C. obscuripes has been suggested as a possible host for P. lamellidens (Sakai 1996; Furukawa et al. 2012), but there was no conclusive evidence. This paper is the first record of a mixed colony of C. obscuripes and P. lamellidens in the field, and P. lamellidens parasitism of C. obscuripes colonies was confirmed in a rearing experiment.
In the behavioral tests, P. lamellidens and C. obscuripes workers from the same mixed colony showed aggressive behavior towards workers from other colonies. Nevertheless, they recognized each other as nestmates, even though they were different species (Table 2). However, non-parasitized C. obscuripes exhibited distinct aggressive behaviors towards P. lamellidens. These results suggest that both species inhabit the mixed colony as nestmates. It is thought that a newly mated P. lamellidens queen will kill the host queen in the early stages of social parasitism according to the previous knowledge of C. japonicus as its host (Japanese Ant Database Group 2003). We carried out rearing experiments to verify that C. obscuripes is affected by social parasitism. In our rearing experiment, we observed that the P. lamellidens queens started laying eggs after killing the host queen (Fig. 2). Thus, we anesthetized the C. obscuripes queen before contacting the newly mated P. lamellidens queen in the rearing experiment. However, since the newly mated P. lamellidens queen demonstrated the same killing process toward C. japonicus, as shown only in the case of a C. obscuripes queen in previous studies (Japanese Ant Database Group 2003), we believe anesthesia does not affect the hypothesis verification of this experiment. Host-queen killing and oviposition in the host colony are common features of temporary socially parasitic and dulotic ants, not xenobiosis and inquilinism (Hölldobler and Wilson 1990; Buschinger 2009; de la Mora et al. 2020). P. lamellidens persisted after the death of the host workers in mixed Colony A. Temporary coexistence with the host and sustaining the colony after the host’s death are unique characteristics in temporary socially parasitic ants, not dulotic ants (with some exceptions, e.g., facultatively dulotic ants) (Hölldobler and Wilson 1990; Buschinger 2009; de la Mora et al. 2020). Based on these observations, the two colonies obtained from the field provide evidence of P. lamellidens completing the temporary social parasitism cycle in a C. obscuripes colony, and we have clear evidence that C. obscuripes is a natural host for P. lamellidens. The above experiments and observations verified that P. lamellidens is not an obligatorily dulotic ant, as they could persist after the death of the host workers. However, the possibility of facultative dulosis has not yet been rejected. None of the reports of raiding behavior in the field or a few records of the mixed colony with its hosts suggests that P. lamellidens is an unlikely facultatively dulotic ant; thus, further verification is needed.
The balance of worker numbers in the two mixed colonies was different (Table 1). We speculate that this reflects the difference in the time that has elapsed since the host queen was killed. Probably, Colony A had been around longer than Colony B since the killing of the host queen; therefore, it had fewer workers of C. obscuripes than P. lamellidens. The left and right mid- and hindlegs of the P. lamellidens queen in Colony A were all severed at their bases (Fig. 1b). In past fieldwork (unpublished) in another region [Nirasaki City, Yamanashi Prefecture (N 35°42′49″, E 138°29′12″)], we observed an injured P. lamellidens queen from a free-living P. lamellidens colony. After the nuptial flight, a newly mated P. lamellidens queen invades a host colony to establish a new colony, but she is often attacked by some host workers at first contact, as observed in rearing experiments in previous studies (Kohriba 1963; Sakai 1996, 2000). In this rearing experiment, a newly mated P. lamellidens queen, probably killed by an attack from its host workers, has been confirmed. We believe that the P. lamellidens queen of Colony A succeeded in usurping the host colony before being killed by attack by host workers in the early stages of social parasitism.
