How do ants sense imminent rainfall?

I have always been told to watch to see if ant-hole mounds are built up as a sign of imminent rainfall. My questions are,

if what I have always been told is true, then by what mechanism do ants 'sense' the imminent rainfall? Is it a case of sensing air pressure drop, increasing humidity or both?

This article claims that ants do sense approaching rain and modify their activities in preparation. The claim is not sourced. This weather site also speaks of ant mound-building before a rain but frankly places it in the "some folks say" category. The AntBlog is associated with AntWeb, a large multi-university-affiliated database. The author of the linked blog states that ants can sense humidity with their antennae, which strikes me as a plausible means of anticipating rain.

That ants might sense changes in barometric pressure is an intriguing idea but has not to my knowledge been demonstrated.

A 2010 article in Journal of Neurophysiology reports an almost unbelievable sensitivity to temperature in ant antennae, allowing them in principle to sense minute temperature changes ($0.005^o$C) over a wide range of temperatures and over 0.2 second time intervals (5Hz). This is said to assist them in orientation in their microenvironment but I think it goes a long way to accounting for an ability to detect looming weather fronts. Humans can sometimes 'smell' rain and we can detect gross temperature changes that almost always accompany rain, but to be able to detect humidity and micro-scale temperature changes would give the ants a real advantage in forecasting. After all, not all rainstorms are preceded by a dramatic drop in temperature but probably the majority are preceded by minute step-wise drops in temperature that evidently the ants can sense.

So while I think the building activity may be anecdotal (and probably true for some species) the ability to sense humidity changes and micro-scale temperature changes accounts for their ability to sense rain ahead of time.

Ants are very well-studied and it's hard to rule out that one has missed something. There are a few "ask the expert" questions on this topic online and no one seemed sure.

An article suggests that many animals and plants have an instinctual response to seasonal cues like light levels or day length (photoperiod). Also they may be influenced by circannual clocks which depend on factors like temperature, the photoperiod and the increase or decrease in food levels available. There is also some speculation that ants may be influenced by barometric pressure changes though there is no literature available for the same (reference). There are methods that ants have devised to escape rain some of which are mentioned in this article. You might be interested in reading this article on the spread of Argentine ants being influenced by rain.

Six amazing facts you need to know about ants

Charlie Durant receives funding from the Biotechnology and Biological Sciences Research Council (BBSRC).

Max John receives funding from the Biotechnology and Biological Sciences Research Council (BBSRC), and the Genetics Society.

Rob Hammond receives funding from NERC, BBSRC, The Genetics Society.


University of Leicester provides funding as a member of The Conversation UK.

The Conversation UK receives funding from these organisations


Have you have seen ants this year? In Britain, they were probably black garden ants, known as Lasius niger – Europe’s most common ant. One of somewhere between 12,000 and 20,000 species, they are the scourge of gardeners – but also fascinating.

The small, black, wingless workers run around the pavements, crawl up your plants tending aphids or collect tasty morsels from your kitchen. And the flying ants that occasionally appear on a warm summer’s evening are actually the reproductive siblings of these non-winged workers. Here’s what else you need to know:

Can ants predict rain?

Friday, September 7, 2018, 1:20 PM - It’s often said that ants can predict impending rain and respond by changing their behaviour.

Some people say that if you see ants building their mounds higher, or building them from different materials, this might signal the coming of rain.

But is there any scientific evidence to support this piece of folk wisdom?

The short answer is “no”, although it is a difficult question to answer partly because of the sheer diversity of ants – there are 13,000 named species on the planet!

Ants can deal with rain – but can they predict it?


Ants are equipped with a full array of senses that could, in theory, give them clues about imminent rainfall.

Ant antennae are sensitive detectors capable of picking up minute chemical traces.

One species, the Florida carpenter ant (Camponotus floridanus), has more than 400 genes for detecting odours – the largest number of any known insect species.

Ant antennae can also detect tiny changes in temperature, which might allow them to sense and react to the drop in temperature that usually accompanies a rain storm.

