To learn more about the occurrence and behaviour of a recently discovered population of blue whales, passive acoustic data were collected between January 2012 and April 2013 in the Chiloense ecoregion of southern Chile. Automatic detectors and manual auditing were used to detect blue whale songs (SEP calls) and D calls, which were then analysed to gain insights into temporal calling patterns. We found that D call rates were extremely low during winter (June–August) but gradually increased in spring and summer, decreasing again later during fall. SEP calls were absent for most winter and spring months (July–November) but increased in summer and fall, peaking between March and April. Thus, our results support previous studies documenting the austral summer residency of blue whales in this region, while suggesting that some individuals stay longer, highlighting the importance of this area as a blue whale habitat. We also investigated the daily occurrence of each call type and found that D calls occurred more frequently during dusk and night hours compared to dawn and day periods, whereas SEP calls did not show any significant diel patterns. Overall, these findings help to understand the occurrence and behaviour of endangered Chilean blue whales, enhancing our ability to develop conservation strategies in this important Southern Hemisphere habitat.

The study of mammal acoustic communication was revolutionized by the application of the source–filter theory, originally developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates a structural link between phenotype and acoustic formants, providing a basis for honest signaling. The phenotype–formant link was validated in many species, but the phenotype–vocal tract link was rarely assessed. We used two dimensional (2D) videogrammetry to estimate the vocal tract length of wild southern elephant seal ( Mirounga leonina (Linnaeus, 1758)) males during their normal vocalization behavior. We showed that ( i) the vocal tract can be measured noninvasively in a large wild mammal; ( ii) the vocal tract depends on the structural phenotype (age, body length, and skull size); ( iii) the nasal tract is more related to the structural phenotype than the buccal tract; and ( iv) the dependence on size, and body length in particular, is stronger than the dependence on age. Altogether, the phenotypic constraint on the vocal tract provides the anatomical basis for honest signaling in elephant seals.