Can Puppies in the Same Litter Have Different Fathers?

Research shows under which conditions a litter can have mixed parentage.

Posted Sep 02, 2020

Michael Peligro (CC BY-ND 2.0)
Source: Michael Peligro (CC BY-ND 2.0)

While it is usually the case that all of the puppies in a litter look very similar, occasionally you will find a litter with some puppies that look different and out of place. It turns out that there is an interesting reason for this.

I found myself pulling into the driveway of a farm owned by a friend of my wife. I was there to pick up a couple of boxes of blueberries since my wife wanted to make blueberry pies and to freeze them so that we could have them over the winter. Marion, the farm's owner, was standing in the yard next to another woman with a long ponytail. She waved me over and introduced me to her companion: "This is Jess, and she's our veterinarian. She was here to look at one of my cows, but since Kate [Marion's dog] had a litter a few weeks ago, Jess thought it might be convenient to give the puppies their first round of shots while she was here."

We walked into the mudroom next to the kitchen where Marion had set up a wire pen for Kate and her puppies. Kate was some kind of poodle cross (the sort that designer dog breeders call "doodles") and she had blonde curly hair and medium-sized V-shaped ears. So it was not surprising to see that four out of her six puppies also had light-colored curly coats and similar ears. However, two of the pups looked quite different, with short hair, a sort of Beagle-like parti-coloured coat, and ears that appeared as if they would ultimately develop into the pendant shape ear flaps common to hounds.

Jess smiled and commented, "It looks like Kate is a bit of a party girl. She must've been carousing with the neighborhood dogs because I'd bet that this litter is the result of matings with more than one sire."

Marion looked surprised and asked, "You don't mean to say that the puppies in this litter have more than one father, do you? Is that really possible?"

"Well, each puppy only has one mother and one father, of course. However, female dogs produce multiple eggs at one time and that's why they typically give birth to litters rather than to single puppies. Furthermore, bitches remain in heat (meaning that the eggs are available for fertilization in the uterus) for around 10 days.

"You've got to understand that dogs are not the best representation of 'fidelity.' I mean they generally are loyal and devoted to their human family, but when it comes to mating, they are much more like free-spirited 'swingers.' Since there are a bunch of eggs that are released at the same time, that means that if a bitch has multiple partners different eggs can be fertilized by different sires.

"However philandering with multiple partners doesn't always result in a litter with mixed parentage. If a female dog is inseminated by more than one sire, especially if the semen from both sires is deposited at nearly the same time, the likelihood that the puppies in the resultant litter will be born by different fathers is less than 1%. That is because when semen from one male is mixed with semen from another dog sperm competition results. Usually, one sire's sperm wins and he successfully fertilizes all of the available eggs. So for a litter to have more than one sire each insemination needs to be spaced out by 24 or 48 hours, and even then the results are not always predictable."

At that point, I pointed out that I had just read a recent article done by a research team headed by Fiona Hollinshead from the College of Veterinary Medicine and Biomedical Sciences at Colorado State University in Fort Collins, Colorado. They were interested in the issue of multiple sires and explored it using both natural and artificial insemination. They did space out the inseminations and found that when there were two sires 90% of the bitches actually gave birth to one or more puppies. (This is, in fact, significantly higher than the whelping rate of 76% when there is only one sire.) For the successful breedings after insemination by two sires, it turns out that only 31% of the litters showed evidence of mixed parentage, meaning that in the remaining 69% of the litters the sperm competition resulted in only one successful father. One odd finding in this study is that when you look at the mixed parentage litters, you find that the sire that inseminates the female second, compared to the first sire, is apt to father a larger proportion of puppies (73% versus 27%). A final interesting fact which came out of that study was that the litter sizes were actually significantly larger when there were two sires.

Jess nodded and noted, "You know some breeders actually purposefully breed a female dog to two different studs. They argue that it's beneficial because it will result in additional genetic diversity in a single litter, and I would expect that the increased litter size is another positive outcome (especially if there is a concern that one of the sires has low sperm count or poor quality semen). Most of the kennel clubs traditionally did not allow the registration of puppies coming from multi-sired litters. However, in 1998 the American Kennel Club decided that Multiple Sired Litters (MSL) could be AKC registered if the mother and both sires were registered purebred dogs and other kennel clubs soon followed suit. Nevertheless, the path to registration is a lot more complicated for the MSL pups than for those born into the more common single fathered litters. It requires the DNA profiles of all of the parents and all of the puppies so that the specific sire and dam can be noted on the pedigree form for each pup."

Jess laughed, tousled Kate's head, and said to her, "But of course our Katie is not going to tell us who she had her amorous affairs with. However, if we could identify the guys involved and test their DNA, we could then tell which of each of her pups belongs to which father, and afterward, we could unveil that information on a salacious episode of our very own canine Jerry Springer Show!"

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Hollinshead FK, Ontiveros M, Burns JG, Magee C, Hanlon DW (2020). Factors influencing parentage ratio in canine dual-sired litters, Theriogenology, doi: