Effect of a foreign male’s age on acceptance into a conspecific colony in a Neotropical wasp

INTRODUCTION

Nestmate recognition is predicted by the inclusive fitness theory (Hamilton 1964) and is well developed in many social insect species. Foreign individuals are either accepted or rejected by guarding individuals based on a comparison of chemical labels with a recognition template (Lacy & Sherman 1983). Cuticular hydrocarbons have long been implicated in nestmate recognition, with recent evidence provided for ants (Lahav et al. 2000), termites (Clèment & Bagnères 1998) and wasps (Gamboa 2004). Early in adult life, each colony member must learn these chemical cues and act to determine the colonial membership of other individuals encountered (Gamboa et al. 1986a). Recent studies using chemical analysis of cuticular hydrocarbons have revealed that female social wasps acquire colony-specific odors shortly after emergence (Panek et al. 2001; Lorenzi et al. 2004). Thus, only newly emerged females can be accepted from alien colonies. To our knowledge, however, there has been no study showing that male social wasps acquire distinct odors after emergence. In the present study, we examine whether male wasps of different age cohorts are accepted from alien colonies in Polybia paulista (von Ihering), a common swarm-founding wasp in Brazil. If acceptance of young males into alien colonies decreases with age, it suggests that males also acquire distinct chemical odors after emergence.

MATERIALS AND METHODS

Field experiments were carried out at the campus of Universidade de São Paulo (USP) in Ribeirão Preto City (21°11′S, 47°48′W) during the dry season between July and August 2005. We introduced adult males from three donor colonies into four recipient colonies nesting in the Departamento de Biologia on the campus of USP. The distances between recipient colonies ranged from 24.4 to 72.9 m (mean ± SE = 56.7 ± 8.3 m). The distances between recipient and donor colonies nesting in two locations were 52 km (Cajuru City, colonies 1 and 2) and 2 km (Departamento de Farmacia on the campus of USP, colony 3), as measured by geographic positioning system.

We placed combs with numerous pupae, which had been obtained from the three nests, into plastic containers (25 cm × 20 cm × 10 cm). We checked the containers daily at 08.00 hours, and when newly emerged male wasps were present, we marked day-specific marks on each wasp’s thorax. We defined newly marked males on the day of emergence as cohort I and males that had emerged for more than 2 days as cohort II. Male wasps of both cohorts were introduced into the four recipient colonies.

All males used in the present study were marked with waterproof ink (Magic Opaque Color; Teranishi, Osaka, Japan) to identify them in the recipient nest. Prior to introduction, each male was placed into a plastic tube (15 mL) and immobilized by cooling with a portable freezer bag. Introduced males were held with forceps and placed gently onto the surface of an envelope. An observer noted the behavior of host residents toward the introduced male and classified it into three categories: (i) tolerant behavior (residents groomed the introduced male); (ii) intermediate behavior (residents bit or lunged at the introduced wasp and then groomed it); and (iii) aggressive behavior (at least one resident bit or stung the introduced male, and the introduced male was ultimately killed or fell down due to wing damage). In the present study, categories (i) and (ii) were regarded as “acceptance” and category (iii) was regarded as “rejection.” Field experiments were carried out between 08.00 and 17.00 hours on days without rain. Each introduced male was used no more than twice, and males were introduced into the recipient colonies at intervals of at least 60 min. When an introduced male was successfully accepted by host residents, we immediately removed the male with an aspirator in order to prevent him mating with a queen in the population.

RESULTS

The percentage acceptance of introduced males by members of alien colonies was considerably different between the two cohorts (Fig. 1). Fifty-nine out of 100 introduced males in cohort I were accepted (59%), whereas 18 out of 96 introduced males in cohort II were accepted (18.75%), and the difference in acceptance between the cohorts was statistically significant (Fisher’s exact probability test, two-tailed, P < 0.00001). To evaluate the effect of colony on acceptance of introduced males by members of alien colonies, we compared the difference in acceptance between the cohorts using Fisher’s exact probability tests in each colony. The probabilities were combined across colonies using a Fisher’s combined probability test. We found that there was a strong significant difference in the percentage acceptance between the cohorts ( χ²₈ = 56.846, P < 0.00001).

DISCUSSION

This is the first study in social wasps to show that acceptance of young males into alien colonies decreases as a function of their age. Frequent acceptance of newly emerged females into alien colonies has already documented in some genera of social wasps (Polistes: Panek et al. 2001; Lorenzi et al. 2004; Ropalidia: Arathi et al. 1997; Polybia: K. Kudôet al., unpubl. data). Panek et al. (2001) and Lorenzi et al. (2004) recently demonstrated that colony-specific cuticular chemical profiles mature between 1 and 3 days after emergence in female Polistes species wasps. However, there appears to be some confusion as to whether recognition in Polistes wasps is mediated by endogenous odors, acquired odors or both. Gamboa et al. (1986b) and Panek et al. (2001) demonstrated that Polistes fuscatus utilizes endogenous (genotypic), colony-specific recognition odors in nestmate recognition. In contrast, Lorenzi et al. (2004) concluded that female Polistes dominulus (L.) wasps passively acquire odors from the nest comb in the first hours after emergence, with such a process allowing wasps to achieve a chemical profile that matches that of their nest and nestmates. Gamboa (2004) hypothesized that the recognition odors acquired from a wasp’s natal comb are similar to the endogenous odors produced by individual wasps. It is likely that newly emerged P. paulista males also acquire distinct odors (labels) shortly after emergence in a manner similar to that of Polistes females. However, given that in the present study newly emerged males of P. paulista stayed on their natal nest combs until they were introduced into alien colonies, it is impossible to determine whether they used environmental (e.g. nest combs) or genetic odors as recognition cues.

Acceptance of 1-day-old males (cohort I) in the present study was much lower than that observed for females of this species. It has been shown that 64 out of 73 introduced females of the same age cohort were accepted from alien colonies in P. paulista (K. Kudôet al., unpubl. data). The acceptance rates for males and females for this species are significantly different (Fisher’s exact probability test, two-tailed, P < 0.00005). Likewise, in R. marginata (Lep.) all 1-day-old females (<24 h) were accepted from alien colonies (Arathi et al. 1997). The lower acceptance rate of newly emerged males by alien colonies may reflect a difference in cuticular chemical profiles between males and females. Layton et al. (1994) showed using discriminant analysis that cuticular lipid profiles of Polistes metricus (Say) males differ from those of females (queens and workers). If the cuticular chemical profiles of newly emerged P. paulista males and females are significantly different, the identity of introduced males in the present study seems likely to be discriminated by recipient females. This hypothesis could be simply tested by analyzing the cuticular chemical profiles of newly emerged males and females and comparing them.

Our present results indicate that mating of reproductive individuals is not likely to occur inside the nest in P. paulista. In swarm-founding wasps, it is unknown where mating of reproductive individuals occurs. One possibility is that a mature male enters a non-natal nest and mates with a new queen in the nest. However, only newly emerged males were accepted by alien colonies with a relatively high frequency, suggesting that mature males may be only rarely accepted by alien recipients.