dog breeds in a dog's purpose

monogamy --the practice of mating with a singleindividual for an extended period of time--isn't that popular in the animal kingdom. only about3% of mammals are monogamous, and, although 95% of birds pair off (at least for one breedingseason), paternity tests have revealed that the avian world is chock-full of cheaters. the least loyal bird species might be thesuperb fairywren: they form lifelong bonds and, if you watched a pair of them from morning'til night for an entire breeding season, you'd think they were perfectly faithful.but that's only because female fairywrens cheat under cover of darkness. using radiotransmitters to track their movements, researchers discovered that fertile females make daily,pre-dawn flights to other territories. these

trips only last about 15 minutes, but apparentlythat's more than long enough-- dna tests show that just 25% of baby superb fairywrens aretheir father's biological children. so modern genetics might be deflating ourromantic notions about lovebirds, but from a biological standpoint, social monogamy withoutsexual monogamy--that is, pairing up with one individual and then copulating with otherson the side--makes a lot more sense than absolute sexual loyalty. for birds, pairing up is agood strategy because their young require a ton of care, so males increase their chancesof successful reproduction if they stick around and lend a beak. on the other hand, puttingall of one's eggs in a single basket is a risky proposition, so it also makes sensefor males to try and slip some of their genetic

material into a few other nests if they can.females, of course, can't have more than one nest, but for their part they can try to sneakin some variety. cheating might also help explain the otherwiseunexpected physical differences between males and females in apparently monogamous species.we've long had a solid explanation for male/female dimorphism in explicitly non-monogamous species:if a male plans to mate with many females, he needs to win their affection and fend offother suitors. over thousands of generations, the traits that help him successfully matecan become more and more pronounced, even if they serve absolutely no other purpose. for example, male gorillas--who fight eachother for exclusive mating rights with the

females in their clan-- are much larger thanfemale gorillas, while male and female gibbons, which are monogamous, are the same size. which brings us to our favorite primates,homo sapiens. there are undeniable physical differences between males and females--butit's unclear whether they're pronounced enough to suggest that our ancestors lived in haremslike gorillas or whether our differences stem from a monogamous but adulterous society likethe superb fairywren's. one thing is clear: among all the specieson earth, monogamy is rare, and sexual monogamy is rarer. there is, however, at least one known exampleof perfect, lifelong fidelity, and its name

is diplozoon paradoxum. when two of theseyoung flatworms find each other, they literally fuse together to form what looks like a singleorganism, and this adultery-free union lasts for their entire long and amorous lives...which they spend sucking blood from fishgills. a truly romantic attachment!