The Omura’s whale (Balaenoptera omurai) was described as a new species in 2003 and then soon after as an ancient lineage basal to a Bryde’s/sei whale clade. Currently known only from whaling and stranding specimens primarily from the western Pacific and eastern Indian Oceans, there exist no confirmed field observations or ecological/behavioural data. Here we present, to our knowledge, the first genetically confirmed documentation of living Omura’s whales including descriptions of basic ecology and behaviour from northwestern Madagascar. Species identification was confirmed through molecular phylogenetic analyses of biopsies collected from 18 adult animals. All individuals shared a single haplotype in a 402 bp sequence of mtDNA control region, suggesting low diversity and a potentially small population. Sightings of 44 groups indicated preference for shallow-water shelf habitat with sea surface temperature between 27.4°C and 30.2°C. Frequent observations were made of lunge feeding, possibly on zooplankton. Observations of four mothers with young calves, and recordings of a song-like vocalization probably indicate reproductive behaviour. Social organization consisted of loose aggregations of predominantly unassociated single individuals spatially and temporally clustered. Photographic recapture of a female re-sighted the following year with a young calf suggests site fidelity or a resident population. Our results demonstrate that the species is a tropical whale without segregation of feeding and breeding habitat, and is probably non-migratory; our data extend the range of this poorly studied whale into the western Indian Ocean. Exclusive range restriction to tropical waters is rare among baleen whale species, except for the various forms of Bryde’s whales and Omura’s whales. Thus, the discovery of a tractable population of Omura’s whales in the tropics presents an opportunity for understanding the ecological factors driving potential convergence of life-history patterns with the distantly related Bryde’s whales.

No abstract

Omura’s whale is a recently described tropical Balaenopterid whale with virtually nothing known about their acoustic behavior. Recordings have revealed a stereotyped 15-50 Hz amplitude-modulated vocalization, rhythmically repeated in a typical Balaenoptera song manner. In order to describe the characteristics of the song, continuous recordings were made using archival recorders during 21 days at 4 sites off the northwest coast of Madagascar in documented Omura’s whale habitat. A total of 926 hours of recordings were manually browsed to identify all occurrences of the song vocalizations, logging 9117 individual song units. Occurrence varied among sites spread across 40 km of shelf habitat, indicating heterogeneous distribution of whales and use of habitat over space and days. Diel variation indicated higher incidence of song during daylight hours, counter to trends found in other Balaenopterid whales. A total of 215 different individual series were identified ranging from 3 to 252 consecutive song units. For 121 individuals with more than 20 consecutive song units, the interval ranged from 147.4 s to 289.0 s with a mean of 200.3 s (s.d. 25.9), and recorded song duration ranged up to 13.33 hr. This represents the first description of singing behavior for this species, suggesting a time-intensive behavioral display likely related to breeding.