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! 


  1. Great challenge. If we weren't traveling we would have spent more time digging deeper.

  2. You might want to look at the new children's book Looking for Marla, which was written by biology grad students with correct information about sex in clownfish.

  3. Ironically, when I randomly googled this question this was the first link to come up. All of the searching was done for me. I thoroughly enjoyed reading through this, I'm going to have to look through the rest of your blog

  4. Was taken to this site after clicking a link from the new post. Thank you, really useful, I Googled this after having an epiphany about how wonderful the world is, with every person having plenty of possible partners, and wondered whether Nature had arranged it so across all species- turns out that yes, it basically has, with a few exceptions. What a wonderful world :D