It's possible to visit the Pantanal year-round, but it's best to go during the dry season (May to September) as the wildlife concentrates around the reduced water. This is peak jaguar-spotting time. The best time to watch birds is from July to September, when the waters have receded and the bright-green grasses pop up from the muck.
Flooding, incessant rains and heat make travel difficult during the wet season (November to April), though this time is not without its special rewards – this is when the cattle and wildlife of the Pantanal clump together on the cordilheiras. However, these islands are covered with dense vegetation that can make spotting wildlife difficult. The heaviest rains fall in February and March. Roads become impassable and travel is a logistical nightmare.
The heat peaks from December to February, when temperatures higher than 40°C (104°F) are common, roads turn to mush, and the mosquitoes are out in force. Many hotels close at this time and in the northern Pantanal tour companies effectively take a break.