If you thought that echolocation or vegetarianism were the only options available to vision-impaired animals, you’re in for a surprise.
Platypuses (the plural “platypus” is also correct, but technically “platypi” is not) have almost 40,000 special cells in their bills called electroreceptors that are activated by the electric fields created by other marine animals’ muscles moving. This is quite useful for the platypus, who tend to live in murky or dark waters but are able to follow the electric fields of their prey to catch their dinner.
While platypuses might not be blind, they are functionally blind when hunting. They close their eyes, noses and ears whenever they dive, and then swing their heads back and forth to sense electrical currents and move towards them (here’s a great video of that!)
But why are there only 3 species of mammals with these electric abilities? Well, it’s partly due to ecological niches. If a species develops a method to hunt where no others can (like in dark murky water) they flourish, but since that niche is now filled, the new skills don’t extend beyond that species.
Otherwise, you can thank evolution for non-electric animals. Sensing electric fields is only really useful if you live in the water (like electric fish) or at least hunt in it (like platypuses). Once you live and eat on land, there’s no real reason to keep your electroreceptors, and with less water comes even fewer reasons. This is reflected in echidnas: Western long-beaked echidnas have about 2,000 electroreceptors on their beaks (20x less than platypuses), and the short-beaked echidnas, who tend to live in even drier climates, have only about 400.
It’s worth pointing out, though, that all animals use electric signals to make their muscles work, so in a sense, we’re all electric!