On an individual level, hybrid animals sometimes have reproductive, health or behavioral issues due to their genetics. Ecologically, hybrids blur the lines of species and pose problems for conservation.
A hybrid is the offspring of parents from two distinct species or subspecies. Many species, because of genetic or behavioral differences, do not or cannot interbreed to produce viable offspring. However, in some cases two species are similar enough genetically to create hybrid offspring. Some hybrids, like mules, are sterile. Mules result from a cross between a female horse and a male donkey. While the mule itself can reach healthy adulthood, it is typically incapable of producing gametes of its own and does not reproduce.
A liger is a hybrid between a female tiger and a male lion, and it is often larger than either parent. Ligers are prone to health problems and birth defects, and many do not survive into adulthood. It is also difficult for the tiger to give birth to her hybrid cubs; their large size may necessitate a C-section for the mother.
Hybrid animals may also exhibit unpredictable behavior. One famous example is the wolf hybrid, which is a cross between a domestic dog and a wolf. Few peer-reviewed papers have been written on the topic of wolf hybrid behavior, and the animals are surrounded by a great deal of controversy. Some owners claim their hybrids are gentle, shy animals, while other anecdotes point to wolf hybrids being unpredictable and dangerously aggressive. Some of the problems arise from the fact that dogs have been domesticated for 15,000 to 30,000 years and, in that time, have been selectively bred for specific behavior patterns that are beneficial to owners. The conflicting genetics of a wolf-dog cross leads to unpredictability. Wolf hybrids are also likely to see their owners as their pack, and care must be taken on the part of the owner to assert himself as the dominant pack member.
A larger scale problem of hybridization comes from wild hybrids. Some species of wild waterfowl, for instance, can easily interbreed with domestic counterparts. This can muddy the genetic waters of a species, in some cases to the extent that a species is in danger of extinction through hybridization. This issue is not by any means limited to waterfowl. The Scottish wildcat is in danger of extinction because of hybridization with feral domestic cats.