Three female lions have died at a zoo in Japan after suffering heat-related illnesses, staff at the facility said. The country is grappling with a blast of extreme heat that has led zoos to rethink how to protect their animals.

The lions, named Mugi, Ichigo and Ruena, died within five days of one another at Tokyo's Tama Zoological Park, according to a statement from the facility released on Monday.

The zoo said it was too soon to determine the animals’ exact cause of deaths, but that initial postmortem findings indicated that all three had suffered from systemic dehydration and multiple organ failure.

“We believe heat stress was a contributing factor,” the zoo said in the statement.

Japan has been gripped by soaring heat and last month, temperatures were higher than 104 degrees Fahrenheit in some places. Health authorities declared July 23 to be a kokushobi, a “cruel heat day.”

In mid-July, the zoo’s lions began exhibiting symptoms of appetite loss and reduced physical activity — possible symptoms of heat stroke. By July 22, 10 of the facility’s 16 lions were showing such severe symptoms that they required treatment.

Despite the zookeepers’ efforts to cool the lions with sprayed water and to replenish lost fluids with intravenous drips, three-year-old Mugi died on July 28, followed by 11-year-old Ichigo on July 31, then 15-year-old Ruena on Aug. 2.