Readers ask about bird droppings, Ice Age cave art and more

Bodily waste is cool, actually

Snot bubbles may help a short-beaked echidna cool off by coating the critter’s nose with moisture, which evaporates and draws heat from a blood-filled sinus, helping to cool the blood, Elise Cutts reported in “Adorable spike-balls beat the heat with snot bubbles” (SN: 2/11/23, p. 32).

Other animals also have creative ways of using fluids to stay cool. Some birds, for example, urinate on themselves to survive hot days, Cutts wrote. Reader James Wilson noted that the word “urinate” might be misleading, since bird waste is typically a mix of urine and feces.

It’s true that bird droppings are a mix of urine and feces that goes through one opening called the cloaca. So “excrete” might be a more appropriate term, says ornithologist Julián Cabello-Vergel of the University of Extremadura in Badajoz, Spain. For birds that cool off via waste elimination, a phenomenon called urohidrosis, the liquid component of bird droppings is key. When storks, herons, boobies and some other types of birds excrete extra-juicy waste on their legs, “it is the evaporation of the water contained in the excreta which produces heat loss from the [body] to the environment,” Cabello-Vergel says.

When gene flow runs afowl

About 20 to 50 percent of modern jungle fowl DNA comes from domesticated chickens due to interbreeding, threatening the genetic diversity and long-term survival of the wild birds, which live in South and Southeast Asia, Jake Buehler reported in “Chicken DNA runs amok in wild birds” (SN: 2/11/23, p. 14).

Reader Van Snyder asked if wild jungle fowl transfer any of their genetic material to chickens.

Evolutionary biologist Frank Rheindt has “no doubt that gene flow between domestic chickens and wild jungle fowl is bidirectional.” Many free-ranging domestic village chickens in South and Southeast Asia are probably regularly exposed to genes from wild red jungle fowl that they encounter near the boundaries between villages and forests.

But the flow of genetic material from wild red jungle fowl to chickens overall would be marginal, says Rheindt, of the National University of Singapore. “The vast global majority of domestic chickens are battery chickens kept in tight enclosures, mostly outside tropical Asia, and those chickens would remain unaffected.”

Defining depression

Depression is often blamed on a chemical imbalance in the brain. In reality, scientists don’t have a great explanation of what depression is or what causes it, despite decades of research, Laura Sanders reported in “No simple answers” (SN: 2/11/23, p. 18).

Many readers expressed their appreciation for Sanders’ reporting.

“It’s a fantastic example of popular science writing for the public about an urgent and difficult issue concerning human suffering,” reader R. Michael Johnson wrote. “I was particularly impressed and gratified by your a­ssurances that depression isn’t one-size-fits-all, and that the geographical/cultural nuances are not to be slighted.”

Why “Y”?

Ice Age hunter-gatherers may have used cave art to track the mating and birthing seasons of local fauna, Anna Gibbs reported in “Ice Age cave art may be a calendar” (SN: 2/11/23, p. 16). For instance, “Y” symbols in the art could signify birth, researchers suspect.

Reader Richard Delaware wondered why the “Y” symbol developed that meaning.

“We will probably never know,” says Ben Bacon, an independent researcher based in London who helped decipher the cave art. One possible explanation could be that the symbol represents the female pubic area. In Paleolithic art, including in small carved figurines and wall paintings, this area was often depicted with a pronounced Y-shaped outline, Bacon says.

Another possible interpretation relies on the fact that the symbol is a line that splits into two. “One becomes two, which describes perfectly the process of birth,” Bacon speculates.