There鈥檚 more than one way to cut emissions. Swapping conventional cars for electric ones might cut emissions from cars, but it may also help cities like Beijing beat the heat, further lowering emissions of carbon dioxide by cutting the use of air conditioning.

The role of electric cars in combating climate change is hotly debated, because although they might emit less greenhouse gas directly, they are more expensive and cause more pollution than conventional cars during the manufacturing process. But it seems there may be hidden benefits to them.

Electric cars emit less than 20聽per cent of the heat that conventional cars do, thus contributing less to the phenomenon of cities being warmer than the surrounding areas, known as the urban heat island effect.

If this lowers temperatures, use of air conditioning would also drop, and so would the energy used to power it, argues a team led by from Michigan State University in East Lansing.

Cooler summer

Using summer 2012 in the Chinese capital of Beijing as an example, the team estimate that replacing conventional cars with electric ones could have reduced the heat by nearly 1聽掳C. That in turn would result in a reduction in air conditioning use in the city, leading to a drop of 10,686 tonnes in daily CO₂ emissions.

鈥淚 was surprised by the level of decreased energy consumption of air-conditioning,鈥� says Li.

The team says that there are several factors contributing to the urban heat island, and not all have been addressed in the study. This makes some people sceptical about the estimates.

鈥淚nvestigating a mitigation strategy is much more complicated than just considering the heat flux,鈥� says of the Heat Island Group at Concordia University in Montreal, Canada. 鈥淭he sophisticated relation between moisture balance, effect of wind, and interactions of buildings and atmosphere should be accounted for.鈥�

And of the Swiss Federal Institute of Technology in Zurich says that our heat emissions are only partly responsible for the heat island effect, it is also down to the city retaining that heat thanks to tarmac surfaces, which don鈥檛 cool down quickly.

Cars of the future

Regardless of how big any cooling effect might be, the climate effects of urban transport merits further study with more sophisticated models, say other researchers.

鈥淚t is an interesting study, as the issue of heat generated by vehicles isn鈥檛 something that has been thought about in great detail,鈥� says of Duke University in Durham, North Carolina.

It could become an ever bigger influence as electric cars catch on. Johnson expects the vehicles to dominate the transportation market in a few more decades once they overcome the technical challenges of driving range and battery charging infrastructure.

Unlike with conventional cars, which depend on non-renewable fuels, the disadvantages of electric cars can be overcome, says Li.

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Clarification, 20 April 2015: The amount of heat generated by electric cars has been clarified since this article was first published.

The smaller male placates his mate to avoid becoming her post-coital dinner (Image: Matja啪 Gregori膷)

Species: Caerostris darwini
Habitat: Across bodies of water in Madagascar

Mating can be a deadly act for spiders 鈥� females often end it by eating their partners.

But the Darwin鈥檚 bark spider may have found an unusual way around this risk: keeping the juices flowing. During copulation, males orally lubricate females鈥� genitals. The reason may be to prevent themselves from becoming a meal.

Darwin鈥檚 bark spiders build some of the largest webs, and they do so using silk that is ten times as tough as Kevlar, but their mating behaviour was largely unknown. So, during a two-week survey in Madagascar, a group of researchers led by Simona Kralj-Fiser at the Slovenian Academy of Sciences and Arts tried to demystify the spiders鈥� sex lives.

鈥淒arwin鈥檚 bark spiders are a rather enigmatic species, but even we were in for a surprise when we observed their peculiar courtship behaviour,鈥� says Kralj-Fiser.

Spider kiss

The team observed that females mate with multiple males, and that males are ultra-competitive as a result. They also found that 76 per cent of females overall were aggressive towards males, with 35 per cent cannibalising them after copulation. So males need a tactic to prevent getting eaten 鈥� and one way is to linger around younger females.

鈥淢ales cohabitate with young females until their final moult to adulthood. While a female is young, soft and defenceless, the guarding male can copulate with her for a long period of time. When a female鈥檚 cuticles harden and she can move and attack, she is able to prevent long copulations,鈥� says Kralj-Fiser.

