Californian authorities say that eight people have died in five years in accidents in the state’s 47,000 abandoned mines. Now state authorities are determined to make them some of them safer while providing a novel ecological niche for bats. Sited due east of San Francisco, the small town of Tracy is the home of several long-abandoned coal mining caves. Vandals recently made one of them unsafe by burning the beams supporting the entrance. Now authorities have barred off with a gate. It is the first step in a program to keep prying humans out and provide an ideal habitat for bats. The slates are wide enough for Townsend's big-eared bats and pallid bats to fly through. Both species hibernate and roost in the dank humidity of the Californian caves.
Cave colonies can become very large with many bats loving the stable microclimate. It is not unknown for colonies of hundreds of thousands of different species to share a cave site. Young bats congregate to in thick clusters to form crèches of 3,000 square meters. In Winter time hibernating bats reduce their body temperature to within a couple of degrees of the site temperature and live on their accumulated body fat. They cannot stand on their legs and so hang upside down, held in place with no effort by the clinging shape of the tendons of the foot.
The reason they cannot stand on their legs is that they have evolved into wings. The scientific name for bats is Chiroptera. The word is Latin for “hand wing” and describes the bats’ fingers which are connected by a stretchy membrane which make up the wings. The fragile nature of bats has not left much data in the fossil record so science does not give a good accounting of how the only flying mammals evolved. The likeliest explanation is that they were originally insect-eating tree creatures who scurried around on all fours until they became airborne. However the theory does not explain how partially successful fliers could survive well enough to produce another generation yet incredible this unlikely event happened twice.
The fact remains that the two types of bats evolved separately. The majority are the mostly insectivorous microbats and the rest are mostly fruit-eating megabats (or fruit bats). Megabats range in size from the Indian giant flying fox with a human-sized wingspan of 1.8 metres to the petite Malaysian flower bat with its 21cm wingspan. The splendidly named false vampire of American tropics (so called because they prefer to bite the prey's head and crush its skull rather than suck its blood) is the largest of the microbats with a 1 metre span which dwarfs the rare and tiny 1.5 gram Thai bumblebee bat, possibly the world’s smallest mammal. The microbats evolved from shrews possibly in a period of global warming 50 million years ago and the megabats evolved later either from microbats, or, more controversially, from early primates.
One of the shrew’s lesser known characteristics is echolocation which they passed on to the microbats. When Britain successfully developed radar in World War II, some scientists were scathing when it was suggested that nature had already beaten them to it. But by 1944, Donald Griffin at Harvard proved bat echolocation existed. Bats send bursts of high-pitched sounds as they fly. These sounds emerge from the larynx and are mostly emitted by mouth. The Egyptian rousette bat clicks its tongue whereas others such as horseshoe bats emit the noise through a complex noseleaf around the nostril that focuses the beam of sound.
Whatever way they are emitted, the sounds travel as air waves until they strike an object. Some of the energy in the sound wave is returned as an echo and the amount returned depends on such factors as distance, durability of the object and whether the object is moving or not. Bats quickly analyse the returned data and identify the object. In order to catch prey they need to locate the target precisely in three dimensions and they do this by measuring the time delay between signal emission and echo reception As it closes in, the bat increases the pulse rate of sound to track it down using the Doppler shift. At the point of contact, the calls are so fast they are known as a “feeding buzz”. Some bats can make an incredible 200 calls per second at the feeding buzz.
Megabats don’t use echolocation but contrary to the ‘blind as a bat’ myth, they have large well developed eyes for night vision and can see as well as owls or cats. In Australia, flying fox colonies are enormous and the animals are highly sociable. Bats can also claim to be the most successful mammals, representing a fifth of all mammalian species. As Sue Churchill says about them in “Australian Bats” they live in a dimension so different from human experience that “we cannot escape a sense of wonder of the precision of even the simplest aspect of their biology”.