07/16/17
Litter Size - Can we predict it?

Litter Size – What the research says

Litter Size - Can we predict it?

There is an ongoing paucity in the literature surrounding most dog matters, and that means dog knowledge is often based on anecdotes and experience instead of facts and figures. Dog breeding is no exception. Breeders will tell you that they get bigger litters if x, smaller litters if y, that they’ll never mate a bitch if z. While personal experiences can provide case studies, I am interested in much bigger data.

Enter a Norwegian study looking at 10,810 litters.

This study used data held by the Norwegian Kennel Club to look at every litter registered in 2006 and 2007, across 224 breeds. Statistical analysis was then done to determine all the wonderful figures summarised below.

The aim of this study was to determine what actually makes a difference in litter size. (Litter size being the number of puppies born, alive or dead.)

Firstly, an overall average had to be determined. For this data set, the average litter size, considering all breeds, was 5.4 puppies.

For those interested in average litter size by breed: The largest average litter size was in the Rhodesian Ridgeback with an average of 8.9 pups per litter. The Toy Poodle and Pomeranian had the smallest average litter size – 2.4 pups per litter. And the Border Terrier (because I’m biased) had an average litter size of 5.1 pups per litter.

This study found that litter size was influenced by the size of the breed, the method of mating, and the age of the bitch. Litter size was not affected by season of birth, or the number of litters a bitch had had.

Size of the breed

This study examined dogs based on average breed size. They were classed as miniature breeds (<5kgs), small breeds (5-10kgs), medium breeds (10-25kgs), large breeds (25-45kgs), and giant breeds (>45kgs). Bitches were recorded against the average size of a dog for their breed and not specifically on the size of that given bitch.

“When looking at all the … litters…, mean litter size increased with the size of the breed. The mean litter size was 3.5 in miniature breeds, 4.2 in small breeds, 5.7 in medium breeds, 6.9 in large breeds, and 7.1 in giant breeds.”

The feature of larger dog breeds having larger litters is not a new thing – this phenomena is consistent across other studies. But this study is different as it found that it wasn’t just size of the breed that mattered…

Age of the bitch

The first analysis of this data showed no significance with the age of the bitch, however, once breed size was taken into account, there were two trends apparent:

  • In small and miniature dogs, young and old bitches had smaller litters than the ages in between.
  • In larger breeds, increasing age corresponded with decreasing litter size. (Young bitches of larger breeds produced the largest litters – unlike small and miniature breeds.)

Predicted litter size by the age of the bitch for the five different breed size groups from Borge et al. study.These results are a little different to other studies, which have shown smaller litter sizes as bitches get older. (As in, the results this study got for larger breeds was seen in all breeds in previous studies.) It could be that smaller sample sizes in other studies may have missed this, or that other studies used larger breeds as their data set instead of small or miniature breeds. One suggestion is that small breeds may not mature as quickly as previously believed, and so they’re not able to reproduce until they reach that mark. (To me, this kind of makes sense – considering small breeds often live longer, then it’s likely that they mature more slowly, too.)

Method of mating

Like the age of the bitch, the first analysis on the data didn’t show a significance change in litter size based on mating method. However, when the data set was adjusted for breed, breed size, and age, naturally mated bitches had significantly larger litters than those who had been AIed (either fresh or frozen).

A decrease in mean litter size of 0.4 puppies would be expected for litters conceived with AI with fresh semen and 1.3 for AI with frozen semen, both compared to natural mating.

Things that didn’t matter

The number of litters a bitch had previously didn’t influence the size of her litter. (However, older bitches normally had had more litters – and their age did influence litter size.)

The season the litter was born in did not influence litter size.

Conclusion

Size of the breed, age of the bitch, and the method of mating are three factors work together in determining litter size. It’s not one thing – it’s all three.

“… the size of the breed, the age of the bitch and the method of mating were found to influence litter size in purebred dogs when controlling for breed, with the size of the breed as the strongest determinant.”

This study is better than past studies in this area for its huge sample size, its variety of breeds, and the fact that it considers all puppies in a litter (not just those registered, as some past studies have done). In this way this study is unique. It is probably a pretty reliable data pool for purebred dogs, too, as 90% of purebred dogs in Norway are registered with their kennel club.

Things to consider regarding the vigour of the results:

  • It’s a retrospective study.
  • It does not include data from litters where all pups were born dead.
  • Calculating mean litter size is hard, because of % made up of small or large breeds. Small breeds are currently popular, so could perhaps pull down the mean litter size seen in this study. Studies done in different countries, with different breeds being popular, is likely to result in different results.
  • There is a relatively small sample of bitches who were mated not-naturally in this study, so the results should be interpreted cautiously, however the results are consistent with previous findings.
  • This study only groups dogs by weight – not by body shape. This might yield differences. (A neapolitan mastiff is really different to a greyhound.) And they group them by weight average of breed, not the weight of the individual bitch
  • This study looked at only two years and the researchers wonder if there might have been more variation in season of birth if there was more time taken into consideration.

In conclusion: Based on this study, breed size is the strongest determinant for litter size in a dog. The age of the bitch and the method of mating were also significant predictors of litter size. These three things interact, making litter size predictions difficult!

