Saturday, October 22, 2016

BPA (Bisphenol A) and Hummingbirds

Note: This program first aired on October 22, 2016.

Earlier this fall, a listener contacted me suggesting a topic for the show. He had just replaced his glass hummingbird feeder with a polycarbonate plastic one, in a successful attempt to thwart the yellow jackets that were frequenting the feeder. He wondered though, about the possible contaminants, especially BPA, that the birds might be exposed to. Anticipating the arrival of the ruby throated humming birds in the spring and watching them feed in our yards all summer long rank high among summer pleasures for many Mainers. But with growing awareness that problematic plastic additives show up anywhere, and everywhere, it has been only a matter of time until some one put two and two together and asked this question.

Plastics are polymers, chains of individual units (monomers) strung together chemically. Most plastics are a mix of different kinds of hydrocarbons, with various additives to give them specific physical attributes. These additives have various levels of fidelity to the plastic they are part of, and some readily leach out of the plastic into the environment. Awareness has grown in the past 10 years of this potential problem with the consumer goods and food packaging we contact on a daily basis. 

Some chemical additives may be inert, other are quite biologically active and that is the crux of the problem. Bisphenol A, or BPA, the additive my listener asked about, is used in polycarbonate plastic, the kind that lexan water bottles, protective eye wear and DVDs are made of. It has proven itself to be, as many plastic additives are, a potent estrogen mimic, meaning, it binds to the same receptive sites in cells that naturally occurring endogenous estrogen does.

Estrogen is the female sex hormone in all vertebrates, from fish to mammals. It plays the same role throughout the vertebrate group, in carefully timed pulses it guides the development of the reproductive system. The biological or anatomical sex of an individual is the result of the relative balance of estrogen and male sex hormones like testosterone, and the timing of the exposure of cells to these hormones. Through the study of developmental biology, we’ve learned that the critical period for this exposure is very early in embryonic development.

Having an environmental estrogen out there can mess up this system, throwing off the balance of hormones, and the timing of exposure. And that is where most of the permanent impact of chemicals like BPA lies, by mimicking estrogen and flooding estrogen receptors in the cells of vertebrate embryos BPA can interfere with the normal development of the reproductive system of exposed organisms, be they fat head minnows, Japanese quail, or human beings.

Most of the research on the impacts BPA on wildlife has been on freshwater aquatic vertebrates, as it is easy for BPA to get into surface water through municipal water treatment facilities and industrial run off. The research on birds is much more limited, but that which is out there points to embryonic exposure leading to persistent malformations of oviducts and the shell gland (leading to thin and weak shells) in female birds, and changes to brain development in male birds leading to reduced copulatory behavior. These are problems, that, while initiated when the birds were embryos, don’t show up until they reach sexual maturity.

All of this bird research has been on model organisms like Japanese quail or domestic chickens, using exposure vectors like injecting BPA directly into eggs, or dipping eggs in an aqueous solution containing BPA. No one has looked at BPA’s effect on wild birds like humming birds, exposed through the parent’s ingestion of BPA laden sugar water from your new plastic hummingbird feeder. All we can say is that there is a demonstrated estrogenic effect in some birds in experimental conditions, but that the impacts on wild populations with more natural exposure are unknown.  If there were negative effects to hummingbirds, I would expect them to be reproductive.

And before you all start writing me telling me that you can get BPA free polycarbonate and other plastics, yes, you can. It turns out that many of the chemicals used to replace BPA are simply other bisphenol chemicals, or are less well studied, and when they are investigated, turn out to have similar biological actions. So just because it says BPA free, doesn’t mean it is necessarily great.

If you are worried about the reproductive health of the hummingbirds who visit your yard, you may want to continue your search for the perfect glass feeder, or better yet, cultivate the original hummingbird feeder, a yard full of flowers.


Excellent review article in Dose Response, focusing on aquatic vertebrates

Full text of a Swedish PHd dissertation from the University of Uppsala on environmental endocrine disruption in birds:

Saturday, October 1, 2016

Kill all the mosquitos?

