Bioaccumulation & Biomagnification: A Heavy (Metals) Topic

In 1962 Rachel Carson published what is now recognized as one of the most important books in conservation literature, Silent Spring. Based on original research conducted in response to reports linking an observed decline in bird populations with widespread use of DDT as a pesticide in the 1950’s, the book broadly asserted that liberal use of DDT was contributing to considerable detrimental impact on the environment through trophic interactions. Although the book was met with considerable opposition from the chemical industry and lobbyists at the time, the academic community, along with the general public, defended the work. The book also emboldened the environmentalist movement, and eventually resulted in a ban on DDT in 1972, following president Nixon’s creation of the Environmental Protection Agency (EPA).

Bioaccumulation is a phenomenon that occurs when chemicals or toxins build in the tissues of organisms over time. One of the key problems with DDT is that at low concentrations it appeared to be harmless to vertebrate organisms, while being lethal to common pest invertebrates such as mosquitoes and flies. Thus, the negative effects of DDT on larger organisms such as mammals and birds were not initially apparent. DDT is not easily broken down through the metabolic pathway, yet readily passes across the gastrointestinal barrier. Organisms that consume organic material containing DDT will therefore accumulate the chemical in their fat stores, until eventually concentrations become lethal. However, such compounds can also be “passed up” the food chain in a process known as biomagnification. Insects such as mosquitoes that die from DDT exposure may find their way into water, where they are consumed by fish. Those fish then sequester the chemical in their tissues, and may then be consumed by larger predators such as eagles, osprey and falcons (Carson, 1962).

Although DDT has been banned for use in the U.S. except for in emergency situations, there are a number of other chemicals and compounds that have been known to bioaccumulate and biomagnify. Heavy metals such as mercury (Hg) are highly toxic to most living organisms. In particular, mercury causes permanent damage to a class of molecules called thioredoxin reductases (Carvalho, et. al., 2008), enzymes that are essential for proper cell growth and in counteracting oxidative damage from metabolic activity (Linster & Van Schaftingen, 2007). In an ecosystem-level study in Connecticut, it was shown that increases in mercury concentrations of fish were correlated with body size and age. Additionally, predators occupying the top of the food chain accumulated mercury the fastest, regardless of species identity (Neumann & Ward, 1999). Similar patterns of bioaccumulation and biomagnification have been observed in sharks in marine ecosystems (Maz-Corrau, et al., 2012), as well as montane stream ecosystems (Chasar, et al., 2009).

However, bioaccumulation of pollutants in aquatic ecosystems may have potentially devastating consequences for non-aquatic organisms as well. In particular, fish-eating sea birds are very susceptible to poisoning from industrial pollutants, as some compounds such as polychlorinated biphenyls (PCBs) are remarkably persistent in tissues (Walker, 1990). In a study conducted  at Kesterson Reservoir in California, bioaccumulation of selenium (Se) was observed in several species of birds, and correlated with reductions in adult body weight, and embryonic mortality (Ohlendorf, et al., 1990).

But bioaccumulation doesn’t just stop at birds and fish. Contamination of fisheries with metabolically stable compounds and chemicals has been reported across the globe, from Romania (Bravo, et. al., 2010) to China (Feng, et al., 2007). Even in the United States there is mounting concern that deposition of atmospheric heavy metals from industrial manufacturing and coal-fired power plants can reach fisheries via hydrologic processes such as runoff, and eventually affect human health (Driscoll, et al., 2007). After all, humankind has in many ways found its way to the top of the global food chain. So while the effects of bioaccumulation may seem a distant or alien concern in our isolated human ecosystem, it may not be long before heavy metals make their way onto our dinner plate.


Bravo, A. G., Loizeau, J. L., Bouchet, S., Richard, A., Rubin, J. F., Ungureanu, V. G., … & Dominik, J. (2010). Mercury human exposure through fish consumption in a reservoir contaminated by a chlor-alkali plant: Babeni reservoir (Romania). Environmental Science and Pollution Research, 17(8), 1422-1432.

Carson, R. (1962). Silent spring. Houghton Mifflin Harcourt.

Carvalho CM, Chew EH, Hashemy SI, Lu J, Holmgren A (2008). “Inhibition of the human thioredoxin system: A molecular mechanism of mercury toxicity.”. Journal of Biological Chemistry. 283 (18): 11913–11923.

Chasar, L. C., Scudder, B. C., Stewart, A. R., Bell, A. H., & Aiken, G. R. (2009). Mercury cycling in stream ecosystems. 3. Trophic dynamics and methylmercury bioaccumulation. Environmental science & technology, 43(8), 2733-2739.

Driscoll, C. T., Han, Y. J., Chen, C. Y., Evers, D. C., Lambert, K. F., Holsen, T. M., … & Munson, R. K. (2007). Mercury contamination in forest and freshwater ecosystems in the northeastern United States. BioScience, 57(1), 17-28.

Feng, X., Li, P., Qiu, G., Wang, S., Li, G., Shang, L., … & Fu, X. (2007). Human exposure to methylmercury through rice intake in mercury mining areas, guizhou province, china. Environmental science & technology, 42(1), 326-332.

Linster, C.L.; Van Schaftingen, E. (2007). “Vitamin C: Biosynthesis, recycling and degradation in mammals.”. FEBS Journal. 274 (1): 1–22.

