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The Age of Parents Predicts the Sex of Puppies in a Litter

Breeders can affect the sex distribution in an unborn litter of puppies.

Cartman0052007 at English Wikipedia [CC BY 3.0]
Source: Cartman0052007 at English Wikipedia [CC BY 3.0]

A few years ago I found myself leaning against a wall watching a friend of mine sitting in a large wire exercise pen playing with a litter of eight 7-week-old Beagle puppies. She had asked me to come along while she picked out a puppy, so that I could test the temperament of her chosen dog. While she interacted with the adorable young dogs I chatted with the breeder, a woman who had a fine reputation for producing sound, sociable Beagles.

She gazed at the squiggly group and said to me, "There are six girls and two boys in this litter. This is what we would call a 'breeders litter,' because the majority of people who show up looking for a puppy claim that they are looking for a 'sweet little girl.' Actually in my experience I find that the males are a wee bit more social and easy-going when it comes to Beagles, but nonetheless that preference in people allows us to charge a small additional premium for the females. Since there is not that much profit in dog breeding, that's important. I wish there were some kind of recipe which I could use to make it more likely that I would end up with more females in the litters that I breed, but as it is now the sex breakdown in any litter is just a matter of chance."

I recalled this conversation when a colleague, who is a geneticist at my university, recently forwarded me an article from a journal with the intriguing name Theriogenology. It took me a minute or so to tease apart the roots of that word, but it finally occurred to me that it was composed of three Greek components: therio, which means "beast" but which is usually taken to mean animals in general; genos, which means "birth"; and logos which means "study of." So theriogenology refers to a branch of veterinary medicine concerned with animal obstetrics and related matters.

The research report that I received comes out of Brazil, and the investigative team was headed by Anna Carolina Lopes Martins, of the Department of Veterinary Medicine at the Pontifical Catholic University of Minas Gerais. In effect, these researchers were trying to see if there was a particular mix of specific characteristics in sires and dams which might produce litters of puppies with a bias toward a greater proportion of males or females.

This study was based on a retrospective analysis of 602 German Shepherd puppies that belonged to 101 litters derived from the mating of 45 dams and 37 sires in a private breeding facility from 1996 to 2016. The dams were mated naturally according to the interests of the kennel and no litters that resulted from artificial insemination were included.

The results that these scientists obtained do in fact seem to map out that "recipe" that my Beagle breeder had hoped for some years ago. The first important component is the age of the dam. Based on a complex statistical predictive analysis these investigators found that the greatest single effect on the sex breakdown of litters was the mother's age. Roughly speaking, for each increasing year of the dam's age beyond the age of two, the percentage of males in the litter is expected to increase by 15 percent.

However, if we take into account the sire's age, our ability to predict the sex composition of the litter becomes more precise. Specifically, they found a higher percentage of males when older dams are mated with young sires, and a higher percentage of males for young dams mated with old sires. Conversely, a higher percentage of females seems to result when young dams are mated with young sires, as well as when old dams are mated with old sires. In other words, if there is a large age gap between the sire and dam, you get litters full of males, while if the sire and dam are approximately equal in age you get litters containing more females.

Why this change in the proportion of male and female puppies occurs is unclear. However, it has been suggested that the chemical composition of vaginal fluids changes as a dog ages, as does the chemical composition of seminal fluid in which the sperm are delivered by the male. It is possible that the mixture of these two fluids, depending upon the age of the dogs that have been mated, can make the vaginal environment more hostile for either the sperm containing the X chromosome (which would produce a female puppy) or the sperm containing the Y chromosome (which would produce a male pup if it successfully fertilized an ovum).

These results seem to suggest that an awareness of some easily manipulated factors, such as the age of the sire and dam, should allow breeders increase the probability of getting puppies of a particular sex. However there is also a random factor that the investigators found, and that involved the size of the litter: They found that a higher percentage of males were predicted for young dams delivering big litters and old dams delivering small litters. Conversely a smaller percentage of males were predicted for young dams delivering small litters and old dams delivering big litters. Obviously litter size is not easily controlled, but ignoring that random factor, these data do suggest that by choosing appropriately aged dogs for mating breeders can increase the probability that puppies of a specific sex will appear in the litter.

I think that I will send a copy of this research report off to that Beagle breeder to see if she would like to try to use this "recipe" to cook up some more heavily female laden "breeders litters." Although not too many, I hope, since over my lifetime with dogs (except for the dog of my college years) I have always owned male dogs, and I would hate to have them become rarer and more expensive.

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Anna Carolina Lopes Martins, Marília Andreia Vaz, Max Mendes Macedo, Renato Lima Santos, Conrado Aleksander Barbosa Galdino, Raphael RochaWenceslau & Guilherme Ribeiro Valle (2019). Maternal age, paternal age, and litter size interact to affect the offspring sex ratio of German Shepherd dogs. Theriogenology 135, 169-173,

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