From my experience as a Drum Maker and as Percussionist as well I realized that animals that work under pressure(ex.carrying weights over their natural ability or being used for ploughing)get imperfections on the skin like bruises,marks,scratches,rubs e.t.c.,which affects negatively the quality of the skin,therefor to the quality of the sound too.
All these years I have been looking to find the best among a big diversity of skins.Animals that have been raised in different climates,
animals lived in stables or free pasturage in interelation with different ways of skin tanning has been my object of investigation.
Just in order to find the Best way of choosing the correct skin for an instrument.
I also have skins from Goat for Darabuka,Goat For Djembes(Original African Way of Tanning along with My Secret),Thin-Medium Mule Skin For Djembe(tottaly diferent way of tanning),Cow,Steer,Fish (For Darabuka,Riq,Bendir).Every single instrument has its own "character" which is achieved with a unique way of making it.
As about your question those animals were used for transporting people and other activities but never harshly used.THIS THING ASSURES THEIR HIGH QUALITY.
When I purchased the leathers, had a velvet touch on the hair with no imperfections at all.In the underside of the skin I've noticed some cutting marks(I suppose from the knife.The same is for cow,steer,deer,goat e.t.c.).These parts are not used at all.If you check any,will be only on the grip side(the exterior part of the diameter that you grab the skin to mount it on the drum).
I can make you sure that these skins are excellent for Bongos-Congas with a very nice look,rich colour,super strong and velvet touch.My 20 years experience and research on skin treatment makes me confident for that.I have many years that I ended up in the way of choosing my skins and the way of treating them each time,as I have told you already above.
Thank you for your interest and for any further questions I will be happy to answer you.
Goza La Sincopa Porque No Hay Ma'Na!!!
El Greco.