October 2, 2009
We are talking mainly about bovids (cattle and antelope), which grow horns over their lifetime, and deer, which grow antlers every year. In most well known bovids and cervids, only the males grow the horns or antlers, but there are a few species where the females do as well.
For example, male and female cattle (including the many wild versions such as the African Cape Buffalo) and wildebeest (a kind of antelope) have horns, while in most other bovids only the males have horns. Both male and female caribou (a kind of deer) grow antlers each year, while in most other deer only the males do so.
This is actually a very complicated issue, and a new study of this question offers a new possible answer. But first, what did we think before this study?
There is one factor that explains most instances of female horns or antlers. The tiny monogamous deer and antelope tend to be much more “monomorphic” (that is, males and females look similar) than larger deer and antelope. These are small, pair-bonded, forest-dwelling species, and their horns or antlers are effective tools for defending territory or defending the young against small forest predators such as cats. Both the males and females have the horn or antler because they both use them, and for similar purposes. That is not particularly enigmatic.
It is also not hard to explain why in the vast majority of large cattle, antelope and deer species males and females are dimorphic (that is, males and females look different) in this trait, with only the males having the big appendages on their heads. In most of these species, males compete with each other, either in direct male-male competition or using a more show-off strategy to impress the females, in which the horns or antlers play an important role.
What’s harder to explain is this: In a small number of these large species, where the males compete over females, why do females also grow horns or antlers?
One early theory suggested that females in larger species could use these appendages for anti-predator defense. In other, smaller, species the females are better off hiding or running away. In my personal experience with wild Cape buffalo, this makes sense. On many occasions while working in the Semliki Valley in the Congo, I encountered small herds of female buffalo with their young. As I would draw nearer in my vehicle, they would gather more closely and form a circle with the young in the center, watching me suspiciously and looking rather formidable, and the horns were very much part of that look. However, this does not seem to hold true for deer. In the largest deer species, females do not have antlers.
Another previous hypothesis, proposed by Richard Estes, who works with wildebeest in East Africa, suggests that horned or antlered females benefit by confusing adult males as to who the young males in the group are. This is a strategy to keep the young males in the group longer, so they can grow bigger before heading out on their own. Essentially, this is a trait that benefits mom (it makes her son more successful) but is manifest in her daughters. According to this idea, female horns or antlers should be found in species where competitive males are forced to hang around with each other more than in other species because they live in large herds that consist of “family” groups. This is, in fact, what is found in caribou and wildebeest, two of the prime example of antlered or horned females.
The new theory, proposed by Ted Stankowich of the University of Massachusetts and Tim Caro of the University of California at Davis, is that females benefit from having horns or antlers if they are of a body size or live in a habitat that makes it hard for them to hide. The more conspicuous the female, the more benefit they gain from horns or antlers, which would be needed for defense against predators. (They may also benefit from competition with members of their own species for grazing spots.) This would explain caribou and wildebeest nicely, as they both live in very open country, as well as a lot of other species. This study was done by looking at a large sample of animals for traits related to body size and vegetation cover in the habitats they live in. The sample included 82 species with female horns or antlers, of which 80 were “very conspicuous.” According to the authors, who feel the two species that did not fit for reasons that can probably be explained, that is a nearly perfect match between theory and data.
More information on this story can be found here.
October 1, 2009
Darwin proposed that all species arose from a single common ancestor, and that this process involved almost uncountable branching events over eons of time. Working backwards, this means that an analysis of all of the living species should provide a “family tree” of life, showing, for instance, how all the monkeys are related to each other, and how the monkeys fit into the broader mammalian tree of life, and how the mammals fit as a branch on the vertebrate tree of life, and so on.
This is, of course, one of the main things scientists since Darwin have been working on, first using the physical appearance of living animals and fossils, and later using DNA. With DNA, however, it becomes difficult to unravel the details of the tree of life the farther back in time you look. This is because as parts of the DNA code change over time, it can randomly change back to an earlier code, which confuses the situation. This can be overcome by using a very large amount of data and a great deal of computer power and applying some powerful theories.
