OFFICIAL49 Why does the professor say this:

[00:00.00]NARRATOR: Listen to part of a lecture in a Biology class.
[00:05.19]FEMALE PROFESSOR: Okay, so that's how the arctic ground squirrel is able to cope in this extreme environment…[00:11.54]Now let's talk about your reading assignment, about reindeer… also typically found in Siberia and other far-northern regions. [00:19.43]Who’d like to start off? Yes, Mike?
[00:22.46]MALE STUDENT: Well, for one thing, they've got thick hair all over their body, even on their noses.
[00:26.75]FEMALE PROFESSOR: Yes. They're very well insulated. And the thickness of their fur varies depending on the season. Good. [calling on another student] Yes.
[00:34.87]FEMALE STUDENT: Um, newborn reindeer are very adult-like, like they can stand as soon as they're born, and by their second day they can already run as fast as a human.
[00:46.26]FEMALE PROFESSOR: Critical. Food is very scarce in the far north, so reindeer herds have to cover lots of ground, every day. [00:53.95]And in the fall they might easily trek a thousand kilometers or more to get to their winter feeding site. [00:59.92]So if you’re a newborn, you’ve gotta get up to speed fast. Okay. [01:04.04]Other adaptations?
[01:06.76]MALE STUDENT: Also, reindeer don't have to keep their legs as warm as their main body, so they don't have to use up as much energy keeping them warm.
[01:14.06]FEMALE PROFESSOR: Yes, so that means they can allocate less energy to heating their extremities, and more energy to maintaining a stable temperature in their body core, where their vital organs are located. [01:25.44] And you know, I don’t think it’s mentioned in your textbook, but even different parts of a reindeer's leg are adapted for optimal cold weather performance: [01:35.49]the fat in the lower part of their legs—um, the part that gets coldest—that fat has a different chemical structure from the fat in the upper parts of the leg, [01:45.30]so it doesn't get hard; even at temperatures down around freezing; it stays kind of gel-like, kind of oily. [01:52.47] Okay, good. [01:54.59]What about food? [01:56.21]What do you remember about that?
[01:58.32]FEMALE STUDENT: Well, they're pretty flexible.
[02:00.74]FEMALE PROFESSOR: Okay. Can you explain that a little more?
[02:03.38]FEMALE STUDENT: Well, they can eat a lot of different kinds of plants, so that improves their chances of coming across something they can eat. [02:10.43]I think they said that they found that the reindeer in one herd had eaten something like 37 different kinds of plants.
[02:16.89]FEMALE PROFESSOR: Okay, yes.[pleased] You’ve really done your reading. [02:20.41]And reindeer also eat a number of different plant species that most animals are not very interested in. Which means…
[02:28.37]FEMALE STUDENT: …they don't have a lot of competition when it comes to that food.
[02:31.89]FEMALE PROFESSOR: That's right. [02:33.00]In particular, your reading mentions lichens.[02:36.83]Lichens are plants you'll find growing on rocks in the far north, sometimes referred to as “reindeer moss.” [02:44.00]They look pretty basic, you know, just a little moss on a rock.

[02:48.33]But lichens are actually quite complex; [02:50.77] they’re not just a single organism, [02:52.62] they're actually a kind of combination of some sort of a fungus and some sort of algae that live together in a symbiotic relationship.
[03:01.30]Anyway. Okay, reindeer. Um… Oh, yes, and one more thing about lichens; [03:07.24]they crank out a lot of chemicals, which is probably at least part of the reason why they're not considered all that tasty by most animals. [03:15.09] Anyway. Does anyone remember what your reading said about them?
[03:19.28]MALE STUDENT: Yeah, somehow, when reindeer eat lichens, they're able to draw a lot more nutrients from them than other animals. [03:26.60]Like if a cow or a sheep eats lichens, they're only going to get like half as much nutrition out of them as a reindeer would.
[03:33.21]FEMALE PROFESSOR: That's right, and in winter, Lichens are crucial for reindeer because they supply energy. But they don't have all the proteins and minerals the reindeer need. [03:43.43]Um, so when reindeer get to the end of the long winter, they’re often very thin, with low levels of minerals.
[03:50.29]In spring they have to eat different plants and replenish what they've lost over the winter. So what reindeer have done is, they’ve developed the ability to digest different plants in different seasons by adjusting the microbes in their digestive systems. [04:04.96]As you know, microbes are microorganisms, like bacteria, that help to digest or break down food.
[04:11.70]And well, what’s interesting about reindeer is that they change the proportion of different microbes in their digestive system. [04:19.17]Uh, so you…so the reindeer might have more of one kind of microbe in winter to help digest the plants it eats then, and in the summer, uh, it would have more of another kind of microbe to help it digest summer plants. [04:32.61]That way the reindeer gets more nutrition out of different foods at different times of the year.