Baiji

The baiji (Lipotes vexillifer) is a possibly extinct species of river dolphin native to the Yangtze river system in China. It is thought to possibly be the first dolphin species driven to extinction due to the impact of humans. This dolphin is listed as "critically endangered: possibly extinct" by the IUCN, has not been definitively seen in over 20 years, and several surveys of the Yangtze have failed to find it. The species is also called the Chinese river dolphin, Yangtze river dolphin, Yangtze dolphin, and whitefin dolphin. The genus name Lipotes means "left behind" and the species epithet vexillifer means "flag bearer". It is nicknamed the "Goddess of the Yangtze" and was regarded as ...

China

Baiji were thought to breed in the first half of the year, the peak calving season being from February to April. A 30% pregnancy rate was observed. Gestation would last 10–11 months, delivering one calf at a time; the interbirth interval was 2 years. Calves measured around 80–90 centimetres (31–35 in) at birth, and nursed for 8–20 months. Males reached sexual maturity at age four, and females at age 6. Mature males were about 2.3 metres (7 ft 7 in) (7.5 ft) long, females 2.5 metres (8 ft 2 in), with the longest specimen 2.7 metres (8 ft 10 in). The animal weighed 135–230 kilograms (298–507 lb), with a lifespan estimated at 24 years in the wild. The Yangtze river dolphin was pale blue to grey on the dorsal (back) side, and white on the ventral (belly) side. It had a long and slightly upturned beak with 31–36 conical teeth on either jaw. Its dorsal fin was low and triangular in shape and resembled a light-colored flag when the dolphin swam just below the surface of the murky Yangtze River, hence the name "white-flag" dolphin. It had smaller eyes compared to oceanic dolphins. When escaping from danger, the baiji can reach 60 km/h (37 mph), but usually stays within 30 to 40 km/h (19 to 25 mph). Because of its poor vision, the baiji relies primarily on sonar for navigation. The sonar system also plays an important role in socializing, predator avoidance, group coordination, and expressing emotions. Sound emission is focused and highly directed by the shape of the skull and melon. Peak frequencies of echolocation clicks are between 70 kHz and 100 kHz.

The World Conservation Union (IUCN) has noted the following as threats to the species: a period of hunting by humans during the Great Leap Forward, entanglement in fishing gear, the illegal practice of electric fishing, collisions with boats and ships, habitat loss, and pollution. Further studies have noted that a lack of information on the baiji's historical distribution or ecology, the environmental impact of the construction of the Three Gorges Dam on the living space of the baiji, and the failure to act for the protection of the baiji are also threats to the species. During the Great Leap Forward, when traditional veneration of the baiji was denounced, it was hunted for its flesh and skin, and quickly became scarce. As China developed economically, pressure on the Yangtze river dolphin grew significantly. Industrial and residential waste flowed into the Yangtze. The riverbed was dredged and reinforced with concrete in many locations. Ship traffic multiplied, boats grew in size, and fishermen employed wider and more lethal nets. Noise pollution caused the nearly blind animal to collide with propellers. Stocks of the dolphin's prey declined drastically in the late 20th century, with some fish populations declining to one thousandth of their pre-industrial levels. A range of anthropogenic-led causes (e.g. boat collisions, dam construction) which also threaten freshwater cetaceans in other river systems have been implicated in the decline of the baiji population. However, the primary factor was probably unsustainable by-catch in local fisheries, which use rolling hooks, nets (gill nets and fyke nets), and electrofishing; similarly, by-catch constitutes the principal cause of mortality in many populations of small cetaceans worldwide. Although there is relatively few data available on baiji mortality, at least half of all known baiji deaths in the 1970s and 1980s were caused by rolling hooks and other fishing gear, and electrofishing accounted for 40% of baiji deaths recorded during the 1990s. Unlike most historical-era extinctions of large-bodied animals, the baiji was the victim not of active persecution but of incidental mortality resulting from massive-scale human environmental impacts, primarily uncontrolled fishing. Its extinction merely reflects the latest stage in the progressive ecological deterioration of the Yangtze region. In the 1970s and 1980s, an estimated half of baiji deaths were attributed to entanglement in fishing gear and nets. By the early 2000s, electric fishing was considered "the most important and immediate direct threat to the baiji's survival". Though outlawed, this fishing technique is widely and illegally practiced throughout China. The building of the Three Gorges Dam further reduced the dolphin's habitat and facilitated an increase in ship traffic. These impacts were thought to make it extinct in the wild. There are some scientists who have found that pollution has resulted in emerging diseases caused by parasitic infection in the baiji population. The baiji's reliance on aquatic environments could have resulted in interaction with both terrestrial and marine pathogen risks. Since the baiji has a limited distribution endemic to the Yangtze River, the freshwater environment may have a higher pathogen level than marine waters (although systematic environmental studies have yet to be conducted). The pathogens in these waters could lead to viral infections that can result in epizootics, which has caused the deaths of thousands of marine mammals over the last 20 years. There have also been captured/killed individuals that have had helminth infestations in the stomach, which leads scientists to believe that parasitic infections could be another cause of decline amongst the baiji. It has been noted, however, that the declining geographical range that baiji have been spotted in is not connected to the population loss of baiji. A model provided by Yangtze fishing communities show that the baiji populatio…

