In general, just about every mammal (哺乳动物) relies on a community of gut bacteria (肠道细菌) for health and survival. Many animals have even developed to the point that closely related host species typically share more similar gut bacteria. But a new study has identified a converse phenomenon on one group of mammals — bats. The gut bacteria of closely related bats can be totally distinct from each other.

To learn about the relationships between bats and their gut bacteria, post-doctoral researcher Holly Lutz and her co-workers took samples of bacteria from the skin, tongues, and guts of 497 bats from 31 different species in Kenya and Uganda. They discovered bats have fewer bacterial species living in their guts than in their mouths and on their skin. What’s more, gut bacteria living in different bats are individually distinctive, following no apparent pattern. This struck the scientists as strange, since for most other mammals that have been studied, closely related hosts share more similar gut bacteria.

“There’s essentially no relationship between bats’ gut bacteria and their evolutionary (进化的) history,” says Lutz. While host identity is still an important factor in predicting the species of gut bacteria, this may simply be tied to where those bats live and what they eat. The fact that bats’ gut bacteria are more closely tied to where bats live than where those bats fall on the bat family tree indicates that developing a special relationship with just-right gut bacteria may not have been as important for bats as it has been for other mammals.

Lutz suspects that bats’ unique relationships with gut bacteria are related to the feature that sets them apart from other mammals: their ability to fly. “Bats have extremely shortened guts,” she explains. “Food takes just fifteen to thirty minutes to pass through a bat’s digestive system, a third as long as it would take for a similarly sized animal. That’s likely because a long, winding digestive canal would weigh the bats down.”

Erin Alexander was having a hard day. A waitress working at Starbucks was too. Erin ordered an iced green tea and told the waitress to cheer up. After picking up the order, she noticed a message on the cup: “Erin, your soul is golden.” Erin Alexander recalled the incident, “Of course, I was still really sad, but that little thing made the rest of my day.”

New findings, published in the Journal of Experimental Psychology in August, prove just how powerful experiences like Erin’s can be. Researchers found that people who perform a random act of kindness tend to underestimate how much the recipient will appreciate it. And they believe that miscalculation could hold many of us back from doing nice things for others more often.

A recent experiment recruited 84 participants at an ice skating rink in Chicago, where they were given a hot chocolate and asked to keep it or give it to a stranger as an act of kindness. The 75participants who gave away their hot chocolate were asked to predict how the recipient would feel on a scale from 0(very small) to 10(very large), and the recipients were then asked to report their actual feelings on the same scales.

In that experiment, the people doing the kind thing consistently underestimated the appreciation they receive, according to Amit Kumar, a psychology professor at the University of Texas. “People tend to think that what they are giving is kind of little, maybe it’s relatively inconsequential,” Dr. Kumar said. “But recipients are less likely to think along those lines. They consider the gesture to be significantly more meaningful because they are also thinking about the fact that someone did something nice for them.” Dr. Kumar continued, “With an experiment like this, I hope it will inspire more people to actually commit random acts of kindness.”

Crying is often associated with emotions like anger, frustration, or sadness. 【小题1】 But have you ever heard of the term “happy tears”? You might have even experienced it! In a 2018 study, 68% of participants reported that a happy or positive event caused them to tear up. Over the decades, researchers have put forward theories to explain why happy crying occurs.

【小题2】 This meant that people who cried when experiencing something joyous had hidden feelings of sadness. Many researchers have been against the theory over the years, but it appears to be one of the earliest attempts to understand why we cry when we are happy.

Tears contain enzymes, lipids, electrolytes, and metabolites. But emotional tears might also include other proteins and hormones (荷尔蒙). It is assumed that release of stress hormones may help regulate the body’s physical and emotional homeostasis (体内平衡). 【小题3】

Crying in any scene could also be our way of showing weakness as human beings. 【小题4】 This happens when we cry because of a happy or sad event. Scientists believe that crying is a way of establishing a social connection with other people.

Yet other scientists suggest that all types of crying come from feelings of frustration and them coming on. When you cry, it feels like you are releasing some of these emotions. Similarly, helplessness. 【小题5】 While you might not always allow your tears free fall, you often feel while you might be experiencing a happy or joyous occasion, you might find the emotion hard to control. Crying helps you release some of this emotion.