Soldier neural architecture is temporarily modality-specialized but poorly predicted by repertoire size in the stingless bee Tetragonisca angustula

Individual heterogeneity within societies provides opportunities to test hypotheses about adaptive neural investment in the context of group cooperation. Here we explore neural investment in defense specialist soldiers of the eusocial stingless bee (Tetragonisca angustula) which are age sub-specialized on distinct defense tasks, and have an overall higher lifetime task repertoire than other sterile workers within the colony. Consistent with predicted behavioral demands, soldiers had higher relative visual (optic lobe) investment than non-soldiers but only during the period when they were performing the most visually demanding defense task (hovering guarding). As soldiers aged into the less visually demanding task of standing guarding this difference disappeared. Neural investment was otherwise similar across all colony members. Despite having larger task repertoires, soldiers had similar absolute brain size and smaller relative brain size compared to other workers, meaning that lifetime task repertoire size was a poor predictor of brain size. Together, our results are consistent with the specialized but flexible defense strategies of this species, broadening our understanding of how neurobiology mediates age and morphological task specialization in highly cooperative societies.

Song complexity in relation to repertoire size and phonological syntax in the breeding song of Purple Sunbird

There are multiple measures for bird song complexity such as repertoire size, phonological or compositional syntax and complex vocal mechanism (CVM). We examined these in an old-world passerine, Purple Sunbird. First, we identified and acoustically characterised the repertoire size (of notes and phrases). We then assessed positional fidelity and ordering of notes within phrases. We found 23 distinct notes by aural-visual inspection of spectrograms which was validated using a Classification and Regression Tree based on 5 acoustic parameters. These notes combined in different iterations to form 30 different phrases. Phrases comprised of an overall structure with an introductory note (prefix) at the onset, followed by notes occurring at specific positions within the phrase body, and terminated with a trill (suffix syllable(s)). Prefix was present in 93% of phrases whereas suffix syllable(s) occurred in 27% of phrases only. We found that notes exhibited positional fidelity and combined in specific order to form a phrase. This is indicative of underlying phonological syntax that limits the ways in which notes combine to form phrase and offers insights to song complexity. Finally, we found that suffix syllables exhibit the presence of mini-breath (very short inter-note interval) which are known to be produced by CVM.