Being a polar bear is harder work than biologists thought. As the bears roam the Arctic sea ice hunting seals, they use more energy than expected. That has ominous implications for a future in which sea ice is ever scarcer.

of the US Geological Survey in Anchorage, Alaska, and his colleagues studied 9 adult female polar bears on the Beaufort Sea ice north of Alaska. They fitted the bears with collars carrying GPS trackers, accelerometers and video cameras. They also injected the bears with water labelled with stable isotopes of hydrogen and oxygen.

When the team recaptured the bears 8 to 11 days later, changes in the ratio of isotopes in their blood gave a measure of metabolic rate. The researchers could compare this with the bears’ activity and hunting success.

Researchers already knew that polar bears are inefficient walkers, but many assumed that they also saved energy by sitting quietly waiting for seals, and by reducing their metabolic rate when fasting. However, it seems these savings are small at best. The bears’ average metabolic rate during the study period was 1.6 times more than most previous estimates.

Not so thrifty

This means the bears must kill and eat more seals over the course of a year, to pay for this higher metabolic rate.

But as climate change melts ever more sea ice, the bears are likely to have to walk farther to find prey. “That has a cost, and the animal has to find the energy to do that – or take it out of growth, reproduction, or survival,” says polar bear biologist at the University of Alberta in Edmonton, Canada.

Though Pagano’s study focuses on just one population of polar bears, many of the other populations around the Arctic likely face the same challenges, says Derocher.

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Many seemingly different species actually live very similar lives. This convergence suggests that it may someday be possible to predict how many species live in a particular habitat, and even to identify the holes left by missing species.

For more than half a century, ecologists have tended to describe ecological roles, or “niches”, as though they were properties of individual species. For example, are camouflaged, tree-dwelling lizards that ambush insects, while are ground-dwelling desert creatures that eat ants and bear protective spines. The diversity can seem overwhelming.

But , an evolutionary ecologist at the University of Texas in Austin, has long wondered whether there might only be a certain, limited set of niches.

To test the idea, Pianka and his colleague at the University of Oklahoma decided to pool their decades of experience studying lizards in the field. “Our entire life, basically, has been going out and collecting data on lizards,” says Vitt. “So we have a huge data set.”

The pair looked at 134 species on four continents. For each one, they examined more than 50 features of their niches, such as habitat type, hunting style, reproductive output and defences against predators. Then, with colleagues, they crunched the numbers.

Over and over, they saw pairs of unrelated lizards converge on similar niches. Out of the 134 species, 100 belonged to a convergent pair, far more than could have happened by chance. For example, African chameleons have ecological equivalents in the Americas called , and Australia’s fills almost precisely the same niche as North America’s horned lizards.

If lizards could evolve into an unlimited number of niches, this convergence would be unlikely. Instead, their result suggests that lizards are constrained to live particular lifestyles. For instance, there are no lizards that behave like elephants. “There’s only a certain number of ways to be a lizard,” says Pianka.

Laws of nature

“This is beautiful,” says evolutionary biologist at Rutgers University in New Jersey. “It’s astonishing the number of species that have converged into ecological roles.”

Ecologists could apply the same approach to other groups, like birds or rodents. However, each group is likely to have its own unique set of niche features – for example, many birds make long migrations, and some rodents hibernate – which would complicate the analysis, says Pianka.

The limited number of niches implies that ecologists may someday be able to construct a “table of niches”, somewhat analogous to chemistry’s periodic table of elements. “If we constructed this table, and we thought it was fairly complete, then we could go into places and look at the structure of the habitat, the temperatures and so on, and say ‘this place ought to be able to support 10 species of lizards’,” says Vitt. Ecologists could then predict far more about the natural world than they can today.

Moreover, such a table would highlight “empty” niches where species ought to occur but do not. These gaps could point to niches that were once filled by a species that has died out, but so recently that evolution has not yet refilled the niche, says Vitt.

However, the findings do not necessarily mean that extinct species can be easily replaced by their ecological equivalents, says Vitt. Such “plug and play” replacements have been attempted on Indian Ocean islands, where . The problem is, even if the new species fills the same niche, it may respond differently to competitors, predators and prey – leading to unpredictable changes in the ecosystem.