Evolution and Biodiversity

Category: warm, cold

Desert ant builds landmark

Nest hill helps ants to return home in barren salt flats

desert ant Cataglyphis fortis has outstanding navigation skills

Often nothing is visible around a nest of the desert ant Cataglyphis fortis that could help foraging workers to return to the nest. In that case, the ants make a landmark themselves, Marilia Freire and colleagues show.

A foraging trip is a survival trip for the desert ant Cataglyphis fortis, which lives in salt pans in Tunisia; salt pans are vast bare plains where there was once water, but now only a salt crust remains. Ant workers venture out individually to search that barren plain for insects and other small critters that have succumbed to the relentless desert heat. After founding something, they must return to the nest with the loot between their jaws as quickly as possible, otherwise they will succumb themselves.

But the entrance to the underground nest is barely visible. That is why the ants build a landmark, when necessary, Marilia Freire and colleagues discovered.


Food is scarce, so foraging desert ants often must move far from the nest to find something. They venture up to 350 meters away. Because they have excellent navigation skills, they usually return safely.

When going out foraging, a worker constantly uses an internal sun compass to keep track of the direction in which she is walking and with a kind of pedometer she measures the distance she covers in that direction. When she finds food, she has usually followed a tortuous path, but thanks to this so-called path integration she can walk back to the nest in a straight line, i.e., via the shortest possible route. At least: she closely approaches the nest.

When within a few meters, she needs visible clues to find the exact place of the nest entrance, because path integration doesn’t work perfectly. The farther an ant has gotten from the nest, the more uncertainty creeps into the route back and thus the greater the chance is that she has to search for too long and succumbs. For the very last bit of the trip homewards, she relies on nest smell.

But in the middle of a salt pan, there is nothing to be seen at all. What to do in this case?


desert ant builds nest hill when no other visual landmarks are aroundFreire and the other researchers had noticed that the desert ants often build a hill at their nest, and that nest mounds in the middle of a salt pan are higher than on the edge, where some shrubs grow. A nest mound in the middle of a salt pan is 12 centimeters high on average (the highest they found was 30 centimeters), a nest mound at the edge only 5. So, they wondered if the mounds might serve as visual landmarks for workers returning from a foraging trip.

To find the answer, they captured ants at the nest and placed them at a distance of a few meters. Since the ants had not walked themselves, they could not use path integration. But they were placed at distances where they normally must be guided by landmarks to find the nest entrance anyway. The researchers had removed the mounds at some of the nests to see if that made a difference.

That turned out to be the case, especially for nests in the middle of a salt pan. Without a mound, ants were not able to walk directly to such nest and more often failed to find it at all. The hills therefore serve as landmarks. Next question: do ants build them specifically for that purpose? It is possible that the mounds have another main function, such as regulating the nest temperature.

Only when needed

But the researchers show that the desert ant does build its mounds as landmarks by conducting experiments in which they removed the mounds at sixteen nests in the middle of a salt pan. At eight of those nests, they placed artificial landmarks, namely two black cylinders. Three days later, the ants were found to have built a new mound at some of those sixteen nests, especially at nests without artificial landmarks, and at those nests, the mounds were taller.

Conclusion: desert ants build mounds near their nest as landmarks for foraging workers. But they only make the effort if there are no other landmarks visible, such as bushes or, in the trials, black cylinders.

Willy van Strien

Photos: ©Markus Knaden
Large: Cataglyphis fortis
Small: nest mound in the centre of a salt pan

Freire, M., A. Bollig & M. Knaden, 2023. Absence of visual cues motivates desert ants to build their own landmarks. Current Biology 33: 1-4 (31 May online). Doi: 10.1016/j.cub.2023.05.019
Steck, K., B.S. Hansson & M. Knaden, 2009. Smells like home: desert ants, Cataglyphis fortis, use olfactory landmarks to pinpoint the nest. Frontiers in Zoology 6: 5. Doi: 10.1186/1742-9994-6-5
Wittlinger, M., R. Wehner & H. Wolf, 2007. The desert ant odometer: a stride integrator that accounts for stride length and walking speed. The Journal of Experimental Biology 210: 198-207. Doi: 10.1242/jeb.02657
Wehner, R., 2003. Desert ant navigation: how miniature brains solve complex tasks. Journal of Comparative Physiology A 189: 579-588. Doi: 10.1007/s00359-003-0431-1


Ants translocate their larvae and pupae to a warm bird’s nest

Myrmica ruginodis translocates brood to a wood warbler's nest

In the nest of a wood warbler not only young wood warblers may grow up, but also ants, as Marta Maziarz and colleagues discovered. Ant larvae and pupae probably survive, grow and develop better in the bird’s nest than in their own nest.

