What They Did
The researchers incubated chicken eggs with half of them kept
in 12-hour periods of darkness and green light and the others kept in total
darkness. After hatching, the brains of the male chicks were dissected to compare
the effects of the incubation condition on the distribution of particular
proteins in the brain. In natural conditions, unequal light exposure between
the two eyes is believed to affect the protein distribution, leading to greater
specialization of the brain hemispheres. The brain dissections, however, did
not reveal significant differences between the chicks from eggs in the two
incubation conditions.
The female chickens were used in additional experiments as
adults. In the first experiment, chickens had to move to the left or right to
pass a barrier in the center of a pen. The chickens from eggs incubated with
light were more likely to pass the barrier consistently on the same side,
suggesting greater brain hemisphere differentiation.
In the second experiment, chickens were individually offered
a grid of nine cups containing mealworms. Chickens that finished the test more
quickly with less time spent revisiting empty cups were believed to have better
working memory. Finally, chickens were individually placed in Y-shaped pens,
with each branch of the Y housing a familiar or unfamiliar chicken. Test
chickens that spent more time with either the familiar or the unfamiliar
chicken were believed to be able to tell the two chickens apart. Neither of the
final two tests showed significant differences between chickens from light-incubated
and dark-incubated eggs.
Further Exploration
Even though this study had few statistically significant
results, I still found it interesting. I wouldn’t have even predicted that egg
incubation with light or darkness would affect the chickens, but other studies
have shown effects of early experience on brain development. In one of my
undergraduate psychology classes, I learned that kittens raised the first few
weeks in an environment with only vertical lines would grow up unable to see
horizontal lines and vice versa (see https://www.psychologytoday.com/intl/blog/brain-food/201404/the-cat-nobel-prize-part-ii.)
Of course, a chicken in the egg is more like a mammal in the
uterus, but there’s some evidence that human fetuses, particularly in the third
trimester, have some sensitivity to light and dark, sounds both inside and
outside the mother’s body, and flavor chemicals from the mother’s food (see https://www.webmd.com/baby/features/in-the-womb).
For the chickens, there’s concern over their welfare because incubation in
total darkness doesn’t reflect the natural condition; the mother hen
occasionally gets up off the nest. But a human fetus is unlikely to experience
sensory deprivation in the womb, so parents don’t need to worry about that.
The results of the chicken experiments above didn’t meet the researchers expectations, and they suggest that the “working memory” test may have been too easy to distinguish between different levels of cognitive ability. The whole structure of the test seems to assume that chickens understand that mealworms won’t spontaneously reappear once they’re removed from a cup, and I don’t know whether that understanding has ever been tested, but that’s a rabbit hole for another day!
https://commons.wikimedia.org/wiki/File:Egg_incubator.jpg
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