Sleeping Jellyfish, Discreet Cannibals, Conscientious Parrotfish and Other Amazing Animals

A quick review of interesting research on living things from the last month.

Jellyfish prefer to sit upside down on the floor of their tank. Credit: Caltech

Jellyfish prefer to sit upside down on the floor of their tank. Credit: Caltech

Even jellyfish go to sleep

Although not much is known about why we need to sleep, some scientists think it helps the brain rest. Recently, researchers discovered jellyfish that have no brains also sleep. Cassiopea, a group of upside-down jellyfish, sits with its bell touching the bottom and upward-pointing tentacles waving in the water. It pulsates its bell once a second during the day to push water through its arms. But at nights, the pulses slowed down to about 40 a minute. When the researchers dropped sleeping jellyfish into the water, instead of swimming to the bottom as it would when awake, it floated for a few seconds before waking up. If the researchers deprived the aquatic animal of sleep by squirting water at it, it made up for the lack during the day.

If jellyfish – said to have been around for more than 500 million years – sleep, then it pushes the behaviour farther back in evolutionary history than scientists realised until now.

Why do female cuckoos call loudly after laying eggs in another bird’s nest?

The behaviour of female common cuckoos (Cuculus canorus) has long puzzled biologists. They sneakily and swiftly lay their eggs in the nests of reed warblers when the parent birds are away and then call loudly as if to attract attention to what they’ve done. It doesn’t make sense. Why would they alert the host species to the presence of their eggs?

Female cuckoos call kwik-kwik-kwik while the males make the classic cuck-oo often heard as clocks chime the hours. Males call to announce and defend their territories. But what purpose does the chuckling calls of the females serve?

Using playback recordings, biologists from the University of Cambridge say the female calls sound like predatory sparrowhawks’ kiii-kiii-kiii. When cuckoos are done laying eggs, they are likely to be noticed by returning reed warblers that may then shove cuckoo eggs out of the nest. When they call, the panicky sparrow-sized warblers intent on escaping the talons of the predator may not notice the cuckoo eggs in their nests. Mimicking a predator gives the cuckoo progeny a chance at survival.

Even cannibal fish have their boundaries

Most mangrove killifish (Kryptolebias marmoratus) of the Americas are hermaphrodites and can self-fertilise their eggs. They are also cannibals but ones with discretion.

When researchers from the University of Guelph, Canada, dropped unrelated and their own embryos into aquariums of adults, the fish were quick to investigate the unrelated embryos than their own. Compared to well-fed fish, hungry ones approached the embryos seven times faster. Despite their eagerness for a meal, they didn’t eat their own embryos. This is unusual since male bluegill sunfish, for instance, eat fry of other species but cannot distinguish between their own and other bluegill fry. It’s possible killifish recognise the smell and taste of their own embryos when they nip them. This is the first discovery of a fish that can sense if a single embryo is its own.

Cephalopods borrowed genes to change their colour

Cephalopods, as octopuses, cuttlefish, and squid are called, can change their body colour in milliseconds. The key to this rapid colour transformation is reflectin, an amino acid composition that has the highest refractive index of any protein. Cephalopods’ skin has reflective platelets embedded with these proteins. By searching the genome of the bioluminescent bacteria (Vibrio fischeri) and matching it with genes that activate reflectins in common cuttlefish, researchers from China discovered these proteins are of bacterial origin. Ancient cephalopods borrowed these genes from symbiotic bacteria that lived within them.

When macaques behave like humans

Long-tailed macaques (Macaca fascicularis) living on the island of Koram, off Thailand, love shellfish. Each shucks about 50 a day, deftly wielding stone tools to break them open. Just 30% of the island’s monkeys studied by researchers are estimated to consume almost 450,000 shellfish a year. Since they harvest the largest ones with the most meat, the average size of shellfish is getting smaller as are the stone tools needed to break them. At this rate of consumption, the macaques may soon have no shellfish to eat or any use for their stone tools. The researchers say just as humans over-harvest resources with tools, so do these primates. Once they run out of shellfish, the primates could forget the use of stone tools.

Parrotfish know the rules of sustainability

Unlike these primates, steephead parrotfish (Chlorurus microrhinos) of Palmyra atoll in the central Pacific are sustainable farmers. They graze on a patch of turf algae and wait for it to regrow before nibbling on it again. They even defend recuperating spots from other parrotfish. Algae can smother coral and by keeping it under control, parrotfish perform an essential service to maintain the health of coral reefs.

Why do giraffes have long necks?

Most people would say giraffes evolved long necks to reach leaves beyond the grasp of other herbivores. Others say they have a high ratio of surface area to mass so they can keep cool. By measuring giraffes, researchers found their elongated body plan does not mean they have less surface area. Giraffes have a similar surface area to other animals of the same mass. But their unique body shape does keep them cool in another way. The area of their trunks, from the base of the neck to the base of the tail, is much shorter than similar-sized animals. On hot days, they face the sun exposing less of their bodies to direct sunlight and therefore don’t heat up.

Why do deaf toads call?

We talk because others hear. What’s the point of talking if everyone else is deaf? Two species of orange-coloured pumpkin toadlets of Brazil are in an anomalous situation. Neither the males who call nor females, the intended recipients, can hear. When researchers observed no reaction to playback recordings, they tested their hearing sensitivity. Although the amphibians were not completely deaf, the part of their ears necessary for hearing high-pitched calls has degenerated. This is the first case of an animal continuing to call long after they lost the ability to hear. Typically, evolution would eventually silence the male toadlets since their calls serve no purpose. But researchers speculate that the throat movement may be a visual signal and the sound may be an unintended product.


How do planarians divide into two?

Planarians, or freshwater flatworms, can reproduce asexually. They split in two and become independent individuals. Although this seems simple, the process was notoriously difficult to observe. By using time-lapse photography, researchers now know how this occurs. A planarian forms a waist and a series of contractions pulls it apart. Each piece then regrows the missing head or tail to become whole.

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.