Given the diversity of ant species and their well-developed sensory systems, it’s possible that some ant species have evolved a way to detect rain before it falls. But observational or experimental data showing that ants actually alter their behaviour in anticipation of rain is currently lacking.


While the question of whether ants can predict rain remains unanswered, we do know that ants have evolved some astonishing ways of dealing with the risk of flooding.

One of the simplest ways ants can survive the flooding of their nest is by holding their breath.

When completely submerged in cool (5-7°C) water, workers of four coastal ant species were able to survive for an astonishing eight to nine days!

Soldiers of the mangrove ant (Camponotus andersenii) use their large heads to block the nest entrance and prevent flooding. Other ant species block the nest entrance using rocks, dirt or twigs.

Clever architecture can also be used by ants to survive in areas of high flood risk.

Australian mangrove ants (Polyrhachis sokolova), which live in mangroves subject to daily flooding, build bell-shaped water tight chambers that trap bubbles of air.

During flooding, the ants rapidly relocate larvae and adults to the dry tunnels where they wait for the waters to recede.

Fire ants (Solenopsis invicta) form incredible living rafts by gripping on to one another.

These ants are naturally water repellent – a property that is enhanced when many are packed tightly together. Rafts are assembled rapidly and can stay afloat for days or even weeks.

Bamboo ants (Cataulacus muticus) have perhaps the most entertaining defence against rainwater.

As their name suggests, bamboo ants build their nests in bamboo twigs that are prone to flooding during heavy rain.

At the first sign of flooding, workers run inside the nest and drink as much of the encroaching water as they can. They then proceed outside en masse and collectively urinate, a process fittingly dubbed “communal peeing”.

Contact with only a few drops of water causes worker ants of the (Pheidole) species to run around wildly alerting their nest mates to the oncoming threat. Other workers respond by rapidly and efficiently evacuating the nests, carrying the young and queen to safety.

Ant species living in a flood plain build functional levees by surrounding their nests with high earthen walls these structures are built within 24 hours of a major rain event and prevent flooding by diverting water away from the nest entrance.

So while ants may not be able to predict the rain, they are well equipped to deal with it when it comes.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Show/hide words to know

Caste: class to which an adult ant belongs.

Larva: the second, "worm-like" stage in the life cycle of insects that undergo complete metamorphosis (like caterpillars).

Larvae: plural of 'larva.'

Metamorphosis: dramatic change in body form. more

Molt: to shed the outer layer of the body.

Pupa: resting stage during which tissues are reorganized from larval form to adult form. The pupa is the third body form in the life cycle of insects that undergo complete metamorphosis (like caterpillars).

Pupae: plural of 'pupa'.

Queen: a female ant that lays eggs.

Worker: a female ant that performs jobs other than reproduction.

Ants undergo complete metamorphosis, passing through a sequence of four stages: egg, larva, pupa, and adult.

An ant’s life begins as an egg. Ant eggs are soft, oval, and tiny – about the size of a period at the end of a sentence. Not all eggs are destined to become adults – some are eaten by nestmates for extra nourishment.

An egg hatches into a worm-shaped larva with no eyes or legs. Larvae are eating machines that rely on adults to provide a constant supply of food. As a result, they grow rapidly, molting between sizes.

When a larva is large enough, it metamorphoses into a pupa. This is a stage of rest and reorganization. Pupae look more like adults, but their legs and antennae are folded against their bodies. They start out whitish and gradually become darker. The pupae of some species spin a cocoon for protection, while others remain uncovered, or naked.

Finally, the pupa emerges as an adult. Young adults are often lighter in color, but darken as they age. The process of development from egg to adult can take from several weeks to months, depending on the species and the environment. Did you know that ants, like all insects, are full-grown when they become adults? Their exoskeletons prevent them from getting any larger.

Furthermore, adult ants belong to one of three castes: queen, worker, or male.

Queens are females that were fed more as larvae. They are larger than workers and lay all the eggs in a colony – up to millions in some species! Queens initially have wings and fly to find a mate(s), but they tear them off before starting a new colony. A queen can live for decades under the right conditions.

