Wolves

Nice topic Omshanti!
The word for keeping the same partner for life, in animals, is "monogamy". i.e If an animal keeps its partner for life it is said to be "monogamous".
Birds are the most numerous of monogamous animals, while mammals display this social behaviour in only a few species. I'll begin with wolves. Numerous types of wolves display monogamy, but I suppose you are referring to the general "Grey Wolf". In a pack, there is a dominant male and female, who are the only ones to breed. Thus, they partner for life and produce all the pack's offspring. Other canine examples include the Maned Wolf, Ethiopian Wolf, Jackals and Foxes even. Monogamy is a bit of an interesting concept, so to attain an understanding of why animals would go against natural "law" and aim to spread there genes around a population as much as possible - remaining loyal to one partner - one needs to distinguish the different levels of monogamy.
Monogamy can be referred to loosely as just being periodic, or only applying to the male, or the female. However, by the definition we are using, it refers to partnering for life.
Here is my idea on why the alpha male and female of a wolf pack are monogamous and are the only to produce young. The alpha male and female are in the position they are, because they are the most senior and matured pair. They are meant to have the fittest and strongest genes, most suitable for passing on to the next generation. Hence why these two partner for life.
Alpine Marmots are monogamous I'm pretty sure...Marmots are medium sized rodents. The alpha male and female are the only to produce litters in the group, and they go to pretty drastic extents to ensure this is the case. During estrus (when females are in heat/sexually active) the alpha female beats up the other females (her past offspring) in case they have mated with any males. This will kill the unborn offspring, especially after several weeks of bashing. Other than maintaining the gene pool, the alpha female does this because, during the deep winter hibernation, if too many offspring are snuggled up in the burrow, body heat is spread to sparsely among the tightly snuggled group, so they have a high chance of freezing to death. Anyway, I'm getting a bit off track from monogamy.
Other monogamous mammals include a few types of bats, beavers and otters, dikdiks (a tiny African antelope), titi monkeys, marmosets (and other New World - South/Latin American - monkeys), and lesser apes like Gibbons and Siamangs. Also, Indris (type of lemur) are monogamous, as well as other lemur species I'd imagine...
In the instance of birds, well, there are many monogamous species, such as the Albatross (eg. Wandering, Royal, Sooty.etc) and the Swan. In fact most passerines (loosely meaning "perching song birds") mate for life.
So there we go, monogamy, a very interesting social habit present in few mammals, but a large percentage of birds.

- Knights -

Very true Aelfgifu. Cuckolding is rather common in the bird world. Also, we mustn't forget that rogue males are notorious for "raping" unprotected (and "in-a-relationship") females. 

Wolves work on a caste system I guess you can say, and in that system positions can be challenged. Sometimes the Alpha male may even choose another female to be the new Alpha Female.