Rice's whale (Balaenoptera ricei), also known as the Gulf of Mexico whale, is a species of baleen whale endemic to the northern Gulf of Mexico. Initially identified as a subpopulation of the Bryde's whale, genetic and skeletal studies found it to be a distinct species in 2021. In outward appearance, it is virtually identical to the Bryde's whale. Its body is streamlined and sleek, with a uniformly dark charcoal gray dorsal and pale to pinkish underside. A diagnostic feature often used by field scientists to distinguish Rice's whales from whales other than the Bryde's whale is the three prominent ridges that line the top of its head. The species can be distinguished from the Bryde's whale by the shape of the nasal bones, which have wider gaps due to a unique wrapping by the frontal bones, its unique vocal repertoire, and genetic differences.
It is a medium-sized baleen whale that grows up to in length and weighs up to . The Rice's whale inhabits a restricted stretch along the continental slope in the northeastern part of the Gulf of Mexico between depths of off the coast of western Florida, although some whales have been sighted in the northwestern portions and the species may have inhabited a wider distribution throughout the Gulf in historical times. It does not migrate but remains within this area year-round. Rice's whales employ a diel vertical diving pattern, in which the whale spends most of the day feeding at or near the seafloor at depths at around . At night it feeds closer to the surface. The whale appears to selectively prey on the silver-rag driftfish, which has a very high energy density, although other fishes are suspected to also be consumed.
The Rice's whale is on the brink of extinction and, alongside the vaquita, is one of the most endangered cetaceans in the world. It is listed as critically endangered on the IUCN Red List and protected under the United States Endangered Species Act. The best population estimate is 51, with as few as 26 mature individuals, and the population is continuing to decline. The reasons why the species' population declined to its current state remain poorly understood, but scientists believe that the industrialization of the Gulf of Mexico and the increase of anthropogenic activities within its habitat are primary contributors; unlike most baleen whales it is unlikely that whaling had an impact. Today, the Rice's whale's main threats are related to industrial and commercial activities within its habitat, including oil pollution, ship collisions, and underwater noise from seismic surveys and vessel traffic. It has also shown to be especially vulnerable to local catastrophic events such as the 2010 Deepwater Horizon oil spill, which single-handedly killed nearly twenty percent of the species' population. In 2026, the Trump Administration lifted protections for the Rice's whale which could result in the extinction of the species. In light of the recent lack of sufficient efforts on a U.S. federal level to halt the Rice's whales decline, non-government organization led conservation efforts have been launched to save the species from immediate extinction. This includes the multi-phase Operation Gulf Guardians launched by the Captain Paul Watson Foundation (CPWF).
Taxonomy Discovery and naming
The Rice's whale was likely first documented by the Yankee whaling ship Keziah in 1790, which reported the chasing of a "finback" in the Bay of Campeche. At least 49 additional encounters of "finbacks" were reported throughout the Gulf of Mexico during the 18th and 19th centuries. Scientists generally agree that the "finbacks" were most likely Rice's whales, as it is the only baleen whale that resides in the Gulf year-round. While fin whales (to which the name "finback" applies today), blue whales, sei whales, and common minke whales have been sighted in the Gulf of Mexico, their presence is extralimital and they only appear in the area as occasional wanderers. The Rice's whale was first scientifically recognized by late cetologist Dale Rice in 1965, but as a local population of the closely related Bryde's whale. The extent of its modern geographic isolation was uncovered during the 1990s. During the 20th and early 21st centuries, this group of supposed Bryde's whales was vernacularly referred to as the "Gulf of Mexico whale".
A 2014 mitochondrial and nuclear DNA study by Rosel and Wilcox discovered that the Gulf of Mexico whales belong to a genetically distinct lineage that does not belong to either of the two members of the Bryde's whale complex (B. edeni brydei and B. edeni edeni), demonstrating that they may be a new subspecies or species. However, the study refrained from describing a new taxon, which requires a then-absent holotype specimen to erect. The taxonomic situation remained unchanged until 2019 when a dead individual washed up at Everglades National Park in Florida, providing the needed holotype without having to kill or harm any living members of the critically endangered population. This specimen was buried to decompose into a skeleton, exhumed, and transported to the Smithsonian Institution, where it is now cataloged as USNM 594665. In a 2021 taxonomic study, Rosel, Wilcox, and colleagues demonstrated through USNM 594665 that the Gulf of Mexico whale is also morphologically distinct from the Bryde's whale, thus confirming its identity as a new species. The subsequent scientific name Balaenoptera ricei and common name Rice's whale honor Rice for his role in discovering the whales' existence and to commemorate his 60-year contributions to marine mammal science. The study also commented that the naming was done to complement the trend of naming members of the Bryde's whale complex after people.
