PULLMAN, Wash.–Hormones regulate critical life processes such as growth and reproduction. But now hormones are being found to have impacts beyond the individual animal that produces them. They reach from one generation to effect the very survival of the next. In addition, they communicate messages about the environmental conditions affecting the parent generation to the offspring – which may help the new generation adapt to their environment.
Hubert Schwabl, Washington State University zoologist, is studying this phenomenon in canaries and egrets. It is one of several “maternal effects” that enhance an offspring’s survival and fitness. Parents convey more than genetic information to their offspring. For example, it is well known that well-fed mothers have bigger, healthier offspring. “Maternal effects” also include such things as nest site selection, variation in providing nutrients to their young, the ratio of females to males among the offspring, and now, the transmission of specific, powerful hormones that orchestrate behavioral development.
These non-genetic contributions can have profound ecological, behavioral and evolutionary implications. Animal breeders have been aware of non-genetic effects for decades, but viewed them as “noise” that obscured the genetic variation they were interested in.
Schwabl’s laboratory investigates the transmission of maternal hormones into bird eggs and the effects of the hormones on the progeny. While early ornithologists collected eggs for their variation in color, shape or size, Schwabl collects them to test their maternal hormone cocktails. And, in some cases, to manipulate the hormone levels by carefully injecting additional doses into the yolks with a fine-needled syringe.
Several years ago, while at Rockefeller University, Schwabl was surprised to discover that freshly laid canary eggs contain maternal sex steroids such as testosterone. Fertilized eggs of birds were thought to hold only the genetic blueprint, water, and nutrients surrounded by a shell. Even more surprising, the eggs within a single canary clutch contain variable dosages of maternal testosterone.
An extra boost of this hormone allows a blind and naked, newly hatched canary chick to beg more vigorously for parental food and, as a consequence, grow faster than siblings receiving less of the hormone. Moreover, the extra shot of maternal testosterone in the egg has effects that last into adulthood. As adults, offspring from testosterone-spiked eggs are more aggressive. Thus, bird mothers apparently dole out variable dosages of hormones to their offspring and thereby hormonally “manipulate” their competitive abilities in the nest and, even later, as adults.
It has long been known that eggs in a single clutch may hatch at different times, depending upon whether the mother began to incubate earlier eggs before the last eggs were laid. In such cases, the early-hatching young have an advantage over their later-hatching siblings.
Schwabl’s work has established that in canaries, later-laid eggs receive more hormones, thus partially balancing out the early-hatch advantage. “Because the effects of incubation onset and testosterone dosage work in opposite directions, canary mothers can favor either their early-egg offspring (via early hatching) or those from later-laid eggs (via testosterone in tandem with hatching synchrony),” notes Schwabl in a scientific correspondence in the March 20 issue of Nature magazine.
However, Schwabl and his co-authors Douglas Mock and Jennifer Gieg of the University of Oklahoma have found siblicidal cattle egrets offer a direct contrast to canaries. “Egret mothers deposited more androgens in the first eggs of their clutches, thus adding a potential hormonal boost to the temporal advantage caused by earlier hatch, which helps senior siblings eliminate junior nestmates.”
Schwabl’s team believes hormonal advantages among nestlings become critical in periods of food shortages. Their work helps “buttress the view that parents are not in evolutionary conflict with siblicidal offspring, but may be subtle participants in the process of orderly brood reduction.” Indeed, he observes, ” Parents don’t interfere as they observe their offspring fighting.”
His upcoming projects include comparing siblicidal species (great egrets, blue-footed boobies and cattle egrets) with non-siblicidal (little blue herons and snowy egrets) species to determine possible evolutionary and ecological bases for the phenomena. “Both patterns of behavior must work to optimize output of offspring,” said Schwabl, “but we’re not sure how.”
This fresh look at maternal effects from an evolutionary and ecological point-of-view proposes an adaptive function of the interaction of genes and maternal hormones in the production of future generations. Though still in its infancy, such an approach may reveal a powerful role for hormonal mother/offspring communication in evolution. Cross-generational effects of maternal hormones indicated in Schwabl’s bird studies may have implications for biomedicine and conservation biology.
Schwabl joined the zoology faculty at WSU last spring. To pursue his interest in maternal effects, hormones and sibling competition, he flew in 80 canaries and encouraged several graduate students to investigate aspects of the phenomena. Keith Sockman is using canaries and wild kestrels to study mechanisms of optimal reproductive strategies, including hormone transfer, offspring sex ratios and sibling competition. Jack Small, using free-living house sparrows, is investigating long-term effects of embryonic exposure to maternal steroids on offspring aggression and fitness.
A native of Germany, Schwabl has been an avid bird watcher since his childhood in the Bavarian Alps. His research is funded by grants from National Institutes of Health and the Harry Franck Guggenheim Foundation.