Large Carnivores: Large carnivores differ in their acceptance of wildlife crossings. For example, there is little information about the acceptance of jaguar to various crossing structure types, but 10’h x 20’w structures should be considered minimal. This estimate is based on what mountain lion would likely use, as recommendations for some species are based on known behaviors of similar-sized animals.

Grizzly bear and wolves are the most sensitive carnivore species with respect to wildlife crossing design. If grizzly bear or Rocky Mountain wolves are present, open-span underpasses or wildlife overpasses may be appropriate. The open-span underpasses in Banff National Park are approximately 13’h x 50’w in size and are used by grizzly bear and other large species. Wolves in Banff National Park have a preference for open-span underpasses. Grizzlies in Banff prefer overpasses that are 150’ wide. Black bears used a variety of crossing structures including 150’w overpasses, openspan underpasses, 13’h x 23’w multi-plate arches and even 8’h x 10’w box culverts. Mountain lion, like black bear, used a wide variety of structures in Banff National Park (Forman 2003). In Banff National Park, the consensus of engineers and biologists is that the best overall design, based on a number of ungulate and carnivore species, is the open-span underpass.

Deer and elk are always target species for wildlife crossings where large carnivores are of concern. For most of the large carnivores and deer, 10’h structures should be considered minimal. If elk or grizzly bear are present, 12’- 13’h should be considered minimal. Widths of at least 20’ are recommended minimums for all large species. The least expensive steel multi-plate 13’h x 23’w crossing will likely be acceptable for black bear, mountain lion, and most other common carnivores. Reed et al 2000 recommends underpasses have an openness ratio or index of at least 2.0 to be effective. Openness ratio or index of a wildlife crossing is determined by height x width divided by length. Calculation must be in meters.

Mid-Sized Carnivores: Wildlife crossings for rare, mid-sized carnivores should be designed for larger animals to ensure their use by target species. These include ocelot, wolverine, and lynx. Using 10’h x 20’w structures, or larger, would be prudent until better research is developed (Gordon 2003). These would also be suitable for deer, black bear, and mountain lion. If elk are present, structures at least 13’h x 23’w are recommended. For more common mid-sized carnivores, excluding coyote and bobcat, 36” pipes or box culverts should be considered minimum. If coyote and bobcat are primary target species, 13’h x 13’w box culverts or 48” culverts or pipes would likely be more effective. If deer are target species, structures suitable for them (10’h x 20’w minimum) will suffice for bobcats, coyotes and most other midsized and larger carnivores.

Little is known about river otter. However, there is anecdotal information that otter may avoid narrow culverts or bridges over streams, and elect to move out of the stream course and across roadways. Suitable highway crossings should include a natural stream channel at all flows and an unrestricted bank. Otter mortality has been reduced in the Netherlands where highway crossings are considered important conservation measures (Bekker 1998).

Small Carnivores: Thirty-six inch pipes are commonly used for cross-ditching on large highways. A variety of small and mid-sized carnivores will use 36" pipes if adequate fencing is provided. Generally, species that dig holes, use burrows, or live or hunt in hollow logs or confined spaces will accept 36" pipes or box culverts. These include American badger, raccoon, skunks, American marten, fisher, mink, weasel, and foxes (Clevenger and Waldo 1999). A number of smaller mammals, reptiles, and amphibians also have been documented using culverts this size, or smaller. Cement pipes are preferable to corrugated steel, however, if steel pipes are used a layer of soil or gravel should be placed in the bottom.