Usually, nests of European forest ants, especially Myrmica ruginodis and Myrmica rubra (the European fire ant or common red ant), are so cold in spring that the larvae and pupae do not grow well. Their development only starts at 16°C, but ant nests rarely get that warm before summer. They are located on the forest floor, between fallen leaves of deciduous trees. The ants cannot produce heat, so without sunlight the nests have the same temperature as the environment. A temperature of 20 to 25°C is optimal for raising brood; the nests never reach that temperature in spring.

But warm places are available nearby, Marta Maziarz and colleagues show. On the forest floor, the wood warbler, a songbird that breeds in European forests, makes a domed nest of grasses, leaves and moss. The bodies of bird parents and, later on, their fully feathered young keep the nest warm.


When the bird parents incubate the eggs, in the second half of May, the nest temperature often reaches 16°C or more; especially on cold days, the difference with the ambient temperature is large. Once the young birds are fully feathered, in the first week of June, the nest temperature even rises to 20°C and higher.

This is a nice temperature for the ants, which indeed visit the warm wood warbler’s nest. In cold weather in May and in the first week of June, they move larvae and pupae from their own nest to a bird’s nest and put them in the sidewalls. Translocation is a lot of work, but apparently, it is worth the effort.

After fledging, the vacant nest cools down again. But the ants don’t remove their brood immediately; they delay relocation for up to two weeks. There is no hurry, because the bird’s nest is no longer warm, but it is not colder than the ants’ nest either.

In the primeval forest of Białowieża in Poland, where Maziarz conducted the research, the ants use 10 to 30 percent of wood warblers’ nests as incubators. The birds do not suffer from the inhabitants, but they do not benefit from them either. Therefore, they do not specifically seek the proximity of ant colonies when starting nest building. It is the ants that start the relationship and benefit from it.

Willy van Strien

Photo: Myrmica ruginodis with brood. Jan Anskeit (Wikimedia Commons, Creative Commons CC BY 4.0)

Maziarz, M., R.K. Broughton, L.P. Casacci, G. Hebda, I. Maak, G. Trigos‑Peral & M. Witek, 2021. Interspecific attraction between ground‑nesting songbirds and ants: the role of nest‑site selection. Frontiers in Zoology 18: 43. Doi: 10.1186/s12983-021-00429-6
Maziarz, M., R.K. Broughton, L.P. Casacci, A. Dubiec, I. Maák & M. Witek, 2020. Thermal ecosystem engineering by songbirds promotes a symbiotic relationship with ants. Scientific Reports 10: 20330. Doi: 10.1038/s41598-020-77360-z

A large bill to cool down

Tufted puffin may be overheated after flying

Flying is strenuous in tufted puffin

Flying is strenuous for a tufted puffin and it causes high heat production. After landing, the bird has to cool down. The bill is used to dissipate excess heat, Hannes Schraft and colleagues report.

Puffins get their food from sea. They have relatively short wings that enable them to dive and swim under water, looking for fish and other prey. But the wings are less suitable for flying. To stay airborne and move forward, the wings have to flap in high frequency; the breast muscles work hard and much heat is produced.

Outside the breeding season, puffins usually stay at sea. But when they have a young to raise, they have to travel regularly between nest and sea. When a puffin approaches the nest with a mouth full of fish to feed its young, it is often overheated.

bill helps tufted puffin to cool downThe birds have a strikingly large bill, which helps to get rid of excess heat during and after the flight, as Hannes Schraft and colleagues show. The object of their research was the tufted puffin, which breeds along the northern Pacific Ocean, for instance on the coasts of Alaska and Kamchatka and on the Kuril Islands.

Network of blood vessels

With a special camera, the researchers made infrared images of birds that rested after landing; they did so from a distance of five to ten meters, in order not to disturb the animals. They took images of bill and back every two minutes. From these images, they were able to calculate temperature and heat exchange.

The temperature of the back remained constant, but the bill gradually cooled down; after half an hour its temperature had decreased by about 5°C. Also the heat exchange gradually decreased after landing, and the portion dissipated via the bill became smaller; a puffin also loses heat through legs and feet. Apparently, the bill is most important for cooling shortly after flight.

The bill dissipates heat thanks to an extensive network of blood vessels; warm blood cools down a bit when circulating through these vessels, just as in a cassowary’s helmet.

The tufted puffin bears the white-yellow tufts to which it owes its name only during the breeding season, when both males and females are decorated. A pair produces one young per year, and the tasks are divided: both mom and dad will breed and feed their young.

Willy van Strien

Large: Kuhnmi (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: Matthew Zalewski (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

See how the cassowary manages to stay cool

Schraft, H.A., S. Whelan & K.H. Elliott, 2019. Huffin’ and puffin: seabirds use large bills to dissipate heat from energetically demanding flight. Journal of Experimental Biology 222: jeb212563. Doi:10.1242/jeb.212563

Staying cool

Southern cassowary dissipates excess heat via its helmet

thanks to its helmet, the southern cassowary can offload excess heat

How can the tropical southern cassowary stay cool at high temperatures? Danielle Eastick and colleagues show that it uses its helmet to prevent overheating.

