Lesser Long-nosed Bats:
Nectar-powered Bat Math!
Counting the Calories in Cactus
Contributed by Ginny Dalton, Bat Biologist
Merlin D. Tuttle,BCI
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 many
saguaro flowers does a bat have to visit to sustain it for one day?
- For how
many minutes does a bat have to forage to get the nectar it needs
- How many
saguaro plants do you think would be needed to feed a maternity colony
of 10,000 Leptos?
is the minimum number of hectares of land required to feed a bat
colony containing 10,000
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."
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
8 flowers per saguaro per night, how many saguaro plants would have
to be visited? _______
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.
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? _________
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? _______________