So sticking with young females that can鈥檛 yet turn cannibal is a good way for males to avoid becoming dinner, but what about when they mate with adult females?

During their normal mating routine, the males attach their chelicerae 鈥� the first pair of fang-like appendages near the mouth 鈥� to females鈥� epigyne, the external genital structure. But when males moved their mouthparts away from adult females鈥� genitals during courtship, researchers could see an unusual drop of liquid on the epigyne. This behaviour wasn鈥檛 seen when males mated opportunistically with young females.

鈥淢ales nibble on female external genitals using their fangs, and then we observed that there was a liquid coming out of the fangs. We do not know what this liquid is, but it looks like digestive juices, which they usually secrete when eating,鈥� says Kralj-Fiser, who presented the study at the Ethological Society鈥檚 鈥淐auses and consequences of social behaviour鈥� conference in Hamburg, Germany, last week.

Web of desire

Kralj-Fiser suggests the oral lubrication relaxes adult females so they are less likely to engage in sexual cannibalism 鈥� which would explain why the males don鈥檛 make such an effort with the younger females that are unable to eat them.

Other species have their own strategies to deal with female aggression. For example, males of Nephila pilipes calm females by giving them 鈥渂ack rubs鈥� and depositing silk on their bodies. Male Darwin鈥檚 bark spiders one-up this species: they engage in what researchers described as intensive courtship behaviour, employing mate binding tactics as well as the oral lubrication trick.

鈥淥ral lubrication may be one way the female evaluates the male quality, or it may have to do with the 鈥榩lug鈥� that the male will subsequently place in the female genitalia, making it harder for other males to mate,鈥� says Ingi Agnarsson at the University of Vermont, who discovered the species in 2010.

European rabbits are in decline in rural areas, yet holding strong in cities. But urban living also comes with a changed lifestyle: instead of being packed into complex burrows, city rabbits seem to live a more solitary life in smaller burrows.

from the University of Frankfurt, Germany, and her colleagues studied the burrow structures of several populations of European rabbits from urban, suburban and rural sites in and around Frankfurt.

They found that the closer to the city the burrows were, the smaller, simpler and less populated they became. 鈥淚 did not expect to find such a clear correlation,鈥� says Ziege.

So why are urban rabbits downsizing? For one thing, rabbits usually form larger groups when food and burrowing sites are in short supply. Surprisingly, this is not the case in cities, where rabbits may colonise parks, for example.

鈥淐ities are providing a constant and high food supply through human waste and deliberate feeding, as well as access to vegetation cover, such as shrubs,鈥� says Ziege. 鈥淢any areas in modern cities are often structurally highly diverse and the urban rabbit population could be benefiting from this.鈥�

In contrast, she says, rural areas in Germany are now typically characterised by open landscapes with scarce vegetation cover.

Predator pressure

Living in large groups also helps rabbits conserve heat in winter, but there is less need to do that in cities where the microclimate tends to be warmer. Then there is the threat from predators.

鈥淚n rural areas, rabbits might be exposed to higher predator pressures and are therefore forced to live in large burrows,鈥� says N茅stor Fern谩ndez of the Do帽ana Biological Station in Seville, Spain. Complex burrows with many exits help rabbits survive attack, but are clearly a major undertaking. 鈥淏uilding burrows requires investing a lot of energy, and individual rabbits can increase their fitness living and contributing to expand existing burrows, particularly under high predator pressure.鈥�

It鈥檚 unclear whether similar patterns in burrow types will be found across Europe, but Ziege says more research could give us key insights into successfully managing rabbit populations and avoiding damage they may cause to vegetation and even buildings.

European rabbits are listed as , with populations in decline especially in Spain and Portugal, which they are native to.

