How could a Kradjin change evolutionary if used through millennia as a mount?

Question

I’m writing a story where Kradjin carnivorous flightless birds have been used as mounts by a nomadic steppe living people for millennia and I’m curious how this might have altered their morphology. The setting of the world is earth like in nature.

• I’m basing the design of the body on a terror bird called titanis walleri. So Kradjin would originally have been 2.5 metres tall and weighed approximately 150 kilograms with a heavy build and a large axe like beak.
• They have not been actively kept or bred in a systematic way, but they have a symbiotic hunting relationship with the humans in their native area that has advantaged the birds who are more social with humans.
• Humans will approach a group of birds when they are heading out for hunting and call for them to join. They will also call for them to join on raids or war. It is common to let the Kradjin eat the dead.
• Similar to some parrots they can bond with specific humans and choose to go live with them instead of being with other birds. The humans in their native area do not treat them like property, but rather with great respect and consider them to be equals.

How could living in such a close relationship with humans have changed the morphology, social structures and vocal range of the Kradjin?

Additional information:

• In the native lands of the kradjin human societies are organised in matriarchies. Women are better riders of kradjin because they are smaller and therefore weigh less. It is possible that the matriarchies evolved because of this, that it gave women a privileged position in warfare and therefore in political dealings between tribes.
• the world fauna is based on a remix of various earlier ages like the neogene. Horses do not exist. There are patriarchal societies though, these have larger, but herbivourous flightless birds as mounts and use ox like mamals for transport of heavier loads.

more additional information:

• Kradjin are quite intelligent, on par with grey parrots.

Answer 1

No animal survives extended contact with humans unchanged

The second bullet point states that there isn't any specific breeding program of the Kradjin by humans. This just means that one of the possible evolutionary pressures isn't being applied. There are many others.

The third bullet point does offer an interesting evolutionary pressure. When times are good, the Kradjin who don't hunt with humans will survive just as well as those who do cooperate. However, in times of famine or other ecological crisis, the added advantage from human-Kradjin cooperation means that those Kradjin who do cooperate will tend to survive more often than those that don't. If there are enough narrowing events like this, there won't be any Kradjin who don't cooperate with humans. Or, if there are any left then will move to a different area or exploit a different ecological niche.

Note that humans will also evolve at the same time. Humans who can't ride the Kradjin may not survive as well as those who can leading to selection in the build of humans or there's a strong sexual selection for males who can ride the Kradjin over those who can't.

Answer 2

I like Green's answer. I'll add a few details I think your birds would have.

The way their symbiosis happens with humans looks like what happened between wolves and the first humans in our world. Neither species "needed" the other to survive, but where both met and cooperated, they fared better than they would otherwise.

The difference being that in our case, wolves are docile enough to be domesticated. Your birds seem too large to be easily leashed and dominated by force. Their social behavior may also not include an alpha in a group, so a human wouldn't necessarily be able to command a pack by bonding with its leader.

Bonding with these critters is not going to be as easy as bonding with a wolf. It also won't be the bonding a tamer has with a lion neither. I'm saying this because I think humans don't get to raise them from youth, right?

Still, a relationship like the one you describe, where it seems to me there is no master and no tamed beast, is not unheard of.

In Brazil, wild dolphins have forged an alliance with fishermen where the cetaceans call the shots. This makes the hunt easier for both species. This has been going on for generations, with the adults from each species teaching their young how to play their roles in this partnership.

So there is a model for you to develop on. It could go something like:

• Humans build traps to catch prey, but the catches are few because we are poor runners (when compared to game);
• Birds, on the other hand, cannot build tools and traps, due to an accute lack of opposable thumbs, paired with a less powerful brain than ours. So they have to rely on talons, sharp beaks, maybe even teeh like the most primitive birds, and a lot of blunt force;
• At some point, birds learn that they can save themselves a lot of effort by leading their prey into traps built by humans. This involves some trial and error before they figure it out, but they eventually do;
• For the first few dozens of generations, humans and birds fight for the spoils. But some birds notice that taking some flesh and leaving another part for the humans is less of a hassle than fighting them.
• Humans also notice this. And they tend to set up traps more often where they live close to bird populations that won't fight them for food.

The very first trait the birds have to evolve is the ability to recognize humans as something other than food or threat. If they can communicate with humans somehow, it will also be to their advantage - like a call to tell us that the prey is coming, so we can activate a trap somehow. Or a call to signal which direction they are coming from.

If they can learn to distinguish signals given by humans, i.e.: a hunter saying "nonono" while pointing to where a trap sits, so they can avoid it - that will also be to their advantage. They will learn to read cues from our voice, facial expressions and hand gestures, much like the way that dogs can read us.

This hunting strategy also benefits them more if they hunt as a pack, so that they can coordinate among themselves how to better push prey onto traps. This may actually how and why they evolve to move in packs, with the populations of birds that don't cooperate with humans living a more solitary lifestyle.

