英语时差:珍稀淡水豚
Most of us are familiar with those clever ocean dwelling mammals, the dolphins. But did you know that there are a few rare species of dolphins that live exclusively in rivers?
Sadly, one of the five species of river dolphins, the Chinese, or Yangtze River Dolphin, was declared “functionally extinct” in 2007 after intensive searches failed to locate even a single individual. The other four species, two in southern Asia and two in South America are critically endangered.
Besides living in freshwater, river dolphins have other traits that make them unique among dolphins. The Ganges and Indus River Dolphins are almost completely blind. Their tiny eyes don’t even have lenses.
Like their marine cousins, river dolphins use echolocation to find prey and navigate around obstacles. But unlike oceanic dolphins, the Ganges and Indus River Dolphins rely almost entirely on echolocation to sense their surroundings. In the murky dark waters in the rivers where they live, sight wouldn’t be much use anyway.
The Amazon River Dolphin, or Boto, also has special traits to help it live in rivers. It can turn its head! The neck vertebrae of all other dolphins are fused. If they want to see an object beside or below them, they must orient their whole body in that direction.
The Boto’s flexible neck allows it to hunt and move more easily in narrow waterways. The Boto also has stiff hairs along its long snout. The sensitive hairs help it detect prey in muddy river bottoms.
These beautiful and unique creatures face an ever increasing threat of extinction due to habitat loss, hunting by humans, underwater noise pollution, river damming, and lethal encounters with boats or fishing nets.
