Mothers’ influence on young wildlife more than genetic
By Cody Trotter, English major, and Lynn Davis, College of Natural Resources
Few relationships in life are closer or more important than those between a mother and her offspring. In many wildlife species, mothers raise and nurture their young until they are independent, teaching valuable life lessons along the way. In addition, mothers influence their young based on the habitats they select for nesting, the types of food they supply, the amount of energy and other materials they provide to their eggs, and the behavioral interactions they have with their young during early development.
These non-genetic investments made by females in their young, called maternal effects by evolutionary ecologists, play a huge role in defining offsprings’ physical appearance, health, and abilities later in life, says William Hopkins, associate professor of fisheries and wildlife science at Virginia Tech. Hopkins and his colleagues conduct research to determine how the physiology and behavior of female amphibians, turtles, and birds affect their offspring and the consequences these interactions may have for population health.
The birds
A classic example of maternal effects is incubation behavior in birds. Birds incubate their eggs to maintain an optimal temperature for proper development of their embryos. However, many birds must also leave the eggs periodically for other activities, including feeding. Leaving the nest comes at a cost because it decreases incubation temperatures for the embryos. A decrease of even a couple degrees in incubation temperature can have important effects on the offspring.
Sarah DuRant, a wildlife Ph.D. student in Hopkins’ lab, tests how the incubation behavior of wood ducks affects their development before they hatch and whether it has lasting effects after hatching. She is trying to understand how changes in a female’s incubation behavior might influence development of the ducklings’ behavior, their endocrine system, and their immune system, all of which are critical to their survival. The initiative is a collaboration between researchers from the Virginia Tech Department of Fisheries and Wildlife Sciences and Department of Biological Sciences, Auburn University, and the University of Georgia, funded by a large grant from the National Science Foundation. The research is being conducted in South Carolina, where there is a considerable amount of information about the long-term life history of a wood duck population, making the link between female incubation practices and the effects it has on overall population health easier to identify.
To study wood ducks and other birds that nest in tree cavities, such as tree swallows, Hopkins’ research team places nest boxes throughout the birds’ breeding grounds to attract breeding pairs. Females build their nests and lay eggs inside the boxes, which are monitored throughout the reproductive season. Immediately after the eggs are laid, researchers remove some of them for incubation tests.
The borrowed eggs are incubated at various temperatures that mimic the natural range eggs would experience in the care of the mother. DuRant and Hopkins measure responses to different temperatures during development and examine the health of ducklings after hatching. The ducklings are released near their original nests to determine how they fare when returned to the wild. Auburn and Georgia researchers monitor the behavior of ducklings and determine first-year survival and return rates.
This year, the research team achieved exciting insights into how subtle changes in female behavior may have lasting consequences for their young. “We discovered that less than one degree difference in average incubation temperature is enough to strongly influence growth rates and endocrine responses during early life,” says DuRant. “This work will be important for conservation efforts because human-dominated landscapes can produce suboptimal habitat for nesting birds. When females are forced to spend more time away from the nest to find food, they spend less time incubating their eggs. Our work demonstrates that even moderate changes in a mother’s behavior could be important to the success of her young.” (Update on Sarah DuRant)
Environmental pollution is the focus of another bird project. Haruka Wada, a postdoctoral fellow in Hopkins’ laboratory, is conducting research on tree swallows in Virginia in collaboration with scientists from Virginia Tech’s biology department and The College of William and Mary. Wada and her team are studying the impacts of mercury pollution on adult and nestling tree swallows. Where adult tree swallows choose to build their nests influences the quality of insects they supply to their nestlings. Using hundreds of nest boxes along a mercury-polluted river, she seeks to establish links between local contamination, dietary exposure to mercury, and early development of tree swallow nestlings. “Mercury is one of the most important pollutants around the globe because of its high toxicity,” says Hopkins. “In addition, it contaminates even the most remote polar regions of the world because it can be transported in the atmosphere.”
