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!  


  1. Very interesting and surprising too indeed, Dr. Russell.

    In recent day, I was reading also an interesting article that made me remember previous SRS Challenges: Parthenogenesis, clown fishes and wrasses among the topics. How I find it? Searching for Math Sutras linked me to some hacks article in that site. And after days, returned to read because articles were interesting

    A Plausible Explanation of Sex Change in Fishes

    With [animal museums gender ratio]

    Similar to your article and with some different data: Misogynist museums? New study says galleries are lacking in female animal exhibits

    Smithsonian Mag

    Gender representation in the natural history galleries at the Manchester Museum looks interesting. I couldn't add the link because it downloads. It is from 2008 and maybe they changed something there already.

    Tried [Manchester Museum gender ratio changes] and removed so just search the site of the Museum

    Museums as whole, it turns out, are simply really bad at recording the sex of specimens in their collections. Of the two million that were looked at for this study almost half were unsexed, even in species where the sex is obvious. So one of the most basic things that could be done is to actually record the sex of an individual in the first place. All the recent news based on the same: Journal Proceedings of the Royal Society B.

    1. After searching and reading about the museums, searched for our Challenge.

      [animal gender ratio]

      2015: Why Some Species Have More Females Than Males

      "...An animal's sex is often determined by the sex chromosomes it inherits. The new research reveals that species with X and Y sex chromosomes, including mammals, generally have female-skewed populations, whereas species with the less familiar Z and W sex chromosomes have a sex ratio tilted toward males.

      The proportion of adult males to adult females in a species, known as its adult sex ratio , can vary widely in nature.... Also mention which species are female/male-skewed...One possibility 2 different chromosomes makes the skew to the one with 1

      [animals adult sex ratio] and [animals "adult sex ratio"] Interesting studies that I am reading: Royalsocietypublishing, and others

      2018:A new study from an international team of researchers finds that an imbalance of the sexes leads to single parenting in birds

      Biology Stackexchange: In what animal species is the sex ratio skewed in favour of the fertile female?

      [animals "adult sex ratio" infographic]
      In-depth: How will climate change affect animal sex ratios?

      Next searched (not yet read) [adult sex ratio]

      And [which animals have more females than males] later will try the other way.
      2018:Of the 76 non-human mammal species that exhibit leadership, only seven have females that take charge during conflict
      2013: Mammals can 'choose' sex of offspring, study finds

  2. First, what to search for. A couple of tries ("uneven male female ratio" and similar) gave a Wikipedia article on "sex ratio" which started to give reasons leading to an article on "Operational Sex Ratio" at It also mentioned Fisher's Principle which, as you suggest would assume the split is 50:50.

    WD Hamilton came up with a model on sex ratios:

    1) Suppose male births are less common than female.
    2) A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
    3) Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
    4) Therefore the genes for male-producing tendencies spread, and male births become more common.
    5) As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
    The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

    Thus where you get uneven distributions something else is playing out to disrupt the equilibrium ratio. Parrot Fish are one example. They are an example of Sequential Hermaphroditism where the animal changes sex at some point in its life. So parrot fish are born female but as they get bigger and stronger, they switch to become male. By having more females at an earlier stage means more fish survive to reproduce.

    Other examples where you get an uneven sex ratio is related to size. If males are smaller and more likely to be eaten, for example, you will initially have more males than females - but as the animal ages, the numbers will even out. This relates to bats where some research ("sex ratio" bats) suggest that other factors such as climate change put more pressure on one sex than the other. (E.g. see and among others). Essentially nature works to get a 1 to 1 equilibrium in the end by having more of one sex at certain life stages.

    Finally I thought about social insects (ants, bees, wasps) with worker colonies that are all female and males are a rarity. This may be because the queen can control the sex of her offspring. She needs workers (female) so most births are female. But to mate and spread she'll need males - so these come at the right time. (E.g.

    A Pubmed article summary concluded with a simple explanation of why there are differences: "within species, the sex ratio varies with the costs or benefits of producing male or female offspring."

    Animals with uneven sex ratios include some fish such as the Parrot fish, social insects (ants, bees, etc.), some arthropods (where males seem to be more likely to be killed by a particular bacteria), alligators where sex ratio is temperature dependent (and according to the Wikipedia "sex ratio" article are a 5 to 1 female to male ratio, some birds - and even some human groups where there is a strong cultural preference for males, meaning girls get aborted or killed at birth or are selected against.

  3. I searched for [sex proportions animal species] which led to a Live Science link
    That article gave me an explanation and more importantly the term "adult sex ratio" which I would use for further searching.

  4. meatbag flaws - ended up looking at h_u_m_a_n_s… most museums don't collect specimens… seems frowned on for some reason…
    they don't fit well in those drawers & are expensive to maintain… maybe robots should do the collecting/harvesting…
    ('flawed' - my addition)… "human collecting preferences" (from Ramón's Smith'n article)
    the nuances of search wording and acronyms…
    path - SERP
    part of path
    start of path
    human rights watch…
    more path
    path - formulas & ratios
    path variant
    mbs + tech
    humor or cruelty?… the tech angle

  5. Deb and Anne here. We started by doing a search for gender ratio in animals. The first result was for a wikipedia article on sex ratios. It is a fascinating article. "In most species, the sex ratio varies according to the age profile of the population." There is actually a theory of sex ratios.There is actually a book written on the topic called "Sex Allocation". Interesting that in domesticated animals it has been found that the most economically beneficial ratio is more females to males.
    This is really fascinating and there are more articles for us to look at but we are heading out to the AASL conference in Louisville later today so not sure when or if we will get back to this.