Elsewhere I heard that male Long-tailed Tits will care the offspring of their brother if they're unable to have their own. However, I'd like to know if there is a case of such behavior where the two are unrelated to one another.
Two examples off the top of my head:
Meerkats (Suricata suricatta).
It's well-known that each member of a meerkat group takes turns minding the pups for the whole group, and this includes both males and females. Quote from the website http://meerkats.net/info.htm which states this specifically:
babysitter - Stays with the pups while the gang is out foraging for food. Different gang members take the responsibility different days, this is not domiated by males of females. Generally though the least hungry Meerkat will do the babysitting. The alpha female never baby-sits. This duty is for Meerkats 6 months or older.
I also found this on the same site:
After sixteen weeks [the pups] are on there own to find there food Each pup will be taken on by a adult Meerkat which will act as a mentor, who will take the responsibility to teach the pup necessary skills for foraging for food as well as responding to danger. Male Meerkats tend to mentor male pups and female Meerkats tend to mentor the female pups. Many of skills Meerkats have are taught by the mentors rather then being instinctual.
(Site author says is an enthusiast compiling information from various sources: very detailed and descriptions square with information I've seen elsewhere, so I think this is a reliable source of information despite its dodgy typing.)
Canada geese (Branta canadensis).
Canada geese often have a "creche" system where one pair of geese at a time will babysit the entire group's goslings while the others forage. Here's a discussion of this in the Guardian newspaper's Notes and Queries pages from 2011.
(The first letter states that this only occurs on water. From my own observation, I can say that that's not true. It may be where Mr Dowsett lives, but I've sometimes seen a whole small flock's goslings asleep on a canal bank together with one pair of adults standing guard over them while the rest forage further along.)
Yes most commonly in a reciprocal altruism fashion, when you aid any offspring of your species or any in your herd/pack. Crocodiles are known for this. They respond to any crocodile (of their species) distress call from any baby crocodiles. This tends to be an advantage because proximity usually means relatedness and even when it doesn't crocodiles are high enough on the food chain the risk to the males is low compared to the time and effort that would be needed to identify the offspring first.
likewise in herd animals it can be an advantage because it ends up with everyone protecting everyone else's offspring but usually has to be paired with some ability to punish freeloaders/cheaters (individuals who don't help protect the offspring).
Of course you also have males who are tricked into it, after all a male does not always know what are its offspring.
You also have males that will help with a current brood to court the mother, or even unrelated females. Basically they are showing off how good a parent they can be, some believe this is what humans are doing. This is seen in some social primates. It might happen in some birds too but I am not sure.