Some female dolphins have evolved a secret weapon in their sexual arms race with males: vaginas that protect them from fertilisation by unwelcome partners.

Penises come in a wide variety of shapes and sizes, especially in dolphins and other cetaceans. That seems to imply a similar diversity in vaginas, but of Dalhousie University, Canada, says there is “a huge lag” in our understanding of female genitalia.

That is partly because it is tricky to visualise vaginal structure. To overcome this problem, Orbach has created silicone moulds of cetaceans’ vaginas, revealing complex folds and spirals.

“There’s this unparalleled level of vaginal diversity that we had no idea existed before,” Orbach says.

Similarly complex vaginal structures are found in several species of duck. Orbach’s collaborator of Mount Holyoke College, Massachusetts, has previously found evidence that duck vaginas have evolved to make it harder for males to force copulation. So Orbach wondered if female cetaceans’ unusual vaginas had also evolved to keep out unwanted sperm.

Orbach, Brennan and their colleagues obtained genitals from marine mammals that had died of natural causes: and dolphins, and . They inflated the males’ penises with saline to see how they looked when they were erect, and compared them with the vaginal moulds. They also took CT scans of penises inserted into the corresponding vaginas, to determine whether they fitted in easily and the best positions.

Take back control

The vaginas of the common dolphin and common seal seemed amenable to penetration, suggesting their genitalia had evolved to allow males access. But the common porpoise and bottlenose dolphin had extensive vaginal folds that obstructed penis entry.

Male bottlenose dolphins form alliances two to four strong to keep competitors away from females. When confronted by such an alliance, a female has little choice in who mates with her, and may mate with everyone.

However, vaginal folds could grant her some agency, says of Georgetown University in Washington DC. “She may not choose who she mates with, but might be able to choose which male or, more precisely, which sperm, fertilises her egg.”

In all four species studied, the ideal position for successful fertilisation appeared to be the male on top with his penis hooked underneath the female. Any deviation from this tended to result in unsuccessful penetration. That is consistent with Orbach and Mann’s observations of wild porpoises and dolphins mating.

This combination of precise positioning and complex vaginal structures means that subtle mid-copulation movements by females could send the penis the wrong way in their vaginas, preventing fertilisation.

“It might appear behaviourally that females are very passive,” says Orbach. “But looking at the reproductive anatomy, we’re learning that they have all sorts of cryptic ways to control paternity.”

Journal reference: Proceedings of the Royal Society B, DOI:

It’s the aquatic battle royale. A previously overlooked conflict between alligators and sharks has been going on for centuries at least, and it seems the alligators are winning.

at Kansas State University has studied American alligators in marine habitats for the past decade. A keen fisherman, Nifong noticed he was catching a lot of sharks in alligator country, and wondered whether the two predators ever interacted. “Alligators are opportunistic,” he says. “They’re not going to pass up a big chunk of protein that’s swimming by.”

However, the only previous evidence of alligators eating elasmobranchs – the group to which sharks and rays belong – was an individual with stingray spines in its jaw.

Nifong searched the scientific and historical literature, and consulted experts on alligators and sharks. He found confirmed instances of alligators eating , and sharks, as well as an . He says sharks and other elasmobranch fish could be a significant but underappreciated food source for alligators.

“Gators will eat almost anything that will fit in their mouths,” agrees at the Northwestern State University of Louisiana. But he has analysed the contents of many alligator stomachs and never found shark remains.

Gabrey says that might be because the alligators lived in marshes far from salt water, so were less likely to encounter sharks. Alligators also have highly acidic stomachs, which could dissolve the cartilaginous skeletons of sharks with little trace.

Sharks bite back

The only previous accounts of sharks attacking alligators come from credulous late 19th-century newspaper articles. One described hundreds of alligators and sharks assembling in an inlet and “fight[ing] like dogs”.

Though undoubtedly exaggerated, such stories are not wholly incredible, says Nifong. Both sharks and alligators gather in large numbers to feed, and alligators were far more numerous in the 19th century than today. “That increases the probability these feeding congregations would commingle,” Nifong says.

Nifong also scoured the literature for accounts of sharks clashing with other crocodilians. He found plenty. But in many cases the crocodilians were the prey, from great white sharks preying on American crocodiles in Colombia, to tiger sharks eating estuarine crocodiles in Australia.

There are even , suggesting this conflict is truly ancient.

Conservationists should bear the crocodilian-shark conflict in mind, says Nifong, as both groups contain endangered species. A study published in April found that many Australian sawfish – another shark relative – . Similarly, the US nursery grounds of the endangered smalltooth sawfish are known to harbour alligators. “If there are alligators in the area, survival of those juveniles is going to go down,” says Nifong.

