🦁 Lions are supposed to be good at heat. Three of them died at a Tokyo zoo in the space of six days, and the necropsies found dehydration and organ failure in all three. Japan is not the hottest country on earth. It is just that Japanese heat works on a body differently than the dry heat most people picture, and the number that explains it barely changed at all this summer.

What happened to three lions born in Tokyo

On August 3, Tama Zoological Park in Hino, on the western edge of Tokyo, published a full account of the previous two weeks.

On July 18, 2 of the 16 lions in its Lion Park stopped eating and grew listless. Staff began treatment. Other animals followed. By July 22 the number needing veterinary care had reached 10. Some refused food entirely; some could no longer stand or lost consciousness. On July 23 the zoo shut the lion exhibit.

Three females died: Mugi, 3, on July 28; Ichigo, 11, on July 31; Ruena, 15, on August 2. None of them had ever set foot in Africa. All three were born at Tama.

Necropsies by the zoo's own veterinarians found whole-body dehydration and multi-organ failure in every case. As of August 3 the zoo believed heat was a factor and had sent samples to an outside institution to confirm the cause. 3 more lions still could not get up and were receiving fluids, liver medication, vitamins and treatment for pressure sores. 4 had recovered or were recovering, 6 were never affected, and 13 lions remained.

The zoo's Lion Bus is suspended too. Since May 1964 it has carried visitors directly into the enclosure, in what the zoo calls the world's first safari-style exhibit, and there is no date for its return. Tokyo Metropolitan Government officials said that in the more than 60 years since that bus started running, the zoo had never before lost several lions in a row to heat, and that none of the other animals the city manages had shown a similar problem. A zoo representative told FNN that staff had not anticipated that hot weather would make lions ill.

Why being built for heat was not enough

Big cats cannot sweat their way out of a hot afternoon. A veterinary clinic in Shizuoka lays out the anatomy for the family: the eccrine glands humans use to shed heat across the whole body sit, in cats and dogs, only on the paw pads and part of the nose. What is left is evaporation from somewhere else. Fuji Safari Park, which keeps lions in the same prefecture, describes what that looks like on its official account: mouth open, tongue out, breathing hard, heat leaving with the moisture.

And evaporation needs somewhere to go. Water only leaves a surface if the surrounding air still has room for water vapour. The same clinic spells out the consequence: air as hot and wet as a Japanese summer is already carrying plenty of vapour, moisture evaporates poorly, and panting is a poor fit for the season. It also taxes the respiratory system, so hours of it wear an animal down.

The zoo has not blamed humidity, and its own account points to an unusually abrupt rise in temperature. But Japan's Ministry of the Environment states the general principle without hedging in its official heatstroke manual: where summer humidity runs high, as it does in Tokyo, the body cannot shed enough heat by sweating and the risk of heat illness climbs. The manual then speaks directly to people from abroad, warning that anyone raised where summers are hot but dry should take Japan's wet ones seriously.

The 77 percent that never moves

Tokyo's average relative humidity in July 2026 was 77 percent. The 1991 to 2020 normal for July is 76.

That one-point gap is the point. Last month's humidity was no anomaly and never made the news, because it was simply a regular Tokyo July. Set it beside the city's January normal of 51 percent and you can see how far the summer baseline sits above the winter one.

What actually broke records was the temperature stacked on top. Tokyo's mean for July came in at 27.2°C against a normal of 25.7, the city logged 9 days at 35°C or above, and the peak reading was 37.1°C on July 25. The Japan Meteorological Agency reported on August 3 that average temperatures across western Japan for the middle and late parts of July were the highest since national records began in 1946, and cited an analysis by the Weather Attribution Center concluding that the heat could not have occurred without global warming. Humidity, in other words, is the constant in this equation and temperature is the variable. That is what makes a given thermometer reading bite harder in Tokyo than in a dry climate.

The Environment Ministry illustrates Japan's position with a climograph, a chart plotting monthly temperature against monthly humidity, built from JMA and World Meteorological Organization long-term normals for four cities. Tokyo's summer sits close to Manila, deep in the tropics. New York gets summer temperatures not far off Tokyo's but with noticeably drier air. Athens, roughly as hot in summer as Manila, runs a monthly average humidity below 50 percent. Climatically speaking, Tokyo has been filed under the wrong heading for a long time.

A country that does not warn you by temperature

Japan's official heat alerts are not based on the air temperature you see in a forecast.

The measure is WBGT, the wet bulb globe temperature, an index the Environment Ministry has published since 2006 and the basis of the nationwide Heatstroke Alert system that launched in 2021. Outdoors it is calculated as 0.7 times the wet bulb temperature, plus 0.2 times the globe temperature, plus 0.1 times the dry bulb temperature.

Dry bulb temperature is simply what an ordinary thermometer reads. It is the number in your weather app. In Japan's official danger index it carries a weight of 10 percent. 70 percent goes to the wet bulb reading, which rises and falls with humidity, and 20 percent to radiant heat from the sun and the pavement. A country with dry summers could get away with warning people by temperature. Japan decided long ago that it could not.

The vocabulary is shifting too. In February 2026 the JMA asked the public to name a new category of day, one reaching 40°C, on the grounds that such days had stopped being unusual. 478,296 people replied. The winner with 202,954 votes was kokushobi, roughly "severe heat day," made official on April 17. Among the write-in suggestions were "sauna day," "stay-home day" and "boiling day."

The new word was put to work almost immediately. According to the JMA, the Tokai region recorded Japan's first ever run of 5 consecutive kokushobi from July 21 to 25, and Kyushu logged its first on record. By August 3 the Japan Weather Association, a meteorological foundation separate from the agency, put this year's cumulative count of locations reaching 40°C at 34, already past the 30 that made 2025 a record year, and August had only just begun.

Where people actually collapse

Ambulance data suggests the danger is not where most visitors assume it is.

In the week of July 20 to 26, Japan's Fire and Disaster Management Agency counted 18,591 people taken to hospital for heat illness nationwide, on preliminary figures, against a confirmed 11,311 in the same week of 2025. 45 were already dead when first examined, and 57.9 percent of everyone transported was aged 65 or over. The surge belongs to those particular days rather than to the summer as a whole: the running total since May 1 stands at 43,021, below the 52,713 recorded over the same stretch last year.

The largest single location category was not a building site, a sports ground or a street. It was home: 7,600 people, 40.9 percent of the total. Roads accounted for 21.5 percent and workplaces 11.4.

A humid room does not announce itself. It feels like an ordinary summer evening in Japan, right up until it isn't one. The lions at Tama had shade in the outdoor enclosure, ventilation at night, and air conditioning in parts of the building. It was not enough for an animal that cools itself by breathing. Spot coolers and industrial fans have since been added, and the zoo says it is strengthening heat measures in the lion house and the enclosure to prevent this happening again.

By Tama's own count, Japan holds 377 lions across 47 facilities. Every one of them is living through the same summer.

So, a question for you. Does summer arrive dry where you live, or wet? And when the authorities warn you that the heat has become dangerous, what number are they actually looking at?

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