Rabbits and guinea pigs don’t have efficient cooling systems like a dog or a human.
Us humans keep our body temperatures regulated by releasing sweat from over 2 million sweat glands, and dogs are able to pant to release heat and cool down their bodies.
Rabbits have a system that rushes blood to their ears, where it’s quickly cooled over the large, thin-skinned surface area and sent back to the rest of the body. They do this to cool down because they don’t sweat like other animals.
Although this system is quite effective, when temperatures rise and rabbits find themselves without shade, the process can’t always keep up with their rising body temperatures. Guinea pigs also don’t sweat, and their ears are quite small and inefficient for cooling their bodies.
The only way to keep guinea pigs from suffering heatstroke is to make sure the temperature around them doesn’t get above 90 degrees. They prefer it even cooler, with relatively low humidity while you’re at it.
Keeping a lot of fresh water and shade available for both rabbits and guinea pigs is a must.