C. obscuripes workers, members of the collected mixed colony, and those whose queen was killed in the rearing experiment did not show aggressive behaviors towards P. lamellidens, and both species coexisted as nestmates. This phenomenon suggests that P. lamellidens may have some involvement in nestmate discrimination by C. obscuripes. It is known that some socially parasitic ants alter host-like cuticular hydrocarbon (CHC) profiles or express almost no CHCs, which promotes coexistence with their host (Dettner and Liepert 1994; Johnson et al. 2001; Lenoir et al. 2001; Akino 2008; de la Mora et al. 2020). P. lamellidens queens and workers in mixed colonies may utilize similar strategies. The behavior of straddling host workers was observed in P. lamellidens queens. As a previous study speculated (Kohriba 1963), P. lamellidens queens may disguise their CHC profiles by performing this behavior, enabling invasion of the host colony.
The host Diacamma sp. and Rhytidoponera sp. workers (Formicidae, Ponerinae) of Polyrhachis lama and Polyrhachis loweryi do not rear parasites’ brood (Maschwitz et al. 2003; Witte et al. 2009). It is speculated that the reason for the lack of brood care by the host is that the parasite/host is distantly related at the subfamily level, and there are discrepancies in larval feeding methods (Formicinae larvae receive liquid nourishment that is preprocessed by workers, whereas Ponerinae larvae are provided unmodified prey particles by workers); thus, the rearing style of the host species is not compatible with the parasite brood (Witte et al. 2009). Conversely, in the case of P. lamellidens, the hosts C. japonicus and C. obscuripes were observed rearing P. lamellidens brood (Fig. 2c, d; Kohriba 1963). P. lamellidens and its host ants are relatively closely related, and although they differ at the genus level, they both belong to the subfamily Formicinae (Brady et al. 2006) and probably share a common feeding method. The differences in the presence or absence of brood care by the host, even among socially parasitic species in the genus Polyrhachis, may be related to common brood feeding methods between parasites and hosts, as described above.
The nesting styles of P. lamellidens and its host differ. Initial nesting sites of P. lamellidens (P. lamellidens changes its nesting site at the end of the social parasitism cycle) are tree hollows, while C. japonicus nests in soil and C. obscuripes nests in decaying wood (Japanese Ant Database Group 2003). Thus, P. lamellidens is relatively unusual as a socially parasitic ant. The exact explanation for why P. lamellidens utilizes Camponotus as its host is unclear, but it is likely that P. lamellidens has high adaptability to various environments that harbor Camponotus.
Morphological similarities (e.g., body size) between socially parasitic ants and their hosts has been reported; one explanation for this phenomenon is that a socially parasitic ant has to adapt to the host’s colony environment (e.g., tunnel size) (Fischer et al. 2020). This phenomenon is probably also true for P. lamellidens and its hosts. The workers of ants in the subgenus Camponotus (C. japonicus and C. obscuripes belong to this subgenus) are 7–12 mm long, which is similar to or longer than P. lamellidens (7–9 mm for workers, 10 mm for the queen) (Japanese Ant Database Group 2003; Kohout 2014). Conversely, workers of the subgenera Myrmamblys and Myrmentoma (belonging to the genus Camponotus), which are not hosts of P. lamellidens, are less than 5 mm in total body length (Japanese Ant Database Group 2003).
This study revealed that P. lamellidens utilizes multiple ant species as a host (C. japonicus and C. obscuripes). In addition to P. lamellidens, socially parasitic ants that parasitize numerous hosts (e.g., Lasius umbratus) are known regardless of parasitism style. The common feature of these species is that the socially parasitic ants and their hosts tend to be closely related species, and the hosts belong to a specific taxonomic group at the subfamily level (Emery 1909; Hölldobler and Wilson 1990; Japanese Ant Database Group 2003; Huang and Dornhaus 2008; de la Mora et al. 2020). The reason why the hosts are closely related to the parasitic ants and are restricted to few taxa is thought to be because it is easy for the parasitic ants to disguise their CHC profile (Blatrix and Sermage 2005; Huang and Dornhaus 2008; de la Mora et al. 2020); the same is probably true for P. lamellidens. Moreover, as mentioned, another factor in establishing social parasitism is that the host is closely related to the parasite species and can take care of the parasite’s brood.
Our findings provide evidence, for the first time, of temporary social parasitism involving P. lamellidens and C. obscuripes. The methodology presented in this investigation will contribute to, and accelerate, the study of social insects.