The cannot catch them so they are not adapted to them.

Our cat does not eat lamb as he is not adapted to lamb, but he does eat a lot of duck purring to the skies.

Do you have a source for lions not being able to subsist on a diet of poultry?

Seems like it would take a lot of chickens to maintain a lion, and that would possibly require a large amount of effort for little gain compared to larger game. Lions can definitely catch chickens if there are some around and they care to.

I meant in a zoo. Of course it's not realistic for a lion in the wild to live exclusively off poultry.

The person I responded to seemed to seems to believe lions eating only poultry would develop nutritional deficiencies of some kind. Maybe that's true but I'm interested to learn if there are sources. Not just gut feel "they don't eat them in the wild so they can't do it".

Lions can subsist on a diet of poultry, but suboptimally.

If we assume that lions' best diet is beef [1], then chicken [2] would be less optimal for them.

[1] https://tools.myfooddata.com/protein-calculator/171797/100g/...

[2] https://tools.myfooddata.com/protein-calculator/171140/wt9/1

Look at the amino acid ratios. Leucine to valine ratio is about 0.66 for chicken and 0.8 for beef. This means that protein synthesis will be bound by valine in case of chicken and what is not used in the protein synthesis will be converted to glucose and then stored as fat. Chicken will be about 80% (0.66/0.8) as nutritious as beef, judging just by two essential amino acids ratio.

What is not used in protein synthesis will be converted to glucose and then used to power their cells.

You're badly misusing that amino acid data.

> If we assume that lions' best diet is beef

I was asking for a source for this assumption. Lions in the wild eat gazelles, giraffes, zebras, and buffalo, not cattle. I guess there isn't a great source so I'll leave it.

When was the last time you caught a duck?

My dad told me of one Christmas he spent in Sheffield in the early 60s. He'd been ill or something and missed his train back home so he was moping about miserably. Then his Polish flatmate came home, took him to a park and taught him how to catch a duck (he mimed the actions used, with some string as a snare) which they roasted for Christmas dinner. There's something grim, damp, probably illegal, but also convivial and ingenious about the story that makes me think of Withnail and Marwood in Regent's Park.