As Many Exceptions As Rules: Why Do Males And Females Look Different?

Biology concepts – sexual dimorphism, phenotype, evolution, sexual selection, secondary sex characteristics, reproductive success, natural selection

Elephants are an animal that we can picture easily in

our head. But is this a male or a female? Don’t answer

quickly, in African elephants both the males and

females have tusks, but in the Asian elephants, it’s only

the males (usually).

We all know what a hippo looks like, an elephant, a duck. In most cases, if you name a species, you can picture the animal in your head. But are you picturing a male or a female? Sometimes they look the same, we can only tell the males from the females if we get close enough and are socially rude enough.

But in some cases, it’s much easier to tell the guys from the gals, so much so that sometimes scientists misidentify them as different species. The differences between how males and females look and how they look is called sexual dimorphism (di = two, and morph = form) and it can range from the subtle to the fantastic.

We have been talking about bilateral asymmetry in the past few weeks, and our next examples of bilateral asymmetry require a discussion of sexual dimorphism – a subject full of its own exceptions.

The mildest form of sexual dimorphism is when the difference lies in just in reproductive organs. This may or may not be visible to the naked eye. Take the American white pelican (Pelicanus erythrorhynochos). On average, the males are just slightly larger than the females, but you couldn’t tell this by looking at them. Only their reproductive organs tell them apart and the external portion of the cloaca of a male looks just like that of a female. Maybe you could separate them another way – I hear only guys like the Three Stooges.

A better example would be the spotted hyena (Crocuta crocuta). The females probably like the Stooges more than the males, because this species has females that are extremely masculinized. Many studies have been done on just how this species is unique among mammals in its lack of sexual dimorphism.

Is this a male or female spotted hyena? Even experts

can’t tell. The females are just as aggressive as the males

and they could easily chase off a cheetah. Males and

females look exactly alike, but I’m betting a female

wouldn’t let a meal get away so easily.

A 2014 review discusses how the female external reproductive tissues look just like the males. Scientists have studied hyena individuals for years assuming they were males until all of suddenly they give birth to a litter of pups! The review goes over the data that shows that much of the external genitalia are masculinized before the reproductive organs can even start producing hormones, so much of the similarities between males and females is genetically driven. But not all – certain aspects could be stopped with anti-androgen drugs.

A 2012 study showed that spotted hyenas have 5x lower levels of SHBG (sex hormone binding globulin). This protein binds up estrogens and androgens and regulates how available they are to the tissues. The spotted hyena has a slight mutation in the gene. The result is that lower overall levels of that gene product (protein) are made. With less regulating protein, the androgens are free to strongly masculinize both the tissues and the behaviors of the females. They are bigger, stronger, and more aggressive than the males. This, along with their external reproductive organs looking so similar to males makes them a complete exception in the mammals.

But it isn’t always so hard to tell boy from girl. There are several external body features that may help if you find yourself needing to tell, say, a boy wombat from a girl wombat.

Size (mass, length, height, muscularity) is a common sexually dimorphic trait. In mammals and birds, the males are most often larger than the females, but our talk of spotted hyenas from above tells you that isn’t always the case. The exceptions carry over to birds as well. When the gender that is normally smaller in most species of a phylum turns out to be bigger, this is called reversed sexual size dimorphism or just reversed size dimorphism (RSD).

These are southern elephants seas, a mating pair. No, he’s

not a cradle robber, the males are just that much bigger

than the females. The penguins let you know just how

far south we are. Does she look scared to you?

Hawks, owls, and falcons (all raptors) show this RSD, which was investigated in 2005. The study found that the small-male hypothesis was supported – that males got smaller to become better foragers, while the females remained large or got larger as prey for their chicks got larger. The study concluded that RSD was a results of natural selection for resource and niche management rather than a selection based on who to mate with (sexual selection).

Amongst the mammals that follow the rule of larger males, the biggest size dimorphism is seen in the southern elephant seal (Mirounga leonina). The males weigh 8-10x more than the females, and they have a huge proboscis that the females don’t have. When hanging out together, they are often mistaken for an adult and a juvenile....unless she’s a trophy wife and he’s 50 years older than her. Then it’s completely believable.

Outside of mammals and birds, phyla generally have females that are larger than males. That’s if there is a difference in size between the sexes at all - many species don’t have sexual size dimorphism. One that does is the golden silk spider (Nephila clavipes) has a female that 35-70x the mass of the male and is 7-8x longer than he is. Many spiders have larger females.

On the left is the golden silk spider that lives in North

America, from NC to TX. The intruder above is the male,

while the female is hogging most of the picture. On the

right is A. aquatica where the male is bigger and both

males and females live underwater their entire lives.

But even in spiders there is an exception. The water spider (Argyroneta aquatica) is one of the few spiders where the male is larger than the female, but that’s not the weird part. It spins a web under water that acts as a diving bell. The spider pulls down air and holds it under the bell of the web. A 2013 study showed that the web contains a biogel that holds the air in the web. It can pull oxygen out of the water and replenish the air in the bell, so the spider can live and hunt under water without ever coming to the surface again.

Often, male and female animals have differences in secondary sex characteristics – traits that distinguish the two genders but are not related directly to the reproductive organs. Colors or ornaments (like wattles, antlers, etc.) can be used to tell the differences between males and females. These are phenotypic (pheno = observed and type = characteristic) differences; they make the two animals look different, not just be of different size.