Workers are females that were fed less as larvae. They do not reproduce, but perform other jobs, such as taking care of the brood, building and cleaning the nest, and gathering food. Workers are wingless and typically survive for several months.

Males have wings and fly to mate with queens. They live for only a few weeks and never help with the chores of the colony.

Do ants speak to each other? Yes!

Another peculiar way of how ants communicate is by sound. A majority of ant species use it to communicate, although it is commonly unknown to most people because of its low resonance. The ants can procure different sounds by scraping their legs on a washboard-like part of their body, thus accomplishing different sounds. Although we may not hear it, other ants can. The sound is actually possible for us to perceive if we hold an ant very close to the ear, listening carefully.

The sounds are used in different ways, depending on the species. A great example of the use of sound is when a worker ant has been trapped somewhere. Maybe through the collapse of a tunnel or chamber – blocking all the exits. The ant can use sound as a distress call, signaling their location to the other workers through the walls. This could not be achieved by pheromones.

12 different categories of communication

Myrmecologists have mapped out twelve different categories of how ants communicate.

How does an ant decide what to do?

We eat when we’re hungry, but social insects have to make decisions which will support the colony not just themselves. They typically divide labour as well as reproductive duties. Even in ant species such as Lasius niger where workers are not split into different physical ‘castes’, some workers stay in the nest while others leave to forage. It is often the younger ants who care for the nest while older ants leave to collect food.

Information that individuals could use to decide whether to forage includes their own experience of performing tasks, or their own physiology such as fat reserves. This was investigated in a recent paper published in the Journal for Experimental Biology.

The researchers used an ingenious system where worker ants were tagged with RFID chips which operate an artificial ‘door’ to the nest. Once workers had visited a feeder they were not allowed to leave the nest for a week. A week after the start of the experiment the researchers opened the doors to all the ants. The experiment was to discover whether the ants who had been trapped for longest would be more likely to leave than those who had recently been out.

The answer was yes: those who had been trapped for longest, i.e. the ants who are the leanest, were most likely to leave and forage. They therefore concluded that physiological condition was more important than memory of recent tasks in determining the ants’ behaviour. Such rules about when to forage allow ants to gather food efficiently and flexibly.

Our flying ant survey is looking at another act of ant ‘decision-making’. Do flying ants appear at a specific time determined by the ants’ biology, or is there a lot of flexibility in response to external conditions such as weather? This is an extremely interesting question.

Events in nature such as the emergence of flying ants can be triggered by the time of year as well as environmental conditions. Animals often use changes in day length to determine what time of year it is. But we’re pretty certain that flying ants are also flexible in response to environmental factors. They will almost certainly choose suitable weather conditions to begin their flights. Be on the look out, flying ant day is probably not far away.

Photo courtesy of Roger Key and Buglife.

Elva J. H. Robinson, Ofer Feinerman and, & Nigel R. Franks (2012). Experience, corpulence and decision making in ant foraging Journal of Experimental Biology DOI: 10.1242/jeb.071076

Plants’ Reaction to Rain is Close to Panic, Study Shows

Complex chemical signals are triggered when water lands on a plant to help it prepare for the dangers of rain, according to a new study published in the Proceedings of the National Academy of Sciences.

Van Moerkercke et al made the surprising discovery that a plant’s reaction to rain is close to one of panic. Image credit: Anthony, Inspired Images.

In contrast to humans, plants cannot feel pain. However, so-called mechanical stimulation — rain, wind and physical impact from humans and animals — contributes to the activation of a plant’s defense system at a biochemical level. This in turn triggers a stress hormone that, among other things, can lead to the strengthening of a plant’s immune system.

“As to why plants would need to panic when it rains, strange as it sounds, rain is actually the leading cause of disease spreading between plants,” said University of Western Australia’s Professor Harvey Millar, co-author of the study.

