OFFICIAL30 What is the function of the spongy tissue in the bones of a female bird?

[00:00.00]NARRATOR: Listen to part of a lecture in a paleontology class.
[00:04.08]FEMALE PROFESSOR: As we've discussed, birds are apparently descendants of dinosaurs, and share many commonalities with some dinosaur species, like, uh, feathers and maybe even flight. And of course, egg laying.[00:16.35]OK, so, many paleontologists, myself included, have wondered about other similarities between dinosaurs and birds. [00:23.30]Since adult dinosaur fossils have sometimes been discovered near, or on top of, nests, we've been looking into dinosaur parenting behavior.[00:31.65]MALE STUDENT: Parenting behavior? Well, that sounds so gentle and caring. [00:35.12]But dinosaurs were ferocious reptiles—and reptiles don't take care of their young, do they?[00:40.43]FEMALE PROFESSOR: Well, some reptiles incubate their eggs—crocodiles do. [00:44.29] And as for popular attitudes toward dinosaurs ...well, take the Oviraptor for instance.[00:50.44]In the 1920s, a paleontologist discovered the fossil remains of a small dinosaur near a nest containing eggs. [00:59.07]He  assumed the dinosaur was stealing the eggs, so he named it "Oviraptor."That means "egg thief" in Latin...which fueled the generally negative public image of such dinosaurs.
[01:12.62]But by the 1990s, other experts had convincingly made the case that, instead of robbing the nest, the Oviraptor was probably taking care of the eggs.[01:22.34]You see, dinosaurs' closest living relatives—birds and crocodiles—display nesting behavior. [01:29.43]And dinosaur fossils have been found in postures that we now believe to indicate brooding behavior, that is, sitting on the eggs until they hatch.
[01:38.07]So we're curious about the type of care dinosaurs gave to their young. [01:42.13] And we'd like to figure out which dinosaur parent, the male or the female, gave the care.[01:46.65]MALE STUDENT: Shouldn't the behavior of crocodiles and birds give us some clues, then?[01:51.47]FEMALE PROFESSOR: Well, with crocodiles, it's the female who guards the nest. And with birds, it depends on the species. It can be the male or the female that takes care of the eggs. Or both. [02:02.21] In over 90 percent of all bird species, both parents take care of the eggs ...and the young birds.[02:08.64]MALE STUDENT: But sometimes it's just the male?[02:12.49]FEMALE PROFESSOR: Well, exclusive care by the male parent is much less common, but it does occur. [02:16.62]Now for animals other than birds, the care of young by both parents is pretty unusual in the animal kingdom— [02:23.24]males contribute to parental care in fewer than 5 percent of all mammalian species. [02:28.16] It's even less frequent among reptiles.
[02:30.52]And, exclusive care by the male is very rare. [02:34.73]So, researchers have wondered about the evolution of male parenting behavior in birds for quite some time. [02:39.65]And now there's research showing that, for some of the birds' dinosaur relatives, it's likely that the male parent was also in charge of taking care of the eggs.[02:48.34]MALE STUDENT: How did they figure that out?[02:50.51]FEMALE PROFESSOR: Well, first they looked at clutch volume—that's the number of eggs in the nest—of crocodiles, birds, ...and three types of dinosaurs, including Oviraptors, that are thought to be closely related to the dinosaur ancestors of birds.
[03:03.34]So, when researchers examined fossilized remains of nests, they found that the dinosaurs had larger clutch volumes, more eggs in the nest, that is, than most of the crocodiles and birds that were studied. [03:14.86]But, and this is important, their clutch volumes matched those of birds that have only male parental care. [03:22.72] You see, bird species in which only the males take care of the nest tend to have the largest clutches of eggs.[03:29.93]MALE STUDENT: So, what's the connection between bird and dinosaur behavior?[03:34.85]FEMALE PROFESSOR: Well, researchers now believe, because of this study, that the male parenting behavior of these birds might have its origins in the behavior of dinosaurs.[03:43.91]MALE STUDENT: Based only on evidence of clutch volume size? —the number of eggs?[03:48.65]FEMALE PROFESSOR:No, there's more. [03:49.87]They also examined the fossilized bones of those three types of dinosaurs that were found on or near nests ...to determine their sex. [03:57.75]You see, adult female birds, during egg production, produce a layer of spongy bone tissue inside certain long bones. [04:06.19]And so did female dinosaurs of the kinds that were investigated. [04:10.07]This spongy tissue serves as a source of calcium for eggshell formation. [04:14.88] But when the dinosaur fossils were examined, there were no spongy bone deposits.[04:19.55]MALE STUDENT: Meaning that those dinosaurs on the nests were probably adult males ...who wouldn't have needed calcium for making eggshells.[04:26.92]FEMALE PROFESSOR: Exactly. And then there's this. Birds like the kiwi, the ostrich, and the emu—they share certain physical characteristics with these dinosaurs, [04:36.08]and, interestingly, they also show a consistent pattern of nest care by the male.