Along the river banks of the Amazon and the Orinoco there lives a bird that swims before it can fly, flies like a fat chicken, eats green leaves, has the stomach of a cow and has claw s ( 爪) on its wings when young. They build their homes about 4.6m above the river, an important feature for the safety of the young. It is called the hoatzin.

In appearance, the birds of both sexes look very much alike with brown on the back and cream and red on the underside. The head is small, with a large set of feathers on the top, bright red eyes, and blue skin. Its nearest relatives are the common birds, cuckoos. Its most striking feature, though, is only found in the young.

Baby hoatzins have a claw on the leading edge of each wing and another at the end of each wing tip. Using these four claws, together with the beak (喙), they can climb about in the bushes, looking very much like primitive （远古的) birds must have done. When the young hoatzins have learned to fly, they lose their claws.

During the drier months between December and March hoatzins fly about the forest in groups of 20 to 30 birds, but in April, when the rainy season begins, they collect together in smaller living units of two to seven birds for producing purposes.

“The ant and the zebra” sounds like the title of one of Aesop’s fables (寓言). Like all good. fables, this one has a moral, which is that human intervention in nature has unpredictable consequences. Unlike the Greek originals, though, this fable is real.

The story plays out in Laikipia county, Kenya, where the big-headed ants, an invasive (入侵的) species have gradually been replacing the native acacia ants. As Mr Kamaru, a Kenyan biologist from the University of Wyoming, and his colleagues report in Science, the an t invasion has triggered a complicated chain of consequences which has helped zebras at the expense of buffaloes, thus neatly illustrating a phenomenon called trophic cascade (营养级链).

It works like this. A kind of tree — whistling-thorn trees — provides the ants with shelter and food. The ants, meanwhile, protect the trees by seeing off the chief threat to them, the local elephants, which are not keen to eat trees, crawling with biting insects. However, big-headed ants are not so good at keeping the elephants at bay (陷入困境). The elephants move in and chew the trees, removing much of the area’s cover. That throws the local lions into confusion, which often use this cover to hide when hunting zebras. To compensate (补偿), the lions switch to hunting buffaloes, which are more dangerous, but run slower.

Mr Kamaru has put numbers to the process too. In invaded areas, elephants break trees five to seven times as often as in uninvaded ones. And in those uninvaded places, zebra kills are almost three times as frequent as those in the invaded ones. Indeed, between 2003 and 2020, as the big-headed ants spread, the proportion (比例) of local lion kills where the victim was a zebra fell from 67% to 42%. On the contrary, over the same period, the proportion of buffalo kills rose from zero to 42%. Whether the buffaloes blame the ants for their misfortune, no one knows.

Like humans, being overweight can have consequences for our pet's health, both short term and long term. Studies have shown that overweight dogs do not live as long. 【小题1】 . There is a risk of killing your pet with kindness.

Using special calorie-controlled foods from the vet is usually the most effective way of losing weight and well worth considering. 【小题2】 . Many vets run weight control clinics providing regular check-ups. They are often free of charge, and help to ensure that the diet is working at the correct rate.

With calorie-controlled food from the vet, you will be advised how much to feed daily. Many dry diets come with calibrated scoops (带刻度的勺子) but it is easy to overfill these. You can reduce hunger by feeding two or three times a day, so long as you don't go above the daily recommended amount. 【小题3】

Make a record of everything—including table scraps (剩饭) and treats—that your dog eats for a few days. This is often valuable in highlighting "extra". A dog that does not seem to eat much dog food probably still has an appetite for sausages!

Increasing exercise alone is not enough, although it is helpful. Start gradually, and be careful with how much you exercise elderly pets. 【小题5】 . Consult your vet right away if you notice your dog is limping or struggling with exercise. Try to take your dog out at least twice a day and increase the activity level at home as well.