[ 2204听力 ]

Posted 2023-01-30 we1less

[ 第一次课 生物学 ]

Listen to part of a lecture in a Zoology class
(female professor) OK, last time we started our discussion on horns and antlers, and we mostly focused on antlers. Um, would someone remind us of the differences between horns and antlers?

(male student) Well, antlers are made of bone, and they have to be regrown every year, that is, they fall off in the winter and start growing again in the spring. Horns are permanent. They don’t fall off, and they are not made of bone. They are actually made of the same stuff that fingernails are made of.

(professor) Good, um, what else? OK, Sharon?

(female student) Antlers are found on members of the deer family, like deer, moose, elk and so on, and it’s usually just the males that have them. But many different kinds of animals have horns and they can usually be found on both males and females, you know, depending on species.

(professor) Um, and what are they used for?

(female student) Well, both horns and antlers can be used for competing for mates, defending territory, and even protection from predators.

(professor) OK, good, and, um, now, today we’re gonna focus on horns, and horns found on creatures other than mammals.

(male student) Do you mean, like, birds, like, great horn owls?

(female student) Excuse me, but aren’t the horns on a horned owl really just tufts of feathers that stick up and only look like horns?

(professor) Yes, that’s right. Uh, a few birds have the word horn in their names, but, they don’t really have horns.

 But I was thinking about the examples from our textbook.

(male student) Chameleons and, uh, what was the other one?

(female student) Dung beetles

(professor) Right, the dung beetle in our book is one of those species where just the male has horns. Do you remember what they are used for?

(male student) Male dung beetles use their horns to keep away other dung beetles. Uh, they live in tunnels, so their horns are used for keeping competitors away from a female mate. They use their horns to block a rival’s passage through the tunnel.

(professor) Right, yes, to keep competitors away. 

And what about those chameleons? Hmm?

(female student) The Jackson’s chameleon? It has a different reason for having horns.

(professor) OK, go on.

(female student) Well, it’s got those three horns sticking out of its face, and they use them partly to keep competitors away from their mates like dung beetles, but also just to defend their territory.

(professor) That’s right. Yes, chameleons in general, including the Jackson’s chameleon, are not well-known for their ferocity, and they are just as likely to try to outstare a rival chameleon as to try to push it around with its horns. But you’re right, uh, some animals, even the occasional chameleon, use their horns to protect their territory.

(female student) What about defending themselves from predators?

(professor) Good, I’m glad you mention that one, because that reminds me of a recent study on the horned lizard, and how it uses its horns not just to defend itself against attacking predators, but also to frighten predators into not attacking in the first place.

Um, in the study, the biologists compared the length of the horns of living horned lizards with those of dead horned lizards, lizards that had been killed by their main predator, a bird called a shrike which is like a hawk. The study found that the horns on the live lizards were ten percent longer than the horns of lizards that were killed. The study also concluded that any lizards with horns that were at least, uh, if I’m not mistaken, eighty percent as long as its head, seem to be safe from the birds. So, for the horned lizard, it seems that the longer the horn, the better their chances.