Denver Zoo use blood popsicles, “snow” to coax animals through heat waves


The Siberian tigers Yuri and Nikita who are supposed to mate suffer most during Denver’s intensifying 95-degree heat waves, and caretakers supply cooling ice “bloodsicles” from the zoo’s heavily frequented freezers.

Zookeepers also have set up an industrial fan behind a mist-maker, aimed at the bedraggled orange-and-black tigers. Sometimes the tigers plunge into pools and collapse in a loft where there’s airflow. Nikita flops on a mound of wet sand bags.

Penguins seek “snow” made by grinding up cubes from the zoo’s ice maker, the big kind found in motels. These are penguins from South Africa and Peru, better adapted to heat than their Antarctic kin.

Sea lions gnaw on modified versions of the tigers’ bloody bones in ice blocks — salmon-infused “fishsicles.” And many animals take refuge in water. For elephants, zoo keepers toss in apples, melons and carrots as incentives to lumber into pools.

Four Mongolian wild horses — a species that evolved on steppe similar to terrain in western Colorado — have proved able to endure baking sun. Leopard tortoises simply bask.

But, overall, the rising heat in Colorado is creating challenges for zoo operators. They care for more than 3,000 animals, representing 450 species, including many that did not evolve to endure extended heat — let alone the temps topping 100 degrees that climatologists warn will be common in Colorado’s future.

Surviving climate change has become a central challenge at zoos around the planet, just as it is for people living in dense-packed cities such as Denver, where concrete and asphalt amplify heat by up to 20 degrees. On one hand, zoos play key roles in conservation of species as natural habitat disappears or becomes less hospitable. On the other, the burden of ensuring suitable safe space is growing more difficult.

Denver Zoo opened its new African ...

Hyoung Chang, The Denver Post

The Denver Zoo opened its new African penguin habitat on Thursday, Sept. 30, 2021.

Denver zookeepers could just move more animals into air-conditioned buildings.

“Yeah, we could. But our animals really want to be outside. And we want them to be outside. We want them to enjoy those outdoor exhibit areas — and be there for our guests,” said Denver Zoo general curator Emily Insalaco, an animal behaviorist.

“How do we continue to provide that environment for them and for our guests? How do we design exhibit areas that meet all the needs of our animals? We have temperature parameters for every single species we take care of. We know where they are from in the wild and what that environment looks like,” Insalaco said.

“We’ve seen wild animals moving out of their traditional habitat, looking for better spaces, cooler spaces, places with more food. We’ve seen pollinators coming out of their winter hiding places earlier than usual and getting caught in spring snow storms. We can learn a lot. It is all right in front of us. We’re all in this together.”

Beyond the frozen treats, mist and occasional ice baths, the zoo’s long-term strategy calls for creating better shade — similar to what Denver officials say they will do to keep the surrounding city hospitable.

The zoo’s horticultural team has counted 7,500 trees overall on their 84 acres in City Park just east of downtown — a mix of mostly cottonwoods along with pines, poplars, lindens and catalpas. But trees are dying here, as around the city, due to heat, drought and insects. Zoo officials recently began a tree canopy assessment and plan to ramp up tree-planting to increase the shade for animals.

Horticulture technician Patrick Crowell, left, and coordinator Jason Crimmins plant Agabe by the entrance of Predator Ridge at the Denver Zoo. (Hyoung Chang, The Denver Post)

Hyoung Chang, The Denver Post

Horticulture technician Patrick Crowell, left, and coordinator Jason Crimmins plant agave by the entrance of Predator Ridge at the Denver Zoo. (Hyoung Chang, The Denver Post)

They anticipate a hotter future. Federal scientists at climate labs west of Denver in May measured a record-high 421 parts per million global average atmospheric concentration of heat-trapping carbon dioxide. The international efforts to contain warming by cutting pollution have failed as burning of coal, oil and gas hits unprecedented levels. This favors accelerated warming.  State agencies project Colorado’s average temperature will rise by an additional 2.5 to 5 degrees before 2050.

In the past, Denver’s zoo featured polar bears, a species imperiled in the wild as ice melts. Two cubs (Klondike and Snow) born in 1994 became stars. Yet, in 2018, zookeepers decided to send away their last two polar bears (Lee and Cranbeary) as zoologists learned more about polar bear needs in captivity.

Making improvements zookeepers wanted would have cost too much, Insalaco said.

Nationwide, polar bears in zoos, once numbering in the hundreds, have decreased to fewer than 50, according to polar bear monitoring groups.

For now, Denver’s former polar bear facility holds a lone grizzly, Tundra, orphaned as a cub around 2002 in Alaska.

And zoo officials acknowledged tree-planting to boost shade will take years. Surviving this week’s heat wave, and anticipated high temperatures in the coming years, requires a creative and aggressive approach, officials said.

Zookeeper teams assigned to each species confer in quick morning meetings and then monitor animal behavior.

The two Amur tigers from Siberia — 409-pound Yuri and 261-pound Nikita — were matched here for mating as part of an international species survival plan. That hasn’t happened, and zoo officials said heat is a factor, inducing lethargy. Last fall, the tigers tested positive for COVID-19.

“They look hot, definitely,” carnivore keeper Kim Pike said. “They tend to sleep a lot. They’re not very active.” And they’re less interested in food.

“We’re constantly trying to figure out how to keep them comfortable,” Pike said.

Yuri, an Amur tiger plays with ...

Eric Lutzens, The Denver Post

Yuri, an Amur tiger, plays with a plastic open-ended play ball in a small pond while keeping cool at the Denver Zoo on Thursday, June 16, 2022.


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