Why are Two Siblings Out of the Same Mare so Different?

An equine behavior expert addresses contradictory characteristics between foals out of one mare born different years.

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Why are Two Siblings Out of the Same Mare so Different?
Besides the genetic effects on temperament, there are many things to look at in the postnatal environment. How a foal actually behaves, reacts, or displays a personality might start with genetics but is plastic and depends on his experiences. | Photo: Anne M. Eberhardt/The Horse
Q: What factors do you think contribute to a foal’s demeanor? My mare had two consecutive foals, raised at the same farm and handled in the same manner by the same people. One was very social and sweet; the other was ill-mannered and even aggressive at times. Were they just inherently opposite, or could something have caused their behavior differences?

A: It’s really interesting to observe similarities and differences in a mare’s foals over the years. As I’m sure you’re aware, many things can contribute to a foal’s demeanor.

We know genetics play a role in behavior and temperament traits. I should say we know this in broad terms, most obviously in the general temperament differences we see among breeds of horses. We haven’t really fine-tuned the heritability of specific temperament traits in a family line of horses quite as well as we have certain physical traits. Plus, the behaviors that we can best measure that help us understand temperament are going to come from the expression of not one but multiple genes as well as the influence of the external environment and experience.

Scientists are learning that gene expression is influenced even by the cellular environment. So metabolic things that occur in an individual, even prenatally, can affect how genes act to produce the physical and behavioral phenotypes we can see

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Prior to attending veterinary school, Dr. Nancy Diehl completed a master’s degree in animal science while studying stallion sexual behavior. Later, she completed a residency in large animal internal medicine at the University of Pennsylvania’s New Bolton Center and worked in equine practices in Missouri and Pennsylvania. Diehl also spent six years on faculty at Penn State, where she taught equine science and behavior courses and advised graduate students completing equine behavior research. Additionally, Diehl has co-authored scientific papers on stallion behavior, early intensive handling of foals, and feral horse contraception. Currently she is a practicing veterinarian in central Pennsylvania.

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