Carl Zimmer has the lowdown on leeches. The article is right, and they really are beautiful animals—I had a tank full of them last semester (gone now, in a flurry of bloody student labs), and watching them undulate through the water was mesmerizing. They were almost as much fun to watch as my fish.
As a proud native of the great Pacific Northwest, when an article on one of our noblest creatures was mentioned to me, I had to read it. Here’s the center of the story.
In July 2005, nine residents of Teslin, Yukon,
witnessed through a kitchen window a large bipedal
animal moving through the brush. The next morning, they
collected a tuft of coarse, dark hair and also observed a
footprint measuring 43 cm in length and 11.5 cm in width.
That’s right: physical evidence, a footprint and hair, from…Bigfoot. The Sasquatch. A sample captured in the wild and brought into the lab. Pinned against the wall, trapped and unable to escape the probing appendages of an implacable, intrusive Science.
So they extracted DNA from the hair and amplified conserved mammalian sequences. They sequenced fragments of the DNA and compared them against sequences in the databases, and got a shocking answer. Prepare yourself: here is a diagram of the phylogenetic relationship of Sasquatch to other mammalian species.
The scientists squirm and try to avoid the obvious conclusions of their results, inventing foolish excuses rather than facing reality.
There are several possible explanations for these
results. First, as suggested from molecular analysis of
hair from a suspected Yeti, the Sasquatch might be a
highly elusive ungulate that exhibits surprising morphological convergence with primates. Alternately, the hair
might have originated from a real bison and be unrelated
to the Sasquatch. Parsimony would favor the second
interpretation, in which case, the identity and taxonomy
of this enigmatic and elusive creature remains a mystery.
I wonder what Radical Sasquatch will think of this.
*Wait a minute…the scientific name for the water buffalo is Bubalus bubalis? Bubalus bubalis? No wonder they’re so mean.
Coltman D, Davis C (2006) Molecular cryptozoology meets the Sasquatch . Trends in Ecology and Evolution 21(2):60-61.
Hey, you know that adorable teeny tiny fish species, Paedocypris, that I was drooling over the other day, and mentioning how cool it would be to work with?
apostropher has the link to the video: penis fencing flatworms. They are impressively athletic.
Olduvai George does it again: another gorgeous, art-filled post on ancient and modern crocodiles, including a reconstruction of the recently discovered Triassic Effigia.
Here’s a nifty video (mpg) of an octopus confronting an ROV working off Vancouver Island. The poor thing was just trying to crush and eat an interloper (or perhaps disassemble it for spare parts to use in its high-tech scheme to take over the world), and the ROV operator uses its thrusters to fling debris at it and drive it away.
It’s quite a battle, and the octopus holds on for a surprisingly long time in the face of an extremely obnoxious machine.
I saw on Muton, and several readers have mentioned it to me, this article about the world’s smallest vertebrate, fish of the genus Paedocypris. It’s a gorgeous translucent cyprinid, so is somewhat related to my favorite fish, Danio rerio. They live in cool, slow moving water in peat swamp forests of Southeast Asia. One female, only 7.9mm long, contained about 50 eggs, so they know it was sexually mature.
That size isn’t at all shocking—my zebrafish larvae at about that size are active hunters with functioning visual systems, capable of coordinated bouts of swimming, and they’re also very impressive animals…but they don’t have sex. It takes about 6 months for zebrafish to reach sexual maturity, and they are several centimeters long at that point. I would love to know how old these fertile Paedocypris were, but they were captured in the wild and virtually nothing is reported about their behavior or lifecycle. Ah, to have a fish colony that could be raised in a set of beakers, and could produce a couple of generations of crosses in a single semester…
One other clue that these are fully functioning, sexually mature adults are the presence of some pelvic specializations. Males have a hook and flange widget on their pelvic fins, and an odd prepelvic knob. Again, though, without knowing anything about their behavior, we don’t know how these are used in mating and courtship. Wouldn’t it be cool to put a pair under my Wild M3 scope and watch courtship and mating?
Of course, in addition to not knowing their generation time yet, these fish have another drawback relative to zebrafish: tiny eggs. They extracted a range of sizes from the ovaries, but assuming the smallest are immature, they max out at around 0.3mm diameter. That’s respectable, but Danio eggs are about 1mm in diameter.
Can you tell I’d love to get my hands on a bunch of these little fish? Unfortunately, I’ve heard from fish importers that it is agonizingly expensive and time consuming to bring wild tropical fish into the country, and for good reason: to block invasive species, to prevent the spread of new fish diseases, and also to discourage the plundering of native populations. I may not ever see one of these animals, short of making a trip to Malaysia, and even then I won’t be bringing any home.
