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Salted Frogs

December 15, 2016

No, this article isn't about cooking. I've never eaten frog legs. They say that they taste like chicken, so why not just eat chicken? Of course, frogs won't give you avian influenza; and, it's probably easier to raise frogs than chickens in some environments. Mars is probably better suited for ranaculture than breeding hens, although it's nice to have the eggs. This article is about factors affecting the sex ratio, the ratio of males to females, of frogs.

Humans are instinctively fascinated by sex, so there have been many surveys of the sex ratio for humans. Male births exceed female births by a considerable fraction, with the ratio of male to female births exceeding 1.05. In one study of US births from 1940-2002, the sex ratio was found to diminish with birth order, going from 1.06 to 1.03 (see figure).[1]

Sex ratio as a function of birth order.Sex ratio as a function of live-birth order, 1943-2002, as determined in a study by the United States Department of Health and Human Services, National Center for Health Statistics.

(Data from ref. 1, graphed using Gnumeric.)

There's been much speculation about the overabundance of males at birth, most of it centering around the idea that evolution has compensated for the relative fragility of men. Male infants have more health problems than female infants, and grown men take more risks which cause them to die at a younger age.

As a consequence of the greater mortality of men over women at all ages, the sex ratio of the overall population is just slightly higher than one. In some countries, the birth sex ratio is quite high, a consequence of sex-selective abortion of female fetuses, and outright gendercide of female infants (see figure).

Birth sex ratio, 2012, world map.
World map of birth sex ratio in 2012, as reported in the Gender Statistics database of the World Bank and United Nations. (Modified Wikimedia Commons image by M Tracy Hunter.)

Why does the human sex ratio at birth persist at a value of very nearly 1:1? Statistician, Ronald Fisher, explained the mechanism for this in his 1930 book, The Genetical Theory of Natural Selection. Note that this book on genetics was published long before Watson and Crick's 1953 DNA paper.

Fisher's_principle is that a 1:1 ratio is an evolutionarily stable strategy that proceeds as follows. If male births are less common, then young men would have a better chance of mating than young women, and their presumably male-producing genes will spread widely. The result is a birth ratio tending towards an equilibrium ratio of 1:1. Premature male deaths would shift the equilibrium to the slightly higher ratio that we see.

While rabbits are the animals most associated with sex in the modern world, the ancients considered frogs to be iconic of fertility. The annual flooding of the Nile brought with it millions of frogs, and thus the association of frogs with fertility. The Egyptian goddess of fertility, Heqet, was depicted as a frog; or, as a woman with a frog's head.

The Greeks and Romans associated frogs with fertility, but also Lascivious behavior. The Frogs[2] is a comic play by the Greek playwright, Aristophanes (c.446-c.386 BC), in which the frogs don't ribbit and croak, they βρεκεκεκεξ and κοαξ. Despite its title, there is just the brief presence of a chorus of frogs, and nothing frog-related. It's as if Debbie went to Dallas just to see the Texas School Book Depository.

Figure captionAn 1899 advertising poster for White Star Coffee.

Apparently, in the time before the passage of the Pure Food and Drug Act of 1906 and establishment of the US Food and Drug Administration, consumers were rightly concerned about the purity of food products.

The use of frogs in this case is something that only a Mad Man could explain.

(Via Wikimedia Commons.)

Frogs are the miner's canaries of the general environment; that is, they're a sentinel species that acts as an early warning of ecological problems. The sensitivity of frogs to their environment is exacerbated by their life cycle in which their tadpole larval stage is immersed in water.

A 2002 study, published in the Proceedings of the National Academy of Sciences, showed that parasite-infected wood frogs (Rana sylvatica), had high levels of deformities when exposed to pesticide runoff.[3-4] Tadpoles needed to be exposed also to trematode parasite infection for development of the limb deformities.[3-4] The pesticides studied were atrazine and malathion.[4] Other studies showed that atrazine reduced the levels of testosterone in male frogs below the levels found in female frogs; and, as a consequence, the male frogs did not have normal reproductive systems.[4]

As it turns out, something seemingly innocuous as road salt can affect the sex ratio of frogs. A recent study in the Canadian Journal of Fisheries and Aquatic Sciences by biologists at Yale University (New Haven, Connecticut) and Rensselaer Polytechnic Institute (Troy, New York) shows that not only road salt, but leaf litter, might influence amphibian development.[5-6]

The research team conducted experiments in which wood frogs were reared with salt concentrations of 114 and 867 mg chlorine per liter and tree litter from the species maple (Acer rubrum), and oak (Quercus spp.).[5] They found that salt masculinizes tadpole sex ratios; and, that oak litter, but not maple litter, feminizes populations, with maple-litter-reared tadpoles producing larger females.[5]

Wood frog (Rana sylvatica), photo by Brian GratwickeThe wood frog (Rana sylvatica).

(Wikimedia Commons photo by Brian Gratwicke.)

In North America, maple and oak are the predominant trees, and more than 22 million metric tons of road salt are applied in the United States each year.[6] The proportion of tadpole females was reduced by 10% when exposed to road salt.[6] In the absence of salt, frogs reared in oak leaf litter were 63% female, but the female population was reduced to 53% with the addition of salt.[6] Females are typically larger than males, but added salt caused female tadpoles to be smaller in size.[6]

Says Max Lambert, the lead author of the paper,
"So you're not only seeing fewer females but smaller females that may not be able to produce as many eggs... And the eggs are probably going to be lower quality. The health and abundance of females is obviously critical for the sustainability of any population because they're the ones that make the babies... So if you have a population that is becoming male-based, the population might be at risk."[6]

The masculinizing effect of salt has been seen in previous experiments in which sex reversal during development is caused by elements, such as sodium, that bind to cell receptors to mimic the action of testosterone or estrogen.[6]

References:

  1. T.J. Mathews and Brady E. Hamilton, "Trend Analysis of the Sex Ratio at Birth in the United States," U.S. Department of Health and Human Services, National Center for Health Statistics, vol. 53, no. 20, June 14, 2005 (PDF File).
  2. Aristophanes, "The Frogs," The Harvard Classics, Charles W Eliot, Ed., via Project Gutenberg.
  3. Joseph M. Kiesecker, "Synergism between trematode infection and pesticide exposure: A link to amphibian limb deformities in nature?" Proc. Natl. Acad. Sci., vol. 99 no. 15 (July 23, 2002), pp. 9900-9904, doi: 10.1073/pnas.152098899.
  4. Stentor Danielson, "Pesticides, Parasite May Cause Frog Deformities," National Geographic News, July 9, 2002.
  5. Max R. Lambert, Aaron B. Stoler, Meredith S. Smylie, Rick A. Relyea, and David K. Skelly, "Interactive effects of road salt and leaf litter on wood frog sex ratios and sexual size dimorphism," Canadian Journal of Fisheries and Aquatic Sciences, November 22, 2016, 10.1139/cjfas-2016-0324
  6. Kevin Dennehy, "Road salt can change sex ratios in frog populations, study says," Yale University Press Release, November 22, 2016. Here, also.

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