Wednesday, November 13, 2019

Answer: How many species aren't 1:1 in gender ratios?

You'd expect it to be a 50/50 split... 

... that is, the split of sexes in any particular animal population. 

But, as we've noticed before the majority of parrotfish on a reef are female (see our earlier blog post about parrotfish).  

The  Atlantic Monthly article that started off this Challenge points out that the majority of bats in museums are females, but other species (like bison or bears) tends to skew primarily male.  Really?  Is this due to the way animals are collected, or do the animals really have an intrinsic difference in their sex ratios?  


There are more female bats than males bats in museum collections, but more bison and bears are male in those collections.
Can that possibly be true in reality?


Thinking about this made me wonder about this topic more generally: 


1.  Do most animal populations consist of males and females split roughly 50% and 50%?  

2.  If that's not true (and it's clearly not for parrotfish), what causes a species to have a non-equal split between males and females?  What kinds of animals have very different splits?  


I started with a basic query: 


     [  sex ratios animals ] 

(I used "sex" rather than "gender" because the term gender typically implies some kind of social agreements, and for animals I just wanted the biology.) 

But even before I read the SERP, I realized as I was typing the query that I already know about the changing sex of parrotfish AND the fact that something like 90% of all honeybees are female.  So when I saw this set of results, I wasn't terribly surprised... 


There are scholarly articles about sex ratios, and 4 different results at the very top of the results (indicating that these are especially relevant to the topic of the query).  

To do a quick scan, I did a bit of parallel browsing (see our earlier discussion), opening the  top is 7 links, as well as all of the Scholarly articles, the Wikipedia article on sex ratio, and that LiveScience article about "Why some species have more females than males."  

My goal in opening all of these parallel tabs was to get a quick broad scan of the topic (rather than just trusting the first result I saw).  

Sure enough, I quickly learned that: 

- sex can be determined by environmental conditions; it's not always genetically determined!   Korpelainen, Helena. Sex ratios and conditions required for environmental sex determination in animals. Biological Reviews 65.2 (1990): 147-184.
- the sex ratios for sex-changing animals are different in female-first animals (vs. male-first) Allsop, D. J., & West, S. A. (2004). Sex‐ratio evolution in sex changing animals. Evolution, 58(5), 1019-1027.
- there's value in having more males in animal populations that are environmentally stressed McGinley, M. A. (1984). The adaptive value of male-biased sex ratios among stressed animals. The American Naturalist, 124(4), 597-599.


So my naive notion that animals come in roughly 50/50 male/female splits went out the window in a hurry!  

That was a great start to my Research Question, preparing me for reading the Wikipedia article on Sex Ratios among animals.  I read this article and clicked through to several of the articles that were linked there.  (It's got a pretty great set of links.)  

Here's what I found:  

Among many species of animals, in general, the ratio is 50/50.  Why?  

According to Fisher's Principle, i.e., with all other things being equal, it's most beneficial to produce a balanced sex ratio in the children because it leads to the maximum chances for reproduction in future generations.  (It's an interesting game theory article--worth a read.)  

But the catch phrase here is "all other things being equal."   I learned there are lots of special cases here.  

The other hits on that first SERP told me that 

* the Seychelles warblers are 90% female.  When females are in high quality territory, they primarily produce female chicks. They seem to have pre-ovulation control of offspring sex ratio. Likewise, when they move into low quality territory, the chicks become much more male.  
* likewise, Wood lemmings are 66% female.  They have two different types of X chromosomes, the normal X and a mutated X*. This leads to three genetic types of females: XX, X*X and X*Y and one genetic type of male: XY. The X*Y females are fertile, but only produce X* ova, which means they only produce female offspring.
* the wasp Nasonia vitripennis has few or no male offspring.  Ever.
  
This is pretty amazing stuff.  This tells me that while the majority of species have a 50/50 split, there are lots of animals that have REALLY variable sex ratios.

Of course, now I'm interested in bats.  What is their sex ratio?   

     [ sex ratio bats ] 

leads quickly to articles that tell us: 

* bats in the Thar desert (on the border of India and Pakistan) come in all three conditions depending on species:  there are several male-dominated species, several female-dominated species, and a few 50/50 splits. 
* hibernating bats (Pipstrellus subflavus) in the northeast United States are predominantly female
* meanwhile, the Big Brown Bats (Eptesicus fuscus) in Michigan are predominantly male (60/40 split), apparently because the males live longer.  
You see the point.  Bat sex ratios vary by species, by region, and by time of year.  If you collect hibernating bats (which is really easy--they don't fly around much), you'll get a very different result!  


Our other Research Question for this Challenge is WHY?  

