Sunlight & Seasons

Credit: Merlin D. Tuttle,BCI

Did you ever wonder: How much energy does it take to operate a bat? How many flowers must a bat visit to stay alive?
Get ready to count the calories in a cactus flower! After reading this page, you'll know everything necessary to answer these questions:

How Much Energy to Operate a Bat?
Let's look at how much energy is required by an adult Lepto. Here's an animal with a big energy demand for its size! (Now, nobody has ever measured this exactly for Leptos, but we can use information about other bats and extend the results logically.) It takes roughly 20.2 kcal to maintain one of these bats for a day. A Lepto uses about 100 times less energy in a day than a human does, as you can see on the chart below. But it weighs 2,000 times less. Obviously it takes more energy to operate an ounce of bat that an ounce of human! Why? Well, for one thing, bats fly--and the energetic cost of flight is high.

*NOTE ABOUT CALORIES: There is confusion in the layman's literature regarding calories. The calorie in everyday use is actually a kilocalorie (kcal, also designated Calorie, with a capital "C"), 1000 times larger than a calorie with a lower case "c"). In fact, the makers of labels on food boxes and cans in the grocery store are careless in their representation of this energy unit. The labels correctly use the upper case when stating total Calories of the food within, but then many of them say "based on a 2,000 calorie diet." Note the lower case "c." If literally interpreted, it is based on a 2 Cal (or kcal) diet, which doesn't make any sense. You and I can't live on 2 Calories a day!! They actually meant to write "Calorie;" instead they wrote "calorie."

Counting Calories in Cactus Flowers
How much energy is available in the nectar of the flowers Leptos visit? (Lepto is short for Leptonycteris curasoae, or lesser long-nosed bats.) It has been calculated that the nectar in saguaro flowers is about 24% sugar. This nectar if very sweet: For comparison, Classic Coke is 10% sugar! Each flower holds about 1.0 ml (milliliter) nectar. A single bat only takes about 0.1 ml with each visit to a flower. A bat's stomach can hold about 4 ml of fluid when full. Of those stomachs measured, 3 ml are sugar water and the remaining 1 ml was pollen.

There are about 4 calories (= 0.004 kcal) in a mg (milligram) of sugar. There is 1.0 mg sugar in each microliter (.001 ml) of nectar. So how many calories in 1.0 ml, the amount a flower holds? If a bat drains an entire flower, how many visits would the bat have to make to the flower? And how many total calories would it get from that single flower? (Don't forget to multiply your answer in cal/ml by 0.24 since the nectar contains only 24% sugar.) I got 960 cal (0.960 kcal) in a single flower that contains 1.0 ml nectar. So, since a bat takes about 0.1 ml for each visit, a bat would have to visit about 10 flowers to get those 960 cal.

Q. How many flowers total does a bat have to visit to sustain it for one day (20,200 cal = 20.2 kcal)? ______

Assuming 8 flowers per saguaro per night, how many saguaro plants would have to be visited? _______

Now, assume it takes about 30 seconds per visit to a flower (that includes transit time to the flower) for one sip (0.1 ml) of nectar. That makes us wonder:

Q. For how many minutes does a bat have to be flying to get the nectar it needs each day (24 hours)? _____________

More Bat Math: Mama and Baby Bats
Each female gives birth to a single young each year. Just think how much energy will be needed by the pregnant bats after they give birth and start nursing! Not enough information is available for calculating the exact requirements for pregnant females, but a near-term fetus of a 22- gram female bat can weigh as much as 8 grams. A pregnant bat carrying that heavy a load can require about 40% more power for flight. Of all the calculations conducted on females during the various reproductive stages, lactation (when the young are nursing) is the most energetically demanding on the female.

Let's say that, for the two months a female is pregnant, she requires an average of 27 flowers per night, since pregnant females require more energy. During pregnancy, how many flowers will one female require? ______ How many flowers will be required by 10,000 females? _______From studies conducted around Tucson, Arizona, the average saguaro produces 295 flowers per plant per growing season. (A single plant blooms from 27 to 61 days and each plant can produce from 82 to 980 blossoms!)

Q. How many saguaro plants do you think would be needed to feed a maternity colony of 10,000 Leptos? _________

Q. There are 6 saguaro plants per hectare on average. Using the figure of 295 flowers per plant per nearly 2-month season, what is the minimum number of hectares of land required to feed a bat colony containing 10,000 pregnant bats? _______________

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