In “Wooing the Fence-sitters” [Science Agenda], the editors suggest that rather than “strong-arm tactics,” we should use a more subtle social strategy of “little nudges” to convince parents to vaccinate their children. But strong-arm tactics are nothing new to public health. How is a law that mandates vaccinations for school-age children, such as the one recently passed in California, any different from, say, fluoridating drinking water? Although forming peer-advocacy groups and promoting provaccine interactions with providers are part of the solution, I do not think these strategies alone are aggressive enough for a time-sensitive and life-threatening public health issue. California's law is a necessary measure to shift the immunization rate above the safety threshold.

University of California, San Francisco, School of Nursing


Stephen Brusatte refers to the extinction of the dinosaurs in “Rise of the Tyrannosaurs.” What was so vulnerable in the dinosaurs that ensured their extinction while allowing mammals to survive and thrive?

via e-mail

BRUSATTE REPLIES: The end-Cretaceous extinction is often viewed as a catastrophe that killed dinosaurs but spared mammals, allowing our ancestors to take over. But it wasn't so simple. Some dinosaurs did survive: birds. Yet it is a mystery why some (but not all) birds did so, but numerous very birdlike, feathered dinosaurs such as Velociraptor and its kin died. Many mammals did survive, particularly those that were smaller and had more general diets. Yet a number of other mammals perished. The late Cretaceous was the heyday of metatherians (living marsupials and close relatives), but this entire group almost went extinct when the asteroid that triggered the dinosaurs' demise hit. In the ensuing Paleogene, it was the placental mammals that took advantage of the metatherian demise and blossomed into the many familiar groups we know today, including our primate forebears.


In “The Glue That Binds Us,” Rolf Ent, Thomas Ullrich and Raju Venugopalan state that “physicists think that when protons and neutrons reach extreme speeds, the gluons inside the protons split into pairs of new gluons.”

But that would violate special relativity's tenet that the laws of physics are the same for all observers. Consider the perspective of a tiny physicist riding a proton in a vacuum, surrounded by a tube that races by at ever faster speeds. Our little physicist monitors his proton from time to time and always finds it the same. Which physicist's gluons are splitting—the tiny one or one observing from outside the tube?


THE AUTHORS REPLY: Einstein's special theory of relativity holds true for the quantum foam of quark-antiquark and gluon pairs that continually pop in and out of existence inside the proton. Thus, in a proton moving at 0.99999 times the speed of light, the lifetime of this quantum foam is dilated long enough that its feature of splitting gluons is captured by an observer's quark-gluon femtoscope. Conversely, the physicist co-moving with the proton cannot observe these quantum fluctuations, because they are short-lived, relative to him or her. This person is therefore impervious to the seething cauldron of quarks and gluons existing within the proton (and all of us). Because the fluctuations exist in both frames, special relativity is upheld.


After reading “Why Girls Are Starting Puberty Early,” by Dina Fine Maron [The Science of Health], I was surprised that there was not a mention of the impact of growth hormones in milk production. Can these hormones bear some of the responsibility for the early onset of puberty in girls?

St. Petersburg, Fla.

Although it is clear that obesity is part of the picture—fat cells secrete estrogen, which is the major hormone involved in puberty in girls—I was astounded that Maron ignored what is most certainly the cause of both obesity and early-onset puberty: a diet rich in high-fat animal products, including dairy foods, which are themselves rich in estrogen.

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MARON REPLIES: It is not simple to identify any one factor responsible for earlier puberty in girls. The bulk of evidence points to those outlined in the article (such as obesity), yet other theories abound.

Milk does contain some substances that have weak estrogenic effects in humans, but they probably are not big drivers of earlier puberty. Meanwhile although some cows are also treated with a hormone related to human growth hormone, there is little reason to think that it would affect humans, and according to pediatric endocrinologist Paul Kaplowitz, naturally occurring and added hormones in these products are quickly degraded in the stomach. Instead environmental chemicals that act like hormones after being ingested appear to be of much greater concern.

Slashing consumption of red meat and high-fat dairy products is a good idea for a variety of health reasons. Yet it is hard to discern whether a more plant-based diet would influence puberty because that dietary change could also reduce obesity.


My daughter and I were fascinated by the gap junctions—structures connecting cells to one another—described by Dale W. Laird, Paul D. Lampe and Ross G. Johnson in “Cellular Small Talk.” Does cell communication via gap junctions occur with blood cells that are circulating through the body? And what about unicellular organisms, especially those that colonize?

Camas, Wash.

THE AUTHORS REPLY: Developing blood cells in the bone marrow make gap junctions and communicate by sharing small molecules with their neighbors. Yet for a long time we thought circulating blood cells did not do so: with gap junctions, groups of blood cells would stick together and potentially block small blood vessels. We now know that lymphocytes “activated” to combat an invading bacterium can form gap junctions with other cells. This might be an early step in blood cells crawling out of a vessel to fight infection.

Regarding unicellular organisms: single cells and even colonial organisms such as Volvox primarily communicate by releasing chemical signals. Gap junctions became necessary when cells began to develop different roles as they lived together with other cells. For example, cells in the coelenterate Hydra form them. Plant cells don't have gap junctions, but they do form connections that pass much larger molecules and complexes.


The Search for a New Machine,” by John Pavlus, refers to Moore's law as indicating that halving transistor size doubles computing performance. It should have referred to doubling the number of transistors on a chip to increase performance.