Sex ratios in zoo populations.

P

Pertinax

Well-Known Member
15+ year member
I've just read Kiang's post about the Somali Ass- facing problems because too many colts are being produced. Its a subject that's always interested me.

In many zoo populations, Births are skewed towards males. In others the sexes seem about eqiual. In a few, such as the Pygmy Hippo I believe, there is actually a shortage of males.

I know many theories are put forward to explain why animals consistently produce offspring mostly of one sex only- temperature, diet and condition, stress, environment etc.
It seems that zoos still haven't got to the root of the problem yet.

What do others think....??
 
there was a time when the eep was chronically short of male malayan tapirs but it seems to be sorting that one out edinburgh and i think antwerp both bred males this year also the hamlyns mokeys were also in that position.
 
I've always wondered if there was a natural bias in some species to produce more males than females- a sort of genetic adaptation that made male offspring more likely. You could see how it would be a benefit some species where there is more pressure on a male; as well as having to contend with finding food and evading predators he might also have to maintain a territory and is more likely to succumb to injury whilst fighting rival males. If males were more likely to die, more would have to be born to keep numbers even.

The only problem with that theory is its pretty impossible given gender has to be a 50% split due to the whole XY chromosome deal (unless you're a croc or a turtle.) I guess its just something we've yet to fully understand.
 
The red birds of Paradise are only producing males. This wouldn't be that bad, but they are particularly prone to iron storage, and this leaves Chester zoo with one of the last breeding red BoPs in Captibity.
 
^Chris^ said:
I've always wondered if there was a natural bias in some species to produce more males than females- a sort of genetic adaptation that made male offspring more likely.
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The reason for a higher male ratio is because of the 'y' chromozone faulty genes on the 'x' are not back up and so the males have a higher immortality rate in the young and so nature has made it more likely for a boy to be born, but with modern science this problem is now less fately and so the population gets scewed.
 
taun said:
The reason for a higher male ratio is because of the 'y' chromozone faulty genes on the 'x' are not back up and so the males have a higher immortality rate in the young and so nature has made it more likely for a boy to be born, but with modern science this problem is now less fately and so the population gets scewed.
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Aah. That makes sense- thanks Taun!
 
Yup, even among humans the ratio is about 52/48 i believe? towards males. But at the age of around 5/6 the ratio is allready turned towards more women because of the higher mortality ratio among men.

That wouldn't explain the even higher ratio's some animals show, but i guess you could still blame some of it on chance. Like the Sifaka's and Elephants not that many are born yet and it could be chance that there are more males. But i wouldn't find it hard to believe that influences like food offerings, temperature or others would influence the ratio.
 
kiang said:
there was a time when the eep was chronically short of male malayan tapirs but it seems to be sorting that one out edinburgh and i think antwerp both bred males this year also the hamlyns mokeys were also in that position.
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kiang,

Nope, Antwerpen Malayans had a female calf. Actually, it is a good sign that several females have been born into the EEP population lately. They have helped to re-invigorate the breeding programme.

As several folks have already mentioned there is a genetic predisposition towards male-biased sex ratios in newborn mammals. In particular in large hoofed and territorial species it is an advantage to have male offspring. However due to the great pressures males are under after weaning as a calf and into subadulthood, more a prone to perish before reaching sexual maturity (and even then stallion\bull fighting accounts for more male losses). Likewise good old Homo sapiens .......!
 
Chester's Chimpanzee group consistently produced female babies over very many years, without a single male bewing born. I believe a male birth broke the run a few years back.

I think Indian rhino's are short on males too- or is that adult bulls as opposed to calves born, or is one the result of the other here?
 
kiang said:
it must have been amsterdam that had the other male
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Caught out again! Artis Amsterdam Zoo's birth this January 2007 was a female ... also! lol ...

It is Edinburgh you are probably thinking of. They had a male calf born in March 2007. Nuernberg Zoo's pair in Germany had a male calf in late 2005. Belfast Zoo had a male born February 2006.

As for other recent births: I forget London Zoo and Port Lympne here ....!
 
grantsmb said:
I think Indian rhino's are short on males too- or is that adult bulls as opposed to calves born, or is one the result of the other here?
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Now this is my area of expertise. The Indian rhino EEP is really at loggerheads and actually enjoys the luxury of having a dirge of females to breed from. Now all is not well that sounds well here! The EEP population is still heavily skewed in favour of a few founders, mostly Basel Zoo born individuals that have spread out their genes over other collections.

Genetically important bulls are hard to come by with those from the Whipsnade, Planckendael (WhipsnadexBasel) and Berlin Tierpark lines most favoured as totally unrelated to the Basel stock. Less desirable are the San Diego born males Gujrat, Jaffna and Gangtok (partially inbred) who tie 2 additional founders between themselves. And then the long line of Basel line males (includes Stuttgart who bred from Basel animals).

Genetically important on the female side are the wildcaught females at Berlin Zoo (no surviving offspring yet, started breeding only 2006), Stuttgart Zoo, Muenchen Zoo (no surviving offspring yet, last bred 2004) and the 2 Whipsnade young Nepali cows (one giving birth 2006, another due 2007). Also several Planckendael females (Lisboa and Lisieux) are sought after. The rest is again made up of Basel or Stuttgart originals.

Now, the EEP breeding programme could actually do with a few more founding animals. And I live in hope that all will see the light of day and acquire some of the mostly wild-caught males languishing in some Indian zoos without mates (an animal for technical/scientific knowhow deal) and no chance of a short-term break. Perhaps, I will personally do something about that as well. Who knows!

I have moved this on to the European zoos thread!
 
I've noticed that captive monkey groups tend to start producing mostly males if no new blood is brought in after some time has passed. Whether this is a way of shutting down the breeding potential of an inbred group/ or a group where there is genetic over-representation, I don't know.
 
Yes, I've heard that theory too- producing all males effectively 'cancels' the inbreeding over time. HOw valid it is, I've no idea. Primates in zoos do seem to suffer from a preponderance of male births, at least in some situations. However, maybe it just seems that way. Some groups like the Chester chimps and Liontailed Macaques, have produced a high proportion of females. Lincoln Park, Chicago, did the same with Gorillas some years ago too. Port Lympne's Drills have produced 3 female young in a row( another new baby was born recently- I don't know its sex...)
 
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