We're starting to see some hints:  

- group organization:  parrotfish live in harems with 1 male and many females (with the largest female converting to male when the pod loses the male).  Likewise, ants and honeybees live in eusocial groups which have a single female (as a queen) and many others in sex-defined roles, often as females, 

- genetic mechanisms: such as the Wood lemmings which have 3 different sex chromosomes, or parthogenesis among other animals (primarily insects, fish, and lizards) where all members of the species are female (and therefore genetic clones),  

- environmental conditions:  temperature of the eggs plays a big part in determining the sex of American alligators and other reptiles (see: temperature-dependent sex determination)

- infection:  Infection by the Wolbachia bacteria causes skewed sex ratios in some arthropod species as the infection selectively kills males.  As a result, some isopod species start off at 50/50 male/female, but the males die off due to the infection before they're born.  



Other things found while researching... 

* among mallard ducks, males are around 60% of the total population, with lots of local variation (probably due to different predation patterns), 

* at the National Zoo in DC, they have an Asian Water Dragon (Physignathus cocincinus) that had a parthogenetic birth.  See the video below for mother and daughter... 



There is a significant shift in sex ratios among some animals by age.  The most interesting example is that of humans.  (These numbers are male/female.)  

     At birth:  1.05  (according to WHO, 2019
     At age 50:  1.00 
     At age 90:  0.50

Here's that data graphed out (ibid):  

Data from WHO. World health statistics, p. 4.  

This tells us that men die at a faster rate than women once they get beyond age 60 or so.   


Research Lessons 


1.  Use parallel browsing.  Parallel browsing is a great technique for getting a lot of different perspectives all at once.  Especially for this kind of topic, I was able to quickly see that my assumptions were NOT correct, and then able to drill down into the different leads that I had.  (This is a great way to avoid confirmation bias!)  

2. Search for mechanisms.  It's pretty simple to find the sex ratio differences, but looking into the causes underlying the differences opens up an entirely new set of issues.  I could have easily written a MUCH longer (and fascinating) blog post just about why these ratio differences occur.  In general, looking into the underlying mechanisms is a great strategy for understanding any topic.  


Hope you had fun with this! 

Search on! 






Wednesday, November 6, 2019

SearchResearch Challenge (11/6/19): How many species aren't 1:1 in gender ratios?


I read an interesting article... 

... in The Atlantic Monthly.  To wit, The Quirk of Collecting That Skews Museum Specimens Maleand had a sudden insight... 

Even though I know that the majority of parrotfish on a reef are female (see our earlier blog post about parrotfish), I'd somehow assumed that in most animals, the number of females and males would be about equal.  

So... I'd also assumed that animals collected in a museum would be roughly 1:1 as well.  

It was pretty surprising, then, to read that in many museums collections, males outnumber females by a significant margin.  

It was truly surprising to learn that bats that ended up in museums are mostly female!  



Thinking about this made me wonder more generally: 


1.  Do most animal populations consist of males and females split roughly 50% and 50%?  

2.  If that's not true (and it's clearly not for parrotfish), what causes a species to have a non-equal split between males and females?  What kinds of animals have very different splits?  


I haven't done any searching on this, but I suspect the answer will be extraordinarily interesting!  

Let us know what you find... and be sure to tell us HOW you found it!  

Search on!  


Monday, November 4, 2019

Answer: Where's this lobster from? What's the story?

This wasn't that hard... 

... but the backstory is DEFINITELY a surprise.  Well, it was to me.    


Remember: I found this cute little lobster image: 


and wanted to know.... 

1.  Where is this lobster image from? 


2.  What's the backstory of the source?  (IIRC, it has a kind of crazy story.  But I don't remember what it is!) 


As most of you did, I did a Reverse Image search (aka, "Search By Image"). Like most of you, I found that it was an image from Description de l'Égypte (1822).  


Like Luis Miguel, I found it on the Paulus Swaen gallery site (an antiquities seller).  From that site I found the full image of the page of that book: 



Lobster (Homard) from the book Description de l'Égypte. (P/C Swaen website)

On that site they say that this lobster is... 



Decorative engraving of a lobster, by Tresca. 
Marie Jules César Lelorgne de Savigny (April 5, 1777 - October 5, 1851) was a French zoologist. In 1798 he traveled to Egypt with the Emperor Napoleon as part of the French scientific expedition to that country, and contributed to the publication of the findings of the expedition in 1809 (Description de l'Égypte published more fully in 1822). He wrote about the fauna in the Mediterranean Sea and the Red Sea and discovered that mouth parts of arthropods were transformed extremities.
Engraved by Marcet and Leleu.
Savigny was responsible, along with Geoffroy Saint-Hilaire, for the zoology sections of the Description de l’Égypte. The plates were engraved between 1805 and 1814, and Savigny contributed all of the ornithology sections and supplemented other sections on vertebrates. The invertebrates are represented on 105 plates with thousands of drawings, all of them from Savigny’s research.