Journal reference: Southeastern Naturalist,

A newly discovered species of prehistoric “hell ant” had anatomy that lived up to its demonic name, including a lethal feeding apparatus reinforced with metal.

Hell ants are an extinct lineage from the Cretaceous Period. Instead of regular mouthparts, they had upward-facing blades.

No living species have such facial anatomy. However, the hairs around hell ants’ mouths are reminiscent of hairs on modern trap-jaw ants that cause their mouths to snap shut when triggered. This has led to speculation that the hell ants’ mouthparts worked in a similar way.

Some also had a horn-like appendage that jutted out over their tusk-like mandibles. This includes the new species, Linguamyrmex vladi, which at the New Jersey Institute of Technology in Newark and his colleagues found preserved in 98-million-year-old amber.

Springing the trap

It may be that when another insect brushed the trigger hairs, the blade-like mandibles flipped up and impaled the prey against the horn, punching through its outer layer. “You have this sort of stopping plate, made to accommodate the mandibles closing and capturing prey,” says Barden.

That’s not all. CT scans revealed that L. vladi’s horn was reinforced with metal.

“Probably the metal helps to keep the horn undamaged,” says at the University of Rennes 1 in France. In 2016, he of another horned hell ant, which he called a “unicorn ant”.

“It makes sense to reinforce that [appendage],” agrees Barden, since the horn must have had to withstand repeated impacts from the mandibles. Some modern insects reduce wear and tear in a similar way, by .

Metal vampires

As well as being a metal-reinforced unicorn, L. vladi may have been a vampire. When their mandibles moved upwards, they formed a “gutter”. “That might be something that developed to funnel haemolymph – insect blood – down through the mouthparts,” says Barden.

Next to the ant, Barden’s team found a preserved beetle grub – exactly the kind of “squishy, haemolymph-laden insect” that could support a vampiric lifestyle. Perhaps it was next on the menu.

But the metal-reinforced horn suggests that the ants’ jaws moved with enough power to penetrate the tougher cuticles of adult insects as well.

“Until we find a specimen with the prey item trapped, which is probably a matter of time, we’re left to speculate,” says Barden. However, the Myanmar amber deposits where he found his specimen are so rich that more detailed observations are likely to emerge.

Journal reference: Systematic Entomology, DOI:

Hundreds of marine species are known to eat plastic – including those that regularly end up on our dinner plates. But why? It now seems that ocean-borne plastic has a smell that marine animals find appealing.

at the NOAA Southwest Fisheries Science Center in Monterey, California, explored the dietary preferences of marine life while he was a researcher at the University of California, Davis. He and his colleagues exposed schools of anchovies to seawater that contained odours from plastic. To make this, they left plastic beads in the ocean for three weeks, then stirred the beads into seawater samples before filtering them out – leaving just the associated odour chemicals.

In the ocean, plastic quickly becomes covered with a layer of algae that releases smelly sulphur compounds. Foraging fish such as anchovies, which feed on algae-munching marine crustaceans called krill, are thought to use these compounds to help them locate their prey.

When analysing videos of the anchovies, the researchers noticed that the fish reacted to the fouled plastic solutions as if they were their crustacean prey. The decision to use solutions that smelled of plastic rather than actual pieces of plastic meant the fish weren’t responding to visual cues; the fish must have smelled the odours. They did not respond to clean plastic.

The work builds on Savoca’s earlier research, which suggested that similar sulphurous odours lure tube-nosed seabirds – which are also krill-feeders – into eating plastic.

Fish aren’t stupid

“This problem of animals eating plastic has not been investigated as completely as it should have been,” says Savoca. Seabirds and fish aren’t stupid, he says, and generally they are good at locating the correct prey. Savoca thinks we might have overlooked the role of odour because we are such visual animals. “That’s how we perceive the world most readily.”

This explains why, for example, it is often suggested that marine animals mistake plastic bags for jellyfish, or plastic pellets for fish eggs. “This is mostly anecdotal,” says at the University of Toronto. She thinks Savoca’s experiments provide the most rigorous explanation yet for the high frequency of plastic consumption among marine species.

This dietary confusion is a problem for fish, but it could also be an issue for animals higher up the food chain, including us.

“There’s no doubt that we eat microplastics when we eat seafood,” says Rochman. What we don’t know, she says, is how much of the chemicals leaching out of the plastic then enter our systems – and whether it matters if they do.

An released last week highlights how much we don’t know about plastic consumption by animals and the possible impact on human health. While Savoca’s work doesn’t provide solutions, he feels it is important to establish why animals are eating plastics in the first place.

“For any problem, if you want to effectively mitigate it or solve it in any way, you have to fully understand what’s going on,” he says.

Read more: Plastic in the food chain: Artificial debris found in fish

Blind-sided

Species: A nest-brooding cichlid (Neolamprologus furcifer)
Habitat: Rocky areas in Lake Tanganyika, East Africa

Appearances can be very deceptive. Look closely at brown striped lake snails and you may spot one that is actually a fish.