An international team of researchers has just come out with such a study of early bilaterians (bilaterally symmetrical animals, such as humans, fish and worms) that solves a long standing question in biology: Where in the evolutionary tree of life do we put a particular group of worms called the Acoelomorpha?
These very small flatworms are like the bilateral animals in many ways but lack some of the most important features that bilateral animals have … such as a gut. All bilateral animals have a gut lined with a specific kind of cell that facilitates digestion. Acoelomorpha, which is an entire phylum including about 350 species, “digest” food in an entirely different way. Some species take food into their body via a mouth, but that food does not enter a proper gut. Instead, pieces of food enter a sack full of special cells which then surround pieces of the food. The food is then broken down inside the cells. In some species, there is not even a space for the food to go into, though there is a mouth. In these species, the food is more or less shoved between the body cells of the organism where it is then digested.
Because of the lack of some of the key features of other bilateral animals, it has been difficult to place these creatures with certainty on the tree of life, so over the years this branch has been moved now and then from one place to another.
Casey Dunn at Brown University and sixteen colleagues from around the world claim that they have finally grafted Acoelomorpha where it belongs on the tree of life. Using a detailed and extensive analysis of DNA, they have placed Acoelomorpha just outside the other bilateral animals, as a sister clade to all other bilaterians (but still within the bliaterian group).
This is important for several reasons other than just putting Acoelomorpha in its proper place.
For one thing, it places the first split in the linage of bilaterians in its proper place. This, in turn, allows a better reconstruction of the last common ancestor of the bilaterians. Reconstructing the last common ancestor of any group of species is very important because differences between that ancestor and all of the subsequent species represent evolutionary events (or sequences of events). For example, Acoelomorpha lack a gut lined with special cells, lack two sexes, have sperm with two tails instead of one and have muscle tissues that are different from later bilaterians. One of the best ways to understand the evolution of key features of bilaterian guts, sexual reproduction and muscles would be to directly compare the early forms of these adaptations, as represented by Acoelomorpha, with the later forms.
Also, this finding might say something important about the evolution of the early bilateral animals. If it can be confirmed that Acoelomorpha truly existed back then as gut-free, using the method of enveloping its food that it is known to use today, then this indicates that a key evolutionary event at the origin of bilateral animals may have related to a change in how food was used as an energy source. It could be that the invention of the bilaterian gut is the very reason for their evolutionary success.
It is possible that this strange gut-free form of digestion, or any of the other traits that are unique to Acoelomorpha, evolved within that group early on in Acoelomorha history. The mere fact that a trait is simpler in one kind of animal than another does not guarantee that it represents the ancestral form. (For example, tapeworms pretty much lack a brain but evolved from ancestors that had brain-like structures.) Additional analysis would be needed to make it more certain, for instance, that this method of digestion represents the original, pre-bilateral (pre-gut) adaptation. But it probably does.
The work was published in the Proceedings of the Royal Society B.
September 30, 2009
The world’s largest living lizard is the Komodo dragon (Varanus komodoensis), a type of “varanid” lizard. Despite the fact that Komodo dragons are very interesting and widely known, there is a lot missing in our understanding of their natural history. Now a study of fossil evidence from Australia, Timor, Flores, Java and India shows that Komodo Dragons most likely evolved in Australia and dispersed westward to Indonesia. Some of the fossils that have been studied are newly described, including a species from Timor, and some are material known for a long time.
Here’s the most important finding: The two main hypotheses for the origin of the Komodo dragon have been brought into question and replaced with a new and better hypothesis.
