Robert L. Pitman

The distinctive Subantarctic (Type D) killer whale (DKW) ( Orcinus orca ) is a highly divergent and apparently inbred form previously known only from three mass strandings and several dozen at‐sea sightings, primarily from Patagonian toothfish ( Dissostichus eleginoides ) longline vessels in the southern Indian Ocean and from tourism vessels in the Scotia Sea. Its range appears to be circumpolar throughout the Subantarctic, but very little is known about total numbers and overall movements. We summarize photo‐identification data from 352 of 406 sightings collected between 44° S and 62° S from 2003 to 2024. Among 207 photo‐identified individuals there were two distinct clusters of 78 and 29 toothfish depredators, one in the southern Indian Ocean and the other off southern Chile, respectively. An additional 100 DKW were photo‐identified throughout the wider Subantarctic. Discovery curves indicate that DKW may be more abundant than some less divergent or inbred killer whale populations. Depredating DKW showed seasonal and interannual site fidelity, but 10 depredating individuals identified in the southern Indian Ocean were also resighted in the southern South Atlantic, 4428 km away. Probable re‐sightings of two other DKW occurred in the Scotia Sea over distances of 1070 and 41 km, respectively. These latitudinally narrow, long‐distance inter‐ocean movements are unique among killer whales. During the study period, the entirely pelagic distribution of encounters appears to have shifted significantly southward.

Relatively little is known about the ranging patterns, diving behavior, or prey preferences of killer whales ( Orcinus orca ) in tropical latitudes. However, they are suspected to be prey‐generalists rather than the prey‐specialists that are typically found in high latitudes. In February 2025, we satellite‐tagged three killer whales from a pod of seven in Bahía de Banderas, Mexico. The tags transmitted for 8–41 days. The tagged whales traveled a total of 4002 km along a narrow, 1000 km stretch of coastline. While tagged, they remained within 50 km of shore 92.5% of the time and in water < 500 m deep 69% of the time, but ranged up to 87 km offshore, to water deeper than 1000 m. Dives were slightly deeper during the day (mean dive depth 44 m) than at night (34.5 m), and they dived deeper (max 528 m) in deeper water. The group was observed attacking/consuming marine mammals, including a humpback whale ( Megaptera novaeangliae ) calf, a dwarf sperm whale ( Kogia sima ), common bottlenose ( Tursiops truncatus ) and spinner ( Stenella longirostris ) dolphins, and attacking/harassing two species of sea turtles. These data provide the first high‐resolution records of movements and dive behavior of killer whales in the Eastern Tropical Pacific and support the existence of a unique ecotype in the region.

In 2024, an expedition was conducted off northwestern Baja California, México, to find and identify the beaked whale species that produced the BW43 echolocation pulse previously recorded in this area and elsewhere in the North Pacific. There were five Mesoplodon sightings and 21 BW43 acoustic detections on both a towed array and drifting pole buoy recorders over the course of the survey. Three of the sightings had concurrent acoustic detections, and a biopsy sample and environmental DNA were also collected from one of the sightings. The genetic identification confirms that the Mesoplodon sighted and acoustically recorded was the ginkgo‐toothed beaked whale ( Mesoplodon ginkgodens ), and the co‐occurrence of these sightings with the BW43 acoustic detections definitively links the species and its echolocation pulse. This is the first time that genetically confirmed ginkgo‐toothed beaked whales have been observed at sea and definitively linked to the BW43 pulse. This paper details the encounters, acoustic behavior, genetics, coloration, and external morphology of this species, including a comprehensive review of its distribution using historical sightings, strandings, and acoustic detection data from the North Pacific Ocean.

