Wednesday, April 22, 2015

Boys Will Be Boys… And Then Girls

Biology concepts – botany, monoecious, dichogamy, imperfect and perfect flowers, self-pollination, cross-pollination, self-incompatibility, heterostyly

This clip shows the mating of hermaphroditic
leopard slugs. Each may provide male gametes
for the other, or it may just go one way. They hang
from a branch to do this, and the male reproductive
organs spiral around one another. The trait has gone
mad – in some species, the male organ has reached
92 cm long!
There are a few ways for animals to make new animals. Asexual reproduction is possible in a few species, while sexual reproduction is much more common. In between, there are the hermaphrodites. These animals carry both sets of reproductive organs.

Some gastropod (snails and such) breeding is strictly sexual; they have male and female snails (marine), while most terrestrial snails are hermaphrodites. But even then, most need another gastropod to mate with. Each hermaphrodite fertilizes the eggs of the other. There are the rare cases where a hermaphroditic slug will self-fertilize and produce clones, but this is the exception, not the rule.

These examples show that weird reproduction does take place in animals, but the plants have us beat by a long shot. Even the simple types of flower breeding systems turn out to be not so simple. Sometimes being both sexes is the easy part.

If a single flowering plant (angiosperm) can produce both male and female reproductive cells (pollen and egg), the plant is called monoecious (one house). This represents a nice tight bundle, reproduction wise. One plant can make the male gamete cells (pollen) and the female gamete (egg) – it’s a hermaphrodite. Let’s look at the types of monoecious flowering plants, maybe it's not so simple:

Monoecious perfect - These plants have flowers that contain both the male reproductive structures and the females reproductive structures, so they are said to be perfect flowers. Because they occur on the same flower, any time the plant flowers, the blooms have both female and male structures and qualities. These are true hermaphrodites.

The king flower of the apple blossom is the key to
getting good apples. It has to be pollinated for the
cluster to all produce apples. The king flower opens
first, is pretty, and smells good.
Apple trees are monoecious perfect, although most are pollinated by other apple trees, not themselves (see below). Interestingly, in order to get the most and best fruit, the king flower (the largest and first bloom of a cluster) must be pollinated and be pollinated first.

Monoecious imperfect – In the simplest form, monoecious imperfect plants have some male flowers and some female flowers, but they both bloom on the same plant. The flowers themselves aren’t hermaphroditic, but the plant is, since it has both male and female structures.

A single American chestnut tree will have both male and female flowers at the same time. Some long catkins (an arrangement of small flowers on a single stem) have only male flowers while others have male flowers at the tips and female flowers at the base.

Male and female flowers on separate individuals at the same time like with the chestnut is one form of monoecious imperfect, but there are others as well.

Gynomonoecious or andromonoecious – These are a mixing of perfect and imperfect flowers on the same plant. Gyno- means female, so these plants have imperfect female flowers AND perfect flowers at the same time. Andro- means male so, you guessed it, they have male flowers and perfect flowers.

A 2003 study of four Solanum (a large genus that includes potatoes and tomatoes) species showed that the number of male flowers compared to the number of hermaphroditic flowers can vary greatly. Some species were about 7% male flowers (weakly andromonoecious), while others were 69% male flowers (strongly andromonoecious). Weak species would change the number of male flowers produced according to how much fruit was produced, but strong species made the same number of male flowers no matter what.

The American chestnut was a towering species until
the late 1800’s when a nursery owner imported some
Japanese chestnuts that had a fungal parasite. The
American version had no immunity, and we lost these
huge trees in short order.
The small Spanish flower Silene littorea was recently (2013) found to be mostly gynomonoecious. Before this, it was thought that this species had two populations of plants, but the seed numbers and variable numbers of female flowers show that being gynomonoecious helps significantly in producing more seed with less flower investment.