Cities have the potential to serve as new habitats for wildlife and in the case of Germany they might even play an important role by acting as a 鈥渟ource鈥� from which rabbits can recolonise rural areas, Ziege says.

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Video: Judo spider tackles armoured foe

Harvestman (right) versus predator

Species: Loxosceles gaucho
Habitat: Human-made or disturbed natural environments in Brazil and Tunisia

In a fight between an armoured soldier and an unarmed opponent, who would win? When it comes to the recluse spider the answer might surprise you. Using its wits and speed, it can kill and eat one of its toughest prey: another arachnid, the armoured harvestman.

Harvestmen have a hard exoskeleton that protects them against several spiders, which are their main predators alongside birds and amphibians. from University of S茫o Paulo, Brazil, and his colleagues found that even large predator spiders have difficulty piercing this armour.

For the attacker to have a half-decent chance, it needs to pin its prey between its fangs. 鈥淏ut this rarely happens, and usually both fangs slide on the surface of the harvestman鈥檚 body,鈥� says Willemart.

Spiders have been seen attacking and eating harvestmen in the wild, but there are so few observations that harvestmen are generally left off lists of their frequent meals.

Dressed like Batman

鈥淗arvestmen are somehow successful in avoiding predation from spiders, and it is well known many harvestman species exhibit several lines of defence,鈥� says , also from University of S茫o Paulo.

Those defences range from feigning death, which puts off most predators that feed on live prey, to releasing chemical irritants to repel attackers.

But Willemart and his colleagues found that armoured harvestmen seldom go for the chemical option, presumably because these compounds are costly to make. Instead, they rely on their exoskeleton for protection. But this backfires when recluse spiders exploit flaws in the armour鈥檚 design.

Machado likens it to a Batman suit. When Batman asked for a more flexible suit, his business manager, Mr Fox, told him that more movement would imply more exposure to weapons such as knives, he says. 鈥淥f course, this lesson is relevant not only for the prey, but also for the predators,鈥� says Machado.

Willemart and his team found carcasses of harvestmen in the webs of recluse spiders in Brazil. These successful hunters were then brought to the lab, where 31 out of 38 spiders found a way to kill and then eat the harvestmen offered. So how do they do it?

Judo manoeuvre

The recluse spider carefully approaches and repeatedly feels out the harvestman with its own legs, looking for weak areas. The recluse spider can outrun any escape attempt, and may then do what Willemart likens to a judo move, pinning the harvestman鈥檚 back to the ground. Finally, it delivers the death blow: a series of poisonous bites in the exact areas not shielded by armour.

鈥淩ecluse spiders are exceptional in that they do not try to pierce through the armour. They simply avoid it and bite the soft parts of the harvestman,鈥� says Willemart.

Although we now know how recluse spiders prey on harvestmen, much remains to be discovered.

鈥淒o they use this strategy for all arthropod prey?鈥� asks at the University of Nebraska, Lincoln. 鈥淚s this targeted biting behaviour learned? Is it innate?鈥�

If this is a learned behaviour, Hebets suggests researchers can explore how they learn it in the first place and remember it for future use. She says spiders are great subjects for lab study of mechanisms of complex behaviours in animals, as they are easy to catch and exhibit complex behaviour themselves.

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Hearing people munching on a snack and rustling the packet in a cinema is annoying, but for some bats the sound of other bats about to eat is a vital clue to locating food.

Detecting tasty insects by echolocation limits bats to 鈥渟eeing鈥� to a distance of nearly 10 metres. But if they plug into the sonars of bats around them, they can hear when another bat has found insects up to 160 metres away. This is because loud bat hunting calls carry much further than sound reflecting off a tiny insect.

鈥淲hen a bat is attacking prey it emits a typical sequence of calls so any other bat within 100 metres knows that someone found food. I call this the bag of chips effect, because when someone in a dark cinema opens a bag of chips everybody knows it,鈥� says Yossi Yovel from Tel Aviv University in Israel.

