Mixed-species groups and the question of dominance in the social ecosystem.

Dominance interactions and hierarchies are of long-standing interest in the field of animal behaviour. Currently, dominance hierarchies are viewed as complex social structures formed by repeated interactions between individuals. Most studies on this phenomenon come from single-species groups. However, animals are constantly surrounded by and interact with individuals of other species. Behaviour and social interactions of individuals can be shaped by the presence or behaviour of other species in their social ecosystem, which has important implications for social behaviour in groups. Given how ubiquitous mixed-species animal groups are, deeper study of the relationships between mixed-species group (MSG) structure and dominance will be key to understanding constraints on individual behaviour and decision making. Here we call for more research into dominance interactions among individuals in MSGs. Greater understanding of the dynamics of dominance relationships among individuals in MSGs, whose size and composition can change considerably over shorter and longer term time frames, will be crucial to understanding their structure and functioning. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Male mealworm beetles increase resting metabolic rate under terminal investment.

Harmful parasite infestation can cause energetically costly behavioural and immunological responses, with the potential to reduce host fitness and survival. It has been hypothesized that the energetic costs of infection cause resting metabolic rate (RMR) to increase. Furthermore, under terminal investment theory, individuals exposed to pathogens should allocate resources to current reproduction when life expectancy is reduced, instead of concentrating resources on an immune defence. In this study, we activated the immune system of Tenebrio molitor males via insertion of nylon monofilament, conducted female preference tests to estimate attractiveness of male odours and assessed RMR and mortality. We found that attractiveness of males coincided with significant down-regulation of their encapsulation response against a parasite-like intruder. Activation of the immune system increased RMR only in males with heightened odour attractiveness and that later suffered higher mortality rates. The results suggest a link between high RMR and mortality and support terminal investment theory in T. molitor.

A comparative study of avian auditory brainstem responses: correlations with phylogeny and vocal complexity, and seasonal effects.

We conducted a comparative study of the peripheral auditory system in six avian species (downy woodpeckers, Carolina chickadees, tufted titmice, white-breasted nuthatches, house sparrows, and European starlings). These species differ in the complexity and frequency characteristics of their vocal repertoires. Physiological measures of hearing were collected on anesthetized birds using the auditory brainstem response to broadband click stimuli. If auditory brainstem response patterns are phylogenetically conserved, we predicted woodpeckers, sparrows, and starlings to be outliers relative to the other species, because woodpeckers are in a different Order (Piciformes) and, within the Order Passeriformes, sparrows and starlings are in different Superfamilies than the nuthatches, chickadees, and titmice. However, nuthatches and woodpeckers have the simplest vocal repertoires at the lowest frequencies of these six species. If auditory brainstem responses correlate with vocal complexity, therefore, we would predict nuthatches and woodpeckers to be outliers relative to the other four species. Our results indicate that auditory brainstem responses measures in the spring broadly correlated with both vocal complexity and, in some cases, phylogeny. However, these auditory brainstem response patterns shift from spring to winter due to species-specific seasonal changes. These seasonal changes suggest plasticity at the auditory periphery in adult birds.

Cultures, genes, and neurons in the development of song and singing in brown-headed cowbirds (Molothrus ater).

In brown-headed cowbirds, Molothrus ater, as in many songbird species, vocalizations are fundamental to reproduction. In our studies, experiments utilizing different social housing regimes and geographic comparisons have indicated the social learning of males' vocalizations and associated abilities to use vocalizations effectively during the breeding season. Here, we describe studies indicating roles of cultural and genetic background, and of social influences from females, on male vocal development. These influences can interact with neural regions, including song learning and song control nuclei, but also visual-processing nuclei, in the development of signaling. We argue that a developmental systems approach to the study of vocal behavior provides a structure to organize these different influences and how they may interact with one another over development. A systems approach requires that researchers study the social context in which signals and signalers develop - both the ontogenetic arena in which young animals learn their signals from older animals, and the functional arena in which young and older animals socially interact with one another.

Cultural transmission of vocal traditions in cowbirds (Molothrus ater) influences courtship patterns and mate preferences.

In this study, the authors tested the cultural transmission of vocal traditions in cowbirds (Molothrus ater). Young cowbirds from a South Dakota (SD) population were housed over winter with adults of the SD population or with adults from an Indiana (IN) population. Song differences between the original SD and IN adult models were acquired by South Dakota culture (SDC) and Indiana culture (INC) males, respectively, and were transmitted to a 2nd cultural generation of birds. During playback tests of SDC and INC songs, SD females gave more copulatory responses to SDC songs. Finally, males with SD-like songs courted SDC females preferentially in breeding season tests, whereas males with IN-like songs courted INC females preferentially. These results indicate that the transmission of vocal traditions plays a fundamental role in the courtship patterns and mating decisions of cowbirds.

Social malleability in cowbirds: new measures reveal new evidence of plasticity in the eastern subspecies (Molothrus ater ater).

The social experiences of young Molothrus ater ater cowbirds were manipulated in a 2-year study. In the 1st year, males were housed with pairs of canaries. The males were tested in 3 social contexts. Also, vocal repertoires were recorded and played back to females. In contrast to a previous study of the M. a. artemisiae subspecies, the males did not vocalize to the canaries in courtship tests (T. M. Freeberg, A. P. King, & M. J. West, 1995) but showed incompetent courtship of female cowbirds. In their 2nd year, half of the males were housed with older males and female cowbirds, and half were housed with only females. Those exposed to older males courted much more successfully than did those deprived of such experience. All males developed new repertoires, and song potencies did not correlate across years. The data reveal intraspecific variation in the ontogeny of mate recognition but intraspecific dependence on social learning to acquire courtship skills.