Problems with Monoecy - With either perfect flowers or imperfect flowers one a single plant, it’s possible for the pollen of an individual to fertilize an egg on the same individual – self-pollination. However, self-pollination isn’t always a good thing. Self-pollination produces clones of the parent that provides both the pollen and egg. Pollen from one plant fertilizing the egg of a second individual is called cross-pollination.

Cross-pollination promotes genetic diversity. Clones tend to build up genetic mistakes, while cross-pollination help to spread genes through the population and makes the species more likely to possess genes that might help them withstand changes in the environment. Therefore, many plants take steps to prevent self-pollination and promote cross-pollination.

Heterostyly (hetero = different, and style = part of the female reproductive organ) prevents “selfing” in many animal pollinated plants. In this case, a certain species will have two morphs (shapes) of flowers. One will have a long anther (pollen producer) and a short style (where the pollen is deposited and grows down to egg). The other will have short anthers and long styles (see picture and caption to below). 

Morph 1 and Morph 2 are the same species, but different
individual plants. The large insect pollinator can easily
get pollen from A anthers (right) and deliver it to A pistils (left), but
how would it get pollen to the B pistil (right)? The reverse is true
for the smaller pollinator. Therefore, Morph 1 can pollinate
Morph 2 and vice versa, but neither can pollinate their
own morph. This is heterostyly.
A pollinator well-designed to gather pollen from a long anther would be poorly designed to accidentally (it’s almost always an accident) deliver that pollen to a short style. So it is unlikely that self-pollination will take place.

However, that same pollinator would be well-designed to deliver its pollen load to a flower with a long style, the kind found on the other morph of individuals of the species. This would promote cross-pollination. The strategy is equally successful for those pollinators best prepared to gather pollen from short anthers.

It is not known whether the move to heterostyly in some plants has been driven by genetics to avoid inbreeding or by pollinators and the need for efficient fertilization. A 2006 study in Narcissus flowers looked at both genetics and pollinator efficiency in breeding and concluded that the pollinator driven evolution was supported to a greater degree. This agrees with the pollinator hypothesis that Darwin proposed almost 150 years ago. He was pretty smart.

In other cases, the position of the flowers may discourage self-pollination. For instance, some wind pollinated fir trees have female cones up high and male cones down low. The pollen from the male flowers might travel on the wind and gain altitude to fertilize female flowers on adjacent trees, but it is extremely unlikely that the pollen would be blown straight up to fertilize the female flowers of the same tree.

The most common mechanism to prevent selfing is self-incompatibility. There are two main mechanisms of self-incompatibility; they both work at the genetic level to make sure that the pollen of a particular individual will not successfully fertilize an egg of the same plant.  

The blue pollen has one rearrangement of the
compatibility genes (S3, S4), while the red pollen has a
different rearrangement (S1, S2). If the ovule genome has
S1, S2, then S1, S2 pollen landing on the stigma will be
destroyed. S3, S4 pollen won’t be recognized and can grow
pollen tubes to fertilize the S1, S2 egg. Self pollen is
incompatible with the same egg; this promotes
Both mechanisms involve genes that can rearrange to form many slightly different gene products. One individual will have the same rearrangement of the gene in its pollen and its egg. Pollen of one type will not work with an egg of the same type. It works in the exact opposite fashion as self-recognition proteins in humans. In that case, tissues with different HLA markers are attacked as foreign; in plants, pollen and egg of the same rearrangement will be shut down.

So these are ways to prevent self pollination in perfect and imperfect monoecious plants. But monoecy can get weird on its own in an attempt to prevent selfing:

Dichogamous monoecy – This breeding system probably evolved as a way to prevent self-pollination in monoecious plants. The pollen and ovule mature at different times. This is equivalent to having a flower (plant) that can change its sex in just a short period of time, and these count as additional monoecious breeding schemes. Some animals can do this, but the change takes place over the period of a lifetime. Here were talking about in the period of a few hours.

