(1) Birds, mammals, and many other animals require sleep. (2) But when an animal is asleep, it’s more vulnerable to predators. (3) As a result, they have systems. (4) Their sleep habits make our typical “climb in bed and pull up the covers” behavior seem boring by comparison.

Question

(5) Some animals, such as dolphins and whales, can actually rest half of their brain at a time. (6) As one half of the animal’s brain enters a sleep state, the other half of the animal’s brain stays alert. (7) That way the animal can continue to watch for predators and come to the surface to breathe even as it sleeps.
(8) Other animals, such as horses, have the ability to lock their legs. (9) This allows them to remain standing while they’re asleep. (10) If a predator comes near, they can make a quick escape. (11) Horses have bigger eyes than most other animals, too.
(12) Birds are some of the most unusual sleepers in the animal world. (13) Some have to migrate thousands of miles every year, and it leaves very little time for sleep. (14) The thrush is one bird that has found a way to survive with minimal sleep. (15) This bird lands briefly for short “micro-naps,” each lasting mere seconds, which is just long enough for it to re-energize for the next phase of the journey. (16) Some birds can even enter a form of sleep while flying, similar to the way underwater mammals can let half of their brain sleep while the other half is awake.
(17) Other animals require deep sleep like humans do. (18) These animals need to be picky about where and how they sleep. (19) Most sleep in a hidden location that provides protection from predators. (20) For example, a bat sleeps in the seclusion and shelter of a cave. (21) A sloth sleeps high in the trees of a rain forest. (22) Other animals, such as chipmunks, rabbits, and groundhogs, sleep in burrows that they dig in the ground.
(23) Why does sleep seem to be an essential part of life for so many kinds of animals? (24) Scientists aren’t sure. (25) However, they aren’t even sure that every species needs sleep. (26) What is certain, though, is that animals that do sleep have found some clever ways to do so and unique.

Go back into Derek’s essay. With your partner, talk about what the most effective transition would be to replace However in sentence 25.
F As a matter of fact
G For example
H To conclude
J In contrast

Answer 1

In sentence 25, the word "However" introduces a contrast to the previous statement regarding scientists not being sure about why sleep is essential for many animals. The best transition to maintain coherence and provide a logical flow would be "In contrast" (option J). This is because it emphasizes the difference between the uncertainty of the necessity of sleep for every species and the certainty that some species do require sleep.

So, the most effective transition to replace "However" in sentence 25 is J: In contrast.