Howler monkeys are dropping dead in Mexico and authorities said on Monday that they were investigating whether extreme heat was killing the endangered species.

According to the Ministry of Environment and Natural Resources, causes under consideration include "heat stroke, dehydration, malnutrition or fumigation of crops with pesticides."

It said studies would be carried out to rule out a virus or disease.

Temperatures of up to 45 degrees Celsius have been recorded in the southern states of Chiapas and Tabasco, where the deaths were reported.

While the government did not say how many primates had perished, Tabasco-based wildlife preservation group COBIUS reported "mass deaths."

"It's very likely due to climatic reasons but we cannot rule out other important causes," it said in a statement.

"If you see monkeys that are weak and apparently suffering from heat or dehydration, please try to hoist a bucket of water by rope for them to drink," it added.

Authorities and conservationists were carrying out patrols to provide water and food, mainly fruits, to help monkeys stay hydrated, the Tabasco Institute of Civil Protection said.

(Cover image via CFP)