If an individual plant can change its sex over a short period of time within one growing season, then it is called dichogamous (dicho = in two, apart, and gamous = gametes), also called sequential hermaphroditism or temporal dioecy. But which comes first male or female? If the plant first produces male flowers, then it is termed protandrous (proto = first). Protogynous is name for those that are female first. This is a great way to prevent self-fertilization and there are a couple of ways plants can employ dichogamy.

This chart will help explain the different monoecious
breeding systems. Each large circle is a population of
plants of the species. The circle with cross means female
flower, the circle with arrow means male flower, and the
circle with both means perfect flower. The line arrow
with a “t” means a change as time passes. For dichogamy,
the flowers may be perfect or imperfect, but they function
as male or female at each time point.
One system of dichogamy comes about if the flowers of the monoecious plant are perfect. In this case, the structures are all there, but the timing for maturity is different. The flowers of Scyphiphora hydrophyllacea, a mangrove shrub, are perfect and protandrous, while Cenchrus clandestinus, a Hawaiian grass, has perfect, protogynous flowers. The flowers are structurally perfect, but functionally imperfect.

In perfect protandrous plants, the pollen matures and is carried away (ind, insects, animals, etc) before the ovules mature on the same flower - so no selfing. In perfect protogynous, the opposite is true. The early maturing eggs must procure pollen from individuals who have already had mature eggs and have changed to produce pollen.

The other dichogamous possibility for monoecious plants is when they have imperfect flowers. This means that the plant would make flowers of one sex first, and then grow separate flowers of the other sex later on. The separate flowers are still on the same plant, but self-pollination isn’t possible because they aren’t there at the same time.

Corn is an imperfect, protandrous plant. This is why country kids detassel in the summer. The tassel is the male flower. If you remove it (de-tassel), it will prevent possible selfing when the female flowers come out. You can create hybrids by planting a few rows of a specific breed of male corn at the end of the rows.

It’s much harder to find an example of an imperfect, protogynous plant. In general, doesn’t it seem silly for a species to produce their female flowers first? They need pollen from the males in order to be fertilized, but if they’re all female, who provides the pollen? It doesn’t seem logical.

The left image is the female flower of corn, every silk is a
flower that can be pollinated. Each one that is will produce
a corn kernel on the ear. The right image is the male
flower, the tassel. This get pulled off by teenagers trying to
make money for that prom dress or new stereo.
The key is in the timing. Not all the individuals of a protogynous perfect population will flower exactly at the same time. So some will have moved on to being male while others are still female. This would then provide pollen for them without resorting to self-pollination.

But why no protogynous imperfect plants? The wasted energy in making purely female flowers very early when little pollen is present probably dooms this breeding system. At least with perfect protogynous, they get some benefit by dispersing pollen later from the same flower. No extra energy is consumed in producing an entirely different flower.

Duodichogamy - This system can help with the timing issue above; it’s dichogamy taken a bit further. Instead of being one sex then the other, they go back and forth and back and forth – like Mystique of the X-Men. In Bridelia retusa, a tropical tree from India, the switching between male and female occurs several times within a single week! To my mind, this is like sexual dimorphism in antlered males of some species - the antlers just keep getting bigger and bigger. Where will it all end - how many times will this plant change sex?

Next week - another mechanism for preventing self-pollination is to separate the reproductive parts to different plants. These are the dioecious plants.

Casimiro-Soriguer I, Buide ML, & Narbona E (2013). The roles of female and hermaphroditic flowers in the gynodioecious-gynomonoecious Silene littorea: insights into the phenology of sex expression. Plant biology (Stuttgart, Germany), 15 (6), 941-7 PMID: 23174011

Pérez-Barrales, R., Vargas, P., & Arroyo, J. (2006). New evidence for the Darwinian hypothesis of heterostyly: breeding systems and pollinators in Narcissus sect. Apodanthi New Phytologist, 171 (3), 553-567 DOI: 10.1111/j.1469-8137.2006.01819.x

Miller, J., & Diggle, P. (2003). Diversification of andromonoecy in Solanum section Lasiocarpa (Solanaceae): the roles of phenotypic plasticity and architecture American Journal of Botany, 90 (5), 707-715 DOI: 10.3732/ajb.90.5.707

For more information or classroom activities, see:

Monoecy –

self-pollination and cross pollination –

Wednesday, April 15, 2015

Boy Plants Are From Mars …..