That's an interesting story, but let's check it.  They're also giving a certificate of authenticity, so I'm disposed to believe that this is accurate. (But... As you know, we SRS'ers always check these things.)   


It's easy to check names, so let's start with Marie Jules César Lelorgne de Savigny. A quick search shows us that de Savigny is the person (with the history) as is claimed above.  What about the other people?  


HOWEVER... who's Tresca?  And why does it say that it was engraved by Tresca, and then later in the description that it was engraved by Marcet and Lelu?   

The obvious query for Marcet and Leleu have MANY hits, revealing them as professional engravers.  (Which, I learned, is a very specific skill about translated a drawing, as done by Savigny, into a copperplate for the printing press.)  Tresca was apparently another engraver, one who did "decorative" engravings.  It seems as though Marcet and Leleu were engravers for books, while Tresca was an engraver for "decorative editions."  

So, I believe Savigny created the original sketches, while Marcet, Lelu, and Tresca all made various engravings.  

But what about this book in which those engravings appear?  The text described in the book is given the full title of:  

Description de l'Egypte, ou Recueil des observations et des recherches qui ont ete faites en Egypte pendant l'Expedition de l'Armee francaise, (Paris, 1809-1830). 

That is, this book, ('Description of Egypt, or collection of observations and research that have been done in Egypt during the French Army Expedition, Paris, 1809-1830)' in English), documents Napoleon's  military and scientific expedition to Egypt.  The Wikipedia Descriptioarticle says:  

...[the book ] was a series of publications, appearing first in 1809 and continuing until the final volume appeared in 1829, which aimed to comprehensively catalog all known aspects of ancient and modern Egypt as well as its natural history. It is the collaborative work of about 160 civilian scholars and scientists, known popularly as the savants, who accompanied Napoleon's expedition to Egypt in 1798 to 1801 as part of the French Revolutionary Wars, as well as about 2000 artists and technicians, including 400 engravers, who would later compile it into a full work.

By digging a bit deeper, I found that the book was: 

...in 13 volumes included 892 plates of which 72 were colored, among which 9 volumes concerned Antiquity. The other volumes dealt with the Natural History and modern Egypt because Napoleon Bonaparte had brought with him a commission of scholars of all disciplines so that, it was said, in his description was stored 'the richest museum of the Universe'. 
This work was written in part by Baron Dominique Vivant-Denon, before the latter was appointed Director General of the Musée Napoleon at the Louvre. More than 80 artists and 400 engravers were engaged for this titanic project. The dimensions of the exceptionally large boards required the creation of a special press and a specific piece of furniture to preserve them.. 

Now that's crazy.  (Books so large that special furniture needed to be made?)  

But what strikes me as even crazier-crazier is that Napoleon would embark on a military expedition with 160 scholars ("the savants") as part of his military expedition to occupy lands in Egypt.  What?  And not just scholars, but 80 artists (including Savigny) and 400 engravers.  

As it turns out, Napoleon was never one for mild ambition.  His goal was to defend French trade interests in the region, seek further alliances with Tipu Sultan, weaken Britain's access to India, and to establish scientific enterprise in the region.   Hence, the need to bring the Savants along.  

Among other interesting things (like this book),  the expedition eventually led to the discovery of the Rosetta Stone, creating the field of Egyptology and allowing the direction translation of Egyptian hieroglyphics for the first time.  


SearchResearch Lessons


1.  While reverse image search got us the answer fairly quickly, it's surprisingly hard to find an original scan!  I know, the notion of an "original scan" is a bit odd, and although I can find MANY copies of the text of the books, but finding a decent scan of ALL the images in the Description is really hard.  (At least I haven't found one yet!)  

2. Even simple Challenges can have untold depths.  While it is simple to find the source, and verify it, the story behind the book is remarkable.  As I said, when I started doing this bit of research, it didn't take long to get to the original artist, or even places where you can buy a modern reproduction.  But once you read that, and then discover that this work was done by a naturalist who was part of the military expedition, the mind boggles.  What an astounding thing!  Hundreds of people traveling along with a major army (and navy) from France to Egypt, and then doing careful, detailed research in a broad number of areas--that's pretty amazing.  

I thought you'd find it an interesting story.  I certainly did.  


Postscript:  Writing this didn't take too much time, and the research was actually fairly simple.  But these weeks in October and November are fairly full.  If you've been watching my personal web site, or my JoyOfSearch.net site, you'll see that I'm giving a LOT of talks these days.  Yes, this is the Joy of Search World Tour, and while it's a lot of fun, it wreaks havoc with my writing schedule.  (Last Wednesday, when I should have been writing this post, I was giving an invited lecture at Yale.  Tons of fun, but it also takes a lot of time.)   
I hope you can forgive me if the next couple of weeks are a bit erratic!  


Search on!