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Thirteen years ago the Natural History Museum in London acquired a remarkable fish specimen recently collected off the Cape Verde islands in the tropical Atlantic. It was a hairy angler, a very rare and extremely ugly species, but what most impressed ichthyologists was its hugely distended stomach.
Feeling around the belly of the fish, which was preserved in alcohol, they could tell that it had swallowed a meal considerably larger than itself, shortly before its capture. But what was its prey?
The curators did not want to spoil the specimen by cutting it open to find out. So for several years the hairy angler sat undisturbed in its jar of alcohol – until imaging technology offered a non-destructive way to look inside.
The museum’s computed tomography (CT) scanner, a non-medical version of the CT X-ray machines that are revolutionising hospital diagnosis, showed up the prey in 3D with remarkable clarity.
The 20cm-long angler had evidently caught another fish about twice its length, which lay curled around inside its belly. The prey’s big round eyes are clearly visible and so are its otoliths, stony structures in fish ears which are an important distinguishing feature between species.
“I think we have a good chance of identifying the prey from its otoliths in the scans,” says James Maclaine, fish curator. “If we can work out what species it is, that would be exciting information because no one knows what these fish eat.”
With digital imaging, experts around the world can join in the challenge of identifying puzzling features such as these otoliths. Farah Ahmed, CT specialist at the museum, says “digital loans” now make it possible to share detailed images and other data with colleagues in other institutions, without having to send out specimens physically.
Altogether the Natural History Museum has archived about 4,000 CT scans covering a wide variety of fascinating specimens, she adds. Projects range from analysing the internal structure of minerals and meteorites to looking into fossils and scanning the brains of bees. A “virtual dissection” of a mummified cat from ancient Egypt showed that the animal was about 16 months old when it died of a broken neck.
But the bloated hairy angler has particularly caught the imagination of museum staff. The scans and their interpretations will feature in a scientific conference on tomography at the Natural History Museum next month. Visitors will be able to see them in an interactive display in the museum’s Darwin Centre during the autumn.
Anglerfish are fascinating for several reasons, including their hideous appearance, their extraordinary method of fishing for food and their bizarre sex lives.
The young male is free-swimming but as he reaches maturity he latches on to a (much larger) female with his teeth. The pair then form an indissoluble life-long union. A special enzyme enables the male literally to fuse into the female. His organs gradually disappear as he taps into her bloodstream and lives as a parasite on her. Only his testes remain in full working order, providing a ready supply of sperm when she is ready to spawn. Some anglers of other species have been observed with multiple mates attached; the record is seven, says Maclaine.
The hairy angler at the Natural History Museum has no male attached but it does have the characteristic angler’s lure – a fleshy growth on the end of a filament adapted from a dorsal fin, which sticks out of the head above the eyes. The fish waves this around in front of its mouth, while keeping the rest of its body perfectly still. When prey approaches, its huge jaws snap shut. Aided by a powerful swallowing mechanism and lines of sharp backward-pointing teeth, the angler can force prey twice its size into its highly elastic stomach.
The feeding strategy resembles that of some snakes: to eat huge meals infrequently. “Where the anglers live, food is relatively scarce,” says Maclaine. “They can go for long periods without eating, then eat a huge fish that might keep them going for months until the next meal.”
What made fathers become husbands?
The origins of social monogamy have long puzzled evolutionary biologists. What makes male animals take a single mate for life, or at least for several breeding seasons, when they could father many more offspring through polygamy or unrestrained promiscuity?
Two UK-based research groups tackled the question, in papers published in top scientific journals last week. Unfortunately they came up with different answers.
Evolutionary theorists have suggested three main explanations for monogamy: natural selection favours pair-forming because offspring are more likely to survive if both parents help rear them; or males bond to a particular female to prevent her mating with a rival; or males pair up to prevent rivals killing their offspring.
A group led by Kit Opie of University College London, which analysed data from 230 primate species, came down firmly in favour of infanticide – or its prevention – as the driver of monogamy. The study appeared in Proceedings of the National Academy of Sciences.
The other team, from Cambridge university, published in Science the results of a wider study of 2,500 mammal species. They concluded that mate-guarding – preventing other males having access to the female – was the main factor. This conclusion applied to the primates in their study too.
“We must clearly get together with Opie et al and work out where the difference lies,” says Tim Clutton-Brock of Cambridge. There was some common ground, however. Care by both parents is a key factor behind pair bonding in birds, for which the care of eggs and rearing of chicks is generally more demanding than bringing up baby mammals. But both teams decided that this was a secondary effect in mammalian evolution.
“Paternal care evolves after monogamy is present and seems to be a consequence rather than a cause of the evolution of monogamy,” says Dieter Lukas of Cambridge. “It appears to occur in about half of all socially monogamous species and, once it does evolve, it provides a clear benefit to the female.”
The apparent conflict between the conclusions of the two studies shows how hard it is to pin down the evolutionary drivers of behaviour. Think for example of the unresolved debate over such issues as the origins of language and culture in our human ancestors.
Whether this social monogamy research would have implications for human evolution – even if the two teams agreed on the cause of monogamy in primates and other mammals – is far from clear.
Susanne Shultz of Manchester University, who is part of the Opie group, is optimistic. “What makes this study so exciting is that it allows us to peer back into our evolutionary past to understand the factors that were important in making us human,” she says. “Once fathers decide to stick around and care for young, mothers can change their reproductive decisions and have more, brainy offspring.”
But Clutton-Brock is not so sure. “Reliance by humans on cultural adaptations means that it is difficult to extrapolate from ecological relationships in other animals,” he says.
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