 Cows calve , or give birth , at the beginning of the wet season . The onset of the rainy season signals the departure of the Monarch butterfly from Mexico . Tropical species of butterflies show larger dot markings on their wings to fend off possible predators and are more active during the wet season than the dry season . Within the tropics and warmer areas of the subtropics , decreased salinity of near shore wetlands due to the rains causes an increase in crocodile nesting . Other species , such as the arroyo toad , spawn within the couple of months after the seasonal rains . Armadillos and rattlesnakes seek higher ground . 