Yovel鈥檚 team mounted tiny GPS devices and ultrasonic recorders to track greater mouse-tailed bats (Rhinopoma microphyllum), which roost in colonies of hundreds or even thousands. They found that the bats spent about 40 per cent of their time hunting within 150 metres of other bats, even though getting too close hampered their chance of catching prey as they then had to focus on collision avoidance instead.

Why then do they hunt in such close proximity? To test this, the team modelled bat behaviour and found that the most likely explanation is the bag of chips effect. The queens of flying ants 鈥� the bats鈥� main prey 鈥� are so sporadically clustered that it may be much easier for a single bat to locate them by eavesdropping on others in a group, essentially expanding their echolocation range more than tenfold.

Trade-off

But there is a point at which this strategy becomes a hindrance. When the number of bats in the group becomes too large, they start to get in each other鈥檚 way. 鈥淲e see by analysing their sonar calls that they respond to the presence of a nearby bat as if it was an obstacle,鈥� says Yovel. 鈥淭here鈥檚 a trade-off.

鈥淏ats might suffer as they constantly need to track other bats, and this makes tracking food difficult. Imagine that you are tracking a fly and a baseball is thrown in your direction 鈥� you will have to stop tracking the fly.鈥�

And some bats may be using sonar calls even more selfishly. A recent study found that Mexican free-tailed bats (Tadarida brasiliensis) could be intentionally jamming each other鈥檚 sonar to prevent rivals from reaching prey.

Yovel doesn鈥檛 think that鈥檚 the case with the greater mouse-tailed bat. An earlier study he carried out suggests that even when two bats were operating at the same frequency, they .

Unjammable bat signal

鈥淚 believe it鈥檚 very difficult to jam a bat. We have tried doing so in the lab and it鈥檚 almost impossible. Bats are highly tuned to recognising their own signal,鈥� he says.

The role and significance of eavesdropping has been difficult to study, as it鈥檚 tough to investigate bats in the wild. In the new study the researchers say they used the smallest GPS trackers available, but still found they were prone to falling off.

鈥淭his interesting study basically puts us for the first time virtually on a bat鈥檚 back,鈥� says Stefan Greif at the Max Planck Institute for Ornithology in Seewiesen, Germany. 鈥淓avesdropping might well be more widespread than we know at the moment and accumulating evidence is supporting this idea.鈥�

Bats could also be eavesdropping to learn , or .

Yovel and his team are now studying five other bat species to see how they forage.

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Whether it鈥檚 making snow angels or building a snow man, most of us love playing in a winter wonderland 鈥� and it turns out some birds do too.

It usually starts with one bird that burrows in the snow, creating tunnels. Another joins in, then a few more, until the entire group is doing it, leaving behind a maze of burrows and furrows.

, professor emeritus at the University of Vermont, studied the peculiar behaviour in Western Maine. He observed a flock of about 150 redpolls (Carduelis flammea) making at least 252 cavities and short tunnels between November 2012 and February 2013, and has just published his findings (Northeastern Naturalist, doi.org/xvj).

But exactly why they do it 鈥� or what triggers the behaviour 鈥� is still a mystery.

Birds dig it

鈥淚 have wondered a lot about what the trigger is, and I have not seen anything obvious,鈥� says Heinrich. Although, he says, there seems to be a social aspect to it. 鈥淲here one does it then a lot of others do too.鈥�

He didn鈥檛 see any evidence that redpolls are seeking food in the snow and there wasn鈥檛 any vegetation near the tunnels. The birds were also unlikely to be bathing, as they were very clean, and they didn鈥檛 appear to be taking shelter 鈥� so were they just having fun?

鈥淧lay is defined as behaviour with no immediate function, so in that sense, yes, it is 鈥榡ust鈥� play,鈥� Heinrich says.

But he suggests snow tunnelling may be adaptive.

鈥淪now tunnelling may have a function way up north in the high Arctic 鈥� their home: the shelter from cold,鈥� he adds.

