According to today's report, our bats have returned to the southwestern United States. Ginny Dalton is also back today with more bat math. By the time you finish this report you'll be counting the calories in cactus flowers again! Don't get too busy to report the first bat you see this spring to: jn-report@learner.org
To: Journey North
From: Ginny Dalton, Bat Biologist
Tim Tibbits, biologist with the National Park Service, visited one of the Lepto roosts near Ajo, Arizona on Wednesday, April 24 and discovered that the bats had begun to arrive. He sat outside the entrance and counted the bats backlit against the sky as they emerged to forage for the night. There were about 2,000 bats. When the colony reaches its maximum size, there will be about 15,000 bats at this particular roost.
The bats' food, the saguaro cacti, are just beginning to bloom at this site. These bats are pregnant and will give birth to their young starting mid-to late May. This leads us to the question which I asked you to consider last week, "How many saguaro plants do you think would be needed to feed a maternity colony of 10,000 Leptos?"
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!) There are 6 saguaro plants per hectare on average.
Let's say that, for the two months a female is pregnant, she requires an average of 27 flowers per night. During pregnancy, how many flowers will one female require? How many flowers will be required by 10,000 females? Use the figure of 295 flowers per plant per nearly 2-month season and see if you can answer today's Challenge Question:
Challenge Question # 115 "What is the minimum number of hectares of land required to feed a bat colony containing 10,000 pregnant bats?"
(To respond to these questions please follow the instructions at the end of this report. )
Just think how much energy will be needed when the young are born and start nursing! Remember, if each bat produces one young bat the colony will swell in size to 20,000 individuals. Consider how much food they will need when the young become volant and fly out of the roost on their own to forage!
By the way, here are the calculations I did for last week's Challenge Question # 104.
Part I:
Q. "How many saguaro flowers does a bat have to visit to sustain it
for one day?"
A. According to my calculations, 21 flowers.
This is because a bat needs 20,200 calories per day and there are 960 calories in each flower. (20,200/960 = 21)
(Since pregnant females require more energy, in my discussion above I estimated each pregnant bat would require 27 flowers.)
Part II:
Q. "For how many minutes does a bat have to forage to get the
nectar it needs each day?"
A. The way I figure it, about 1 hr and 45 min flight time.
Here's why: It takes 10 visits per flower to get the 960 calories available. Since the bat must visit 21 seperate flowers in a day, that's 210 total visits. (21 x 10 = 210). Now, since 210 visits are required, and each visit takes 30 seconds, that's 6,300 seconds or about 1 hr and 45 min flight time. (210 x 30 = 6,300 seconds. 6,300/60 = 105 minutes, which is about 1 hour and 45 minutes.)
We have focused on energy from carbohydrate, but bats cannot live on sugar alone. Nor can you! We all need other nutrients, especially protein. It has been estimated that the average adult mammal requires 10% protein in the diet; young and growing mammals require 20%.
Nectar-feeding bats get their protein from pollen. The plants upon which these bats depend have on average 15%-30% pollen -- plenty for even the growing youngsters.
Interestingly, when scientists compared plants upon which bats fed and closely related plants that bats did NOT visit, there was a significant difference in sugar and protein content. The bat- visited plants had much more of each. In contrast, hummingbird plants also have high sugar content but not protein. Hummingbirds get their protein from insects, not pollen.
After a bat has visited a flower, it is so covered with pollen that it looks yellow in color, not its natural brown color, because of the dense covering of yellow pollen. The bat will stop foraging for anywhere from several minutes to several hours to lick off the pollen (and to rest).
Over the eons, the lives of flowers and their pollinators have evolved together. Compare these lists of their characteristics and note the similarities.
Characteristics of bat-pollinated plants include:
Characteristic of nectar-feeding bats include:
Not only do Leptos and other nectar-feeding bats pollinate saguaros, but they also pollinate other culturally important plants such as bananas, avocados, agaves (from which tequila is made), dates, figs, mangoes, and peaches. The wild plants are used for improving the cultivated stock. The fruits of the columnar cacti (such as the giant saguaro seen many of the cowboy movies) are an important food item for Native Americans.
How to Respond to Challenge Question # 115:
1. Address an e-mail message to: jn-challenge@learner.org
2. In the Subject line write: Challenge Question # 115
3. In the Body of the message answer these questions.
"What is the minimum number of hectares of land required to feed a bat colony containing 10,000 pregnant bats?"
The Next Lesser Long-nosed Bat Migration Update Will be Posted on May 16, 1996.
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