Animal acoustic communication often takes the form of complex sequences, made up of multiple distinct acoustic units. Apart from the well-known example of birdsong, other animals such as insects, amphibians, and mammals (including bats, rodents, primates, and cetaceans) also generate complex acoustic sequences. Occasionally, such as with birdsong, the adaptive role of these sequences seems clear (e.g. mate attraction and territorial defence). More often however, researchers have only begun to characterise – let alone understand – the significance and meaning of acoustic sequences. Hypotheses abound, but there is little agreement as to how sequences should be defined and analysed. Our review aims to outline suitable methods for testing these hypotheses, and to describe the major limitations to our current and near-future knowledge on questions of acoustic sequences. This review and prospectus is the result of a collaborative effort between 43 scientists from the fields of animal behaviour, ecology and evolution, signal processing, machine learning, quantitative linguistics, and information theory, who gathered for a 2013 workshop entitled, “Analysing vocal sequences in animals”. Our goal is to present not just a review of the state of the art, but to propose a methodological framework that summarises what we suggest are the best practices for research in this field, across taxa and across disciplines. We also provide a tutorial-style introduction to some of the most promising algorithmic approaches for analysing sequences. We divide our review into three sections: identifying the distinct units of an acoustic sequence, describing the different ways that information can be contained within a sequence, and analysing the structure of that sequence. Each of these sections is further subdivided to address the key questions and approaches in that area. We propose a uniform, systematic, and comprehensive approach to studying sequences, with the goal of clarifying research terms used in different fields, and facilitating collaboration and comparative studies. Allowing greater interdisciplinary collaboration will facilitate the investigation of many important questions in the evolution of communication and sociality.
Soil organic carbon (SOC) is the largest carbon sink in terrestrial ecosystems and plays a critical role in mitigating climate change. Increasing reactive nitrogen (N) in ecosystems caused by anthropogenic N input substantially affects SOC dynamics. However, uncertainties remain concerning the effects of N addition on SOC in both organic and mineral soil layers over time at the global scale. Here, we analysed a large empirical data set spanning 60 years across 369 sites worldwide to explore the temporal dynamics of SOC to N addition. We found that N addition significantly increased SOC across the globe by 4.2% (2.7%–5.8%). SOC increases were amplified from short‐ to long‐term N addition durations in both organic and mineral soil layers. The positive effects of N addition on SOC were independent of ecosystem types, mean annual temperature and precipitation. Our findings suggest that SOC increases largely resulted from the enhanced plant C input to soils coupled with reduced C loss from decomposition and amplification was associated with reduced microbial biomass and respiration under long‐term N addition. Our study suggests that N addition will enhance SOC sequestration over time and contribute to future climate change mitigation.
Bird song is the primary animal model system for cultural evolution. Longitudinal studies of bird song across many generations can provide insights into patterns and mechanisms of change in socially transmitted traits. In this study, we conducted a comparative analysis of songs of the House Finch (Haemorhous mexicanus) across an interval of 37 years (in 1975 and in 2012). Recordings from both years were collected in western Long Island, New York, which is thought to be the initial site of introduction of the House Finch around 1940 from the West Coast. Song types experienced a complete turnover during this period, although half of the syllable types were represented in both samples. Song length, frequency bandwidth and several other spectrographic features were the same in both years, and no structural features predicted recurrence of individual syllables. Consistent with the fact that our study population expanded substantially following a cultural founder effect, song and syllable sharing and similarity between individuals were lower in 2012 than in 1975, reflecting an increase in song diversity at the population level. However, in the more recent sample individual songs had fewer syllables, and were sung with less sequence stereotypy across renditions than in the earlier sample. Syllable prevalence in 2012 was associated both with complexity (as gauged by frequency excursion) and increased minimum frequency. Thus, over nearly 4 decades, Eastern House Finch songs remained structurally similar at the whole-song level, diversified between individuals, but became simpler and less consistent within an individual.
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