The idea of going for days, weeks, or even months without food is a nightmare for humans, but for bats it’s just part of their yearly routine. Like other animals that hibernate for part of the year, they spend part of the year sleeping away without doing much of anything so as to conserve energy.
That may seem like a pretty pure and simple, straightforward answer, but as Oscar Wilde so perfectly put it, “The pure and simple truth is rarely pure and never simple,” and such as the case with bats and their eating habits.
With that in mind, let’s take a closer look at when bats eat, how long they are able to go without food, and how that’s even possible.
What Bats Eat and When
First and foremost, it’s worth stating that, with more than 1,300 different species of bats, there is actually a lot of variation in what bats eat and when. Bats can typically be broken down into three main categories – those that eat fruits, those that eat insects, and those that are carnivorous or blood-sucking and thus eat or suck the blood of animals. In each of these cases, what bats eat and when depends on the seasonality of their food source as well as its abundance. When insects, fruit, or animals are abundant, bats typically eat twice per night. When they are not, bats must go without.
How much this is the case, however, and thus how long bats can reasonably be expected to go without food, can be harder to determine.
For example, one popular myth is that bats can eat thousands of mosquitos per day. This claim stems largely from decades-old studies that were not even centered on bat eating habits and were centered around bats eating nothing but mosquitos within a controlled environment – conditions that were not likely to be easily replicated. Likewise, the causes that may leave bats without food are not likely to be uniform. Barring hibernation, cases of bats going foodless is less a built-in survival technique and more them simply having to do more with less, or nothing at all.
Hibernation, Migration, and Going Without Food
Before we get into hibernation and bats’ response food-wise, it is worth noting that while many bats eat a lot before hibernating, not all bats do this. On the contrary, there are plenty of bats that migrate instead. This complicates the question of how much bats eat, when they choose to eat, and how long they might go without food at times.
Consider things from the bat’s point of view.
On the one hand, if you are going to go on a continent-spanning journey, it is understandable why you might want to eat something first. You’re in for a long trip, and you need your strength. That’s even truer if, like many bat species, you migrate in a group. You can’t be the one bat out of hundreds or even thousands who stays behind the group to eat – once they move on, you must too, or they’ll leave you behind, which leaves you vulnerable to predators.
On the other hand, however, when bats do eat, their small bellies can become so engorged that they can no longer fly. A satisfying meal? Yes. But would you want to fly for hundreds of miles on an empty stomach? No. Besides, sticking with the group can mean a huge haul later on – colonies of Mexican bats millions strong can collectively eat as much as 200 tons of beetles per night.
But what about at the opposite end of that extreme, when there isn’t food available?
It takes a lot of energy for bats to fly, which is why they must eat around 100% of their body weight each night under normal conditions. This is especially true given the fact that bats have high metabolisms, which means that they burn energy faster than most other mammals. That means they need to eat all the more regularly to stay healthy, which is why, if they’re going to be flying and living their usual lives, bats should not go more than 24 hours without eating or having food.
They should be able to survive such a drought, but they’ll be a lot weaker.
The Importance of Fat Reserves
While a sudden lack of food due to poor conditions or human involvement may leave bats hungry for an evening or two, there is a situation where bats can live just fine without food for months on end – hibernation. As in the migration case, the key here is to eat early and live off of fat reserves. Migrating bats need to strike a balance between having enough strength to take them through the journey and not being too fat or docile to keep up with the colony; however, hibernating bats are all about building up fat reserves.
Gender also plays a role here – females build up greater fat reserves and rely on them more than male bats during the hibernation period. Not only do they have greater fat reserves for winter, but they also consume them slower than the males on average. This is due in part to the fact hibernation season often comes right after mating season, and so females will either already be or are preparing to take care of offspring. This and other factors make having additional fat reserves for hibernation especially important for the female’s reproductive success. In addition, females need greater fat reserves once they exit hibernation between February and March to mitigate ovulation and subsequent fluctuations in their hormones. By contrast, males build up a greater store of fat reserves for May to August, during which time the most hunting and then mating occurs.
In short, these bats may not be doing a lot of eating during these periods, but their bodies are doing a lot, so they need to be well-stocked with fat reserves to ensure that they have the energy necessary to carry out these essential bodily functions.
How can bats survive without food during the several months they are hibernating? Again, metabolism is key. Just as bats’ metabolism is extremely high for mammals, they can drop it to extremely low levels for winter hibernations. Their overall metabolism can drop as low as 99%, drastically lowering their body temperature, with their hearts beating as little as five beats per minute.
How Long Can Bats Last Without Food?
The answer to our question, as demonstrated, is a lot more complex than it first appears.
On the one hand, bats can go for months without food while hibernating – in the most ideal conditions imaginable, six months is the outer limit. On the other hand, bats obviously don’t tend to hibernate for six months but rather far shorter periods. To keep their body temperature and metabolism that low for that long would be difficult at best and disastrous at worst, to say nothing of how it would leave them vulnerable to predators (who could hardly go six months without noting unmoving meal-ready bats) and throw their reproductive and hormonal cycles out of whack. Much more realistic is the timetable put forth by the Bat Conservation Trust in the UK, which estimates part of four foodless months in hibernation living off fat reserves.
The lives of bats are anything but “pure and simple,” and the same holds for how long they can go foodless and the delicate balancing of their fat reserves that accompanies hibernation.