It's National Bat Week, and what better way to celebrate than with an appreciation of the flying mammals of the Golden State? California is home to 27 species of bats, more than a third of them of conservation concern, and they occupy just about every available habitat in the state that isn't underwater.
In addition to providing a persuasive Hallowe'en motif, bats serve Californians by eating insects, pollinating increasingly rare plants, and dispersing seeds of some of those same plants. They also add something to being out at night, where they still live: a reminder that the wild world still exists.
But many bats are in trouble in California. The dreaded white nose syndrome, a fungal disease that's killed at least 5.6 million bats since it was first detected in 2006, hasn't made it to California yet. But California bats face other threats, and it's worthwhile to remind ourselves just how much worse off we'd be without them.
If you were surprised to read that California has 27 species of bat, don't be: these semi-cryptic critters make up about 20 percent of mammal species worldwide. It takes some time and expertise to be able to tell them apart, mainly because they tend to come out into the open when we humans have the most trouble seeing, and they move pretty quickly. A bat field guide would be little help: even if they show clear photos or drawings of the bats, it'd be hard to compare those images to the dark, fast-moving blurs most bats present out in the wild.
That said, there are a range of forms and behaviors that can help the bat enthusiast distinguish among groups of species. One of the biggest differences is in the bats' choice of roosting habitat: a little less than half of California species regularly use caves or mines as places to sleep. The remainder prefer either narrow crevices in rocks -- especially some of the desert species -- or trees, hanging from tree branches or sheltering under sections of bark.
There's some thought that tree-roosting species may be less susceptible to white-nose syndrome; should that disease ever make it out to California, since they don't congregate in huge numbers the way cave bats can. (Caves can be really good environments for spreading fungal diseases, with relatively constant temperature and humidity and not much in the way of air circulation. Being a cave bat back east is a lot like being a passenger on a crowded subway car during flu season.)
That's not to say that tree bats are necessarily in less danger of outside threats. Some researchers postulate that tree-roosting bats may perceive wind turbines as potential habitat, and that often doesn't end well for the bat. The precise mechanism by which wind turbines injure bats is still a matter of conjecture, but injure them they do: a 2013 study suggested that wind turbines killed at least 600,000 bats in the U.S. in 2012, and the authors say that given their margin of error the actual toll could easily be more like 900,000. Even at the more conservatively estimated mortality rate, that's still about as many dead bats per year as caused by white nose syndrome since 2006.
In California, migratory tree-roosting bats seem to be the most susceptible to injury from wind turbines, with hoary bats, silver-haired bats, and red bats among the most frequent victims. Little brown bats are vulnerable as well, as are California mastiff bats and Mexican free-tailed bats.
Other threats to bats in California include disturbance of roosts by recreational exploration of caves and mines, pesticide use that makes bats' insect prey toxic in the long-term, habitat destruction of forests and rock crevices, and drought. That last is especially troubling in California these days. Bats drink while in flight by skimming water from the surface of ponds, pools, and rivers. Less available surface water means the bats go thirsty, and with their large surface area to volume ratio -- all that wing skin -- they can lose a third of their body weight to dehydration in 12 hours.
Losing bats means losing one of humanity's most effective allies in keeping insect populations under control. A single little brown bat, for instance, can eat as many as 500 mosquitoes in an hour, and little brown bats can assemble in colonies of hundreds. Pallid bats, like the one pictured above, usually feed on the ground -- which means they help control flightless insects such as roaches, as well as crickets and grasshoppers. Those last two may be more popular, but they're still agricultural pests. If pallid bats are around and eating them, it may mean less pressure on farmers to use insecticides.
Pallid bats are also known for eating scorpions, which makes it safer to put on your shoes where I live.
As for the main fear people seem to hold of bats -- that they're a reservoir for rabies -- it's true enough. Of 19 cases of human rabies in the United States documented by the Centers for Disease Control between 1997 and 2006, 17 involved a strain of the virus associated with bats. It's an unfortunate occurrence, when it happens, and you should seek medical attention immediately if you come into any direct physical contact with a bat. Rabies shots are no big deal these days.
That said, a death rate of about two American a year is so low as to be statistically negligible. Bees kill 50 times as many Americans, for instance.
In other words, we pose much more of a threat to bats than they do to us. And that damage can take a long time to heal. Bats are enough like rodents that it comes as a surprise to many people that they don't reproduce all that quickly. But bats aren't all that closely related to rodents. Many evolutionary biologists actually suspect bats are more closely related to primates than to rodents, and like many primates bats' litter sizes are actually rather small. Most bats native to California have just one or two pups each year.
That fact has serious conservation implications: it means that if something kills off a majority of a bat population, that population isn't going to breed itself back into its former numbers all that quickly. In the absence of continued threats, bats' relatively long lifespans -- little brown bats, for instance, can live 30 years in the wild -- mean an individual female bat can rear enough young to maturity to replace herself and her mate several times over. But that takes time, and it takes a commitment on the part of human primates in bat country to protect their flying kin in the meantime.
If you'd like to help, the group Bat Conservation International has lots of great information on how to make your contribution to bat survival, from places to see thriving colonies of bats in the wild to plans for bat houses.
In the meantime, if you're lucky enough to live in a place where there are still bats, be sure to get out and enjoy them.