Since a pack is more efficient in killing prey than a single individual, the members of a pack don't have to be as large and strong, so they may evolve to a smaller size - which may be part of the reason that pack hunters like velociraptors tended to be an order of magnitude smaller than solitary hunters like the T-Rex. In your case, it could be that two and a half meters being the final size of your human-friendly terror birds, their closest cousins may be four-meter tall solitary terror birds (or whatever larger size you decide to settle on, if you wish to include these details in your world);

Now say we cooperate to catch the bigger, meaner beasts out there (mostly bull-sized or bigger mammals), but your birds can catch rodents, lizards, small cats and birds etc. on their own. They will lose some traits they need to kill off the larger, harder game on their own. They may evolve to lose their teeth (if they have it), and their talons and beaks may lose some sharpness. They will still be sharp, just not as much as originally. Maybe they originally had claws like a velociraptor, and now they have claws like an ostrich. Their beaks will still be sharp, though, at least as a vulture's, so as to cut flesh in smaller, swallowable pieces.

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As to how the birds become mounts. This may be awkward.

For an animal the size and shape you describe, the only reasons another creature may want to climb its back and cling there would be to kill it or to mate with it. We know that the riders don't want that, but how did they convince the birds to allow themselves to be mounted in the first place?

It may be that the mother birds carry their young in their backs. That's why they would allow a human, specially a small one, to climb there - they have the instinct to carry another creature there (maybe only fmales are rideable? Maybe that's why women bond with these birds more easily?). At some point someone had the idea to mount a bird to use it for speed on a hunt, while taking along some javelins or a bow. This becomes a successful manner of hunting, so the birds evolve into a more ostrich/chocobo body shape (i.e.: more saddle space, wings don't interfere with leg riders so much).

But for this to happen, it may be necessary for them to learn how to live along with humans first. Such as in the way described above in which they approach humans for an easier, but with no mandatory close contact.

Answer 3

There are 2 types of artificial selection: selection for and selection against. Selection For is what modern animal breeders use, and requires knowledge of heredity etc. You have a litter of 10 puppies - some soppy, some average, some aggressive. If you want to breed a tame, docile dog, then you only permit the mild mannered puppies to breed - you are selecting for docility.

Selection Against requires no knowledge of genetics, heredity and the like. You have that same litter of 10 puppies and do not control their breeding at all. Except that whenever an aggressive pup bites your child, you kill it. It will take far, far, longer than Selection For, but eventually you'll get to the same docile dog breed.

You Kradjin are being encouraged to eat human dead after war/raids. This sets up a very unfortunate equation of Human = Food. And human children are far more easy to overpower than adults and don't tend to carry those nasty pointy, sharp things to fight back with.

So I'd say that in the early days of this symbiosis, there would have had to be some culling of Kradjin going on. A 'rogue' kradjin hunts a lone adult or takes a human child when hanging out in the village, then the village retaliates and kills it. Gradually, through Selection Against, the local kradjin become less inclined to view people as prey.

(I know a guy who owns a harpy eagle and says he's seen it eyeing up small children in a speculative "Hmm, lunch!" way, which makes him very nervous. Harpy eagles are tiny compared to terror birds, so half the village will be on the menu of 'pre-symbiosis' kradjin).

On the other hand, from their use in war, I guess you want the kradjin to attack people? At least to attack the enemy side? So to solve that contradiction, have your riders use a battlecry that mimics the squawk/whistle of a parent kradjin protecting its chicks. In the wild a sentry kradjin would squawk and the pack would rally to stamp and peck the encroaching predator to death. In battle the riders trigger this instinctive behaviour. (Downside, anyone who can do the right squawk can cause chaos in the ranks).

Eating the dead... perhaps it is a cultural thing and it is all dead, not just the war dead? The deceased are chopped into bits and left for carrion birds of all sorts, not just the kradjin. Like some kinds of Tibetan sky burials. If the kradjin are never given whole bodies, maybe the Human = Food link is broken? Most non-carrion birds have a rubbish sense of smell, so kradjin may not be able to identify the 'meat' as human flesh.

Edit: oops, almost forgot the social stuff. You don't say what the original social structure of the birds is, so here's a suggestion. They form 'packs' because they are cooperative breeders (like humans, wolves, scrub jays). The environment is tough enough that Mum and Dad alone cannot successfully raise a chick. They require help from siblings, last year's chicks, mates of previous broods, etc. The animal equivalent of "It takes a village to raise a child".

Humans have plugged into this social system as part-time helpers. Parent birds can drop off their chicks with human 'babysitters' when they go hunting. Or humans will kill a sheep from their flocks and feed it to the birds in hard times.

The bit I have trouble with is the sheer number of kradjin packs a decent sized human nomad band would have to be on good terms with. This is fine for people, but the kradjin packs should be in competition with each other, like wolf packs or prides of lions. Perhaps human interference has created 'superpacks'? So many kradjin chicks now survive, that instead of a pack of 10 that sometimes splits into 2 groups of 5, the rider-kradjiin pack numbers 50 or more, in a complex fission-fusion group with multiple breeding females.