||Illustration by Uko Gorter |
Discovery and Taxonomy
Discovery: The vaquita first became known to the scientific community after the discovery of a bleached skull on the beach north of Punta San Felipe in Baja California Norte, Mexico, on 18 March 1950. The following year two additional skulls were found and these three skull specimens formed the basis for the description of a new species of porpoise, Phocoena sinus, (Norris & McFarland, 1958).
Nomenclature: In Latin, sinus means “gulf”, referring to the occurrence of the species in the Gulf of California. The Spanish common name for the species is vaquita (“little cow”). Vaquita (or sometimes “vaquita marina”) is the most commonly used name for the species. An alternative English name for the species is the Gulf of California harbor porpoise, the common name ascribed in the original species description.
Taxonomy: The porpoises, Phocoenidae, are one of ten families that comprise the suborder Odontoceti, the toothed whales. Of the seven living species of porpoises, four comprise the genus Phocoena, including two in the Northern Hemisphere (the vaquita and the harbour porpoise, P. phocoena) and two in the Southern Hemisphere (the spectacled porpoise, P. dioptrica, and Burmeister’s porpoise, P. spinipinnis). There are three additional species in the family: the Indo-Pacific finless porpoise, Neophocaena phocaenoides, found in coastal waters of the Indian Ocean (Persian Gulf) to South and East Asia, the narrow-ridged finless porpoise, Neophocaena asiaeorientalis, found in the coastal western Pacific and the Yangze river, and Dall’s porpoise, Phocoenoides dalli, in the colder waters of the North Pacific.
Evolutionary Relationships: The vaquita is believed to represent a relict population of an ancestral species, most closely related to the two southern hemisphere species (Burmeister’s and spectacled porpoise), that crossed the equator from the Southern Hemisphere during a period of Pleistocene cooling (Norris & McFarland, 1958). Genetic analyses have corroborated this interpretation (Rosel et al., 1995; Morin et al. in prep).
Size and Shape: At 1.5m (about 5 feet) in length, the vaquita is among the smallest cetaceans in the world. As with other porpoises, the vaquita is small but robust (Figure 1). It has a rounded head (no “beak”) and, as in most other porpoises, the teeth are flat or spade shaped (teeth are conical in shape in dolphins). The most conspicuous features of the coloration pattern are the relatively large black patches around the eyes and lips. The most striking external character is the shape and height of the triangular dorsal fin, which is proportionally much higher and wider than in other porpoises except for the spectacled porpoise. With its relatively large surface area, the vaquita’s dorsal fin is suggested to be an adaption for removing excessive heat in warm upper Gulf of California waters via a vascular counter-current heat exchange system. Females are larger than males in total length but adult males have proportionally higher dorsal fins and wider flukes (Torre et al. 2014).
Genetic Diversity: No polymorphism has been found in the mitochondrial DNA control region (Rosel & Rojas-Bracho, 1999) and in the DQB locus of the Major Histocompatibility Complex (MHC) and limited diversity was found in the DRB locus of the MHC, (Munguía-Vega et al., 2007) of 43, 25, and 29 vaquita specimens, respectively. These findings are consistent with the hypothesis that the evolutionary history of the species included a bottleneck or founder event. Judging by the results of simulations of plausible population dynamics, vaquitas have probably always been rare, and their loss of genomic variability likely occurred over evolutionary time (Taylor & Rojas-Bracho, 1999; Munguía-Vega et al., 2007).
Distribution: The vaquita is known to occur only in the extreme northern Gulf of California, Mexico, mainly north of 30° 45’N and west of 114°20’W (Brownell, 1986; Gerrodette et al., 1995; Figure 2). The ‘core area’ of the vaquita distribution is centered near Rocas Consag (31˚18’N, 114˚ 25’ W), east of the town of San Felipe, Baja California (Rojas-Bracho et al., 2006). The vaquita has the smallest geographical range of any marine mammal. Nearly the entire population lives within a 4000 sq-km (1519 sq-mile) area, an area less than 1/4 the size of metropolitan Los Angeles. The range of the vaquita coincides with most of the Upper Gulf of California and Delta of the Colorado River Biosphere Reserve, one of the earth’s most extraordinary marine habitats supporting a diversity of macro-invertebrates, fishes, birds, marine reptiles, and marine mammals.