Note: This program first aired on October 1, 2106.

I was asked by a listener recently about the place of mosquitos in the world. The combination of news stories about the Zika virus and developing a vaccine on the one hand, and rampant species extinction on the other got him wondering—why don’t we solve the Zika problem by trying to extinct the mosquito? At least that would be an extinction that solves a human problem. If things are going extinct anyway, why not try to get some benefit from that? The real question here is: are mosquitos good for anything?

And the answer is of course, it depends on who you ask. The typical answer to this oft asked question (why can’t we just kill all the mosquitos) is that they provide critical ecosystem services, in the form of being food for other animals in various ecosystems. Scientists are not unified on the impact of eliminating mosquitos (of which there are over 3500 known species, only a couple of hundred of which bite humans). Many say that the positive public health benefits far out weigh any temporary ecological disruption, and that any unoccupied former mosquito niche would be filled immediately by another organism. There are varying opinions and findings about the true role of adult or larval mosquitos in various food webs; they have been on the scene for over 100 million years, allowing plenty of time for elaborate co-evolution with predators, yet food web studies often demonstrate that mosquitos do not make up a large percentage of the food items in the stomachs of insectivorous predators.  Male mosquitos do not bite, and feed on the nectar of flowers, and thus serve a pollination role, though none of the plants typically serviced by them are of any economic importance to humans. So the summary on this commonly held wisdom is maybe, perhaps there would be some impact to various ecosystems, and some specialized predators would go extinct, but nothing that we know as of now that would negatively impact humans.

On the other side of this question asks why would we kill all the mosquitos, if we could? Mosquitos are flies, in the order Diptera, a group of insects with mouthparts specialized for piercing and sucking. Their life history requires a blood meal for development of the eggs, hence only the females bite. It is this biting habit that makes them an annoyance, a public health problem and an excellent means of transportation, again, depending on who you ask.  Many of the diseases that affect millions of people throughout the world, particularly the economically developing, tropical and sub tropical world, are spread by mosquitos. Malaria, dengue fever, west nile virus, triple E, chikungunya virus, yellow fever, a host of encephalitises, zika, all these are spread either from human to human, or from zoonotic (or animal) host to human, through mosquito bites.

The problem is that mosquitos exploit an ecological niche that includes us warm blooded nutritious humans, and are at the same time exploited by pathogens that use mosquitos to transport them around. Mosquitos are really just a proxy species for the pathogens we would like to rid the world of.  A patsy. Purposeful extinction of mosquitos would be an attempt to extinct the pathogens that we would like to avoid. It is these pathogens that kill hundreds of thousands of people, mostly children a year, and sicken millions more. To return to that original question, are mosquitos good for anything—if you were to ask a malaria plasmodium, or a west nile viron, the answer would be a resounding yes.

It was John Muir who once said “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” * And Aldo Leopold said “To keep every cog and wheel is the first precaution of intelligent tinkering.” ** This classical American environmental thinking argues strongly against taking out an entire family of organisms, it's flipside is the same logic that hunted large predators to near extinction throughout North America.  But I doubt the Muir and Leopold were thinking about malaria, and deep human suffering.

I think what my listener wanted to know is if we did succeed in taking out all mosquitos, would it initiate some kind of ecological collapse? Are mosquitos a keystone species? The answer is potentially no, especially when colored with the anticipated reduction in human suffering. With that card on the table it may be difficult to get a truly unbiased assessment. Which is doesn’t even take into account if purposeful extinction would be possible, though people are working hard on this front all the time.

I’d like to see us use our ingenuity to find a way to prevent contact between the disease spreading mosquitos and humans, rather than pursue what are likely to be toxic extermination methods. I think we can go a lot further to reduce human suffering and eliminate, what one scientist called, collateral damage. These are difficult philosophical questions—I appreciate you asking them. Keep them coming.


I include this for the table of mosquito borne diseases part what down the page:

The journal Nature has addressed this very same question:

*From My First Summer in the Sierra

**From Round River: From the Journals of Aldo Leopold