Maz-Courrau, A., López-Vera, C., Galvan-Magaña, F., Escobar-Sánchez, O., Rosíles-Martínez, R., & Sanjuán-Muñoz, A. (2012). Bioaccumulation and biomagnification of total mercury in four exploited shark species in the Baja California Peninsula, Mexico. Bulletin of Environmental Contamination and Toxicology, 88(2), 129-134.

Neumann, R. M., & Ward, S. M. (1999). Bioaccumulation and biomagnification of mercury in two warmwater fish communities. Journal of Freshwater Ecology, 14(4), 487-497.

Ohlendorf, H. M., Hothem, R. L., Bunck, C. M., & Marois, K. C. (1990). Bioaccumulation of selenium in birds at Kesterson Reservoir, California. Archives of Environmental Contamination and Toxicology, 19(4), 495-507.

Walker, C. H. (1990). Persistent pollutants in fish-eating sea birds—bioaccumulation, metabolism and effects. Aquatic Toxicology, 17(4), 293-324.


Images (In Order of Appearance):

1st edition copy of Rachel  Carson’s Silent Spring. (

Illustration of the process of bioaccumulation and biomagnification. World Wildlife Fund. (

American Coot, one of the many species detrimentally impacted by bioaccumulation of selenium (

Coal fired power plant. Shutterstock. (


Finding Love in the Animal World: Multiple Cues and Sexual Selection

Dating in the modern age can seem rather convoluted. Refer to any guide or book on the subject and you’re like to come across massive laundry lists detailing everything you must be mindful if you wish to snag the guy or girl of your dreams. Or at least, to make sure your outing isn’t a complete disaster. You must look good, smell nice, be confident, be a good listener, mind your posture, and as a beloved Disney villain once put it, never underestimate the importance of body language.

However, we are not alone. Other animals that seek to reproduce have interest in ensuring that they are as successful as possible. Choosing the best mate fundamentally requires some way of distinguishing good suitors from poor ones. Although the dynamics of mate choice in humans is considerably more complicated, across the animal kingdom it is typically the females that make these decisions. This is because reproduction is often more costly for females than it is for males in terms of producing gametes (eggs), birthing, and rearing young. Thus, females require as much information as possible about their potential mates to make good decisions. Males therefore communicate with females by producing a variety signals that serve as the basis for female mate choice decisions. This provides the foundation for preferential female selection for these male traits, which over time has resulted in a stunning array of adaptations and behaviors in males that in some cases serve no other purpose than to attract mates (Andersson, 1994).

But sometimes one signal isn’t enough. Different kinds of signals can communicate different kinds of information based on context. Carotenoids, a colorful pigment that is acquired in the diets of many birds, are a classic example of this principle in action. The length or complexity of a male bird’s song might indicate general healthiness or vigor to a potential mate. But bright, showy plumage may provide additional information on an individual’s ability to acquire nutritional resources, or the quality of territory he defends and forages on. Male house finches vary in coloration from yellow to red based on the amount of carotenoids in their diet. Although females do prefer that males have longer songs than shorter ones (Nolan & Hill, 2004), females will frequently choose to ultimately pair with the most colorful male regardless of other characteristics (Hill, 1990).

One question that this might raise is, if color seems the most important for females, why bother singing at all? The answer may be the medium your signal travels through. Auditory signals like song can be heard across long distances, while visual signals such as plumage and mating displays are relatively short-distance singals. A study on tree frogs suggested that this might be the case for some species. Tree frogs chorus at night to attract mates, and females show strong preferences for males that sing fast and at high frequency. However, when dummy frogs were presented to females along with recordings of a male chorus, females preferred males that had a prominent stripe on the side of their body (Taylor, et al., 2007). Thus, sexual selection in some species might be a two-step process. Traveling to meet a potential mate is costly, so females decide whether or not to make the trip depending on the quality of long-distance song. But anyone who has used a dating app would probably tell you that sometimes a once-promising suitor isn’t quite what you expected when you finally see them up close.

This issue of sending and receiving signals through the environment raises some potential concerns for conservation biologists. As urbanization, habitat fragmentation and human-induced climate change continue to alter and threaten habitats, so in turn does this fundamentally alter the physical mediums that signals are transmitted through. For example, excessive road noise may make it difficult for females to locate potential mates. Likewise, changes in the availability of nutrients like carotenoids as a result of urban development may also indirectly change male plumage coloration in some birds, thereby altering sexual selection dynamics. A great deal of research has been done on this kind of multi-modal communication in invertebrate models such as wolf spiders, jumping spiders and crickets. However, a great deal of resolution in how multiple cues interact in mate choice decisions, and how these sexual selection dynamics might potentially be affected by human disturbance, is still lacking in vertebrate systems.



Andersson, M. B. (1994). Sexual selection. Princeton University Press.

Hill, G. E. (1990). Female house finches prefer colourful males: sexual selection for a condition-dependent trait. Animal Behaviour, 40(3), 563-572.

Nolan, P. M., & Hill, G. E. (2004). Female choice for song characteristics in the house finch. Animal Behaviour, 67(3), 403-410.

Taylor, R. C., Buchanan, B. W., & Doherty, J. L. (2007). Sexual selection in the squirrel treefrog Hyla squirella: the role of multimodal cue assessment in female choice. Animal Behaviour, 74(6), 1753-1763.


Images (In order of appearance):

Screen-Capture: Ursula, The Little Mermaid (1989), Disney.

Illustration: Male and Female House Finch, Diane Pierce, National Geographic.

Photo: Hardin Waddle, USGS.