Spending the night in snow tunnels may help them survive the freezing nights in the Arctic tundra, even though it would also make them vulnerable to burrowing predators, such as short tailed shrew, and could potentially leave them in an ice tomb.

Beaks under blankets

In the Arctic, Redpolls can be quite certain when they duck down under the fluffy snow blanket that it will still be fluffy the next morning because it rarely thaws.

But the snow regularly thaws in Maine, and the icy crust that develops as temperatures drop overnight would trap the birds. 鈥淭he best adaptation becomes the best death trap, if conditions change,鈥� Heinrich says.

The fact that redpolls in Maine don鈥檛 go under the snow at night suggests they have some inhibition to chance it.

鈥淭hey may recognise the forest here is not the Arctic tundra, and maybe also they have some temperature or hunger threshold before they would risk all by spending the night in the tunnel,鈥� says Heinrich.

Reference: Northeastern Naturalist, DOI:

It lurked in wait for unsuspecting prey on the swampy Baltic coastline 35鈥�47 million years ago. Now the first fossilised specimens of a carnivorous plant are helping scientists probe the organism鈥檚 early evolution and its Eocene habitat.

Researchers from the in Germany found the fossil of two leaves from the plant in the Jantarny amber mine near Kaliningrad, Russia. It seems to be related to plants from the Roridulaceae family, which catch their prey using long, sticky hairs.

鈥淲e were all so excited when we discovered it because it鈥檚 very beautiful and striking,鈥� says lead researcher . 鈥淚t鈥檚 amazing to look at something so old, yet so well preserved.鈥�

The fossils were a long way from where this family is endemic: South Africa. 鈥淚t was surprising to find the fossils in Europe. It suggests they were probably more widely distributed than initially thought and later restricted to a few places,鈥� says co-author .

Supercontinent

This plant family is thought to have originated in Africa and became isolated there after the Gondwana supercontinent 鈥� comprising modern-day Africa, South America, Madagascar, India, Australia, the Middle East and Antarctica 鈥� broke apart about 180 million years ago.

The leaves have hairs that could have been used to capture prey. With only two leaves preserved in the amber, it鈥檚 not yet possible to know what the entire plant looked like and what its diet could have been, but the plant family today catches a wide range of arthropods.

The Roridulaceae genus Roridula is a quirk of botanical carnivory, as the plants depend on relationships with other organisms to successfully digest their prey. They trap prey using sticky hairs on their leaves, but depend on a symbiotic species of capsid bug to digest them and then consume their droppings instead. One bug鈥檚 demise is another鈥檚 gain.

鈥淚t really is remarkable to consider that this genus was once so much more widespread, and this seems to indicate that it really is a relic today, hanging on against extinction,鈥� says , a senior botanist at Kew Gardens in London.

鈥淚t gives us a great insight into how much things have changed. It is inconceivable today that oaks and Roridula should co-occur in nature, but obviously they did,鈥� Cheek says. 鈥淭he specimen is so well preserved that it is as if someone had nipped back in a time machine into the Eocene and got it. Who knows what other revelations are on their way through this fossil source?鈥�

Eocene forest

It鈥檚 rare to find plant remains in amber that are well-preserved enough to be studied in fine detail.

Most fossils found in amber are insect remains because they tend to be quite small, whereas it鈥檚 harder to preserve large plant remains, says Schmidt.

鈥淚f we look at amber from many localities and many geological time horizons, we get unique access to groups of organism that would otherwise be hardly preserved,鈥� says Schmidt.

While it鈥檚 difficult to tell from a single plant what the environment would have been like 35 million or more years ago, the study describes coastal areas with mixed forests of carnivorous and flowering plants with open, patchy habitats. The carnivorous plant would have been able to survive in the carbonate-free, nutrient-poor soil.

The carnivorous-plant fossils therefore provide important clues for reconstructing the habitat, but many more pieces are needed to get a full picture of what this Baltic Eocene forest looked like.

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