The First Heat Breeding Controversy: A Science Smackdown!

Clementine

Well, today’s topic certainly does have some claws.

Few subjects in the dog world stir as much emotion as first heat breeding.

The moment anyone mentions it, conversations can go from polite to volcanic in seconds.

I understand why.

We love our dogs, and we want to protect them.

But passion and biology aren’t always the same thing.

And pretending otherwise only hurts the animals we’re trying to help.

In this Inner Circle, my goal is to give you the clearest possible picture of the facts so you can make informed decisions.

Emotions don’t erase reality.

When it comes to reproduction, reality is written in bone, muscle, and hormones, not hashtags or hurt feelings.

So today I’m stepping into a thorny subject with calm eyes and steady hands.

I’ll address real questions (and some sharp accusations), explain what the science actually says, and put the myths out to pasture.

Whether you agree with me or not, you’ll walk away with knowledge, not rumor, and that’s what our pack deserves.

Now, let’s open the den and tackle the first question from our first Inner Circle packmate…

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“Hi Jennifer,
Does this mean Clementine won’t have more litters? I understand what you are saying when you say the growth plates of American Dirus dogs close at 16-18 months of age and I wonder about the role of Relaxin in pregnancy in dogs. I know dogs have this hormone. Also, if the first five passed through Clementine’s birth canal alive, are you saying the fixation of Clementine’s pelvis did not affect the first five births? If so, why? Why didn’t Relaxin come into play here, relaxing the ligaments and joints to open and expand her birth canal?
Genuinely curious”

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Those are great questions.

Let’s take them one by one.

• Will Clementine have more litters?

Yes. If her recovery is good, we can plan future breedings. Once a female has had a first delivery, regardless of age, future births are typically easier.

• What’s the role of Relaxin in pregnancy?

Relaxin relaxes and increases the laxity of cartilage, collagen, tendons, and joints throughout the body. Relaxin does not, however, make bones more pliable. It is often confused with an effect on bone pliability itself, but it does not widen the bony pelvis. PMC+1

• Why did the first five live and the later ones die?

Because birth is a mix of timing, size, position, and the mother’s stamina. The first five may have been smaller, perfectly positioned, or simply lucky with timing and contractions. The later pups arrived after long pauses (3–5 hours), when the uterus can tire and placental blood flow drops. That causes fetal hypoxia and stillbirth even when earlier pups survived. Add a pelvis that’s already tight because it’s “set,” and the odds stack against the later pups. That pattern is well described in veterinary studies of whelping. ScienceDirect+1

So, relaxin helps, but it can’t overcome a narrow bony canal or the lethal effects of prolonged inter-puppy intervals.

We’ll do what’s best for Clementine going forward, but she can absolutely have more puppies with good planning.

In fact, as we’ll see down below, the next time may likely be easier for her.

Now, let’s turn our attention toward another Inner Circle question about yesterday’s email.

Here’s exactly what she wrote:

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“Where are you getting this information? That is not how bodies work. Having a first litter at an older age, in their prime, without having another litter previously doesn't make it more difficult. Science doesn't back this statement. You are claiming things that are simply not true as if it is a fact and that is concerning.”

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That’s a heck of an accusation to lob at someone who’s been in the  dog breeding business for 20+ years.

Let’s walk through the cold, hard biology she said doesn’t exist:

• Relaxin relaxes and increases the laxity of cartilage in the presence of estrogen.
  Relaxin is a hormone released in pregnancy that breaks down collagen, and turns the pubic symphysis and other pelvic connective tissues more flexible. This is well shown in rats and mice and is supported by many reviews. PubMed
  
• Younger animals have more cartilage and less calcified bone.
  Bone development follows a universal pattern: epiphyseal plates and joint surfaces begin mostly as cartilage and ossify progressively with age, including the pelvis. Elsevier eLibrary
  
  In a sexually mature dog under 16 months:
  
  
  • Rough estimate: ~25–35% of the pelvis is still cartilage (fibrocartilage + growth plate cartilage + articular cartilage).
    
    In a sexually mature dog over 16 months:
    
    
  • Older, post-ossification: <5–10% remains as articular cartilage only; growth plates are closed.
    
• Across mammals, the parity effect is real.
  Farmers and vets have known for generations that first births, regardless of age, are higher risk, but once a female has delivered vaginally, many of the mechanical and physiological hurdles for later births are reduced. This pattern is seen in cattle, goats, sheep, and in observational data in dogs as well. MDPI+1
  
  So… if young animals have significantly more pelvic cartilage that relaxin affects during birth and delivering vaginally the first time results in subsequent easier births, would a young or mature breeding be more beneficial?
  
• There’s actual data showing higher risk when the first litter is delayed past about 2 years.
  One large analysis of thousands of pups found that females whelping their first litter after turning two had roughly 2.4 times the odds of dystocia (difficult birth) compared with females whelping between 12–24 months. That is species-specific evidence, not opinion. YMAWS
  
• My own 20+ years of personal experience.
  In recent memory, these dogs had closed growth plates on their first whelp:
  * Maggie (first litter @ 2 years old): 2 stillborn out of 3.
  * Yeti (first litter @ 19 months old): 3 stillborn out of 4.
  * Jengu (first litter @ >2 years old): 1 stillborn + required c-section out of 9.
  * Clementine (first litter @ 3 years old): 5 stillborn out of 10.
  
  Compared to the countless number of females bred on the first and second heats that had large litters of 8+ puppies with no stillborn births.

So no, I’m not claiming things as fact without evidence.

Here’s the bottom line:

Imagine the young female’s pelvis as a green sapling compared to the older female’s as a seasoned tree branch.

The sapling, still rich with soft tissue and cartilage, can bend, flex, and sway far more easily than the seasoned branch, which has hardened into sturdy wood. When relaxin floods the body during pregnancy, it acts like a warm rain softening the sapling even further, allowing it to bow and flex under the weight of its first litter.

An older female, whose pelvic bones have mostly ossified and whose cartilage has thinned, is more like that hardened branch. It still responds to relaxin, but only with a subtle give rather than a deep bend.

The younger female’s extra cartilage provides more “living hinge” material for the hormone to act on, while the older female’s pelvis must rely on ligament stretch alone.

Thus, both mothers experience pelvic flexibility during birth, but the type and degree of that flexibility differs: the adolescent pelvis bends more like a sapling, the mature pelvis more like a weathered branch.

NOTE: Some anthropological work suggests first births during adolescence can subtly influence pelvic morphology by adulthood. - PubMed

I have an entire chapter on First Heat Breeding with a lot more to say about this highly controversial topic in the Dire Wolf Project book I wrote in 2018.

If you want to have all the answers at your fingertips to all of the arguments we get thrown at us from time to time, then the Dire Wolf Project book is what you need:

https://direwolfproject.com/direwolf-publishing/dire-wolf-project-books/non-fiction-books/dire-wolf-project-book/

P.S. The Fraggle Rock litter will be the only litter this fall, so if you want a puppy, this is your time to reply to this email and let me know. Otherwise, you’ll have to wait until next spring for another chance to bring a loving new companion into your life.

Jennifer Stoeckl is the co-founder of the Dire Wolf Project, founder of the DireWolf Guardians American Dirus Dog Training Program, and owner/operator of DireWolf Dogs of Vallecito. She lives in the beautiful inland northwest among the Ponderosa pine forests with her pack of American Dirus dogs.