Deraniyagala's beaked whale, Mesoplodon hotaula, was previously known from eight published strandings and no confirmed live sightings, making it one of the least‐known cetaceans. We conducted six cetacean survey cruises in the South China Sea between 2019 and 2023, and during 120 sea‐days we recorded 12 sightings of unidentified Mesoplodon. From biopsy samples and acoustic recordings, five sightings were subsequently confirmed as M. hotaula. Using photographs, we provide the first field descriptions and observations of this species. The calf/juvenile has a distinctive facial coloration and dorsal striping pattern that appear to be diagnostic among Indo‐Pacific Mesoplodon; among adult males, the dark rostrum of M. hotaula and the distinctively white‐tipped beak of ginkgo‐toothed beaked whale, M. ginkgodens, distinguish these similar‐looking, partially sympatric species. Mean group size was small (3.0, n = 5), with a preference for deep (2,156 m, n = 5), warm (30.3°C, n = 3) water. We update the known distribution of this species, present evidence of fishery interactions, and document the first reported mixed‐species Mesoplodon group: a juvenile Blainville's beaked whale M. densirostris associated with four M. hotaula. These sightings identify the South China Sea as a potentially important area for this almost completely unknown species.

Although Hubbs' beaked whale (Mesoplodon carlhubbsi) was previously known from over 60 strandings on both sides of the North Pacific, it had been identified alive in the wild only once, off Oregon in 1994. In September 2021, we conducted a search effort for beaked whales off the coast of Oregon using a towed hydrophone array and a visual search team. Approximately 350 km off the Columbia River mouth, we detected the vocalizations of an unidentified mesoplodont whale; we stopped our vessel and waited in the area until two unidentified juvenile Mesoplodon surfaced and stayed near our vessel for almost 2 hr. During that time, we took numerous photographs and videos, made behavioral observations, and recorded their vocalizations. The DNA sequence from a biopsy sample identified them as M. carlhubbsi. In this paper, we discuss our biological observations, including color patterning and acquired markings, behavioral observations, and describe for the first time the acoustic characteristics of this species. We confirm that M. carlhubbsi is the source of a previously unidentified acoustic signal known as BW37V, and we update what is known about the at‐sea distribution of this species based on previous recordings and observational records.

Historically, there has been only one documented fatal killer whale (Orcinus orca) attack on a fin whale (Balaenoptera physalus)—from Baja California, Mexico (BC), in 2005. We compiled records of seven more recent fatal attacks, including four more from BC. Two male killer whales that we identified were involved, separately or together, in at least four of the five fin whale kills off BC as they traveled with their groups the length of the Baja Peninsula on both sides. They were also involved in two Bryde’s whale (Balaenoptera edeni) kills, and we suggest that they might specialize on large whale prey. We discuss offensive and defensive strategies during these attacks and the role (or lack thereof) of adult male killer whales. One of the fin whales taken appeared to be a healthy adult, which raises questions about large whales as prey for killer whales. We also discuss the burgeoning role of citizen science in this type of research, noting that most of our data came from social media postings.

Genome sequences can reveal the extent of inbreeding in small populations. Here, we present the first genomic characterization of type D killer whales, a distinctive eco/morphotype with a circumpolar, subantarctic distribution. Effective population size is the lowest estimated from any killer whale genome and indicates a severe population bottleneck. Consequently, type D genomes show among the highest level of inbreeding reported for any mammalian species (F_ROH ≥ 0.65). Detected recombination cross-over events of different haplotypes are up to an order of magnitude rarer than in other killer whale genomes studied to date. Comparison of genomic data from a museum specimen of a type D killer whale that stranded in New Zealand in 1955, with 3 modern genomes from the Cape Horn area, reveals high covariance and identity-by-state of alleles, suggesting these genomic characteristics and demographic history are shared among geographically dispersed social groups within this morphotype. Limitations to the insights gained in this study stem from the nonindependence of the 3 closely related modern genomes, the recent coalescence time of most variation within the genomes, and the nonequilibrium population history which violates the assumptions of many model-based methods. Long-range linkage disequilibrium and extensive runs of homozygosity found in type D genomes provide the potential basis for both the distinctive morphology, and the coupling of genetic barriers to gene flow with other killer whale populations.