What's the difference?

I thought that the X and Y chromosomes determined gender in animals, but I see no mention of them in "When to Change Sex" (SN: 1/17/04, p. 40: http://www.sciencenews.org/articles/20040117/bob8.asp). Does this mean that on a genetic basis, males and females in these organisms are identical?

Neil H. Murphy
Walnut Creek, Calif.

In a sense, yes, says Philip Hastings of the Scripps Oceanographic Institute in La Jolla, Calif. Sex-changing fish don't rely on chromosomes to determine sex, so there's no special DNA difference between males and females. However, other fish species, such as swordtails and guppies, do depend on chromosomes to determine sex.—S. Milius

Curious exceptions

John Harris is quoted as saying that the absence of opossums is a "curious exception" to the list of current mammals of the Los Angeles Basin preserved in the La Brea tar pits ("L.A.'s Oldest Tourist Trap," SN: 1/24/04, p. 56: http://www.sciencenews.org/articles/20040124/bob9.asp). But the presence of opossums on the West Coast is well documented to be very recent. All current California opossums derive entirely from the liberation of the stock of failed fur farms in the San Jose area circa 1910.

Michael J.P. Nichols
San Francisco, Calif.

Believe it or not, the tar pits of Hancock Park hold sentimental value for me. They provided one of the best, free learning experiences one could find in the 1950s in my then-smog-ridden hometown. They helped spark an undying love of the sciences.

Andrew J. Glick
Centreville, Va.

With its title, I expected the article to report that human remains had been found in one of the tar pits. But the story ended by only implying that human remains had been found: ". . . scientists have identified remnants from every mammal species that lives in the Los Angeles Basin today—with the curious exception of opossums . . . ." So, have human remains been found?

Jim Nottke
Pfafftown, N.C.

Yes, one. The "La Brea woman," trapped about 4,000 years ago.—S. Perkins

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