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Reposted from my blog.

Over the last few months, there’s been a slow-boiling battle underway between Holly Dunsworth and Jerry Coyne about the evolution of sexual dimorphism in humans, surrounding the question of why male and female humans, on average, differ in size. The battlefield ranged from blogposts to twitter to magazine articles. In a nutshell, Coyne argued that “sexual dimorphism for body size (difference between men and women) in humans is most likely explained by sexual selection” because “males compete for females, and greater size and strength give males an advantage.” His whole argument was motivated by this notion that certain Leftists ignore facts about the biology of sex differences because of their ideological fears, and are therefore being unscientific.

Dunsworth’s response to Coyne’s position was that “it’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever. It’s that this point of view is too simple and is obviously biased toward some stories, ignoring others. And this particular one he shares…has been the same old story for a long long time.” Dunsworth went on to propose, seemingly off the cuff, alternative hypotheses for sexual dimorphism in body size in humans that were focussed not on men but on women, as examples of the kind of hypothesis that is relatively rarely considered or tested in this field.

Though on the surface this battle may seem to be about specific biological facts (Coyne certainly tries to win by treating it that way), in reality this disagreement is, as Dunsworth argues, about the process by which hypotheses are tested and about how knowledge comes into existence. About which hypotheses are considered for testing in the first place. As a result, the two ended up arguing past each other quite a bit.

As I followed this whole exchange, I shook my head at the timing–I had a paper in preparation that was SO RELEVANT to the centre of this debate! That paper is now available as a preprint, so I can try to outline why I think that Dunsworth is right, and Coyne is being short-sighted. My argument has *nothing* to do with humans, however–I don’t know the human sexual selection literature well enough to weigh in on that. Instead, my argument is by analogy with our knowledge of mating systems in Anolis lizards.

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It’s currently dewlapping mayhem down here at the moment, with all species except the late-rising Cuban knight anoles (A. equestris) out and showing off!

An adult male Puerto Rican crested anole (A. cristatellus) performing dewlap extension displays in Miami FL

Visual displays such as dewlap extensions are often used to mediate physical interactions by acting as an indication of the relative size, strength, and fitness of each individual. This is beneficial for both parties; dominant individuals do not have to waste energy that a physical interaction would require, and weaker individuals avert the risk of physical injury (of course, both reasons are reciprocal to both individuals also).

However, when two individuals cannot determine dominance through visual communication, for example if two individuals are equally matched in size, then an aggressive and physical confrontation may occur (read a previous account of one such interaction between two equally-sized males here). The results of these interactions are apparent in many injurious forms, for example through extensive bite marks to the body (as previously discussed here and here), or perhaps even to the extent of tail loss (as discussed here).

Yesterday (9 March 2017) I observed this male Puerto Rican crested anole (A. cristatellus) below that looks like another male had taken a good bite at him!

Of course, there are many avenues through which such an injury may appear. However, the presence of a still-erect nuchal crest paired with how fresh the wound looks (and the time of year!) gives me the impression that this was probably the result of an intraspecific male-male interaction.

Note: the watch on the bottom right is not one of ours!

Celebrate Daylight’s Savings time with 40% off the Ecomorph line of watches on Zazzle.com. Sale Code: DAYLIGHTDEAL

And we’re open to suggestions for new species to feature on a lovely wrist fob. Suggest away!

Photo by Rich Glor

Hello everyone,

As part of some ongoing work comparing muscle physiology and performance among Anolis species, I am in search of data on the Field Active Body Temperature (Tb) of Anolis chlorocyanus so that I am sure to perform data collection at relevant temperatures. Unfortunately I have been unable to locate Tb data for this species in the literature, so I hoped one of you might have this information and be willing to share it with me. Any help would be greatly appreciated!

Along with Devi Stuart-Fox, Indraneil Das and Terry Ord, I recently published a paper in Biology Letters showing that arboreal Draco sumatranus lizards orient themselves on the tree trunk perpendicular to the position of the sun during broadcast signalling. This presumably increases the radiance of the translucent dewlap, and likely it’s conspicuousness.

Figure 1. (a) Draco sumatranus male displaying, showing the transmission of sunlight through the dewlap (photo: T. J. Ord). (b) Perch angle for displaying males, and (c) perch angle for non-displaying males, measured in relation to the sun. Both perpendicular angles (90° and 270°) have been transformed to equal 180°.

Draco lizards are ecologically analogous to the anoles and share similar signalling behaviour (see this recent Draco clip from the BBC’s Planet Earth II). They too possess extendable dewlaps that differ in colour and size between sex / species groups, and they also live in many different habitat types throughout Southeast Asia. I’ve written about my Draco research on Anole Annals before, here and here, if you’re interested – I hope they’re now well accepted as honorary anoles!

Like the anoles, the skin of the dewlap for many Draco species is stretched thin when extended and allows light to pass through.  Leo Fleishman published a Functional Ecology paper in 2015 measuring how the dewlap of Anolis lineatopis appears to glow when positioned with the sun behind them, and how this might improve signalling efficacy. Contrary to expectation, they found the transmission of light through the dewlap doesn’t improve the luminance contrast of the dewlap against the background. The radiance of the dewlap is increased by light transmission (radiance is the sum of the light reflected by the dewlap and any transmitted through the dewlap) – but patches of high radiance are very common in Anolis lineatopis forest shade environment, due to many the little shafts of light shining between gaps in the leaves. Instead they showed that due to the higher total intensity of the dewlap colour (thanks to light transmission) it’s probably easier for a conspecific to discriminate the signal from the natural background colours.

