Behavioral Ecology and Sociobiology

Showing traces of eusocial behavior, Zethus miniatus is an emblematic member of the primarily solitary subfamily Eumeninae. The females of this casteless species share a common nest, but each one provisions her own brood in a progressive manner. They express dominance when reproductive (queen-like behavior) and occasionally perform tasks that benefit others when provisioning the brood (worker-like behavior). Hence, the biology of this species has long been considered as harboring traits that could mark the transition from a solitary to social lifestyle. Here, we present for the first time measurements of the juvenile hormone (JH) titer in combination with behavioral observations and data on the females’ ovarian status. Aggressive reproductives seeking to reuse or usurp brood cells were found to possess longer oocytes than provisioners of young larvae. Brood killings by reproductive cell-seekers and JH titers were both higher when the female/open-brood-cell ratio of the nest was high—an indicator of strong competition. Females lacking brood, such as those that are victims of cell usurpation attempts by other females, were found to perform tasks relating to nest defense, nest maintenance, and/or adoption of orphaned brood. Hence, we hypothesize that competition for brood cells may have shaped the reproductive physiology of this group-living species of social wasps, and that JH is critical for this competitive behavior. Furthermore, we hypothesize that JH-mediated dominance interactions in casteless groups may have preceded and contributed to the origin of reproductive castes. How, mechanistically, did caste phenotypes (a polyphenism with egg-layers and non-egg-laying helper females within groups) originate from solitary ancestors? Key to addressing this question can be casteless, group-living wasp species, where the females progressively feed their brood. The eumenine wasp Zethus miniatus has behavioral traits intermediate between solitary and caste-possessing wasps, and hence, has gained a model status for this transition. This is the first study of such a wasp in nature that combines data on behavior, ovarian activity, and juvenile hormone (JH) levels. The results show that the ovarian status of a Z. miniatus female is a good predictor of her behavior, and that JH titers were highest when competition for available brood cells was intense. This suggests that JH may have become linked to the origin of worker caste behaviors through its involvement in competitive interactions among nestmates.

Laboratory studies of nestmate recognition (250.0 h of behavioral observation) were conducted on two species of paper wasps (Polistes). Gynes of P. carolina isolated from their natal nest and nestmates at emergence later failed to recognize nestmates, but gynes previously exposed only to their natal nest for 2 h recognized nestmates. Gynes of P. fuscatus previously (1) exposed to their natal nest and nestmates for at least 5d, (2) exposed only to their natal nest for 4.16–13.5 h and (3) exposed only to their natal nest for 1 h, all discriminated nestmates from non-nestmates. Thus, exposure to the natal nest is a sufficient condition for the ontogeny of nestmate recognition ability. Unrelated gynes of P. fuscatus, previously exposed to different fragments of the same but unrelated nest, also recognized each other. These results, together with the evidence from other recent studies, indicate that the mechanism of nestmate discrimination in Polistes has the following attributes. Recognition cues are (1) learned, (2) learned in the adult stage, (3) learned within a few hours of emergence with memories of these cues being durable if not permanent; thus, learning of recognition cues resembles imprinting; (4) Recognition cues are learned from the natal nest and/or brood and not from adult nestmates, (5) they cannot be visual, tactile, or auditory features of adult nestmates and thus are probably chemical, and (6) they can be both learned and acquired from a nest and/or its brood.

In insects, the last male to mate with a female often gains access to a disproportionate number of subsequent fertilizations. This study examined last-male sperm precedence patterns in doubly and triply mated Tribolium castaneum females. Sperm storage processes were investigated by measuring the quantity of sperm stored within the female spermatheca following single, double, and triple matings. Both doubly mated and triply mated females exhibited high last-male sperm precedence for progeny produced during the first 48 h following the last mating, with females in both groups exhibiting parallel declines in sperm precedence 1 and 2 weeks later. The number of sperm stored by females increased by 33% between singly mated and doubly mated females, indicating that the spermatheca is filled to only two-thirds capacity following insemination by the first male. Based on the proportion of stored sperm from first and second matings, we tested predictions about sperm precedence values based on models of random sperm mixing. High initial last-male sperm precedence strongly supports stratification of last-male sperm. By 1–2 weeks after double matings, sperm precedence declined to levels indistinguishable from values expected under random mixing. These results provide insight into mechanisms of sperm storage and utilization in this species.