“When a raindrop splashes across a leaf, tiny droplets of water ricochet in all directions. These droplets can contain bacteria, viruses, or fungal spores.”

“The sick leaves can act as a catapult and in turn spread smaller droplets with pathogens to plants several feet away. It is possible that the healthy plants close by want to protect themselves,” added study lead author Dr. Olivier Van Aken, a biologist at Lund University.

In lab experiments, Dr. Van Aken, Professor Millar and their colleagues used a common plant spray bottle set on a soft spray.

Arabidopsis thaliana plants were showered once from a distance of 6 inches (15 cm) after which the researchers noticed a chain reaction in the plant caused by a protein called Myc2.

“When Myc2 is activated, thousands of genes spring into action preparing the plant’s defenses,” Professor Millar explained.

“These warning signals travel from leaf to leaf and induce a range of protective effects.”

“Our results show that plants are very sensitive and do not need heavy rain to be affected and alerted at a biochemical level,” Dr. Van Aken said.

The findings also suggest that when it rains, the same signals spreading across leaves are transmitted to nearby plants through the air.

“One of the chemicals produced is a hormone called jasmonic acid that is used to send signals between plants,” Professor Millar said.

“If a plant’s neighbors have their defense mechanisms turned on, they are less likely to spread disease, so it’s in their best interest for plants to spread the warning to nearby plants.”

“When danger occurs, plants are not able to move out of the way so instead they rely on complex signaling systems to protect themselves.”

“It was clear plants had an intriguing relationship with water, with rain a major carrier of disease but also vital for a plant’s survival,” Professor Millar concluded.

Alex Van Moerkercke et al. A MYC2/MYC3/MYC4-dependent transcription factor network regulates water spray-responsive gene expression and jasmonate levels. PNAS, published online October 29, 2019 doi: 10.1073/pnas.1911758116

Staying In

Many insects can sense atmospheric pressure differences. Honey Bees for instance, just stay home if they sense a storm coming. Other bees, like Mason Bees may stay out and forage in light rain but will take shelter when it starts raining too heavily or the wind gets too intense.

Since the water can weigh them down, it’s harder for insects to fly when it’s cold and the rain can damage their wings, many insects just seek shelter. After a particular rainy afternoon in Ecuador, I looked under a small leafy plant and found several butterflies, hunkered down, just waiting for the storm to pass.

Some resting butterflies I found after a big storm in the Ecuadorian rain forest.
PC: Nancy Miorelli

Also, after a particularly wet morning, I found some Red Flat Bark Beetles (Cucujus clavipes) hiding away under some bark, clearly disgruntled that I disturbed their slumber.

Can you put the covers back?
PC: Nancy Miorelli

Where do ants go when it rains?

Have you ever wondered where insects go when it rains? We have all seen a poor unfortunate spider washed down the plughole so we know how vulnerable they are to rushing water. Surely then, isn't rain one of their worst enemies?

Sorry, but this is one of these "it depends" things. It depends on the volume of the rain and the insect. If rain is light to moderate, most insects will take this in their stride. Just like us, they will take shelter. You may find insects under leaves or rock crevices. If the rain is light enough they may be quite happy to stay out and even enjoy it a little.

If the rain is heavy then things can be quite different. Insects that frequent water more often, like mosquitoes and water skaters will negotiate rising, flooding and flowing water with more ease. Those insects that are more used to dry land will be the most affected. Larger insects will cling to whatever shelter they can find until they are eventually washed along by running water. Depending on the insect, they will configure themselves to float on the water whilst protecting themselves. In general is not common for insects to drown. Many will simply be displaced and find themselves in new surroundings. Some, though, will inevitably perish.

Small burrowing insects - like ants - are good at finding air pockets in underground burrows, even during flooding and flowing water. They require very little oxygen and can survive for weeks using air pockets that are always available even in densely flooded areas. Once the waters subside there will be a high rate of survival amongst small insects that have found these air pockets, though ants, for example, will probably go about finding a new drier nest at the earliest opportunity.

