Hybrid vigor, or just chance?

my heterosis dream!

Females, AKC site breed standards.
Labrador retriever: avg 63 pounds, 71 pounds top end.
ACD: avg 43 pounds, 50 pounds top end.
Combination Mean average: 53 pounds

Wanda weight at 16 weeks: 30 pounds

I am enthusiastic about discussing my puppy, so I don’t mind the daily comments and questions about her size. I am curious myself. She is growing really fast, and is likely to be much bigger than I expected! I don’t necessarily accept the various guides for calculating her adult size, but they all suggest she will be larger than her breed mix would suggest.

We are fairly certain Wanda’s breeds are Labrador Retriever and Australian Cattle Dog. A litter of dogs can contain puppies from different fathers, so there is always a chance of something crazy even if one knows the mother, but we met all the littermates, and they looked very much alike. Wanda was the smallest. Why in the heck is she so big?

I have a long-time interest in heterosis, or “hybrid vigor,” which is the occurrence of increased hardiness, size, or strength when two different species, sub-species, or breeds, are bred together. Animal examples include mules (stronger than either horses or donkeys), or ligers (much bigger than either lions or tigers). There are many examples of this phenomenon, however, including most modern domesticated species.

Note: when two different species are bred together, their offspring is infertile (male donkey + female horse = baby mule; mule + mule = a party, but no little mules). Among subspecies, by the way, conception rate goes down relative to the genetic difference from their common ancestor. Dogs + dogs = baby dogs more often than dogs + wolves. Still, dogs + wolves = wolf dogs more often than would, say, dog + jackal = jackal dogs.

The term heterosis also applies to plants, the most commonly studied being corn, and roses –but plant hybridization is a whole other topic!

Breeds of animal count as the same subspecies– they are not “their own animal,” but rather an amalgamation of individual traits selected for through unnatural selection. Oddly, in domesticated species with long enough genetic lines, the hybridization of breeds can count as heterosis! Cattle are a good example of this — by crossing two breeds of cattle from very different ancestral lines, it is possible the offspring may inherent enhanced versions of one of the parent’s breed traits.

This is where it gets really crazy: the strength of the heterosis in a trait is inversely proportional to its heritability. In other words, minor increases in size, or strength, or whatever, are much more likely than major increases. We tend to think only of size regarding hybrid vigor, but it can apply to many different traits. In dairy cattle breeding, for example, they might weigh value of milk production against the value of early weaning. For beef production, it might be adult weight compared to hardiness against cold weather.

Because the odds of a strong heterosis effect is inversely related to its frequency (the larger the effect, the rarer its occurence), responsible intentional selection requires very large populations. One guide I read for the hybridization of cattle recommended at least three herds of 75 individuals each, with 5 separate pastures to rotate them through!! Following these guidelines would allow for a reasonable chance of the resulting offspring increasing their adult weight by a mean average of 5%. 5% increased weight is a huge amount of $, but the investment in resources is also very large — and the outcome is not guaranteed.

Could Wanda’s size be a result of accidental heterosis? Possible, but the effect would be extreme, meaning the odds are extremely low. I admit I want it to be this, and I struggle with the common tendency toward confirmation bias. Wanda’s size is probably not due to hybrid vigor. Darnit. That would be so cool.

In addition to the possibility of hybrid vigor, there are other factors which might increase an individual’s adult size. Unfortunately, there is not nearly as much conclusive information on these factors in dogs. What we even think we know is limited, and is mostly based in common sense (read: without actual peer review), or studies with limited scope.

• Early spaying. Some studies suggest that early spaying causes bone growth to occur faster, and go on for longer. Bone growth is regulated by a hormone which is not produced after spaying or neutering. This makes sense to me, and I do accept it as a factor, but still… 20 years ago experts were pretty sure early castration caused stunted growth, and we now know for sure that isn’t accurate. I question everything. Can’t help it. Even though heterosis is much more interesting, her early spaying probably contributes to her larger size more than hybrid vigor.
• Food. When it comes to food, there is no relationship between the actual expertise that most of us do or do not have, and the passion with which we push our opinions. It is reasonable to me that diet affects overall size, but only in extreme cases of malnutrition or mineral saturation. We aren’t feeding her food marketed for faster growth, or anything terribly unusual. I am following the advice of the best breeder I know. Her dogs are consistently healthy, happy, and long-lived. Wanda’s food seems unlikely as a factor in her size.
• Exercise. Like food, this is a topic people are very passionate about, often in direct disproportion to their factual knowledge. There are many opinions, but few facts. I don’t think her physical exercise is radically high or low. She doesn’t run marathons on concrete, nor does she spend 20 hours a day crated. She has taken a few crazy leaps, but only onto cushioned grass — and if that was a factor, it would make her smaller, not larger. I doubt herexercise is a contributer to her unexpected size.

Wanda could grow up to be 35 pounds, or 350 pounds. We will love and adore her no matter what. It is fun to consider the factors involved in
how she’ll turn out, and interesting to research. Really, though, we just don’t know how big she will be, or why! This is one of the fun things about having a mixed breed dog.