Signs of Spring: Report Your Observations!

If you read Do They Follow the Nectar Trail?, you learned that the bats' arrival at roosts along their migratory route coincides with flowering of their preferred food. Now 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:

Watching the bats as they eat (forage);
Examining contents of bats' stomachs to determine what they just ate;
Examining feces (undigested or partially digested material that made it through the digestive tract), and
Analysis of kinds of carbon isotopes in bat tissue.

Credit: Merlin D. Tuttle,BCI

Watching Bats Forage
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.

Examining Stomach Content or Feces
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.

Analyzing Carbon Isotopes in Bat Tissue
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.