First, a tiny bit of quantitative morphological data you can find in just about any comparative anatomy text:
| mammal | number of vertebrae | ||||
|---|---|---|---|---|---|
| cervical | thoracic | lumbar | sacral | caudal | |
| horse | 7 | 18 | 6 | 5 | 15-21 |
| cow | 7 | 13 | 6 | 5 | 18-20 |
| sheep | 7 | 13 | 6-7 | 4 | 16-18 |
| pig | 7 | 14-15 | 6-7 | 4 | 20-23 |
| dog | 7 | 13 | 7 | 3 | 20-23 |
| human | 7 | 12 | 5 | 5 | 3-4 |
The number of thoracic vertebrae varies quite a bit, from 9 in a species
of whale to 25 in sloths. The numbers of lumbar, sacral, and more caudal vertebrae also show considerable variation. At the same time, there is a surprising amount of invariance in the number of cervical vertebrae in mammals — as every schoolkid knows, even giraffes have exactly the same number of vertebrae in their necks as we do. What makes this particularly striking is that other vertebrates have much more freedom in their number of cervical vertebrae; swans can have 22-25. I was idly wondering why mammals were so limited, and stumbled onto a couple of papers that addressed exactly that question (Galis & Metz, 2003; Galis, 1999). Galis’s explanation is that it is a developmental constraint that may have something to do with the incidence of cancer.
Development is an intricately choreographed process that treads a dangerous line. On one side is stability; but development is in many ways a destabilizing process, in which cells have to change their path and form new tissues, and stability is not compatible with it. On the other side is chaos, unregulated proliferation — cancer. During development, the organism has to foster proliferation and change to a greater degree than it can tolerate later, and that loosening of constraints represents a danger. Galis suggests that one reason we mammals may always have 7 cervical vertebrae is that the regulatory genes that specify the number of vertebrae are coupled to processes that otherwise regulate cell fates, and that modifications to those genes that would cause variation in vertebra number would also lead to unacceptable increases in the frequency of embryonal cancers.
This isn’t at all an improbable idea. Genes exhibit bewilderingly complex patterns of expression, and pleiotropy (the regulation of multiple phenotypic characters by a single gene) is the rule, not the exception. The Hox genes, the particular genes that control the identity of regions along the length of the animal, are known to switch on and off in proliferating mammalian cell lines in culture. Perhaps the Hox genes involved in defining cervical vertebrae are somehow also involved in controlling cell proliferation, making them dangerous targets for evolution to tinker with?
Galis provides several lines of evidence that this is the case. To see whether variation in cervical vertebra number leads to increased incidence of cancer, we need to look for instances of variation in mammalian vertebrae.
There isn’t much variation in cervical vertebra number, though. There is an exception: sometimes, the 7th cervical vertebra is found to undergo a partial homeotic transformation and forms a pair of ribs, which are normally found only on thoracic vertebrae. Humans develop cervical ribs with a frequency of about 0.2%; do they also develop cancers? The answer is yes, with a frequency 125 times greater than the general population.
Another place to look would be in phylogenetic variation — between groups rather than within a population. It turns out that there are two groups of mammals that do have a non-canonical number of cervical vertebrae: one manatee genus and two genera of sloths. No data is available on frequencies of embryonal cancers in either, and Galis reports that manatees at least seem to have a low incidence of cancer. One explanation is that both sloths and manatees have exceptionally slow metabolic rates, which in itself will reduce the frequency of cancer, since it will reduce the rate of oxidation damage; the idea is that this low cancer rate may have made these organisms more tolerant of variation in these genes.
An open question is how birds can have greater variability in the number of cervical vertebrae — they certainly don’t have low metabolic rates. One suggestion is that the coupling between these particular Hox genes and a predilection for cancer is unique to mammals. Another possibility is that birds possess other, unidentified mechanisms that reduce free radical production, reduces oxidative damage, and makes them relatively cancer-free. Galis cites several studies that show that birds do seem to be less severely afflicted with cancers than us mammals.
It’s an interesting idea, but the evidence so far is a collection of correlations. I’d be interested in seeing some direct analyses of the role of patterning genes on carcinogenesis. Still, it’s the first answer I’ve seen to explain why such a peculiar restriction in morphology should be nearly universal within a whole class of animals, when other classes allow so much more diversity.
Galis, F and JAJ Metz (2003) Anti-cancer selection as a source of developmental and evolutionary constraints. BioEssays 25:1035-1039.
Galis, F (1999) Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J Exp Zool (Mol Dev Evol) 285:19-26.