It is thought that insects can "sense" the onset of very wet weather and make plans before us humans do. It is often observed in monsoon and rainy areas, that prior to an onslaught of wet weather, some buildings are invaded by insects looking for shelter. Of course, your house or businesses is an ideal place for ants to invade should some inclement weather come along!

Vernon Stent is the author of this article.

Article Source:

Furthermore, when areas become flooded many ants tend to "hold" onto grass blades ect. and other ants onto them. Almost like a floating ant island. They will move in a rotation, constantly moving. I have personally obseved this and seen hundreds of ants in small bundles climbing ontop of each other making an "ant ball" as they hold onto whatever they can. This was obseved in Florida after a storm in a park where much of the park was flooded.

How Ants Secretly Damage Rainforests: Some 'Predatory' Ants Are Vegetarian Conspirators With Sap-Sucking Insects

Some tree-dwelling ants that were thought to eat other insects and animals actually look like plant-eaters when biologists analyzed nitrogen in their bodies. The study revealed the ants mostly eat plant nectar and the "honeydew" excreted by parasitic insects that suck sap from trees, suggesting the ants indirectly cause far more damage to tropical rainforests than previously believed.

The new study in the May 9 issue of the journal Science explains the mystery of why ants are so abundant in rainforest canopies. And it shows that "we have vastly underestimated the amount of resources that individual trees are losing to these ants and their sap-feeding associates," known as scale insects and treehoppers, said Diane "Dinah" Davidson, principal author of the study and a professor of biology at the University of Utah.

"They [ants] are draining water, carbohydrates and amino acids -- the building blocks of proteins -- out of the plants. The plants don't grow as much and they may die," especially during droughts.

Global warming already threatens to retard plant growth in rainforests, but to fully understand the impact of climate change on shrinking tropical rainforests, "we have to understand all sources of plant loss, and this [plant damage by supposedly carnivorous ants] is a huge loss we haven't recognized before," Davidson says.

The research -- which analyzed ratios of rare nitrogen-15 to common nitrogen-14 in ants, other insects and plants -- provided some surprising answers to the question of who eats what and whom in tropical rainforests on opposite sides of the world in Peru and Brunei, a nation that shares the island of Borneo with portions of Malaysia and Indonesia.

Mass spectrometers weigh atoms, and can distinguish the rare isotope or form of nitrogen from the common form. The process by which organisms change living tissue into energy and waste creates a higher nitrogen-15-to-nitrogen-14 ratio the higher an organism is on the food chain. So plants have the lowest ratios, while predators have the highest ratios.

Yet Davidson and colleagues found the ratios were low in ants known as dolichoderines and formicines -- both of which once were thought to get most of their nitrogen nutrients by preying on or scavenging other insects and animals. But their low ratios did not resemble the higher ratios of truly carnivorous ants that eat prey and scavenge dead animals. Instead, their nitrogen ratios were typical of leaf-chewing and sap-sucking insects and even plants themselves.

The study indicated the dolichoderine ants indirectly feed on trees and shrubs, even though they themselves do not eat plant leaves. Instead, they eat sugar-rich honeydew excreted by parasitic insects known as scale insects and treehoppers that are related to aphids and use needle-like mouth parts to suck sap from leaves and twigs. Scale insects resemble a small circular scar on a branch or leaf. Treehoppers often resemble a thorn on a plant. Others look somewhat like tiny sails.

"Plants have nasty chemicals to protect them against leaf-chewing herbivores like certain insects and monkeys," says Davidson. "But the sap-suckers [treehoppers and scale insects] are unique because they bypass those chemical defenses. They have a little tiny soda straw for a mouth part and they use that to suck sap from plants."

Birds, wasps and flies try to eat the sap-sucking treehoppers and scale insects, but dolichoderine ants protect them, and in exchange eat their honeydew, which is rich in sugar and amino acids. "That means the sap-suckers can become incredibly abundant," Davidson says. "The sap-suckers that are tended by ants are among the most devastating herbivores of the rainforest. They can kill even large trees."