Color is a good example of a phenotypic sexual dimorphism (sexual dichromatism). Cardinals are red (male) or kind of grayish-brown (female), while male and female Eclectus parrots (Eclectus roratus) are both colorful, they just have completely different coloration patterns (see picture below). Mandrill (a type of primate) males have coloration on their face and bums, while the females are basically all one color.

The Eclectus parrots on the left are also a mating pair.

The male is green and the female is red and blue. Why

might this sexual dimorphism have developed. Both are

bright and could be spotted easily, although in a forest

the male is probably hidden better. The right image is the

triplewart seadevil female. I superimposed a male about

the right size and where he would attach (see arrow).

Secondary sex characteristics often work in combination with differences in size. Perhaps one of the most dramatic examples is the triplewart seadevil (Cryptopsaras couseii), a type of anglerfish. The female is huge, up to 10 kg, with a bioluminescent lure and a gaping mouth. But the male is 1/25th her size and only 150 g at most; he looks nothing like her. He exists only as a parasite that attaches to her side and gets nourishment from her body. He is there when it is time to mate because he’s always there, just hanging on.

Why would it be advantageous for species to show a sexual dimorphism – like size, phenotype, or even behavior? There are sexual dimorphic behaviors, like male penguins presenting pebbles to prospective mates or male manakin birds dancing for females. Some birds dance better than others – at least according to the females, so this is a selection criterion just like other sexual dimorphisms, but these are beyond our discussion today.

Sexual selection (mate selection) criteria are good reasons for sexual dimorphisms. If a male (or every once in a while a female) has enough energy to make ornaments (or even better, larger ornaments like horns, wattles, etc.), then they must be good at finding food or have good genes. This would give those with larger ornaments a reproductive advantage and would select for genes that promote larger ornaments. Over time, there would be greater and greater separation between males and females.

Likewise, larger tusks or antlers would allow a male to compete better against other males. This would again help separate those with supposedly stronger genes. Winning a battle might reflect bigger muscles, again a sign of better energy procurement or the ability to resist disease. All in all, he’d be a better mate for a female looking for physical survival traits. The more a species starts to control its environment (ie. humans), the less these survival or strength genes matter.

This is a form of sexual behavior dimorphism in the

manakin bird. The male dances for the female. Now we

know where Michael Jackson got the idea for the

moonwalk.

Then again, sexual dimorphism may be a survival advantage. If the two genders are put together somewhat differently, then perhaps they will exploit different food sources in the same area. This would allow both males and females to get enough energy and more of each gender would survive to reproduce because they aren’t competing with each other for resources. This is what happens with some hummingbird species where the males and females have different bill shapes and lengths that allow them to drink from different types of flowers.

A new paper shows that plumage color in birds is often related to survival advantage - not mate selection advantage. Plumage can be used for camouflage, when males live in slightly different environments than females. The alternative - if they don’t survive, they probably won’t mate. On the other hand, sexual size dimorphisms can promote stronger mate selection if the males are bigger (sexual selection), or may allow for the mothers to hunt better and find more food for offspring if it is they that are larger (natural selection).

In some arthropods, there is often a sexual size dimorphism where the female is larger. This would allow them to lay more eggs – more eggs means more potential offspring might survive to reproduce themselves. Likewise, female humans have a wider pelvis to allow for passage of the baby through the birth canal - a dimorphism not associated with mate selection. Males don’t need that – thank goodness.

We see here that the point of sexual dimorphisms can be for reproductive success or survival advantage. These are what keeps a species living generation after generation. However, evolution has deemed reproductive success even more important than individual life span. In pheasants, the females live much longer, so the males have to make themselves stand out so that they will mate as often as possible in their shorter lives. Therefore, they are colored much more brightly.

Next week, sexual dimorphism isn’t just an animal thing. There are genders in plants too. Sometimes they different sexes have very different characteristics so that they can mate as well, but do plants select mates?

Dunn, P., Armenta, J., & Whittingham, L. (2015). Natural and sexual selection act on different axes of variation in avian plumage color Science Advances, 1 (2) DOI: 10.1126/sciadv.1400155

Neumann, D., & Kureck, A. (2013). Composite structure of silken threads and a proteinaceous hydrogel which form the diving bell wall of the water spider Agyroneta aquatica SpringerPlus, 2 (1) DOI: 10.1186/2193-1801-2-223

Cunha, G., Risbridger, G., Wang, H., Place, N., Grumbach, M., Cunha, T., Weldele, M., Conley, A., Barcellos, D., Agarwal, S., Bhargava, A., Drea, C., Hammond, G., Siiteri, P., Coscia, E., McPhaul, M., Baskin, L., & Glickman, S. (2014). Development of the external genitalia: Perspectives from the spotted hyena (Crocuta crocuta) Differentiation, 87 (1-2), 4-22 DOI: 10.1016/j.diff.2013.12.003

Hammond, G., Miguel-Queralt, S., Yalcinkaya, T., Underhill, C., Place, N., Glickman, S., Drea, C., Wagner, A., & Siiteri, P. (2012). Phylogenetic Comparisons Implicate Sex Hormone-Binding Globulin in “Masculinization” of the Female Spotted Hyena Endocrinology, 153 (3), 1435-1443 DOI: 10.1210/en.2011-1837

Krüger, O. (2005). The Evolution of Reversed Sexual Size Dimorphism in Hawks, Falcons and Owls: A Comparative Study Evolutionary Ecology, 19 (5), 467-486 DOI: 10.1007/s10682-005-0293-9

For more information or classroom activities, see:

Sexual dimorphism –

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