi 1  · G. Santini 1 Received: 30 May 2018 / Revised: 1 December 2018 / Accepted: 4 December 2018 © International Union for the Study of Social Insects (IUSSI) 2018 Abstract One of the main tasks a freshly mated ant queen has to face is to fnd a safe and suitable nest site to start a new colony. Colony foundation by associated queens, also known as pleometrosis, has been described for several ant species and, under specifc selective pressures, represents an alternative to independent colony foundation. Despite most newly mated queens of the common Mediterranean acrobat ant Crematogaster scutellaris generally adopting independent colony foundation inside tree trunks, both feld and laboratory studies have demonstrated that the formation of pleometrotic groups may occur, particularly inside lignifed aphid galls on poplar or oak trees. These associations typically end with the survival of only one queen after the foundation phase, and the benefts they may provide remain unclear. In this study, we investigated how queen density and diferent nest availability may promote the formation of pleometrotic associations in C. scutellaris. We found that occupied nests are not actively sought after by queens, as hypothesised in previous studies, but might be accepted when they are the only safe refugia available. Moreover, the tendency to form groups increases as queen density increases, and nest availability is a limiting factor. Finally, we found no evidence that the size of the queen afects whether to join an already occupied nest. Keywords Mediterranean acrobat ant · Pleometrosis · Colony founding · Nest choice · Queen associations Introduction Colony foundation by associated queens, also known as pleometrosis, occurs in several ant species (Hölldobler and Wilson 1977, 1990; Bernasconi and Strassmann 1999) and, under specifc selective pressures, represents an alternative to independent colony foundation (Tschinkel 2006; Cole 2009). These cooperative associations of unrelated queens (Kellner et al. 2007; but see Nonacs 1990) rarely persist enough to give rise to polygyny (Heinze et al. 2001; Over- son 2011), and generally end with the death of all but one queen after the appearance of the frsts workers (Sommer and Hölldobler 1995; Helms and Helms Cahan 2012). Mor- tality can be due to direct fghting between the queens (Ris- sing and Pollock 1987) or aggression from newly hatched workers (Balas and Adams 1996; Balas 2005). Under such circumstances, only the surviving queen will beneft from this cooperation. Pleometrotic foundation can potentially be favourable when it allows a faster worker production or a larger workforce, and hence the achievement of higher foraging success or provides better defence against preda- tion and brood raiding during the early stage of a colony’s life (Adams and Tschinkel 1995; Bernasconi and Strass- mann 1999). The importance of these benefts is maximal in highly competitive environments and for territorial spe- cies (Sasaki et al. 2005; Motro et al. 2016). Sometimes, the safety provided by a suitable nest site is enough to trigger associative foundation (Pfening 1995; Overson et al. 2014), and the costs of competition are simply counterbalanced by the reduction of desiccation or predation risks (Herbers 1986)—an aspect that is particularly true when queen den- sity is high and nest availability is limited (Tschinkel 1993). In some species, pleometrosis is associated with the density of young queens in the area and the size of the founding group positively correlates with queen density, as observed for example in Solenopsis invicta (Tschinkel and Howard 1983), Veromessor pergandei (Rissing and Pollock 1987), and Messor semirufus (Motro et al. 2017). The real benefts achieved by group-founding queens are, however, not always clear and, at least in some species, these aggregations seem to be motivated more by the scarcity of nesting sites than by Insectes Sociaux * A. Masoni alberto.masoni@unif.it 1 Department of Biology, University of Florence, 50019 Sesto Fiorentino, FI, Italy