Best Time to See: Spring and Summer
Bighorn sheep are iconic to RMNP and Colorado. These deceivingly agile animals—the largest wild sheep in the world—are hunted by cougars, wolves and grizzly bears, but take refuge high in the steep, stony cliff redoubts of the Rocky Mountains. If you want to spot the majestic bighorn, Sheep Lakes in Horseshoe Park is a good bet. Herds come down off the mountains on an almost daily basis during late spring and summer to graze in the vast meadow and eat soil to replenish essential nutrients. To see them in their alpine home, a short, strenuous trail near Milner Pass on the park's west side leads toward Specimen Mountain and to the edge of the Crater (though it's closed during lambing season, from May to mid-July), where you can watch them in what amounts to a natural sanctuary.
Best Time to See: June through October
Katmai is home to the world's largest protected population of brown bears. Most of the year, the massive predators are spread out along the coastal areas of the sprawling, 4 million-acre wilderness, but come summer they head inland to feed on huge runs of spawning salmon. The most notable stretch of water is near Brooks Camp, one of the only developed areas of the park. There, in a 1.5-mile stretch of the Brooks River between Naknek and Brooks lakes, Brooks Falls creates a barrier that slows the sockeye salmon's annual migration, and crowds of bears—as many as 70 have been counted there at once—gather to take advantage of the protein-rich fish. Visitors can safely watch the fishermen…err, bears…from a nearby viewing platform. Katmai is a remote fly-in-only park.
Number: 30,000 worldwide
Best Time to See: May through October
It's not uncommon, while exploring the rocky points and granite peaks of rugged Mount Desert Island, to spot whales spouting offshore. Humpback, finback, minke and right whales all frequent these waters, though humpbacks tend to be the most readily visible and, lucky for whale watchers, playful. Take a boat tour with nearby Bar Harbor Whale Watch Co., and you'll likely get close to the 40-ton creatures, which engage in demonstrative behaviors like spy hopping, flipper slapping and breaching.
Number: More than 600
Best Time to See: Year-round
These hulking icons of the American West once numbered between 30 and 60 million. Today, only about 16,000 roam wild in places like Yellowstone, Theodore Roosevelt National Park and, of course, Badlands. The good news is that these populations are, for the most part, rebounding. Each year, Badlands rounds up "surplus" animals (in excess of 600, which is how many are considered sustainable on those grasslands) and donates them to Native American tribes in order to supplement smaller wild herds around the country. To see bison in Badlands National Park, just head to the more frequented North Unit, and look for the big animals in the vast grasslands along the main road.
Number: ~1.3 million in Florida
Best Time to See: During the warm months, but particularly during May's mating season
Alligators are the Everglades most iconic animal—ancient reptiles that inhabit freshwater sloughs and brackish swamps, living much as they have for the past 150 million years. With their armored bodies, powerful, toothy jaws and thick, muscular tails, alligators are the alpha predators of the park. They can be seen year-round, but the best time to see them is during the height of breeding season, in May, when males are on the move in search of mates, and bellow to attract females. The best ways to get a glimpse of them are to hike the half-mile-long boardwalk of the Anhinga Trail near the Ernest Coe Visitor Center, or to take a two-hour-long mangrove wilderness boat tour, which can be booked at the Gulf Coast Visitor Center.
Best Time to See: Late Spring through October
The elk that summer in Grand Teton are members of the 11,000-strong Jackson Herd, the largest elk herd in North America. During the warmer months, these grazers migrate from the National Elk Refuge, about 60 miles away at the south end of Jackson Hole. These huge ungulates are easiest to see at dawn and dusk, when they emerge from the cool darkness of the forests to graze in lush open meadows all over the park, including along Jackson Lake, the Snake River, Blacktail Ponds, Beaver Creek and Willow Flats. Elk are simply so abundant in Grand Teton (not to mention so big) that it won't be hard to spot them. During fall, you can witness bull elks bugling–a high-pitched whistling sound that carries for miles—as part of their mating ritual.
Number: Tens of thousands
Best Time to See: Summer
Located just 50 miles west of Los Angeles, at first blush the Channel Islands make for an unlikely national park (the five northernmost islands comprise the park). But they're situated at the confluence of two ocean currents—a cold one moving south from Alaska and a warm one moving north from the South Pacific—that afford them an incredible diversity of marine life. In fact, the westernmost point on the islands—Point Bennett on San Miguel Island—plays host to one of the world's largest wildlife gatherings, when more than 70,000 California sea lions, 50,000 northern elephant seals, 5,000 northern fur seals and 1,100 harbor seals come there to breed each summer. It's not easy to get to them, though. After a boat ride to the island, it's a 15-mile roundtrip hike to reach Point Bennett.
Number: 71, with 16 more frequenting the park
Best Time to See: Winter
Wolves, reintroduced to Yellowstone in 1995 and 1996, have made a well-documented—and, to some, controversial—comeback in the park. From just a few dozen released back then, their numbers have swollen to include roughly 480 wolves in 75 packs across the greater Yellowstone ecosystem. Still, canis lupus can be pretty elusive from spring through autumn. But bare trees and a permanent blanket of snow make winter the best time for spotting the top dog, especially in Slough Creek and the Lamar Valley. Wildlife biologists Nathan Varley and Linda Thurston—aka The Wild Side—offer a full day of wolf trekking, including transportation, entrance fees, hot drinks and breakfast, from $480 (1 to 5 people) to $680 (6 to 14 people).
Best Time to See: mid-April through October
The moose—and their mortal enemies, wolves—on Lake Superior's largest island have been studied for more than five decades in what is considered the world's longest-running predator-prey study. The loping ungulates live in a closed system here, isolated from the mainland in all but the coldest of years, when the lake freezes and forms an ice bridge. Moose frequent almost every part of the densely populated island year-round, though the park is closed to visitors during the winter. An extended camping trip, which begins with a boat or seaplane ride to the park's wild shores, is one of the best low-impact ways to view one of North America's largest mammals.
Best Time to See: Winter
It's best to be upfront about this—you'll probably never see a wolverine in the wild. One of the least studied predators in the world, wolverines are incredibly elusive, thanks to their huge home territories. A single adult male might lord over an expanse of wilderness as big as 260 square miles. That makes them difficult to study, though a handful of wildlife biologists set about the work in Glacier, spending seven years tagging and tracking more than half of the Lower 48's densest wolverine population. What they found is incredible: a small, stout animal that can cover as many as 40 miles in a day; its traveling speed—4 mph—is fairly constant, even when running directly up and over 10,000-foot mountains in the depths of a February freeze; equipped with powerful, bone-crushing jaws, a wolverine will stand up to a grizzly bear more than 10 times its size. Your best chance at seeing one is to tag along with a biologist conducting research.