Dogs with floppy ears make them look cute and attractive and a new study has offered an explanation for why this is the case. The domestication of animals occurred thousands of years ago and it is thought that cuter or more friendly and less aggressive animals were chosen to be tamed. Over time, the traits that humans found comforting and useful in animals were inherited and expanded in species that became domesticated. Animals that are domesticated bear a resemblance to their wild ancestors but yet are very different.
In the recently reported study, published in the journal Genetics, the researchers reported that a phenomenon during development gave rise to the characteristics humans love in dogs, such as floppy ears. They suggested that neural crest cell deficits occurred during development that modified morphological traits that made animals such as dogs seem less threatening. The traits include floppy ears ears, smaller jaws and snouts, short curly tails and smaller teeth.
The study was carried out by researchers at various universities including the Stellenbosch Institute of Advanced Study in Stellenbosch, South Africa, Humboldt University in Berlin, Germany, Harvard University in the U.S. and the University of Vienna in Austria. Dr. Adam S. Wilkins was the lead author.
The neural crest is an area of the brain region that develops during embryo growth and it lays the basis for the development of the brain. As an embryo grows, it differentiates into three layers: the ectoderm, mesoderm and endoderm. The ectoderm is the outermost layer and this is the layer that will continue differentiation into the brain and other tissues. At an early stage, a thickening of the ectoderm occurs and this becomes the neural crest. The neural crest is also the source of development for the facial skeletal and connective tissues, teeth, external ears and also some nerves and the adrenal glands. The adrenal glands produce hormones that mediate the “fight or flight response.” Adrenalin is the hormone that is activated and produced during stress and is linked to aggression.
The authors of the report state that the “domestication syndrome” is related to breeding of animals that results in a displacement of early embryonic cells that are part of neural crest development and subsequent adrenal gland development. The result of this displacement would be deficiencies in adrenal gland functions that give rise to less adrenalin and less aggression. While breeding of animals during the initial stages of domestication would have been to make the animals less aggressive, some of the other phenotypic traits would have been unintended consequences. For example, displacement of neural crest cells could disrupt the development of ears, since the external ears have been suggested to have neural crest cell involvement. This disruption in the normal development of ears happened to result in less perky and more floppy ears.
The scientists suggested in their report that domestication set up selective forces for certain gene variants that affect neural crest development. These genetic variations produced a modest reduction in the number of neural crest cells or a reduction in their activities. This results in a broad range of traits that lay the basis for the domestication syndrome. As part of the domestication syndrome, dogs with floppy ears became prevalent.
By Margaret Lutze