It was previously thought that one of the best explanations for the large size of the Komodo dragon was the “island effect.” On islands, some animals may get bigger because of an increasing reliance on lower quality food found on island—the larger body size accommodates a gut that can process the food. In other cases, animals get smaller for a variety of reasons. But mostly, islands have strange effects on many species because evolution in the small population can proceed very rapidly. The animals that are confined to islands for long periods of time may simply evolve into food niches (which often relate to body size) that their sister species on the mainland did not experience.
A second hypothesis for the large size of Komodo dragons is that they were once specialists in the hunting of the pygmy Stegodon (a small elephant). This is a sort of indirect island effect. The Stegodons got small because they lived on islands, and the lizards evolved to be large enough to eat them.
Both of these hypotheses—island effects and specialist Stegodon hunter—now seem unlikely. The new research
indicates that Komodo dragons were really part of a distribution of related species of really large lizards across the region, including Australia. In fact, in comparison to some of these other lizards, Komodo dragons are kind of small.
In the words of Scott Hocknull, Senior Curator of Geosciences at the Queensland Museum and author of the paper, Australia is a hub for lizard evolution:
The fossil record shows that over the last four million years Australia has been home to the world’s largest lizards, including a five meter giant called Megalania (Varanus prisca). Now we can say Australia was also the birthplace of the three-meter Komodo dragon (Varanus komodoensis), dispelling the long-held scientific hypothesis that it evolved from a smaller ancestor in isolation on the Indonesian islands. Over the past three years, we’ve unearthed numerous fossils from eastern Australia dated from 300,000 years ago to approximately four million years ago that we now know to be the Komodo dragon. When we compared these fossils to the bones of present-day Komodo dragons, they were identical. This research also confirms that both giant lizards, Megalania (Varanus priscus) and the Komodo dragon (Varanus komodoensis), existed in Australia at the same time.
This research was published Tuesday in the Open Access journal PLoS ONE. You can access this paper here.
Citation: Hocknull SA, Piper PJ, van den Bergh GD, Due RA, Morwood MJ, et al. (2009) Dragon‚Äôs Paradise Lost: Palaeobiogeography, Evolution and Extinction of the Largest-Ever Terrestrial Lizards (Varanidae). PLoS ONE 4(9): e7241. doi:10.1371/journal.pone.0007241
September 29, 2009
You may know that much of the climate change on earth over the last two million years–the coming and going of ice ages–is caused by the “orbital geometry” of the planet. The amount of planetary tilt and the time of year the tilt occurs change over time. When the Northern Hemisphere is less tilted towards the sun on June 21st, and at the same time the Earth is as far from the sun in its elliptical orbit as it ever gets, ice age conditions prevail. This makes ice ages on Earth pretty regular, cyclic, events.
You also may know that a big chunk of Earth’s water is frozen into the ice caps.
You also may know that the history of Earth climate is preserved, in part, in changes in the ice in those ice caps.
Well, same for Mars!
Previously developed climate models suggested that the last 300,000 years of Martian history experienced low-level swings in climate, while the prior 600,000 years experienced more severe swings, owing to differences in the tilt of the planet. Most of the water we know about on Mars is in the Martian polar caps. And now, we can see, using radar, evidence of climate change reflected in that ice. From NASA:
New, three-dimensional imaging of Martian north-polar ice layers by a radar instrument on NASA’s Mars Reconnaissance Orbiter is consistent with theoretical models of Martian climate swings during the past few million years.
Alignment of the layering patterns with the modeled climate cycles provides insight about how the layers accumulated. These ice-rich, layered deposits cover an area one-third larger than Texas and form a stack up to 2 kilometers (1.2 miles) thick atop a basal deposit with additional ice.
“Contrast in electrical properties between layers is what provides the reflectivity we observe with the radar,” said Nathaniel Putzig…, a member of the science team for the Shallow Radar instrument on the orbiter. “The pattern of reflectivity tells us about the pattern of material variations within the layers.”
Essentially, the radar detects different amounts and/or kinds of dirt, and the ice is dirty in different ways. These vastly different climate periods (of more vs. less severe oscillation in climate change) probably leave behind different amounts of dirt in the ice. The radar can penetrate the ice and “see” these differences, with one period having more dirt than another.
