That is very interesting also for European zoochatters to know what is on the other side of the Atlantic.
It is important to compare with European collections so that American collections from AZA which have more limited space can focus on the right populations.
Gibbons
A transfer of the last pileated and Javan gibbon, or at least, some valuable individuals would be welcome for European populations that are bigger but on the edge of sustainability. On the other side, some golden-cheeked gibbons could be sent there if AZA decide they have sufficient space to house a fourth specie.
Guenons
It is quite impressive how European and American are so well separated, with very little species held in sustainable numbers overlap. The only question opened would be the long-term sustainability of the AZA population of diana monkey. Even if it would be assessed as not sustainable, I do not know if it would be better to send remaining individuals to Europe or on the contrary send there a few individuals to enlarge the founder population.
Macaques
I am surprised AZA zoos do not hold any sustainable population of one of the endangered Asian macaques. The largest, lion-tailed, seem to be managed as a phase-out specie while it is very numerous in Europe (and where other endangered species are also held in significant numbers).
Mangabeys and baboons
Here again, it is astonishing how few zoos are housing some of the endangered taxons. Black crested mangabey would be the biggest program where cooperation between EAZA and AZA would make sense. For red-capped mangabey and drill, the only thing to do would be to send remaining unrelated individuals to EAZA that have much more specimens but would probably benefit from new bloodlines. Of course, gelada baboons can be sent again to USA but the specie is not assessed as endangered and should not be a priority in my opinion.
Golden-bellied mangabeys were already transfered from USA to Europe, probably remaining individuals are related in some way and would not be a huge benefit.
Colobus and langurs
Here, on the contrary, I would advocate that EAZA slowly phase out the Angolan colobus and transfer some valuable individuals to AZA and focus exclusively on the king colobus, more endangered and not present anywhere else.
Dusky langurs will soon be gone from Europe but does not appear to be much more widespread in America. It would be simpler to concentrate Javan langurs in Europe but if AZA have space for them, why not ?
Callitrichids
There is very few species of conservation interest in USA (pied tamarin, Goeldi's monkey and the two lion-tamarin).
I do not know if it is a lack of breeding, interest from institutions or founders, but I would say that expanding the population of pied tamarin should be of highest concern for AZA, otherwise, individuals should be shipped to Europe not to loose their genetics.
Cebids
Why there is so few robust capuchins ? Lack of interest or breeding ? I guess the tentative to build a captive population is now gone. That is really a shame, especially if the cause is that zoos have not shifted from more common species.
With spider monkeys, it appears that the job have already been done by sending some valuable monkeys to European programs while AZA focus on fusciceps and geoffroyi. Maybe some individuals from dying EAZA populations could be of some use ?
Lemurs
Aye-aye, blue-eyed lemur and recently, Coquerel's sifaka are now managed globally. European individuals of collared lemurs are not very numerous and would maybe have a better future in America. For AZA, the collection plan should definitely look like:
-Aye-aye
-Coquerel sifaka
-Blue-eyed lemur
-Collared lemur
-Mongoose Lemur
and of course the numerous ring-tailed and ruffed lemurs.
The only question mark remaining are :
-Is there enough space and genetic diversity into AZA to continue with crowned lemur ? EAZA have now quite a significative population
-What is the taxonomic status of individuals labelled as red-fronted lemur in AZA and EAZA? If they're pure, it would make sense to try to gather them in one region and increase their population size for conservation purpose.
I will conclude that I am fully aware of the complexity and cost of cross-ocean transfers but currently, too much species are housed without a future in AZA because of the competition for space that comes from larger enclosures and higher populations numbers necessary to keep a sufficient genetic diversity in the long-term and for conservation purpose. If some of those animals may help another whole population with a few specific transfers, then it should be