Given this and the similarity between anole and Draco dewlaps, I wondered whether Draco lizards might behaviourally adapt their position on the trunk relative to the position of the sun, to maximise the exposure of the extended dewlap to sunlight. To look at this, I just observed the position of the lizard relative to the sun upon first sighting, and noted whether the lizard was displaying, and if so, whether was it directly to a neighbouring conspecific, or whether it was a territorial broadcast display. We found males were significantly more likely to be oriented perpendicular to the sun when displaying, but not when not displaying (fig. 1).

Of course, signals intended for specific individuals in close-range encounters require the signaller to position themselves such that the receiver is in line of sight – but Draco lizards (and anoles) also give these ‘broadcast signals’ which are not intended for any specific individual, but just as territorial display. For these signals, where there is not another lizard around, they seem to orient themselves perpendicular to the sun, so their extended dewlap is exposed to the most light.

Female D. sumatranus also have dewlaps, but they are small in size and females only very occasionally engage in broadcast display.  I had not expected to see this orientation behaviour in females, as their dewlaps appear opaque and so don’t benefit from light transmission. However, I found the same orientation pattern for females as for males: perpendicular to the sun when displaying, but not when not displaying. This is perhaps because their dewlap reflects UV light (fig. 2) and direct sunlight is richer in UV and shorter wavelengths than light reflected off objects in the surrounding scene. Males have yellow dewlaps, and they too reflect a little UV (though much less than females). Of course, the transmission of light is unidirectional and only increases the radiance of the dewlap for those viewing the dewlap from the opposite side to that of illumination, so the benefit of direct sunlight hitting the UV/yellow male dewlap likely plays a role in this orientation behaviour for males as well.

Figure 2. a) Draco sumatranus male yellow dewlap colour reflectance;
(b) Draco sumatranus female blue dewlap colour reflectance.

What a lovely festive (a.k.a., brown) anole!

For more on the new AMNH exhibit on Cuba, see our previous report.

h/t to Sandra Buckner for notifying us of this magazine cover.

It’s been a while since we updated this montage…and at least a few months since the last anole cover. Get to work, everyone! And let me know if we’ve missed any.

Peter Uetz of the Reptile Database fame sends the following Valentine’s Day greetings:

If you or your significant other loves anoles, you may want to show her/him this hearty Anolis distichus (Figure 2960, above) on occasion of today’s Valentine’s Day. It clearly shows a heart on it’s head. Some other specimens such as the couple in Figure 3297 (right, from locality 1 in the Google map), also show a heart although it’s not as pronounced. Also note their blunt coloration which doesn’t seem to affect their affection.

Anolis distichus is pretty variable and even within this subspecies, A. d. dominicensis Reinhardt & Lütken 1863, to which all these specimen belong, there is considerable variation. By the way, the guy with the heart (Figure 2960) is from the same locality 3 as two other specimens which do not have a heart (Figures 2948 and 2968) although they display a similar shape on their heads. Figure 3087 shows yet another specimen for comparison, this time from locality 2.

2968

2948

3087

Various authors have described a dozen subspecies from Hispaniola (reviewed in Schwartz 1971, see map 2 from that paper). The northern half of Hispaniola is almost entirely in the hands of A. d. dominicensis, hence the specimens on the photos have been assigned to that subspecies.

Note that Glor & Laport 2012 elevated several Dominican subspecies of A. distichus to full species level, namely A. dominicensis, A. favillarum, A. ignigularis, A. properus, and A. ravitergum. The Reptile Database hasn’t followed this yet because their geographic sampling was limited to relatively few localities and they did not provide any updated diagnoses (but their recommendations have been recorded in the database). Also, there seems to be hybridization among several of these populations.

Photo localities:

2948: 3

2960: 3

2968: 3

3087: 2

3297: 1

Acknowledgments

Thanks to Miguel Landestoy and Luke Mahler who helped with the IDs.

References

Glor, Richard E.; Robert G. Laport 2012. Are subspecies of Anolis lizards that differ in dewlap color and pattern also genetically distinct? A mitochondrial analysis. Molecular Phylogenetics and Evolution 64 (2): 255-260.   http://www.sciencedirect.com/science/article/pii/S1055790310004276

Schwartz, A. 1968. Geographic variation in Anolis distichus Cope (Lacertilia, Iguanidae) in the Bahama Islands and Hispaniola. Bull. Mus. comp. Zool. Harvard 137 (2): 255- 309.  http://biodiversitylibrary.org/page/4784182

Schwartz, A. 1971. Anolis distichus. Catalogue of American Amphibians and Reptiles (108)

(used to be available online at ZenScientist, and maybe soon at the SSAR website again).

Anolis distichus in the Reptile Database

http://reptile-database.reptarium.cz//species?genus=Anolis&species=distichus

(an extended synonymy and distribution section will appear in the next database release)

The database entry also has another 43 references most of which are available online.