Science

Sex-Shifting Reptiles, Accessorising Crabs, Loud Horny Fish and Other Amazing Animals

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

Great tits in the wild. Credit: Per Tillmann

Great tits in the wild. Credit: Per Tillmann

Birds learn to avoid unpleasant tastes from others

The bright colours of toxic insects save their bird predators from getting a taste of them. If every bird had to learn from experience, poisonous insects might not have much of an advantage. In fact, their colourful bodies would stand out, and they’d be picked off as soon as they showed themselves. To be effective, they’d have to warn the birds of their distasteful nature before they are injured or killed. How do birds learn to associate colour with foul taste?

Great tits discover by watching others. Researchers filmed one deal with an almond covered in a bitter liquid and played the recording to others. When these television-watching birds encountered a similar item, they knew to avoid it. But other great tits that didn’t have the benefit of the tutorial had to discover the unpleasant taste for themselves.

Unusual mirrors help scallops see

Beads of blue eyes stud the two lips of the scallop’s semi-circular mantle. Some species have as many as 200 eyes. Instead of using a lens to focus an image on the retina, these marine bivalves angle concave mirrors. Light passes through two retinas, one below the other, before it hits a grid of layered reflective material. Made of square guanine crystals, these mollusc mirrors reflect the image up to the retinas. Should the likeness of dark shadowy shapes strike the upper retina, the scallop panics and escapes. The lower retina offers peripheral vision in dim light. The rods and cones of our single retinas function similarly as the scallop’s dual ones. The visual processing centre, the parietovisceral ganglion, receives all the images from the many eyes to produce one seamless perspective.

How bearded dragons change their sex

Sex chromosomes dictate the gender of central bearded dragons. But when the incubation temperature goes over 32º C at a particular stage in development, heat overrides chromosomes, turning males into females. How do the embryos develop if warmth changes gender late in the day?

By incubating eggs under different temperature regimes, researchers observed how the embryos developed. Early in their development, all embryos grow small phallic swellings on either side of their cloaca. These genital bumps get larger and club-shaped at first before taking on the appearance of a hemipenis, as the forked penises of snakes and lizards are called. When the embryos are three-quarters developed, sex organs continue to grow in male dragons, but in females, they shrink. If higher temperature causes males to become females, they follow the course of female development and absorb the budding sexual organs.

Crabs decorate themselves for protection

Most species of a marine crab family called Majoidea decorate themselves with objects they find around them. They use seaweed, tiny shells, gravel, anemones and algae to either camouflage themselves or ward off predators.

The spider decorator crab (Camposcia retusa) even has a Velcro-like surface on its shell and tiny hooks on its legs to holds its decorations – sponges and lengths of algae – in place.

Researchers studying these coral reef-living crabs in captivity placed PVC elbows as shelters in the aquariums of some of their wards but didn’t provide them to others. They gave each group a collection of little pom-poms and timed how long it took them to adorn themselves.

All the crabs armed themselves with pom-poms within 24 hours. The ones that could hide attached the colourful pieces of fluff to their arms that stuck out of the shelter. Others adorned their shells first since they housed their vital organs. Researchers say the crabs want to protect the parts most prone to attack by predators.

The crabs that don’t have anywhere to hide decorate themselves more. But loading themselves with extra weight could work against them since they need more energy to walk and the drag from these appendages could slow them down. So they pack the larger pom-poms on their strong hind legs.

In their natural habitat, their preferred decorations are sponges that may not only hide the crabs but are the weapons with which crabs probably defend themselves against predators.

Insects use leaves as acoustic aids

Male tree crickets rub their forewings together to create a vibrating buzz that attracts mates. But the sound waves generated by each wing clash. The compressed air in front of the wing and the rarefied air behind it cause acoustic short-circuiting that drowns the sound. To be effective noise-makers, the crickets have to separate the two phases. They cut a hole in a leaf, crawl through so their head and front legs are on the other side, and rub their wings against it. Since the leaf acoustically isolates the two phases, the loudness of the insects’ call increases. These are the only insects to use an aid to overcome their disadvantage.

Researchers established the qualities of the best baffles – large leaves, large holes, and positioning these holes in the centre of the leaves.

When given a choice of leaves, they prefer the larger ones. But they didn’t bite a hole through the centre even though that would have made the best design. If they did that, they’d have to chew through the midrib, which would make the leaf flop over and defeat the purpose. The insects don’t learn this by trial and error. Instead, they seem to inherit the template for the best baffles.

Horny fish create a deafening racket

The noisiest fish in the world is the Gulf corvina (Cynoscion othonopterus). When millions of these two-foot-long marine fish gather to spawn in spring, the males croak together. Researchers measured the sound levels of nearly two million corvina that stretched for about 30 kilometres. At peak chorus, it reached 200 decibels, reverberating even through the hulls of small fishing boats. Other marine mammals in the vicinity of the Colorado River delta, where the sex orgy takes place, could be in danger of losing their hearing.

A new species of spider holds its breath underwater

A female individual of the new species Desis bobmarleyi. Credit: R. Raven

A female individual of the new species Desis bobmarleyi. Credit: R. Raven

A new species of spider discovered from the intertidal zone of Australia’s Great Barrier Reef holds tight to coral or rocks when the tide is high. To survive underwater, it weaves air chambers out of silk. When the sea water recedes, this leggy predator hunts small invertebrates that are exposed. This marine lifestyle is rare among spiders that are usually terrestrial.

Researchers named the spider Desis bobmarleyi after the reggae legend Bob Marley, inspired by his song ‘High tide or low tide’.

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.