Because abundant dolichoderine ants use this indirect method of extracting nutrients from trees and shrubs, it means ants damage or kill more rainforest plants than was believed.

"Tropical rainforests are the greatest repository of terrestrial biodiversity on Earth, and yet we are just beginning to unravel the secrets of their inaccessible canopies," Davidson says. "Ants are a great example of this. Ants hold dominion over the canopy in terms of their numbers, biomass and, probably, functional significance. We have long assumed they fed as omnivores, obtaining much of their protein through hunting and scavenging of animal prey -- i.e., though carnivory."

Davidson says that for many years, scientists believed canopy ants primarily helped trees and shrubs by killing and eating leaf-chewing insects. In exchange, the plants secreted nectar and fat-rich "pearl bodies" to feed the ants.

But the study shows "we've assumed too much that ants are helpful to the trees and shrubs by killing and eating other insects that eat leaves," she says. "There is some of that going on. But the vast majority of ants, the ones that are superabundant in the rainforest canopy, are tending sap-feeding insects [by eating their honeydew and protecting them]. The sap-feeding insects and the ants that tend them are parasites of those plants."

Davidson, who is visiting Peru in May, says she has been doing research there for 20 years at the Cocha Cashu Biological Station. In areas with large colonies of particularly damaging ants, "you can just look around and see dead trees everywhere," she says.

The study gave hints as to why some canopy-dwelling ants -- known as formicines because they produce formic acid -- also have nitrogen signatures characteristic of plant-eaters even though Davidson says they do protect trees from leaf-eating insects. These tree-helping ants feed on "extrafloral nectar" produced on leaves, fruits and the bases of flowers. Scientists assumed they also ate insects because the nectar is not rich enough in essential amino acids.

But Davidson speculates that many formicine ants get adequate nutrition because bacteria in their guts upgrade the nitrogen in nectar to essential amino acids that are missing in nectar but are needed for survival. She notes that bacteria in the guts of treehoppers and scale insects do the same thing, producing nutrient-rich honeydew for the dolichoderine ants.

While dolichoderine ants feed indirectly on trees by eating sap-sucker honeydew, the formicine ants eat nectar, so the trees lose resources even to their protectors.

"The total cost to the forest of herbivory is a lot larger than once thought because some of the plants are using a lot of resources [nectar] to feed ants that protect them from herbivores, and the others are losing a lot of resources to parasites [like scale insects and treehoppers] that are feeding ants," Davidson says.

She says the presumption that tropical rainforest canopy ants were mainly carnivorous first was questioned in the early 1990s by a Harvard University student who noted how abundant ants were in rainforest canopies. Ants are far more abundant than any other insects in the rainforest canopy. That seemed to violate a general rule that populations of carnivores should not outnumber prey populations.

Davidson's study also found ground-dwelling, carnivorous army ants and canopy-dwelling weaver ants had nitrogen isotope signature typical of predators.

Davidson notes that weaver ants have been used in China for 1,700 years to suppress herbivores that feed on citrus trees. Because isotope studies of ants and other insects can reveal what species rank where in the food chain, the method shows "who is most predatory," she says. "That's the ant you want to transfer to your agricultural field."

Davidson conducted the study with Steven C. Cook, a biology doctoral student at the University of Utah Roy R. Snelling, an entomologist at the Natural History Museum of Los Angeles County and Tock H. Chua, an entomologist at the University of Brunei Darussalam.

Biologists long have used mass spectrometers to analyze stable isotopes (which do not decay radioactively) and answer ecological questions such as those addressed in Davidson's study. In the last few years, however, University of Utah biologist Jim Ehleringer has pioneered the use of stable isotope analysis to help federal agencies track the sources of counterfeit money, drugs like cocaine and heroin, explosives and biological warfare organisms. Davidson's study used mass spectrometers in Ehleringer's laboratory.

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Materials provided by University Of Utah. Note: Content may be edited for style and length.

Watch the video: Fire Ants vs. Flood. What Happens to Ants When It Rains? (November 2021).