A cosmic quandary, risks of hatching early and more reader feedback

Your letters and comments on the August 6, 2016 issue of Science News

Cosmic mismatch

Researchers used supernovas, cosmic microwave background radiation and patterns of galaxy clusters to measure the Hubble constant — the rate at which the universe expands — but their results were mismatched, Emily Conover reported in “Debate persists on cosmic expansion” (SN: 8/6/16, p. 10).

Reader J.R. Kennedy thought that light-dimming space dust and debris might explain the discrepancy.

Gas and dust in space can have an impact on the brightness of standard candles — objects with known brightness such as type 1a supernovas and some variable stars, Conover says. But astronomers correct for those discrepancies in their measurements.

In the absence of gas and dust, a candle’s apparent brightness should decrease in relation to its distance from Earth. “But if there’s dust in the way, it can make the candle dim more than that,” Conover says. “However, this intervening material doesn’t dim the candle quite in the same way as distance does. It will dim the shorter, bluer wavelengths of light more than the redder ones. Astronomers can look for this effect to identify the impact of dust and correct for it.” So the mismatch stands.

Great escape

High-speed video captured how the o­ffspring of red-eyed tree frogs prematurely break free from their eggs when in danger, Helen Thompson reported in “Under threat, tadpoles make early escape” (SN: 8/6/16, p. 32).

Online reader myndflyte wondered if early hatching had any long-term de-velopmental effects on the tadpoles.

There’s definitely a trade-off involved in hatching early to escape a predator or some other threat, Thompson says. Past work by tree frog researcher Karen Warkentin, now at Boston University, shows that red-eyed tree frog embryos grow tails and mouthparts in the last few days of their roughly weeklong incubation. Those that hatch earlier, up to four days if threatened, tend to be underdeveloped with smaller bodies and shorter tails. “In the short term, this developmental deficit puts early hatchlings at greater risk of getting eaten by pond shrimp and fish than their older brethren,” Thompson says. “But there’s also evidence to suggest that early hatchers compensate down the line and grow at higher rates as tadpoles.”

More to the story

Although the death rate from motor vehicle crashes in the United States has declined since 2000, the country still tops 19 other high-income nations in m­otor vehicle deaths, Alex Maddon wrote in “U.S. still leads in fatal motor vehicle crashes” (SN: 8/6/16, p. 5).

Some readers took issue with the conclusions presented and thought the researchers should have measured fatalities per miles driven instead of per population. “Using a per capita metric makes the U.S. look unsafe when the opposite is true,” John Underwood wrote. “Since A-mericans drive more miles per year than the other countries in the chart, we will have the highest fatality rate per 100,000 population.”

It’s true that fatalities per miles driven changes the ranking. Using the measure “per 100 million vehicle miles traveled,” the United States drops to fifth place, says Deputy M-anaging Editor, Features Cori Vanchieri. When the researchers looked at deaths per 10,000 registered vehicles, however, the United States still topped the list. The researchers’ overall message is that the United States could further reduce crash deaths if seat belt use goes up and alcohol-impaired driving and speeding go down.


“Under threat, tadpoles make early escape” (SN: 8/6/16, p. 32) states that the tree frog embryos gape their mouths to stretch out their egg membranes. Not all embryos gape their mouths, and ultimately, an enzyme secreted from the embryo’s snout breaks open the membrane.

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