Wednesday, March 21, 2007

Burrowing dinosaurs!

Amazing new discovery reported on New Scientist's website today:

Evidence for a species of BURROWING dinosaur. The coyote-sized animals were hipsilophodonts, closely related to the hadrosaurs (my personal favorite class of dinos), and the fossils were discovered by David Varricchio of Montana State University in Bozeman. It appears that an adult and two juveniles were in the burrow together...does anyone else smell parental care? Its name is Oryctodromeus cubicularis, which means "digging runner of the lair."

Excerpt from the news story:
The bones were inside a twisting, worm-like deposit of sandstone that passed through three distinct layers of rock.
Varricchio says the sandstone formed 95 million years ago when sand washed into a burrow measuring more than 2 metres in length, 30 centimetres in width, and nearly 40 cm high. The dinosaurs inside had apparently already died of unknown causes.

Judging from the illustration, it looks like the skeleton was amazingly complete, also:

Just another illustration of how much there is left to learn (and how much we will probably never even guess) about the ecology and diversity of dinosaurs...awesome find!

Varricchio, David, Martin, Anthony, and Katsuro, Yoshihiro. First trace and body fossil evidence of a burrowing, denning dinosaur.
Proceedings of the Royal Society B (DOI: 10.1098/rspb.2006.0443)

Saturday, March 17, 2007

Polygyny Threshold

It doesn't take a scientist to figure out one of the most basic principles about relationships: males and females often have very different mindsets. Any survey of humans could show this, but the principle is universal, with the "battle of the sexes" being fought over and over everywhere from colonies of marine invertebrates to you local singles bar.

The main idea:
1. Males are capable of fertilizing many females, and so it is to their advantage to seek as many mates as possible.
2. Females are limited in their reproductive capabilities and invest more time and energy into offspring, so they are more 'careful' about critical resources, including mate selection.

But, it could be argued, there is a way to make everyone happy! In polygynous systems, several females share a single male mate. This is a widespread mating system, and is even found in up to 2/3 of traditional human societies. Boy gets many girls, many girls get boy's resources, everyone wins, right?

Obviously it cannot be this simple. If polygyny is that universally advantageous, why would any animal be monogamous? There are many examples of strong male-female pair bonds in nature, most classically avians: 90% of bird species are monogamous. Gulls, with a "divorce rate" of 0.3%, put humans and their approximately 50% rate of failed marriages to shame.

So what determines whether a species is polygynous or monogamous? There are many factors, but a particularly instructive case is that of hoary marmots, (Marmota caligota).
Marmots are Northwestern mountain animals, living in alpine meadows from Idaho and Washington up to Alaska. This is not a kind environment, and scrabbling out an existence on the cold mountainsides obviously involves many resource limitations for the animals. They are known to be very territorial, and males will aggressively defend their home ranges.

Those home ranges are the key for the principle of polygyny thresholds. This is basically a measure of how "bad" a territory must get before a female decides that she'd rather be someone's second mate than have a mate all to herself. As long as territory quality is good and plentiful, monogamy will dominate, but if it gets to the point that all the single males have poor territories, a female will choose to share a territory and mate with another female, effectively switching to polygyny. In populations where there is little difference in fitness between being the 1st or 2nd (or 3rd, etc) mate, you will see polygyny more often. This means that you can have a single population of marmots in which both monogamy and polygyny occurs, with high-quality males having many mates and males with pooer territories only being able to support a single one.
That, my friends, is a polygyny threshold. Draw human analogies at your own risk/entertainment.

Friday, March 16, 2007

New species: Bornean Clouded Leopard!

Yesterday the WWF issued a press release announcing that a new species of big cat has been declared, the Bornean Clouded Leopard (Neofelis diardi). It is really rare to find previously undescribed mammal species, this is a HUGE deal in the zoology community. Genetic analysis at the National Cancer Institute has concluded that the genetic differences between this and 'regular' Clouded Leopards (Neofelis nebulosa) are at least as large as those separating jaguars, lions, tigers, etc.

The 'new' species (which is not actually new, persay, the populations have been known about for quite a while but have only now been classified as a separate species) have a darker pelage, with spots in their "cloud" markings and a double dorsal stripe. Also, there is distinct geographic separation between the two species. Only N. diardi inhabits the Malay Peninsula, specifically the Indonesian islands of Borneo and Sumatra, while N. nebulosa lives on the mainland of Southeast Asia.

