Relationship between 22-kHz calls and testosterone in male rats

Ultrasonic calls in rats induced by the presence of a predator, referred to as “22-kHz calls,” are mainly emitted by socially dominant male rats. Testosterone levels are closely related to social dominance in male rats. In the present study, we investigated the relationship between the emission of stress-induced 22-kHz calls and circulating testosterone levels in male rats, using a combination of surgery (castration or sham operation) and chronic steroid administration (testosterone or cholesterol) to modify circulating testosterone levels. We also assessed the effects of androgen and/or estrogen receptor antagonists on the emission of 22-kHz calls in male rats. An air puff stimulus, known to reliably induce 22-kHz calls in rats, was used as a stressor. Castrated rats with cholesterol implants exhibited significantly fewer 22-kHz calls than rats that had received a sham operation and cholesterol implants, and there was no significant difference between castrated rats with testosterone implants and rats that had received a sham operation and cholesterol implants. Only male rats pretreated with a binary mixture of androgen and estrogen antagonists exhibited significantly fewer 22-kHz calls than controls. These results show that testosterone in male rats has a positive effect on the emission of stress-induced 22-kHz calls, and the calls may be regulated by the activation of both androgen and estrogen receptors.     

Song ranging by incubating male Blue-headed Vireos: the importance of song representation in repertoires and implications for song delivery patterns and local/foreign dialect discrimination

ABSTRACT.   Mechanisms used by birds to range their distance from singing conspecifics are being debated. In particular, the idea that an incoming song must be in a bird's repertoire for it to be ranged accurately is controversial, but important to our appreciation of the role ranging plays in song evolution. We tested the relation between ranging accuracy and songs in repertoires in playback experiments to male Blue-headed Vireos ( Vireo solitarius ) whose precise locations were known because they were incubating eggs. Males ranged songs heard while incubating and, when their mates relieved them at the nest, flew directly to the silent playback sites, suggesting that they remembered the locations of simulated intruders. Male vireos approached playback sites of local songs, likely in their own repertoires, more precisely than foreign songs recorded 95–645 km from our study site. Songs included in local and foreign playback tapes differed primarily in frequency modulation, but were similar in other measurements. These results support ranging theory as described by Morton (1986) . If the songs within an individual's repertoire are ranged with greater accuracy, we discuss how the stability of neighborhoods becomes a factor as to whether or not selection will favor repertoire sharing in song evolution. As well, singing style is affected by ranging. Because Blue-headed Vireos present their songs in a stereotyped order, a listener can compare ordered sequential changes in signal degradation. Comparing degradation in a sequence of songs adds a temporal element that should result in more accurate ranging of the singer's location.     

SWIMMING SPEEDS OF SINGING AND NON-SINGING HUMPBACK WHALES DURING MIGRATION

Limited data exist on swimming speeds of humpback whales ( Megaptera novaeangliae ) and none on swimming speeds of singing whales during migration. We tracked humpback whales visually and acoustically during migration from the breeding grounds past our study site on the east coast of Australia (latitude 26°28'S). The mean swimming speed for whales while singing was 2.5 km/h, significantly less than for non-singing whales with a mean of 4.0 km/h but significantly greater than the mean of 1.6 km/h observed for singing whales on the Hawaiian breeding grounds. Between song sessions, there was no significant difference in speeds between whales that had been singing and other whales. Migration speeds were less for whales while singing but increased during the season. Although humpback whales can swim rapidly while singing (maximum observed 15.6 km/h), they generally do not do so, even during migration. Slower migration by singers would delay their return to the polar feeding areas and may be costly, but may be a strategy to provide access to more females.     

Polymorphism and division of labour in a socially complex ant: neuromodulation of aggression in the Australian weaver ant,Oecophylla smaragdina

Complex social structure in eusocial insects can involve worker morphological and behavioural differentiation. Neuroanatomical variation may underscore worker division of labour, but the regulatory mechanisms of size-based task specialization in polymorphic species are unknown. The Australian weaver ant, Oecophylla smaragdina, exhibits worker polyphenism: larger major workers aggressively defend arboreal territories, whereas smaller minors nurse brood. Here, we demonstrate that octopamine (OA) modulates worker size-related aggression in O. smaragdina. We found that the brains of majors had significantly higher titres of OA than those of minors and that OA was positively and specifically correlated with the frequency of aggressive responses to non-nestmates, a key component of territorial defence. Pharmacological manipulations that effectively switched OA action in major and minor worker brains reversed levels of aggression characteristic of each worker size class. Results suggest that altering OA action is sufficient to produce differences in aggression characteristic of size-related social roles. Neuromodulators therefore may generate variation in responsiveness to task-related stimuli associated with worker size differentiation and collateral behavioural specializations, a significant component of division of labour in complex social systems.