The Omura’s whale (Balaenoptera omurai) is a recently discovered species of baleen whale, first recognized as a distinct and ancient species in 2003. It is a Mysticete, or baleen whale, meaning that it has no teeth, but instead feeds by filtering small prey through a sieve-like structure called baleen, that hangs in rows from the upper jaw. Omura’s whales belong to the family of baleen whales called Balaenopteridae, or rorquals. Rorquals include the largest of whales, the Blue Whale at nearly 100ft, and range in size to the smallest, the Minke whale, at about 25ft. The Omura’s whale is on the smaller end of the range at about 33ft in length.
Prior to its discovery, Omura’s whales were confused with the slightly larger Bryde’s whale. In the early 1990’s, scientists started to notice that whales that had been originally classified as “Pygmy Bryde’s whales” were actually quite different in both physical appearance and genetically. By 2003, the Omura’s whale was recognized as a completely different species, with an evolutionary lineage that dates back 10-17 million years. At the time of its discovery, it was known only from a handful of strandings and whaling specimens, all in the western Pacific and eastern Indian Oceans, and never documented in the wild.
The Omura’s whale is a long and sleek baleen whale, with a body plan typical of rorquals and more slender than most. The external appearance of Omura’s whale was first described by Wada et al. 2003 based upon very limited information from the Japanese research whaling expeditions in the Solomon and Cocos Islands, and the Sea of Japan holotype stranding specimen. Our work in Madagascar resulted in the first detailed description of the external appearance and pigmentation pattern, adding substantially to what had been previously reported. Omura’s whales have an unusual pigmentation pattern in that they are laterally asymmetrical, similar to the fin whale, with the lower jaw being white on the right and dark on the left, along with a light blaze and chevron that is more prominent on the right. The ventral grooves extend beyond the umbilicus and number approximately 80-90, which is different in comparison to Bryde’s whales having 42-54 grooves. The top of the head, or rostrum, has a single prominent central ridge; the lateral rostral ridges that are prominent in Bryde’s whales are absent in Omura’s whales, however the head is not entirely smooth. Reported sizes are less than 12m, with physically mature individuals ranging from 10.3 to 11.5m for females, and 9.6 to 10.0m for males.
Images of several Madagascar Omura’s whales displaying details of pigmentation and external appearance. Visible features:
(A) Lower Jaw - asymmetrical coloration of the lower jaw, with lightly pigmented right jaw and darkly pigmented left jaw;
(B) Gape - asymmetrical coloration of the gape (inferred by inner lower lip), with lightly pigmented left gape and darkly pigmented right gape;
(C) Pectoral fin - leading edge of pectoral fin white from tip to shoulder;
(D) Head and rostrum - the apparent absence of lateral rostral ridges, with only faint indications detectable at some angles;
(E) Blaze - lightly pigmented blaze originating in front of the eye, present only on the right side, with dark eye and ear stripe, two additional dark stripes and a light inter-stripe wash;
(F) Chevron - lightly pigmented chevron in front of dorsal fin, present on both sides but asymmetrical, being most prominent on right where it displays a broad double-banded pattern, and a more narrow single-band on the left with different positions relative to the dorsal fin;
(G) Dorsal fin - highly falcate dorsal fin with gradual sloping insertion angle into the back, intermediate between fin whales (which have a more gradual slope) and Bryde’s and sei whales (which are more upright and have a sharper right angle insertion).
Using recordings made from boats while in the presence of Omura’s whales, along with long-term recordings from remote acoustic recorders mounted on the sea floor, the Madagascar Project has documented for the first time the vocalizations made by Omura’s whales. Even more exciting, the vocalizations occur in what we consider to be a typical rorqual whale song. There are three main features that indicate a vocalization is song:
1. Stereotyped vocalization – this means that the sound is a very recognizable pattern that is similar each time it is produced. The main vocalization that we have recorded is a low frequency call, between 15-50 Hz (below most humans’ hearing), about 8-9 seconds long, and pulsative in form, like a long low rumble.
2. Rhythmic repetition – whenever this call is recorded, it is produced in a series with a very consistent repetition rate of 2 to 3 min, and can go on for hours. We consider each series the vocalization of a single animal repeating the call over and over again. This is an individual whale singing.
3. Chorusing of multiple individuals – the remote recorders frequently record many overlapping series of the vocalization, that be believe are multiple individuals singing in close proximity to each other, in what we call a chorus.
Song is known to be a male breeding display, and although we have much more work to do on the breeding behavior of Omura’s whales, we therefore suspect that the song we have recorded is likely a male behavior and is important in the species mating system. You can see what the song looks like in the spectrogram below.