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Counting neck bones, break-neck speed, and speedy urban sprawl

Q: I know that most mammals have seven vertebrae in their necks. Do you know of any mammals with more or fewer than seven? (Dia, Washington DC)

Q: What animals have more than seven cervical vertebrae? (Austin, Colorado Springs, Colorado)

A: Only two mammals have a different number. The slow-paced manatees have six and the lethargic sloths have six or nine. The flexible three-toed sloth has nine vertebrae and the more rigid two-toed sloth has only six. All others have seven — from shrews to whales. Long-necked giraffes and camels have the same seven neck bones as do short-necked mice and men.

Short-neck (6 bones) manatees browse off the coast of Florida [US Fish & Wildlife Service]

We might know why — a genetic danger. The Hox gene (which controls the number of neck vertebrae a fetus sprouts) also maps out the nervous system and cell growth — both normal and cancerous. When Hox gene expression changes, bad things sometimes happen: neural problems, increased susceptibility to early childhood cancer, and stillbirths.

Maybe that’s why natural selection has restrained changes in Hox gene expression and, therefore, in the number of neck bones. In fact, Frietson Galis of the University of Leiden in the Netherlands proposes just that: We are stuck with seven because changes might damage us.

Why only manatee and sloths have a different number of neck bones appears to be related to their slow life style. It’s another example of ‘unintended consequences’ that evolution appears to thrive on.

The manatee can survive on 25% less energy than an animal of comparable size and the sloth on 40 to 45% less energy.

We know that byproducts of normal metabolism cause oxidative damage to DNA. Such damage can cause cancer. So, a slower metabolism might diminish DNA damage and thereby inhibit cancerous growth. That, in turn, could allow Hox gene expression to change and spell out necks with a manatee’s six bones or a sloth’s six to nine.

Further Reading:

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.

Galis, F. and J.A.J. Metz. 2003. Anti-cancer selection as a source of developmental and evolutionary constraints. BioEssays 25: 1035-1039

Adelman, R, R L Saul, and B N Ames. 1988. Oxidative damage to DNA: relation to species metabolic rate and life span

Griffin EB & Gillett M. 1995: Exceptions to the rule of seven: defining "cervical" in mammals with non-traditional vertebral counts. J. Vert. Paleont. 15(3, Suppl.): 31A-32A.

Q: How long does it take to reach the speed of light? (Lara, Alvin, Texas)

A: Forever. Any object with a non-zero rest mass (like a dust mote) can never reach light speed. It takes an infinite amount of time and an infinite amount of work to accelerate an object to the speed of light.

Further Reading:

Georgia State University: HyperPhysics — c as Speed Limit by Rod Nave

University of Virginia: Speed of Light by Michael Guidry

Q: What is the largest city on the planet? (Josh, New Brunswick, Canada)

A: By "largest", you probably mean has the "most people." This is actually a tough question because "city population" depends on how we define a "city." Is a "city" a "city proper", that is, the population within the city limits?

The United Nations chooses this definition and further defines "city proper" as a "locality with legally fixed boundaries and an administratively recognized urban status that is characterized by some form of local government."

With that definition and based on censuses over the period of 1995 to 2004, the UN found that Seoul, the capital of South Korea, wins the title with 10.2 million people.

However, Info Please, using the same definition, comes up with Shanghai, China as the most populous with 13.3 million people.

Tokyo, Japan — the planet’s most populous agglomeration.

Finally, Thomas Brinkhoff says comparing "populations of cities, which have rather arbitrary boundaries, is not really reasonable." Instead he ranks world agglomerations and defines an agglomeration as a central city and neighboring communities linked to it by continuous built-up areas or many commuters.

On that basis Tokyo, Japan is the most populous city on the planet with 34 million. The Tokyo agglomeration includes Yokohama and Kawasaki.

Brinkhoff credits Mexico City with 22.4 million (including Nezahualcoyotl, Ecatepec, and Naucalpan). Seoul comes in third with 22.1 million (including Bucheon, Goyang, Incheon, Seongnam, and Suweon).

Further Reading:

UN Statistics Division: Population density and urbanization

Info Please: Most populous cities of the world: 2004

Thomas Brinkhoff: City Population

(Answered April 8, 2005)