Just because of its size and strong legs – with a dangerous dagger-like claw that can be 10 centimetres long – the southern cassowary is an impressive bird. It also wears a prominent helmet or casque. What could be its function?

Until now, that was a mystery. Perhaps the helmet amplifies the deep sounds that the bird can produce, some people thought. Or perhaps it is a decoration to seek the attention of possible partners, next to the blue head and the blue and red wattled neck, according to another assumption. Otherwise, it may protect the head when the bird is moving through dense vegetation at high speed, or involved in a fight.

But now, Danielle Eastick and colleagues come up with a different answer.

Easily overheated

The southern cassowary lives in tropical forests of New Guinea and Australia. Being a large and dark animal, it can easily become overheated at high temperatures, so it must have the possibility to offload heat. Eastick hypothesized that the helmet might offer that possibility and set out to test this idea. It turned out that she was right.

The helmet consists of fragile, spongy bone and is partly hollow; it is covered with a horn layer and has an extensive superficial network of blood vessels. Infrared images taken by a special camera revealed that the blood vessels dilate at high temperatures, and the helmet gets warm. Heat can be offloaded to the air. But when it is cold, the blood vessel walls constrict and only a small amount of blood flows into the helmet. It cools down while the heat of the animal is preserved.

The legs and the tip of the beak contribute to temperature regulation in the same way, but the helmet plays the most important role. When it is very hot, a cassowary sometimes plunges its head into the water to lose more heat.


The possibility of thermoregulation had already been suggested earlier, but it was never studied extensively. There are some other tropical birds with a helmet that may help to offload heat; for instance, some hornbills have a helmet on the beak. And perhaps the helmet of some dinosaur species facilitated heat loss as well.

The fact that the helmet of the cassowary has a function for thermoregulation does not exclude that it also plays a role in partner choice. Although, to be honest, its design is not very impressive.

Willy van Strien

Photo: Paul IJsendoorn (Wikimedia Commons, Creative Commons CC BY 2.0)

Eastick, D.L., G.J. Tattersall, S.J. Watson, J.A. Lesku & K.A. Robert, 2019. Cassowary casques act as thermal windows. Scientific Reports 9: 1966. Doi: 10.1038/s41598-019-38780-8
Naish, D. & R. Perron, 2014. Structure and function of the cassowary’s casque and its implications for cassowary history, biology and evolution. Historical Biology 28: 507-518. Doi: 10.1080/08912963.2014.985669
Phillips, P.K. & A.F. Sanborn, 1994. An infrared, thermographic study of surface temperature in three ratites: ostrich, emu and double-wattled cassowary.  Journal of Thermal Biology 19: 423-430. Doi: 10.1016/0306-4565(94)90042-6

Cooling down

Blowfly blows bubbles to prevent overheating

blowfly cools down by bubbling behaviuour


A blowfly often extrudes a liquid bubble between its mouth parts and then takes it back. By exhibiting this bubbling behaviour, it gets rid of excess heat, Guilherme Gomes and colleagues show.

How can a buzzing blowfly avoid getting overheated? Few people will ever have wondered, but as it happens, Guilherme Gomes and colleagues did. And they discovered that the oriental latrine blowfly Chrysomya megacephala manages to lower its body temperature by blowing a bubble.

At high air temperatures, a blowfly can expel a drop of liquid out of its oral cavity and hold it with its mouthparts. As some liquid evaporates, the droplet will rapidly cool down, whereupon the fly will take it in. The cycle is often repeated, and a droplet may tidally move out and back up to six times until eventually the fly swallows it and its body temperature decreases. The liquid is a complex mix of fluids derived from ingested meals and saliva.

Daytime and night-time

The blowfly applies this trick during the day when ambient temperature exceeds 25 °C. At that temperature, the animal is warmed-up and busy, so that its muscles produce a lot of heat, which makes cooling necessary. When it gets really hot, above 30 °C, the fly becomes inactive and no longer generates heat. It then stops blowing bubbles.

At night, it bubbles more than during the day to stay cool, in order to decrease its metabolism and save energy.

If air humidity is high, the liquid will not evaporate well and a bubble will not cool down. If the fly still expels a drop, it will not re-ingest it, but spit it out instead.

Small animals only

Cooling down by extruding a droplet is only feasible in small animals, and a number of insect species seem to exhibit such behaviour. For larger animals, it is impossible to produce and handle a liquid drop that is large enough for a cooling effect. To us, it would make no sense to trying it – fortunately. We cool down by sweating, which is impossible to an insect because of its chitinous exoskeleton and wax covering.

Willy van Strien

Photo: Blowfly Chrysomya megacephala. gbohne (via Flickr, Creative Commons CC BY-SA 2.0)

Gomes, G., R. Köberle, C.J. von Zuben & D. V. Andrade, 2018. Droplet bubbling evaporatively cools a blowfly. Scientific Reports 8: 5464. Doi: 10.1038 / s41598-018-23670-2