While there are estimated to be 5,000-11,000 mainland Clouded Leopards, the Bornean species is though to number 3,000-7,000, although it is very difficult to get accurate population information for these elusive species. The region of Borneo where this cat is found has yielded discoveries of 52 new species in just the last twelve months!

Clouded Leopards are ranked as an Appendix I endangered species by CITES, and as Vulnerable by IUCN. Hunting these cats is forbidden in most of its range, but these regulations are rarely enforced. It appears that attention drawn by this new discovery will greatly benefit conservation efforts, however: last month three Bornean governments: Brunei Darussalam, Indonesia and Malaysia--signed an historic Declaration commiting to conserve the region known as the "Heart of Borneo"

You can find out more about Clouded Leopards and conservation efforts to protect them at the The Clouded Leopard Project website.

A couple of interesting leopard facts:
~They have the largest canine teeth of any cat, relative to total body size.
~They are considered to be the "most arboreal" of all big cat species.

Further References:
Buckley-Beason et al., (2006). Molecular Evidence for Species-Level Distinctions in Clouded Leopards. Current Biology 16(23): 2371-2376.

Kitchener et al. (2006). Geographical Variation in the Clouded Leopard, Neofelis nebulosa, Reveals Two Species. Current Biology 16(23): 2377-2383.

Sunday, March 11, 2007

Familial Down Syndrome

Chromosomal abnormalities are the main cause of birth defects afflicting tens of thousands of infants every year. The numbers are pretty scary, about 20% of all conceptions result in spontaneous abortions (usually before the mother knew she was pregnant), and about half of those are due to a chromosomal abnormality. Of all conceptions that do show those abnormalities, 98.4% are aborted, 1% are stillborn, and 0.6% make it to birth alive.

When you consider that between 25 and 30 thousand babies are born with these abnormalities each year in the U.S., that means that there are over a million conceptions with abnormal chromosome configurations! The cellular processes controlling cell division, fertilization, and development are very specific, and the slightest misstep has extremely serious consequences.

The most well-known chromosomal abnormality is Trisomy 21, also called Down Syndrome. Individuals with this disorder have three copies of chromosome 21, instead of the usual two, and this is estimated to be the cause of 1/4 of ALL miscarriages. This results from a nondisjunction when the chromosomes separate in meiosis. Either the sperm or the egg (almost always the egg) ends up with both copies of the chromosomes which were supposed to be divided between the two daughter cells, and after fertilization by the sperm, which has its own Chromsome 21. the baby then has three, giving it a total chromosome count of 47, as opposed to the 46 found in normal humans, resulting in the characteristic abnormalities of the syndrome: epicanthal folds (the politically incorrect reason the disorder used to be called "Mongoloid idiocy"), underdeveloped secondary sex characters, depresssed immune system, heart trouble, simian fold on the palms, and various other health problems resulting in shortened life expectancy.

This is how the vast majority of Down Syndrome cases can be explained. There is, however, a different mechanism that can produce exactly the same disorder. Called "Familial Down Syndrome", it results from a translocation of part of Chromosome 21 onto Chromosome 14. Basically, part of 21 breaks off and attaches itself to 14, but it is a big enough chunk that it retains most of the genes and can function well enough to substitute for the whole chromosome.

So you have a cell with a good 21, and two 14's, one of which has a second 21 latched into it. What happens when the cell divides (goes through meiosis) to make gametes? It is supposed to dole out its chromosomes equally to each daughter cell, but this configuration can't be separated correctly. As a result, you get four kinds of gametes: normal, one with only 14, one with only the 14/21 hybrid, one with a 21 and the 14/21.

Obviously this is a mess, and it gets worse after fertilization. The results are shown below. The blue chromosomes are 21, the pink ones are 14. There are four possible outcomes of a fertilization: you get a normal child, a child that looks normal but carries the mutation, a nonviable zygote (all autosomal monosomies in humans are fatal), or a child with Down Syndrome, which has a 45% chance of surviving to its first birthday.

This fascinating mechanism is the cause of about 3-5% of Down cases, and is an example of the complexities involved in chromosome regulation during development.