Supplement 1 from Maternal responses to pup calls in a high-cost lactation species

Details of study methods. (a) Population. The study site was located at Año Nuevo State Park, San Mateo County, California, USA (37.121 N, −122.340 W), which includes a 2.7 km2 shoreline area with ~ 2000 northern elephant seals during the winter breeding season. Data were collected from January – March 2018 and 2019. The image illustrates typical harem density during the breeding season. (b) Individual identification. Pregnant female elephant seals were marked with alphanumeric codes upon their arrival at the breeding colony by applying black hair dye to their brown coats. Births were directly observed during daytime hours, and the thick black coat of the newborn pups was marked with blonde hair dye following observation of successful nursing. To minimize disturbance, females were marked either while at rest by a hand-held squeeze bottle or with a 1 m pole stamp, while pups were marked with a stencil applied to a 4 m extension pole that was deployed while their mothers were asleep. Image shows marked female with newborn pup. (c) Acoustic recording. To obtain recordings of spontaneous vocalizations without disturbing mothers or pups we used a Marantz PMD 670 or a PMD 661 digital recorder (sampling rate 44.1 kHz) linked to a Beyerdynamic M69TG microphone that was mounted on a 4 m extension pole; the microphone was gradually positioned 0.7 to 4.5 m from the target pup at a height of 20 cm. A lapel microphone was used to record annotations to a second channel; comments included pup identity, pup age in days, distance and orientation of pup relative to microphone, and behavioural context. Vocalizations were screened for quality and catalogued to create a bank of call for each pup. The image shows a typical pup recording session. (d) Field experiments. Playback treatments were created with Avisoft SASLab Pro v5.2.12 software and projected from a portable self-powered speaker (JBL Charge3, +/− 5 dB from 0.1–15 kHz) linked to a Bluetooth digital player (iPhone SE). Each treatment included a call series for the filial pup and the non-filial pup such that pups were close in age (+/− 3 days), the behavioural context of the recordings was similar, and the pups were recorded at > 85 m apart. Each call series contained six call examples from the same pup played at a natural rate (~3 s between calls) and amplitude (90 dB SPL @ 5 m). Field experiments were audio/video recorded from a location out of view of the target seals with a Nikon D700 or D800 camera. The image shows the configuration of a typical playback trial, with the speaker position, the target female, and her filial pup indicated by arrows. Corresponding video data of the playback trial is provided in Supplement 3.