Acknowledgements:

Borge, KS, Tonnessen, R, Nodtvedt, A, & Indrebo, A 2011, “Litter size at birth in purebred dogs – A retrospective study of 224 Breeds”, Theriogenology, 75, 911-919.

Thank-you to Waldwiese Kennels for the cover image – a litter of longhair weimaraner puppies.

02/20/13

Golden Retrievers: Cancer If You Do, Cancer If You Don’t

You don’t need to be in the dog world for very long before you hear about desexing benefiting the health of dogs. These claims talk about reducing cancer (testicular, mammary, prostate, ovarian, uterine, cervix), reducing prostate disease (in boys), and preventing pyometra (in bitches).

What we don’t hear about is the undesirable side effects of desexing, and how desexing is linked to increased risks of some cancers, and an increased likelihood of joint disorders.

Torres de la Riva et al, in their research published just this month, decided to look into the health effects of desexing in golden retrievers; Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers.

They, indeed, hit back at these spruced ‘health benefits’ in the introduction of their piece, saying:

“In contrast to the rather strong evidence for neutering males and/or females as a risk factor for osteosarcoma, hemangiosarcoma, lymphosarcoma, mast cell tumours and prostate cancer, evidence for neutering as protection against a dog acquiring one or more cancers is weak.”

Golden retriever, walking next to flowers and towards camera.

This research set out to investigate spay and neuter in Golden Retrievers from 1-8 years.  They chose goldens because they are commonly used as assistance animals, and so they hoped the implications of this study may have consequences for related assistance organisations (of course, dog science only happens when it helps people!). It makes sense: it’s ‘wasteful’ to invest in a dog becomes invalid for the work they were trained in, especially if that invalidity could’ve been prevented by more-appropriate timing of desexing.

While other research has pooled many breeds and health affects together, this is the first study to look at desexing in just one particular breed.  Prior-analysis determined several conditions to look at: hip dysplasia, cranial cruciate ligament tear, lymphosarcoma, hemangiosarcoma, mast cell tumours, osterosarcoma, and elbow dysplasia.

Dogs were included in the study if they were between 1-8 years of age.  They were put into categories of either ‘early neuter’ (before 1 year of age), ‘late neuter’ (after 1 year of age), and ‘intact’.  Data regarding their health was retrospective, gained from veterinary records.  Any dogs where a health diagnosis was ‘grey’ (non-conclusive), they were excluded from the study.

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11/16/12

Research Finds: Hungry Dogs are Hungry

Some Thoughts About Dogs welcomes guest blogger Michael D Anderson from NerdWallet.

Photo © Ruthless Photos.

Biologists at the University of Vienna published a study last month about dogs’ temperament in relation to their owners. The study hypothesized that without their owners, dogs would be more likely to view ambiguous events as negative ones. This is a common feature of human cognition – you’ll often hear that depressed people “see the glass as half-empty.”

The abstract of the study is available here.

The Vienna researchers found that, unlike in humans, when presented with ambiguous stimuli, dogs don’t have a negative judgment bias when they’re in distress. This is a jargon-loaded, awkward way of generalizing on the following: These scientists found that, when hungry, dogs don’t become emotional if their owners are absent—they go right to the bowl of food because, following one of the experiment’s stipulations, these dogs hadn’t eaten in at least three hours.

Experimenters measured how long it took each of 24 dogs to approach a bowl—the study had initially included 32 animals, but the scientists decided to exclude dogs that had unusually extreme separation anxiety.

In training—before the two testing days—the biologists conditioned the dogs to identify one side of the testing room as positive—where a bowl had food—and the other side as negative—where a bowl was empty.

On testing days, they refreshed dog’s memories about the room, but then they changed up locations a bit. They established near-negative (i.e. closer to the original negative location), middle and near-positive locations. At the beginning of each test, they approached one of these new locations with a bowl.

The experimenters then tested for “latency,” or long it took the dogs to approach the bowl, when the owner was present and when he or she was absent. The owner, they said, had an effect. The dogs took longer to approach the near-negative location and shorter to approach the near-positive; in tests without the owner, they approached at the same rate to each respective location.

What I don’t understand is how the biologists so tightly connect dogs’ approach to food—which they measure as “latency”—to their mood. The idea, I think, was that dogs should take longer to approach a bowl—even if the location is near-positive—when the owner isn’t there. The idea is that the dog is distressed without the owner around—they’ll start barking, toileting, or whatever else instead of going right to the bowl.

But these dogs were hungry: as I mentioned at the beginning of this piece, owners were asked not to feed their dogs in the 3 hours before the study.

The whole premise of this experiment is odd. What they pose is that dogs are less temperamental than humans: emotional distress or not, they’ll logically discern where the food is. What I think they meant to ask is whether or not dogs behave any different after domestication: Are they still primal? The answer, I think, didn’t even require extensive experiments: yes, they’re hungry, owner be damned.

 

Further reading:

“Animal Behaviour: Cognitive Bias and Affective State”

“Bias in Interpretation of Ambiguous Sentences Related to Threat in Anxiety”

“Dogs Showing Separation-Related Behavior Exhibit a ‘Pessimistic’ Cognitive Bias”

 

This article comes from NerdWallet, a consumer-focused, analysis-driven website dedicated to dissecting the data behind the story.