Biology concepts – sexual dimorphism, plants, monoecious, dioecious, pistil, stamen, floral scent, ecology, ecological selection

Charles Darwin missed the boat on linking his sexual
and natural selection in animals to plants as well.
This is odd because he was quite the botanist and
spent many years studying grasses and such with his
son. The parallels between selection in plants and
animals might have strengthened his initial argument,
but there would still be dissent about the descent of man.
Charles Darwin was a smart guy. He had a lot to do with recognizing sexual dimorphism and natural selection and sexual dimorphism to come up with the idea of sexual selection (natural selection based on preference of one sex for certain characteristics in the other) in evolution. But he blew it when it to recognizing sexual dimorphism in plants.

Does it seem weird to talk gender differences in plants? Yes, they do have genders and sex chromosomes, but it's way more complicated in plants than in animals. More complicated probably means more exceptions, but now let’s focus on the main types of reproductive systems in plants and the types of sexual dimorphism they create.

We have talked about how plants come in many varieties-angiosperms, gymnosperms, bryophytes, etc. in previous posts. Many can reproduce both sexually and asexually, but for today, let’s limit ourselves to angiosperms (flowering plants) and sexual reproduction.

More than 90% of flowering plants have flowers with male reproductive organs (stamen) and female reproductive organs (pistil) on the same individual plant. The term in botany for this is monoecy – the plant is monoecious (mono = one and ecious = house).  

The anther produces pollen (the male microgametophyte, see this post) and the carpel produces the ovule with the egg cells (female microgametophyte). How could a plant have differences based on sex if both sexes are on the same flower? Can a flower be dimorphic with itself? Well…. no, but it isn’t always that simple.

Carnovali’s painting of Hermaphroditus and Salmacis
from 1856. The nymph became so smitten with him on
first sight that she prayed they would never be parted.
The wish was granted. Note the soft outlines, this was a
radical move in Italy in the 1800’s and got some of his
paintings rejected by the church.
If one flower has both the male and female reproductive structures on it, it is called a perfect flower - I wonder if they have inflated egos.  This makes them true hermaphrodites. The term comes from the Greek mythology; Hermaphroditus was the son of the Greek gods Hermes (dad) and Aphrodite (mom). Hermaphroditus (male) was fused with the nymph Salmacis (female) so that the result was a demigod with male and female characteristics.

In science we use the term more strictly; it means one individual that has both and female reproductive organs. This happens to rarely in animals. But in flowering plants it’s the rule rather than the exception.

But in some monoecious plants, the flowers aren’t perfect. Individual flowers will have either male or female reproductive parts. These are called…. you guessed it, imperfect flowers.

So, can an imperfect monoecious plant be sexually dimorphic? On one hand, it is a hermaphroditic individual plant, just like a plant with perfect flowers; both male and female reproductive organs are found in one individual. This would argue that it can’t be sexually dimorphic, just like the perfect plant.

But on the other hand, it has two different types of flowers, and they look different because one has staminate structures (anther + filament) and the other has pistillate structures (pistil = ovary + stigma + style). Because of this, the two types of flowers are different morphs (different shaped versions), and that makes them sexually dimorphic. Does it matter that they’re on the same individual plant? I leave that argument to you.

The top line drawing is a perfect flower, both male and
female. The bottom drawings are of a male (left) and
female flower. These can exist on the same plant –
monoecious imperfect, or on dioecious plants. It’s not
quite this simple, but wait until next week for that.
That takes care of the hermaphrodite majority, but about 6% of angiosperms are what is called dioecious (two houses). They have individuals with male flowers and individuals with female flowers. Now we’re talking sexually dimorphic for sure; different individuals of a species with different characteristics based on sex. Sounds just like the animals we talked about last week.

