Birds of a feather flock together — and mammals tend to stick together as well, or reptiles. Or slimy silicon-based hydrogen-breathers. Intelligent species often associate by preference with those who are like them, even though they can't interbreed. What the differences are can depend on what they are contrasted with; anything from closely related species, such as all primates, to oxygen-breathers or carbon-based lifeforms. This may reflect the difficulty of dealing with Starfish Aliens; the biological differences may lead to mental incompatibilities too. It can also be a prejudice to be overcome. In a Fantasy setting, the Five Races usually reflect this, since they resemble each other more than the other humanoid races. If they can interbreed, it's Species Loyalty, or something narrower. The tropes are compatible, in that a race can prefer first its own, and then those that are similar.
- The Insectoid Alien in the first Men in Black movie seems to feel sympathetic for cockroaches while despising humans.
- In David Brin's Uplift books, hydrogen-breathers leaving their own association to deal with oxygen-breathers instead are rare. While the galaxies in general abound with Starfish Aliens, hydrogen-breathers are so extremely alien that oxygen-breathers can't comprehend them, and usually can't communicate coherently or interact safely. Trying to treat with even weirder life, such as the Quantum Order, is mostly futile. Even when oxygen-breathers have achieved communication with quants, they have nothing in common that they can discuss.
Even among the oxygen-breathing species many associate with similarly-minded races. For instance, humanity gets along so well with the Tymbrimi because they're much more flexible than other Galactics and actually have a sense of humor. In many cases this is justified by the fact that most species are uplifted by another and form "clans" with their patrons and "siblings".
- In Jack Campbell's The Lost Fleet series, there is a great deal of anger that human ships fought beside monstrous aliens like wolf-spider hybrids (who were willing to communicate and have always acted reasonably toward human ships) against cute teddy-bear aliens (who attacked instantly, and respond to no communications).
- In the backstory to the Confederation of Valor series fears by the Confederate military that humans, initially the only member species able to fight, would have trouble once the Taykans and Krai started joining up turned out to be unfounded for this reason. Since the Confederation's other members run the gamut from vaguely humanoid to Starfish Aliens, the differences between the three species of similarly sized primates seemed minor by comparison.
- In the Noon Universe, humanity prefers to interact with Human Aliens and humanoids. Nonhumanoid psychology is just considered too different and difficult for any permanent social interaction.
- In one Stainless Steel Rat novel, an alliance of aliens is trying to put down humans as a revolting non-slimy race.
- In Andre Norton's Storm Over Warlock, the reptilian Wyverns are considerably more like Terrans than the insectoid Throgs. Though that the latter are Planet Looters unwilling to even communicate and the Wyverns willing to live in peace doubtlessly helps determine which one Terrans can live with.
- Subverted in the Humanx Commonwealth. Humans at first had a choice of allying themselves with the reptilian-humanoid Aann or the insectoid Thranx, and were leaning toward the Aann. Then the Pitar showed up, very humanlike and charming, and we instantly allied with them. Turned out the Pitar were Absolute Xenophobes (after they'd gotten in position, they launched a war that only ended with their annihilation), the Aann were manipulative sleazebags, and the thranx were the ones who turned out to make an awesome team with us.
- In the Star Trek Verse there have beem many non-humanoid alien races which have appeared over time, but all the ones we see on a regular basis - i.e., the ones the main characters hang out with - are humanoid.
- In the Next Generation episode "Human Soil" tiny Hive Mind crystalline aliens want nothing to do with the humanoids, calling them "ugly bags of mostly water."
- In Star Trek: Enterprise they did a full season arc with the Xindi races as the Big Bad. Xindi had five sub-races: Mammalian, Arboreal, Aquatic, Reptilian, and Insectoid. The Mammalians (along with the Arboreals) were the most reasonable to deal with. The harshest were the Reptilians. Reptilian races seem tend to be the most evil throughout Speculative Fiction.
- Later editions of D&D (and Pathfinder) tend to include an explanation of how a race interacts with other races in their description. This trope tends to dominate since the descriptions emphasise which of the other races they have the most in common with (with the odd exception of, say, mercenary or merchant races who get along with whoever they're trading with).
- Because most audience members are mammalian, this drives the trope Reptiles Are Abhorrent.
Hello, Unknown Troper. You'll need to get known to lend a hand here.