Even experts can be fooled. of Osaka City University, Japan, found it hard to keep count of baby cichlid fish when he was studying cichlid reproduction in Lake Tanganyika. “Often I confused the fish young with snails,” he says.

The cichlid, Neolamprologus furcifer, raises its brood in rocky nests – a habitat shared with various snails. Satoh found that the most abundant snail in the nests by far is Reymondia horei. The baby fish sport the same brown and white stripes as these snails, and resemble them in size and shape (see image below).

The cichlid babies appear to be the first known case of fish masquerading as snails. Where the snail is absent, the young of these cichlids don’t develop the stripes.

So why do they do it? Satoh thought that if he found it hard to tell the fish and snails apart, so too might fish that prey on the young cichlids.

Attacks by mature female cichlids on would-be predators were twice as frequent when Satoh and his colleagues removed the snails. With the snail smokescreen gone, the fish seem to need to work harder to keep their babies alive.

Female cichlids also chase away harmless algae-eating fish as well as predators. The researchers suggest that this could be attracting snails to the nests, by ensuring they have a plentiful supply of algae to eat.

Not all snails have the stripes that the baby fish mimic, however, and the team observed female cichlids carrying non-striped snails away from their nests, effectively giving the striped ones more space to colonise.

“The results strongly suggest the mother fish’s behaviour enhances the chances that the offspring will be mistaken for inedible snails,” says at Newcastle University, UK, who studies animal disguise. He says it is the first example of an animal altering the efficacy of another’s masquerade. The mothers collaborate with their young to optimise their disguise, and consequently waste less time and energy fighting off enemies.

The survival of baby fish wasn’t lower when the snails were removed, but this is because mothers had upped their vigilance.

As the offspring are protected either way, it is the mothers who have the most to gain from the masquerade.

“All of the literature on masquerade investigates how individuals benefit from their own disguise,” Skelhorn says. “The results of this study seem to suggest that masquerade doesn’t directly benefit the masquerader, but rather it benefits the third party.”

MatjaĹľ Kuntner

Species: Dwarf crocodile (Osteolaemus tetraspis)
Habitat: Abanda caves, in the rainforests of Gabon

Caves are scary places – especially ones filled with crocodiles. “I was crawling through the cave and suddenly there were two red eyes,” says explorer . “It was frightening!”

In 2010, he was part of an following a tip-off about a population of dwarf crocodiles living there.

While crocs sometimes retreat underground during droughts, this is the first population documented taking up long-term residence in caves.

The team’s crocodile expert, from the Rare Species Conservatory Foundation, soon realised why they had done so: a bounty of ready-made snacks was falling into the water or lining up to be plucked off the cave walls.

“You walk in and there are just bats and crickets everywhere,” he says. “The crocodiles are pretty good hunters anyway, but even if they didn’t have to pull bats off the walls, there are individuals falling to the floor all the time.”

Thanks to a steady diet of bats and other cave critters, the crocs appear to be in better physical condition than their forest counterparts, especially the young ones.

With at least 5 kilometres of caves to occupy, it’s difficult to precisely know how many crocs are taking advantage of this banquet. However, based on the team’s exploration up to 100 metres into the cave system, Shirley reckons there are at least 50, and probably many more.

Orange is the new black

As the team headed further into the caves, they made an unexpected discovery: in the deeper recesses, the older, dark-coloured males had become paler, turning a bright orange. Were they losing unnecessary pigment in a similar way to other cave-dwelling animals, such as Mexican blind cavefish?

Shirley doesn’t think so. Cave crocs must maintain contact with the outside world for one simple reason. “They cannot reproduce in the caves,” he says.  “It’s a nesting ecology thing: they need big mounds of rotting vegetation to lay their eggs in.”

So while the crocodiles appear to spend the entire dry season in the caves, they emerge during the wet season – at least to breed. Instead of the colour change marking an early stage of subterranean adaptation, Shirley has a more disgusting explanation.

The water these crocodiles swim through is essentially an alkaline slurry formed from bat droppings. “The urea in bat guano makes the water very basic,” he says. “Eventually that will erode away the skin and change its colour.”

Bizarrely, this is similar to the chemical treatment applied to crocodile skin so it can be turned into belts and wallets.

Dwarf crocs in Gabon are in little danger of becoming belts, but they are hunted extensively for bushmeat.

Thankfully, Shirley thinks the Abanda population avoids this threat, citing the difficulty he had capturing them in narrow, often impassable, tunnels – “and I consider myself to be fairly good at catching crocodiles”.

Read more: When crocodiles roamed the Arctic; These baboons and lemurs have left the trees to live in caves

Armed and dangerous