Group Size and Behavior: Vaquitas occur in small groups of 1-3 individuals; often just a mother and calf pair. They are typically inconspicuous at the surface (they rarely splash, jump or leap) and they avoid boats. This behavior, coupled with their small body size, makes them difficult to observe.
Feeding: Vaquitas are generalist feeders, consuming a wide variety of over 21 benthic and demersal (bottom-dwelling) teleost fishes and squids (Findley et al., 1995).
Predators: Large sharks and killer whales, Orcinus orca, are potential predators of vaquitas.
Longevity: The life span of the vaquita is expected to be similar to that of the harbor porpoise, approximately 20 years; the oldest vaquita known to date was estimated to be 21 years of age (Hohn et al., 1996).
Growth and Reproduction: Sexual maturation is estimated to occur at 3-6 years of age. The maximum population growth rate is not likely to exceed 4% annually, a rate commonly used for porpoises (Rojas-Bracho et al. 2006). Most births occur in early March, presumably following a peak in ovulations and conceptions in approximately mid-April. The gestation is estimated to be 11 months. Unlike most other porpoise species which breed every year, vaquitas appear to give birth every 2 years.
Mating system: The mating system and social structure of the vaquita have not been studied. However, it has been inferred from the large testes size (ca, 5% of body mass), reverse sexual dimorphism (females are bigger than males) and small group sizes that sperm competition plays an important role in the species’ reproductive strategy (Hohn et al., 1996). The larger dorsal fin and flukes of adult males are probably related to the swimming capacity (i.e., propulsion, maneuvering, and agility); mature males may swim more rapidly and be more agile than females, which may be an advantage during breeding (Torre et al. 2014).
Findley, L.T., Nava, J.M. and Torre, J. 1995 Food Habits of Phocoena Sinus (Cetacea: Phocoenidae). In: Abstracts, 11th Biennial Conference on the Biology of Marine Mammals, 14–18 December. Society for Marine Mammalogy, Orlando, FL.
Hohn, A. A., A. J. Read, S. Fernandez, O. Vidal and L. T. Findley. 1996. Life history of the vaquita, Phocoena sinus (Phocoenidae, Cetacea). Journal of Zoology, London 239: 235-251.
Munguia-Vega, A., Y. Esquer-Garrigos, L. Rojas-Bracho, R. Vazquez-Juarez, A. Castro-Prieto and S. Flores-Ramirez. 2007. Genetic drift vs. natural selection in a long-term small isolated population: Major histocompatibility complex class II variation in the Gulf of California endemic porpoise (Phocoena sinus). Molecular Genetics 16: 4051–4065
Norris, K.S. & McFarland, W.N. 1958. A new harbor porpoise of the genus Phocoena from the Gulf of Califonria. Journal of Mammalogy 39: 22-39.
Rojas-Bracho, L. and B. Taylor. 1999. Risk factors in the vaquita. Marine Mammal Science 15 (4): 974-989.
Rojas-Bracho, Reeves, R.R. and Jaramillo-Legoretta, A. 2006. Conservation of the vaquita Phocoena sinus. Mammal Review 36: 179-216.
Rosel, E. and L. Rojas-Bracho. 1999. Mitochondrial DNA variation in the critically endangered vaquita, Phocoena sinus. Marine Mammal Science 15(4): 990-1003.
Taylor, B. and L. Rojas-Bracho. 1999. Examining risk of inbreeding depression for a naturally rare species, the vaquita. Marine Mammal Science 15(4): 1004-1028.
Torre, J., Vidal, O. and R. L.Brownell. 2014. Sexual dimorphism and developmental patterns in the external morphology of the vaquita, Phocoena sinus. Marine Mammal Science DOI:10.1111/mms.12106.