Sexual dimorphism in plants comes in two main flavors, just like in animals. One is obvious; differences in reproductive organs (primary sex characteristics) will make the flowers look different. The second type is more interesting. You can have difference in characteristics not directly related to reproductive organs (secondary sex characteristics). Dimorphisms could include the shape, color, number, or smell of the flowers, or even differences in the vegetation of the plants. Who knew that plant sexual characteristics could be so complicated?

In general, male flowers are smaller and more numerous than female flowers. Think about it. Males need to spread as much pollen as possible, whereas females spend much more energy to make fruits and seeds. Just like in animals, males make lots of reproductive cells, and females make fewer – so more, smaller flowers in males makes sense.

The male soapwort flowers are on the left. They have
smaller flowers and more variability in coloring. The
female flower is on the right. The tall structures are the
pistil. Compare them to the anthers in the male flowers.
But there can be differences in individual flowers too. A 2014 study showed that in Saponaria officinalis (soapwort), an insect pollinator can discriminate between male flowers and female flowers based on shape and color (males are a little pinker).  The most successful male flowers (whose pollen got to female flowers and fertilized them) were just a little different from females, so the insect works to keep the dimorphism low.

Scent can be another dimorphism. Most often, the scent of male and female flowers is very similar; this is so they can attract the same pollinators. But an exception is Phyllanthaceae plants as shown in a 2013 paper. A parasitic moth is the pollinator. It gathers pollen from males, offers it to the female flowers where it lays its eggs. The larvae then eat the seeds - so the females need to be fertilized for this system to work. This means that the beetle really needs to find male flowers first.

Mated female beetles prefer the smell of the male flower, so female beetles being mated drives them to collect pollen by attracting them to the male flowers more than the female ones. Then, when it visits the female flower to lay its eggs, it brings along the pollen to ensure a food source (seeds) for the larvae.

The differences between the sexes can be seen in the plants as a whole too. Longer living dioecious plants often have males that are larger. Male seeds will be heavier and germinate earlier than females. The extra endosperm gives them a chance at establishing themselves and growing larger, and the early germination also gives them a head start on the females. So no wonder they are often bigger.

The quaking Aspen is an exception. It is long lived, but
the females are usually bigger and have more clonal
propagation. And the exception to the exception – Pando,
one of the oldest and largest organisms on Earth is a
clonal population of quaking aspens in Utah –
but they’re male!
The investment females make in fruits and seeds also keeps them smaller; they have to conserve energy for building expensive reproductive structures. A 2010 study showed that in many serotinous species (plants that release seeds after a fire), the females branch less than the males and have fewer but bigger leaves. In studying which individuals release the most seeds after a fire, those that looked least like males did the best. Saving energy by making fewer branches and leaves really does pay off, especially because the females have to invest so much in keeping the cones alive until there’s a fire.

Another example of plant size dimorphism is the Rumex hastatulus from a paper from 2012. This is a wind pollinated and wind dispersed (for seeds) plant. The male grows taller than the females early, so it's taller when the pollen is dispersed (better distance). Then the female grows more and is taller than the males when the wind disperses the seeds (better distance again). Neat how that works out.

On the other hand, plants that live shorter lives usually have females that are bigger. In the perennial plant Silene latifolia, growth and survival are same in male and female until reproduction begins, the females grow bigger and live longer. There is a live fast and die young strategy for the males – their job is done first, a lot like female spiders that eat the males after mating. By providing their mate with a meal (himself), the male spider improves the chance she will lay healthy eggs.

