Table 1 in Evohution of the syrinx of Apodiformes * inchuding the vocah-hearning Trochihidae (Aves: ºtrisores)

Table 1. List of causal relationships between pairs of morphoanatomical variables tested using PGLS in this study, with corresponding functional hypotheses. Abbreviations: BM, body mass; DTL, distance tracheolateralis–labia; IMCSA, intrinsic muscle cross-sectional area; LL, labia length; TD, tracheal diameter; TER, tracheal elongation ratio; TLCSA, tracheolateralis cross-sectional area (see text for definitions).
Tested causal relationshipsProposed hypothesesDTL~BM TLCSA~BM LL~BM TER~BM TD~BM IMCSA~BMBody mass has a strong, albeit differential influence on acoustic parameters and morphoanatomical measurements of vocal folds (and associated muscles) in vertebrates (e.g. Riede and Brown 2013, Riede and Goller 2014). We expect morphoanatomical traits to be positively correlated with body mass (Riede and Brown 2013) and acoustic traits to be negatively correlated with it (Mason and Burns 2015, Pearse et al. 2018, Mejías et al. 2020).DTL~TLCSA TLCSA~TER DTL~TER IMCSA~TLCSAHummingbirds have been suggested to use interclavicular air sac, TL, and IM to stabilize the syrinx (Monte et al. 2020). This might result in a TL that is thicker to increase stability, but also shorter to make room for the large IM, resulting in a positive correlation between TLCSA and TL shortening, as well as a link between IMCSA and TLCSA. In this hypothesis, a short trachea (high TER) would result in high TLCSA and a higher DTL.LL~DTL LL~IMCSA IMCSA~DTLIn hummingbirds, IM ensure abduction and adduction of the labia (Riede and Olson 2020), so we expect LL and IMCSA to be correlated. A higher DTL may allow for larger IM, which in turn affects LL; we also test for a direct effect of DTL on LL as a control for that hypothesis.TLCSA~TD LL~TD IMCSA~TDA wider trachea with larger tracheal rings may require a thicker TL to modify its length during vocalization and stabilize its position (Riede et al. 2006, Riede and Olson 2020). Since LL and IM are involved in fine song tuning in hummingbirds (Monte et al. 2020, Riede and Olson 2020), TD may also affect their biomechanical resistance through allometric constraints.IMCSA~TER LL~TERTracheal shortening in hummingbirds has been proposed to be an adaptation for high frequency vocalizing associated with well-developed intrinsic musculature for fine modulation of fundamental frequency (Riede and Olson 2020). The high tensile forces required by this mechanism could constrain the structure of the vocal folds, resulting in thicker and longer lateral labia.