Evolution 2013: How Hormonal Pleiotropy Affects Sex Differences In Body Size

Anolis sagrei mating. Image from Bob Cox's lab website (http://faculty.virginia.edu/coxlab/Cox_Lab/Home.html)

Anolis sagrei mating. Image from Bob Cox’s lab website 

Although the Evolution meetings are coming to a close, we get to go out on a high note. Christian Cox gave one of the last talks of the day discussing the hormonal basis for gender differences in sexual size dimorphism in anoles. Sexual size dimorphism (SSD), or the tendency for the sexes to differ in the size of different traits, has been widely documented in nature. Usually the male exhibits comparatively larger features, such as bigger body size or larger ornaments. Anoles are an intriguing case of SSD, as the traits that can exhibit dimorphism can vary widely among species. Some species, such as Anolis carolinensis, exhibit SSD in multiple traits, including body size, head shape, and dewlap size. In contrast, other species exhibit minimal SSD. As an example, A. distichus from the Caribbean island of Hispaniola tends to show no SSD in body size or head shape, but has strong SSD in dewlap size.

Christian Cox and his collaborators posit that one mechanism underlying SSD may be a pleiotropic regulator that can couple and decouple dimorphism in different phenotypes and their candidate for this study was testosterone. They conducted experiments manipulating levels of testosterone in adult males and females of Anolis sagrei and assessed how body size, head shape, and dewlap traits changed. Anolis sagrei is a particularly good system for assessing the role of SSD in anoles. Male A. sagrei can be up to 50% larger and three times more massive than females.

To conduct the study, they took three year-old male and female lizards and gave them either testosterone or blank subdermal implants. They maintained lizards under laboratory conditions for two months and then gathered information on morphological dimensions and dewlap characteristics. Under testosterone treatment, males and females grew similarly, whereas males grew faster than females in the control group. This merits restating – they were able to make females grow like males just by applying testosterone! Clearly testosterone has strong effects on male-specific growth patterns.

To determine if testosterone affects metabolism, they measured metabolic rate using stop-flow respirometry. They found that testosterone treatment increased metabolic rate for males and females. Correspondingly, they found that visceral fat bodies were lower in testosterone treated animals, suggesting that increased growth is caused by shunting energy towards growth and away from storage metabolism. They further determined that testosterone treatment increased the size of the humerus and femur, but had no significant effect on jaw length and head width. Because this species exhibits little SSD with respect to head dimensions, perhaps this finding is not surprising, but I would be curious to know whether testosterone influences skull growth in species with SSD in head dimensions, such as A. carolinensis.

Finally, the authors found that testosterone led to increased dewlap size in both males and females. In fact, the dewlaps of testosterone-treated females were comparable in size to those of control males and eroded the sex differences that otherwise existed between them. Testosterone treatment decreased the saturation and brightness in the dewlap, leading the authors to suggest that it accelerates its development, as they posit that this color is representative of the fully developed dewlap in the wild.

Thus, they find strong evidence that testosterone plays a large role in modulating SSD in anoles. In particular, it abolishes differences in growth in various traits except for skull shape. And it can create male-like females as well as forge super-males. It would be interesting to see if, in addition to acquiring a male-like morphology, the females would tend to act like males, as well. Their next step is to conduct testosterone manipulation experiments in A. distichus, a species that has low SSD in body size and head shape, but strong SSD in dewlap size, to determine if the effects of testosterone are repeatable in a system exhibiting a pattern of SSD that is different from A. sagrei.

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Extreme sex differences in the development of body size and sexual signals are mediated by hormonal pleiotropy in a dimorphic lizard. Authors: Cox, Christian L.; Hanninen, Amanda F; Cox, Robert M.

About Martha Muñoz

Martha is a postdoctoral researcher in Sheila Patek's laboratory at Duke University. She received her Ph.D. at Harvard University, where she studyied the evolutionary ecology and thermal physiology of anoles, focusing on the cybotoid anoles from the Dominican Republic. Martha serves as Conference Editor for the Anole Annals. Website: www.marthamunoz.weebly.com

5 thoughts on “Evolution 2013: How Hormonal Pleiotropy Affects Sex Differences In Body Size

  1. “They conducted experiments manipulating levels of testosterone in adult males and females of Anolis sagrei…”

    Perhaps patterns of dimorphism are established in a temporally mosaic pattern and focusing solely on adults (albeit more experimentally more tractable) only uncovers a small part of the mechanistic details in this system. I would be VERY curious to see these experiments repeated on animals as they enter sexual maturity and juveniles well before sexual maturity to see if the patterns are consistent. It would also be extremely interesting (to me) to understand how specific tissues are responding to the hormonal signals as body size dimorphism represents a composite of many different traits.

    1. Martha, thanks for the post. A few comments and clarifications for Lars and Thom:

      These results are unpublished and are part of an ongoing series of studies across ages in Anolis sagrei. Although we’ve previously reported effects of testosterone on growth and other traits in A. sagrei adults (though only in males, see Cox et al. 2009 papers in JEB and PBZ), the data in Christian’s talk were from juveniles (4-6 months old at the beginning of the study), not adults. When we use intra-peritoneal implants to elevate testosterone in intact males and females at these juvenile stages, we find that testosterone stimulates growth in both sexes and abolishes the natural sex difference in growth that is present at this time. Other traits, such as dewlap size, are already becoming dimorphic at this stage, though nowhere near their eventual adult levels of dimorphism. Much like it does for growth and body size, testosterone stimulates dewlap development in juveniles of both sexes. Given the short timeline of the experiment (2 months), we can’t comment on eventual adult phenotypes that might result, but our data thus far suggest that a variety of dimorphic traits respond to testosterone in similar fashion in both males and females (though we can’t rule out aromatization to estrogens for any given tissue in this particular study). We’re just finishing a follow-up study on slightly older “sub-adult” animals now, along with manipulations to reduce endogenous androgens, and should have a full story ready for publication soon.

      Also, Thom, I completely agree with your “temporal mosaic” description for the development of various dimorphisms. Let us know if any of our data and/or samples could be useful to you. – Bob

      1. Cool! I am bummed that I needed to miss the meeting.

        Out of curiosity, are you measuring circulating levels of testosterone in “wild type,” control, and experimental animals? Is it possible that the hormone pellets are pushing the lizard’s growth physiology beyond its natural bounds?

        1. Hey Thom,

          Yes, we’ve been measuring circulating androgen levels in our experimental groups and collecting plasma from wild animals to describe natural patterns. The question of physiological relevance is always a tricky one, and although we don’t have all the samples assayed yet, our results thus far are promising. Drop me an email if you want to discuss it further.

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