In the 1950s, the population was estimated at 6,000 animals, but declined rapidly over the subsequent five decades. Only a few hundred were left by 1970. The number fell to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. The baiji was last sighted in August 2004, though there have been many alleged sightings since. It is listed as an endangered species by the U.S. government under the Endangered Species Act. It is now thought to be functionally extinct.

The baiji is not closely related to any living species of dolphin, having diverged from the ancestors of the La Plata dolphin and Amazon river dolphin during the Miocene, estimated to be around 16 million years ago. The closest known relative of the baiji is Parapontoporia, native to the western coast of North America during the Latest Miocene and Pliocene. The baiji was one of five species of dolphins known to have made fresh water their exclusive habitat. The other five species, including the boto and the La Plata dolphin, have survived in the Río de la Plata and Amazon rivers in South America and the Ganges and Indus rivers on the Indian subcontinent. It is well known that the river dolphins are not a natural group. Their mitochondrial genome reveals a split of two separate lineages, Platanista and Lipotes + (Inia + Pontoporia), having no sister relationship with each other, and the Platanista lineage is always within the odontocete clade instead of having a closer affinity to Mysticeti. The position of the Platanista is more basal, suggesting separate divergence of this lineage well before the other one. The Lipotes has a sister relationship with Inia + Pontoporia, and they together formed the sister group to the Delphinoidea. This result strongly supports paraphyly of the classical river dolphins, and the nonplatanistoid river dolphins do represent a monophyletic grouping, with the Lipotidae as the sister taxa to (Iniidae + Pontoporiidae), and is well congruent with the studies based on short interspersed repetitive elements (SINEs). Low values of haplotype diversity and nucleotide diversity were found for the baiji of the Yangtze River. The analysis of molecular variance (AMOVA) supported a high level of overall genetic structure. The males having a higher genetic differentiation than the females suggested a significant female-biased dispersal. The aquatic adaptations of the baiji and other cetaceans have happened slowly and can be linked to positively selected genes (PSGs) and/or other functional changes. Comparative genopic analyses have uncovered that the baiji have a slow molecular clock and molecular adaptations to their aquatic environment. This information leads scientists to conclude that a bottleneck must have occurred near the end of the last deglaciation, a time that coincided with rapid temperature decrease and a rise in eustatic sea level. Scientists have also looked into PSGs in the baiji genome which are used for DNA repair and response to DNA stimulus. These PSGs have not been found in any other mammal species. Pathways being used for DNA repair have been known to have a major impact on brain development and have been implicated in diseases including microcephaly. The slow down of the substitution rate among cetaceans may have been affected by the evolution of DNA damage pathways. Over time, river dolphins, including the baiji, have had a reduction in the size of their eyes and the acuity of their vision. This probably stems from poor visibility in fluvial and estuarine environments. When analyzing the baiji genome, scientists have found that there are four genes that have lost their function due to a frameshift mutation or premature stop codons. The baiji has the lowest single-nucleotide polymorphism (SNP) frequency reported thus far among mammals. This low frequency could be related to the relatively low rate of molecular evolution in cetaceans; however, considering that the decrease in the rate of molecular evolution in the baiji was not as great as the decrease in heterozygosity rate, it is likely that much of the low genetic diversity observed was caused by the precipitous decline in the total baiji population in recent decades and the associated breedings. The reconstructed demographic history over the last 100,000 years featured a continual population contraction through the last glacial maximum, a serious bottleneck during the last deglaciation, and sustained population growth after the eustatic…