There are two distinct models for how the dirt gets concentrated in the ice enough to be distinguished by the radar. One is that ice evaporates away more during some periods than others, leaving behind more dirt when the ice disappears, like the dirty snow during the late winter in northern cities. The other model simply has more dust in the atmosphere, and thus more dust falling on the ice, during certain periods. The present study supports the later model (more dust = dirtier ice). The radar reflectivity signal observed in this study is probably too coarse to link specific features of the signals with specific Martian “ice ages” so far.
“The radar has been giving us spectacular results,” said Jeffrey Plaut of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., a co-author of the paper. “We have mapped continuous underground layers in three dimensions across a vast area.”
The other images are different views of the polar cap using the radar images, and are explained in great detail on NASA’s site.
September 28, 2009
I’m Greg Laden, and I usually blog at here at Scienceblogs.com and Quiche Moraine. I’m a biological anthropologist interested in human evolution, the biologies of race and gender, human hunter-gatherers, science education and African prehistory. I’ve been asked to fill in here at Surprising Science for a couple of weeks, and I promise to try not to break anything while I’m here. On to my first post.
A new species of fish has been named from specimens collected over the last several decades off the coast of California. It is called Hydrolagus melanophasma, and will go by the common name “Eastern Pacific black ghost shark.” This is the first new species of cartilaginous fish to be described from California waters since 1947, and is a member of the Chimaeridae family. Technically, according to ichthyologist Doug Long of the California Academy of Sciences, Hydrolagus melanophasma is “a big weird looking freaky thing. They have some shark characteristics and they have some that are very non-shark.”
Chimaeridae is a family of fish related to sharks. Sometimes they are called ratfish. Sometimes they are called ghost sharks. Some have a venomous spine on their backs. They live in the ocean, usually quite deep, and the most recently discovered species in this family is gaining fame because it is said to have its sex organ on its head.
This “sex organ on the head” is actually quite normal for ghost sharks, though it is one of the big differences this sort of fish has with sharks. The feature in question is a tentaculum. A tentaculum is any of several sensory organs found on fish. In male ghost sharks the tentaculum is specially adapted as a grasping organ used during mating. So it is not the male’s penis, but rather, a grabby thing that the male uses to facilitate copulation with the female. So, referring to the ghost shark’s tentaculum as a “sex organ” on “its head” is a little like calling a finely chosen wine and just the right music a sex organ …. perhaps related to sex, but not sufficient for reproduction, anatomically speaking.
Hydrolagus melanophasma, was described in the September issue of the journal Zootaxa by a research team including California Academy of Sciences David Ebert (also with Moss Landing Marine Laboratories) and Douglas J. Long (also with the Oakland Museum of California) and Kelsey James, a graduate student at Moss Landing Marine Laboratories, and Dominique Didier from Millersville University in Pennsylvania.
The closest living relatives of the Chimaeras are sharks, and the Chimaera-shark split is probably about 400 million years ago, which is a long time ago by any standards. Chimaeras have cartilage instead of bone for skeletons, as do sharks. Chimaeras were once a very diverse and abundant group of species, and today are present in all oceanic waters though rare in any given locality.
The genus Hydrolagus means “water rabbit‚” and is so named because of its grinding tooth plates that resemble a rabbit’s front teeth. The term “melanophasma” means “black ghost” which is a refernce to the common term “ghost shark” as well as its dark, nearly black color. Hydrolagus melanophasma was originally collected as early as the mid 1960s, but went unnamed until now because its taxonomic relationships were unclear. This fish is found in deep water and is believed to range from the coast of Southern California, along the western coast of Baja California, and into the Sea of Cortez (Gulf of California). This species is known from a total of nine preserved museum specimens, and from video footage taken of it alive by a deep-water submersible in the Sea of Cortez.