In the Amazon, thousands of fish and more than 100 dolphins have been discovered dead after temperatures reached a record-breaking 39C.
It is getting harder for wildlife to live in the Brazilian region due to a drought.
The bodies of 120 dolphins have been found floating on an Amazon River tributary in the north.
A recent lack of oxygen in the water resulted in the death of thousands of fish.
Biologists and other specialists working in white PPE and masks performed post-mortems on each carcass on Monday .
The scientists are seeking to rule out alternative explanations, such as a bacterial infection that could have killed the dolphins on a lake formed by the Tefé River.
Amazon, in order to rule out the possibility that drought and heat are to blame for the surge in death.
"It's still too early to determine the cause of this extreme event," scientists at the Mamirauá Institute.