On the left is the male L. xanthoconus plant with flowers.
These provide the Pria beetle in the middle with nectar.
The female plant and flowers on the right are shaped
very differently and provide the beetle with shelter. It
doesn’t mean to pollinate the female, it’s just trying to
get out of the rain.
Ecology plays a role in plant sexual dimorphism as well. The environment and the pollinators can bend plants to their will. The Leucadendron xanthoconus of South Africa is pollinated by a single beetle species (Pria cinerascens) according to a 2005 study. It gets nectar and lays it’s eggs in the male, but seeks shelter from the rain in the differently shaped female flower. It only gets food from the male, and only receives shelter from the female. Even though the female doesn’t offer any nectar – the system works and therefore evolution keeps the males and females from being similar.

The same plant demonstrates another ecologically driven dimorphism.  Males that maximize their number of flowers get visited by more beetles, but the investment makes them die sooner. Bond and Maze in 1999 showed that the males spend more on non-photosynthetic flowers (because they aren't green) and that these flowers cover up more of the photosynthetic leaves. The dimorphism is that the female plants live longer.

In many dioecious plants, the sex ratio is nearly 1:1 male:female.  That makes sense, unless there are longevity issues induced by one sex or the other (like females dying younger because they put more into making fruits or the example immediately above). However, in some plants the ratio may be way off. Males may be bigger, and they may make less defense toxins, therefore, they may get eaten by herbivores more. This example not withstanding, male bias is more common than female bias.

Biologically, an interesting picture. Stephen Colbert is
about six feet tall, sexually dimorphic than most females
based on height and haircut. The bald eagle has reverse
sexual size dimorphism, the females are usually larger.
And the pistachio tree is dioecious and has a sexually
dimorphic ratio; there is about one male for every
10 females.
On the other end of the scale, dimorphism can end up affecting the environment as well.  Spatial segregation of the sexes (SSS) can occur if the resources are spaced differently in the environment and the different sexes need different resources (it happens). This could lead to areas that are mostly male and areas that are mostly female. It’s not a problem unless they get so far apart that the wind or the animal pollinators don’t make it from the males to the females.

A report in 2010 stated that SSS usually works out so the males end up in areas of less resources and females in areas of more. Females need the resources more, and while the less ideal areas mean fewer competitors for the males. That’s the general rule – males are limited by competition, and females are limited by resources – think about that.

Next week, the sexual dimorphism of plants seems strange to us because we don’t really see the difference between males and females (maybe in holly plants). But it does get weirder. Animals have males, females, and hermaphrodites. But plants take it to a whole other level.

Davis, S., Dudle, D., Nawrocki, J., Freestone, L., Konieczny, P., Tobin, M., & Britton, M. (2014). Sexual Dimorphism of Staminate- and Pistillate-Phase Flowers of Saponaria officinalis (Bouncing Bet) Affects Pollinator Behavior and Seed Set PLoS ONE, 9 (4) DOI: 10.1371/journal.pone.0093615

HEMBORG,., & BOND, W. (2005). Different rewards in female and male flowers can explain the evolution of sexual dimorphism in plants Biological Journal of the Linnean Society, 85 (1), 97-109 DOI: 10.1111/j.1095-8312.2005.00477.x

Bonduriansky, R., Maklakov, A., Zajitschek, F., & Brooks, R. (2008). Sexual selection, sexual conflict and the evolution of ageing and life span Functional Ecology, 22 (3), 443-453 DOI: 10.1111/j.1365-2435.2008.01417.x

Pickup, M., & Barrett, S. (2011). Reversal of height dimorphism promotes pollen and seed dispersal in a wind-pollinated dioecious plant Biology Letters, 8 (2), 245-248 DOI: 10.1098/rsbl.2011.0950

Okamoto, T., Kawakita, A., Goto, R., Svensson, G., & Kato, M. (2013). Active pollination favours sexual dimorphism in floral scent Proceedings of the Royal Society B: Biological Sciences, 280 (1772), 20132280-20132280 DOI: 10.1098/rspb.2013.2280

For more information or classroom activities, see:

Monoecious/dioecious –

Spatial segregation of sexes –

Serotiny –

Wednesday, April 8, 2015

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
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 –

sexual selection –