However, some scientists maintain the use of "Gulf of Mexico whale" as the preferred common name. A 2022 letter by Peter Corkeron of New England Aquarium and colleagues argued that most baleen whales have common names based on their geographic origin (with exceptions typically for wide-ranging species), that specifying a geographic origin encourages greater awareness of the species' endangered status, that the trend of Bryde's whale complex members having names honoring people is inconsistent (i.e. the sei whale's common name references a fish and scientific name a geographic region), and that promoting such perceived trend risks glorifying problematic namesakes (i.e. Eden's whale honors Ashley Eden, a colonial officer of British India). Rosel and colleagues published a response letter in the same journal issue presenting counterarguments, namely that "Gulf of Mexico whale" is too ambiguous and may be confused with other local cetaceans, that a region-specific name alienates outsiders and risks reducing global interest, and that encouraging an alternate common name when "Rice's whale" is already widely used will increase confusion.
Phylogeny
The Rice's whale is a type of "Bryde's-like whale", a group of rorquals within the genus Balaenoptera that are distinct genetically and in skeletal morphology but are near-indistinguishable in outward appearance. Other members of this group include the Bryde's whale (B. brydei), Eden's whale (B. edeni), and Omura's whale (B. omurai). However, Rosel et al. (2021) commented that the term may no longer be necessary.
The following cladogram is modified from a phylogenetic tree constructed by Rosel et al. (2021) based on mitochondrial DNA. Based on the study, the Rice's whale's closest living relative is the Eden's whale. The two form a clade that forms a polytomy with the sei whale and Bryde's whale.
Description
It is an intermediate-sized rorqual that is usually about the same size as the Eden's whale, although few adults have been reliably measured to date. The largest measured individual was a lactating female in length; USNM 594665 is the largest known male of the species and is long. Adults may be as small as in length, and many verified and possible Rice's whales do not exceed . One beached calf was measured at in length. In the wild, another calf has been observed to be around half the length of its supposed parent. One whale that washed up in Louisiana in 1982 was estimated at , but whether this is a Rice's whale remains unconfirmed as information on the diagnostic rostral ridges was never documented. When using a length-weight function developed by Ohsumi (1980), a length would correspond to a mass of around . However, the NOAA reports a maximum weight of .
Appearance and coloring
In external appearance, the Rice's whale and Bryde's whale are virtually identical and when compared can only be conclusively distinguished by their genetics, skeletal anatomy, and acoustic signatures. Like the Bryde's whale, the body shape is streamlined and sleek. The dorsal fin is shaped like a large hook that can range between ~ tall in adults and located around two-thirds of the length back from the snout. The smooth-edged tail flukes can be up to in width. The flat and somewhat pointed rostrum (top of the head) shows three prominent ridges: a large ridge along the center and two smaller ridges on the left and right sides. This unique trait only appears in Rice's whales and members of the Bryde's whale complex, making it a diagnostic feature when whale watching.
The dorsal side of the body is uniformly dark charcoal gray, although this is sometimes reported as dark brown. The belly and underside of the tail is pale to pinkish. This light coloring is especially prominent on the ventral side of the peduncle. There is neither asymmetric pigmentation on the lower jaws nor blaze or chevron pattering on the body, which distinguishes the Rice's whale from fin whales and Omura's whales. The flippers are uniformly dark-colored. In some phenotypic variations, a gradient of white also appears around the rim of the dorsal fin and/or along the side of the body, but there is no consistency of the trait between individual whales.
Forty-four to 54 ventral pleats run down the underside of the mouth, which are creases that allows the whale to expand the inside of its mouth when feeding. The pleats usually reach up to the navel, but some pleats extend beyond it. In the holotype specimen, one pleat extended beyond the navel and two additional elongated pleats were observed, although they could not be measured.
Skeleton
Knowledge of the skeletal anatomy is largely inferred from the holotype. However, it is not the only skeletal remains of Rice's whales accessible to science. Rosel et al. (2021) listed three additional specimens that were recovered in earlier years. The first is a skull (LSUMZ 17027) that was discovered in the Chandeleur Islands, Louisiana in 1954, which was initially identified as a fin whale but later genetically confirmed to be a Rice's whale. However, the skull is incomplete. The second is a complete skeleton (USNM 572922) recovered from a long subadult that washed up in North Carolina in 2003. The third is the skeleton (UF33536) of the long lactating female, which washed up in Tampa Bay, Florida, in 2009 and was subsequently disposed of via burial at a nearby park. Prior to the stranding of USNM 594665, this was considered a candidate specimen to serve as the Rice's whale holotype, but upon its excavation in 2018 it was found that the skull was crushed and much of the skeleton corroded as tides submerged it in water over the nine years it was buried.
Skull
The holotype skull is long, making up a little over one-fourth of the total body length, and wide. The upper jaw is in total length and wide at the antorbital notch (the point before the maxillary bone expands to connect with the rest of the skull).