Shrinking trees and tuskless elephants: the strange ways species are adapting to humans | Evolution

From the highest mountains to the depths of the ocean, humanity’s influence has touched every part of planet Earth. Many plants and animals are evolving in response, adapting to a human-dominated world. One notable example came during the Industrial Revolution, when the peppered moth turned from black and white to entirely black after soot darkened its habitat. The black moths were camouflaged against the soot-covered trees, surviving to pass on their genes to the next generation.

As human influence has expanded, so too have the strange adaptations forced on the natural world. We asked researchers around the world for similar changes they have noticed in the 21st century.

Shrinking mahogany trees

Famed for its durability, resistance to rot and deep-red wood, mahogany has become synonymous with luxury. The largest trees, crucial pillars of rainforest ecosystems, were razed for their timber, with populations falling more than 70% in some countries since 1970. While old-growth trees have largely disappeared, the species itself is still widespread in many areas, says Dr Malin Rivers, a conservationist with Botanic Gardens Conservation International. But now, it grows in a different form. “Now considered commercially extinct in many parts of its range in the Caribbean, the species still remains and is even abundant in some areas. But these younger trees no longer take the large towering form of commercial Swietenia mahagoni and will never grow to heights this species had previously been famed for,” she says. While mahogany trees once grew to – and sometimes exceeded – 20 metres in height, says Rivers, “mahogany now exists as smaller, shrubby trees with little commercial value. As the largest trees were cut, they were not able to reproduce and share their diverse gene pool which promoted the tall growth of the species.”

Magpie nests with anti-bird spikes

The sight of birds’ nests built on poorly designed anti-bird spikes is not an uncommon sight in urban areas. But researchers have found that magpies – who cover their dome-shaped nests with branches from thorny bushes to deter egg-thieving predators such as crows – have started ripping out anti-bird spikes to use in their nests. PhD student Auke-Florian Hiemstra found that the use of bird spikes was part of a growing trend of birds using artificial materials for their nests.

Brittle stars that like beer bottles

First described by scientists in 2018, Astrophiura caroleae, a new species of brittle star, is found at depths of about 300 metres off the coast of Curaçao in the Caribbean. The pentagonal creature, a relative of the starfish, was collected in a submersible by the Natural History Museum’s (NHM) longtime curator of echinoderms, Dr David Pawson. Scientists have since found it favours living in curious places. Dr Hugh Carter, a starfish expert at the NHM, says: “Live specimens have only ever been observed on discarded Heineken bottles about 300 metres deep, which had probably been thrown overboard by day fishers with the exception of one specimen that was living on a discarded rubber tyre. “Interestingly, the members of the genus are thought to be hard substrate specialists – so mostly living on rocks – and it seems as though this species is carrying on the tradition quite happily on human-made detritus,” he says.

Snails with paler shells to counter city heat

Using pictures of grove snails collected by thousands of citizen scientists in the Netherlands, researchers found that snails living in the city centres had evolved shells with paler colours. Scientists believe this change is the result of warmer temperatures in cities, which can be up to 8C (14.4F) higher than rural environments. Prof Menno Schilthuizen, a Dutch evolutionary biologist, says: “Snails inside dark shells tend to heat up more, risking death by overheating. Probably, the pale shell colour keeps the snails just cool enough on the hottest days of summer in the city.”.

Swallows with shorter wings to avoid cars

Cliff swallows in south-west Nebraska, which often nest under bridges, have frequently been hit by passing cars. But a long-term study published in 2013 found that the birds have adapted to the risk of being hit by developing shorter wings. Shorter wings made the birds more agile, allowing them to avoid rapidly approaching traffic, while birds with longer wings were more likely to be killed. “It’s like the difference between a U2 spy plane and a fighter jet,” according to the late Mary Bomberger Brown, of the University of Nebraska-Lincoln, who led the study.

Elephants losing their tusks to beat poachers

During the Mozambican civil war, heavy poaching by fighters meant that African savannah elephant numbers plunged by more than 90% in Gorongosa national park. With populations now in recovery and representing one of the most important examples of global restoration, many of the female elephants have no tusks – a consequence of tuskless elephants being less likely to be targeted by poachers, say researchers. The same change has also been recorded in Tanzania.

Tanya Smith, a senior adviser at WWF-UK, says: “One tragic adaptation in response to the devastating pressure of poaching in previous decades has been an increase in the proportion of African elephants born with shorter tusks or no tusks at all. A disastrous example of how human pressure could mean elephants lose one of the things that makes them so iconic.”

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