Preferred shallow-water nursery sites provide acoustic crypsis to southern right whale mother-calf pairs.

Adaptations to sound production behaviour can reduce the detectability of animal signals by eavesdroppers in a phenomenon known as acoustic crypsis. We propose that acoustic crypsis can include selection of locations that affect how sound transmits through the environment: habitats with poor acoustic propagation can minimize the range of detectability of animal signals. We investigated the potential for the preferred habitats of southern right whales to confer acoustic crypsis. We modelled acoustic propagation and range of detection of calls from southern right whales in the shallow, sandy, near shore waters where mothers and calves aggregate during the calving season. At three nursery sites across three continents in the southern hemisphere, results showed that the depth at which right whales are most commonly sighted has the most limited acoustic detection range for their calls. Thus, these habitats allow mother-calf pairs to remain acoustically cryptic from potential eavesdroppers, both predators and conspecifics, when their calves are the most vulnerable. Our results provide preliminary evidence that, in addition to other behavioural strategies, the use of habitats with poor acoustic propagation can contribute to acoustic crypsis. This adaptation may be a widespread and underappreciated mechanism for avoidance of eavesdroppers.

Vocalizations produced by southern right whale (Eubalaena australis) mother-calf pairs in a calving ground off Brazil.

Aiming to gather information on southern right whale (Eubalaena australis) mother-calf pairs' vocal behavior, archival acoustic recorders were deployed at a calving area off Brazil. Manual inspection of spectrograms revealed seven call classes: upcall, downcall, down-upcall, tonal variable, tonal constant, hybrid, and pulsive calls, which are consistent with those previously described for this species in Argentina. Gunshots and warbles, vocalizations described from other right whale species, were not detected. Mean values of start, end, maximum, minimum and peak frequencies, frequency bandwidth and duration were calculated for each call class. Start and end frequencies, frequency bandwidth and duration of upcalls recorded off Brazil were compared to those from other right whale populations and species. Only mean duration of upcalls from Brazil were significantly different from upcalls from all other populations. Differences in call duration may be driven by differences in demographic factors or background noise features among study areas. The repertoire characterization presented in this study will contribute to increase the utility of passive acoustic monitoring as a tool for conservation and research of southern right whales off Brazil as it provides important baseline information on the vocal behavior of this species.