News of the first lesser long-nosed bats' arrival in the U.S. is expected any day. While we're waiting, don't forget to report sightings of bats in your region by sending a Field Data Form to Journey North. Simply press the blue button below labeled "Report Field Observations" or send an e-mail report to: jn-report@learner.org
Have you ever wondered:
Today our bat expert Ginny Dalton discusses these things. Her report is full of math facts which we hope you'll enjoy. Read carefully so you can answer today's Challenge Question:
Challenge Question # 104
(To respond to these questions please follow the instructions at the end of this report. )
To: Journey North
From: Ginny Dalton
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.
ANIMAL......WEIGHT.......ENERGY NEEDED
Human......2,000 oz......2,000.0 kcal/day*
Bat........0,001 oz......0,020.2 kcal/day
(*There is confusion in the layman's literature regarding calories. Please read the explanation below.)
Flower Powered Bats
How much energy is available in the nectar of the flowers Leptos
visit? 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. 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, assuming about 30 seconds per visit to a flower (that included transit time to the flower) for one sip (0.1 ml nectar), how long does that bat have to be flying to obtain the required number of sips to maintain it for a day (24-hr period)?
The information above is all you need to answer today's Challenge Question. Good luck!
There is not enough information 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. Each female gives birth to a single young each year. 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.
Before my next report, think about this:
More About 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."
The Nectar Corridor
From my previous reports you have discovered that the answer is
"yes" or at least "certainly seems possible" to Challenge Question
#78 ("Does there seem to be a nectar pathway the bats could follow
from southern Mexico northward into the southern U.S.?"). See if
the following information supports the idea that the bats do indeed
migrate northward as their food resources begin to bloom.
Bats have been found in the roosts (or captured nearby) in Chapala, Jalisco and Infiernillo, Michoacan in November and December. They are usually gone from those roosts by March. Roosts located at Pericos and Concordia, Sinaloa do not have bats arrive until late February through early March. By April, bats have arrived at in Alamos and Guaymas, Sonora. By late April and early May, bats have arrived at the Bania Kino and Carbo, Sonora roosts, plus the roosts near Ajo, Arizona. In August, bats are found in the western part of Arizona (Portal) and eastern New Mexico (Animas). In September, bats are gone from the U.S. and have been seen in the vicinity of Pericos, Sinaloa.
How Do We Know What Bats Eat?
Now, having seen that the bats' arrival at roosts along their
migratory route coincides with flowering of their preferred food,
let's turn our attention to the relationship of the animals to these
plants. How do we know what the bats eat? In what way do these
foods meet the nutritional requirements of the bats. Do the
nutritional requirements for the same bat change during the year?
Very few studies have been conducted on Leptos, but we do have
information on other species of bats that we can use to help
understand these relationships.
The first piece of information researchers had to obtain was a list of plants the bats visited at night for food. There are several ways to learn about foods bats eat and prefer, including 1. watching the bats as they eat (forage) 2. examining contents of bats' stomachs to determine what they just ate, 3. examining feces (undigested or partially digested material that made it through the digestive tract) and 4. analysis of kinds of carbon isotopes in bat tissue.
Since bats are active at night, it is not easy to watch them as they forage. In one of the two most common methods used to watch specific foraging activity, an observer sits close to a plant he or she thinks the bat will visit. The observer used a light-amplifying device such as a night vision scope, the kind of device we use to count bats when they emerge from their day roost in the evening. If the researcher guesses correctly, bats will be seen drinking nectar from the flowers. The other common method is to put tiny lights onto the backs (or bellies) of bats and watch which plants the lighted bats visit to feed on.
There are always some plant parts that are not digested by bats. By examining stomach contents or feces, the undigested parts can be identified. It is not always easy to do with bats that eat insects because they can be chewed nearly beyond recognition. But some pollen grains pass through the digestive system intact and are thus easier to assign to a specific food item. Food that is in the stomach is easier to recognize because it has not been as thoroughly digested as food that passes through the rest of the digestive process in the small intestine. However, bats have to be killed to obtain stomach contents and so researchers are examining far fewer stomachs than were checked in the past.
The carbon isotope analysis depends upon the fact that carbon has different isotopes that can be recognized in the laboratory. Although carbon has a radioactive isotope, C-14, it is not this isotope that is used. Another isotopes of carbon, C-13 (not radioactive, but heavier than the most common C-12 isotope) is important in this analysis. During photosynthesis, plants take the carbon dioxide from the air and convert it into carbon-containing sugars. Botanists (plant biologists) discovered several different ways plants do the conversion. Two ways, called C3 and CAM pathways, can be identified by the different quantities of C-13 that end up in the sugars. When bats eat the plant parts, the amount of C-13 can then be identified in the bat when a small sample of bat wing tissue is taken (like when a small sample of tissue is taken for human biopsy). Using this technique, it was discovered that the major food plants for the bats in Arizona were columnar cacti and agaves.
How to Respond to Challenge Question # 104:
1. Address an e-mail message to: jn-challenge@learner.org
2. In the Subject line write: Challenge Question # 104
3. In the Body of the message 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 each day?"
The Next Lesser Long-nosed Bat Migration Update Will be Posted on May 9, 1996.
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