A quick review of interesting research on living things from the last month.
The guts of sea spiders pump oxygen
The heart pumps blood and oxygen throughout our bodies. But sea spiders circulate oxygen by pumping their stomachs.
The gut of these invertebrates branches and goes down each of the eight legs. Sea spiders absorb oxygen through their skins, and alternately squeeze and ease off their stomach muscles to shunt the gas-laden hemolymph (invertebrate blood) throughout their bodies. The creatures do have hearts but they beat too weakly to get the fluid moving to the extremities.
Such energetic guts probably ensure the marine invertebrates don’t get a pot belly.
Insects are usually bird food. But one family of insects has turned the tables on its predators. The praying mantis got its name for folding its long front legs in a prayerful posture, while lying in ambush. When it spots insects or spiders, it abandons its seemingly benign preoccupation and grabs them with its legs. Sharp spines hold its struggling meal firmly. Occasionally, mantids snack on vertebrates like frogs, lizards, and even snakes. Nature enthusiasts thought birds figured rarely in the insects’ diet.
A recent review documented at least 12 species of mantids from 13 countries in all continents except Antarctica snatching birds of 24 species. Most of the 147 cases took place in the US where the insects learned to hunt hummingbirds near feeders and flowering plants in gardens. The ruby-throated hummingbird, the most common member of the family east of the Mississippi River, was a frequent casualty. The insects got to feast on most occasions. But humans, perhaps feeling sorry for the birds, intervened to rescue them from their insect captors in 18% of the instances.
In 1899, C.A.R. Browne reported a giant Asian mantis (Hierodula patellifera) preying on a crimson-backed sunbird (Leptocoma minima) in the Western Ghats. Interestingly, there were no other reports from India or even Asia.
A better name for these insects would be ‘preying’ mantids.
Bamboo sharks swallow their prey with a shrug
Fish don’t have tongues to help them swallow food. So they devise other ways of getting it down. The one-metre-long white-spotted bamboo shark scrounges for fish on coral reefs. Immediately after it gets hold of its prey and closes its mouth, it shrugs its shoulders back by 11 degrees to create suction that pulls the fish down into the digestive tract. Researchers from Brown University used a combination of CT scans and high-speed high-resolution X-ray movies to see how muscles and cartilage move in live sharks. Since the shoulder girdle wasn’t connected to the head in any way, the researchers didn’t expect to see it move. They suspect other sharks with similar eating habits may use the same move to get their food down.
Bullfrogs learn to avoid predators even before they hatch
Researchers from Oregon State University conducted an experiment with American bullfrog eggs. They let one lot smell the odour of the frog predator, largemouth bass, for two days. Besides the fishy smell, another lot also sniffed injured tadpoles, while a third didn’t get a whiff of anything. After they hatched, nervous tadpoles from the first two groups ducked undercover faster than the ones that didn’t smell anything as embryos. They seemed to learn what dangers lurked in the environment before they emerged from their eggs. The shy ones also grew longer.
Bullfrogs don’t belong to Oregon; they are natives of eastern US. They eat anything they can stuff into their mouths, posing a threat to smaller indigenous creatures. If tadpoles are savvy about predators such as largemouth bass and take evasive action, they could flourish without control. However, if embryos didn’t smell any fish, the tadpoles seemed to know they had nothing to fear. Biologists working to limit the spread and number of these invasive gluttons have a much harder job.
Thankfully, frog embryos haven’t evolved to smell biologists yet.
How much does the personality of a predator affect its hunting performance?
Although spiders don’t seem great cognitive abilities, they have personalities. Researchers from the National University of Singapore tested two species of jumping spiders. Portia labiata hunt insects and other spiders. Its versatile and adaptable hunting strategy is reminiscent of a cat. Found throughout south and southeast Asia, its frequent prey is Cosmophasis umbratica, another common jumping spider. In any predator-prey interaction, aggressive predators fare better than shy ones, while shy prey survive longer. The dice ought to be loaded against shy predators and bold prey.
The researchers ranked aggression in members of labiata by watching their reactions to their own mirror reflections. If they touched the mirror, they were assumed to be aggressive. If they retreated from the mirror, they were called docile.
The researchers graded members of the prey species by their reactions to an artificial spider predator made of putty with paper clips for legs. The ones that ran away were shy and the ones that jumped on the model were bold. Even though they behaved the same way on average, there were slight variations in their reactions. The more unpredictable these behaviours, the better they would be at dodging predators.
The investigators then pitted prey with predator. Compared to their docile brethren, aggressive labiata, which acted impulsively, performed better at catching unpredictable prey. While timid predators, which spent more time watching prey behaviour, did better with predictable umbratica.
Plants can turn caterpillars on each other
When herbivores chew on a plant, it releases an airborne chemical called methyl jasmonate that alerts its neighbours to take precautionary action such as pumping alkaloids into their leaves to turn them bitter. Besides serving as predator alert, this chemical also turns caterpillars into cannibals.
Researchers from the University of Wisconsin-Madison sprayed three groups of tomato plants with a jasmonate solution of varying strengths – low, medium, and strong. A fourth group got no spray at all. They added eight caterpillars to each group, and after eight days, counted how many were left. They also weighed the plants to calculate how much the worms had munched.
The caterpillars completely ate the plants that got no spray or low concentration of jasmonate. Plants doused with stronger doses of the chemical remained intact.
With nothing to eat, the hungry caterpillars turned on each other. However, the ones that turned carnivorous didn’t become beefy and large. They grew at the same rate as others on a vegetarian diet.
Plants haven’t yet learnt to pump out enough jasmonate to turn herbivorous mammals into cannibals.
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.