As part of her research, Wada collects blood samples from nestlings to monitor their exposure to mercury and how it relates to a variety of physiological parameters important for development and longer-term survival in the wild. The chicks remain unharmed throughout the process and are returned to their nest within minutes after the blood samples are taken. The researchers then take the samples to the laboratory where they run a battery of tests to evaluate the performance of the nestlings’ endocrine and immune systems.
Most previous research on mercury has focused on its effects on the nervous system, not the endocrine system. “Haruka’s work is extremely exciting and has broad implications for understanding the effects of mercury in the environment,” says Hopkins. “Last year she demonstrated that tree swallow nestlings experience very high mercury exposure, a direct result of the prey provided by their parents. Associated with this exposure, she demonstrated for the first time in a wild animal species that mercury may disrupt normal hormone signals critical to survival. Haruka’s work represents a substantial scientific breakthrough,” Hopkins adds.
This year, Wada has expanded her efforts to include studying the immune system. “Because our findings suggested that mercury was interfering with normal endocrine processes in young birds, we also want to determine if other systems, such as the immune system, are affected.” says Wada. “Our colleagues at William and Mary have shown that adult birds breeding at the contaminated site don’t fare as well as they should in the wild. It is possible that problems with their immune or endocrine systems underlie these observations.”
All of the data enable researchers to learn a great deal about the important connections between the nesting and feeding ecology of adults and how it relates to the health of their offspring. Such information has important implications for the health of a wide array of other animals including humans.
And toads
Mercury pollution is also a concern for amphibians. Female amphibians, like other egg-laying wildlife, pass nutrients, hormones, and energy to their eggs to support the early development of their young. Unfortunately, females can accumulate contaminants in their tissues; when they reproduce, these contaminants are transferred to their eggs. Christine Bergeron, a Ph.D. student in fisheries and wildlife sciences, examines this process of maternal transfer of contaminants in American toads to document the effects of maternal transfer of mercury on early embryos, larva, and juveniles, with implications for local population health.
Many of Bergeron and Hopkins’ approaches to studying toads are experimental. On rainy nights in early spring, the research team collects breeding pairs of toads when they congregate in wetlands. The animals are taken back to the field lab and allowed to reproduce there under controlled conditions. The eggs are collected immediately after the mothers lay them, and adults are then returned unharmed to their habitats.
Virginia Tech is collaborating with researchers at the University of Kentucky to quantify the amount of mercury and other contaminants transferred to the eggs. The researchers conduct a wide array of descriptive bioassays and experimental tests to determine the viability of the offspring and how many of the offspring experienced adverse effects from the contaminants.
Data from the past field seasons confirm that female toads do transfer mercury to their eggs. “Even though females pass only a small percentage of the mercury in their bodies to their eggs, we observed lower hatching success from egg clutches with the highest mercury concentrations,” Bergeron explains. “Our next step is to determine if the mercury passed from the mother affects the hatchlings later in life, especially as they undergo metamorphosis [the transition of a tadpole into a toad]. These measurements are important because they may indicate how well toads do once they enter the terrestrial environment.”
The toad research in Virginia is important because mercury is among the most pressing pollution issues faced by the state. Hopkins and his colleagues believe that amphibians, with their permeable skin and complex lifecycles, may be a natural barometer of ecosystem health and can be used to identify threats to other components of polluted environments. Bergeron’s Ph.D. work is just one of several projects in Hopkins’ lab focused on how human disturbance affects these sensitive creatures. Hopkins’ laboratory is the only group in the world examining the important process of maternal transfer of contaminants in amphibians.
Hopkins is also looking at the interaction between pollution and parasitism, the bioenergetics of threatened species, and stress physiology. He aims to use modern physiological tools to provide scientific results that contribute to the conservation of healthy wildlife populations.
Mothers need the help.
Update: In 2009 Sarah DuRant received a National Sigma Xi Research Award for her research on immunological and energetic tradeoffs in young birds, as well as a research award from the Society of Integrative and Comparative Biology.
National Science Foundation video: Make Way for Ducklings