The song repertoires of male song sparrows (Melospiza melodia) are comprised of distinct song types and many overlapping variations within each type. This study examines the sensitivity of territorial male song sparrows to both multiple song types and variation within types. A playback experiment was conducted using stimulus tapes that had either single songs (but not variations), song variations (but not types) or a single song. Response during playback was similar for all three stimulus groups, but diminished sooner after playback for the single-song tape. Thus our results indicate that male song sparrows are sensitive to song variability both within and among song types.

Females in some mountain gorilla groups can be ranked on the basis of decided, non-aggressive approach-retreat interactions. However, data on four gorilla groups show that females do not form clear agonistic dominance hierarchies. Most aggressive interactions between females were undecided, and most dyads had undecided dominance relationships (Fig. 2). Females ignored aggression or responded to it aggressively more often than they responded submissively (Fig. 3). Moreover, directional inconsistency in aggression (including escalated contests) was high, and in some groups females showed significant bidirectionality in initiation of aggression (i.e., the more often they received aggression from partners, the more often they directed aggression at those partners; Fig. 4). Assigning ranks on the basis of aggressive interactions has limited power to explain variation in rates of initiating aggression and in responses and outcomes to aggression. Aggression was most common during feeding, but usually did not interrupt feeding bouts. It was proportionately more common in contexts that seem to involve competition for access to males (Figs 5, 6). This is consistent with the argument that females depend crucially on male services. As predicted by current models of female primate social relationships, agonistic relationships between gorilla females are unlike those typical in cercopithecine primates. They resemble those of some other primates and some equids in which contest competition for food is relatively unimportant and ecological costs of female transfer are low.

Social bees can deposit specialized glandular secretions, or signals, that allow foragers to revisit rewarding and to avoid unrewarding food sources. However, it is not known if bees can orient towards olfactory cues such as excreta deposited near food sources. We report that Melipona mandacaia foragers (stingless bees) deposit an odor cue, anal droplets, and a previously undescribed ventro-abdominal odor on food sources. Surprisingly, foragers deposited attractive odor marks on good food sources to which they recruited and on poor food sources to which they did not recruit. Foragers left the most anal droplets on dilute food sources to which they did not recruit (1.25-M sucrose solution), yet returning foragers were attracted to anal droplets obtained on poor food sources and presented in bioassays. Foragers were attracted to ventro-abdominal odors obtained on good food sources (2.5-M sucrose solution). Chemical extractions suggest that odor marks contain attractive polar compounds. We also provide the first detailed description of forager waggling and spinning behavior on poor and good food sources. Waggling may be a method of dispersing anal droplets and spinning may help foragers learn local landmarks.

Polarized odor-trail communication, in which a receiver can orient towards the correct endpoint from within the trail, is documented in relatively few animals and is poorly understood, although such directionality could significantly enhance resource localization. Among animals, stingless bees exhibit the unique behavior of depositing long substrate-borne odor trails that assist the orientation of flying nestmates to a specific three-dimensional food location. However, relatively little is known about the spatial structure of such odor trails, particularly vertical trails, and whether these trails are polarized to indicate the correct terminus. We show that a stingless bee, Trigona spinipes, can rapidly recruit nestmates in large bursts to a food source at a specific distance, direction, and height. In conjunction with a major recruitment burst, foragers deposited odor marks that attracted nestmates for up to 20 min. Surprisingly, these odor marks formed a short odor trail instead of a complete odor trail extending from the feeder to the nest (the classic description of a meliponine odor trail). The length of the odor trails varied between different feeder locations with different colonies, from a minimum of 3 m to a maximum of 29 m. The odor marks formed a polarized trail that newcomers followed to the end with the most concentrated odor marks (the feeder), even when the entire odor trail was rotated 180° and clean test feeders were set out at locations that foragers had never previously fed at. Thus locale odor or the potential communication of food location inside the nest do not account for the ability of newcomers to find the correct terminus. This result provides the first strong evidence for odor-trail polarization in social insects.