An Introduction on the Mating System of Koopas

By Andrew Anderson (@AndersonEvolve)

As an evolutionary biologist, it’s often a fun exercise to look at fictional creatures and think about how they might have evolved or how patterns in nature might apply to them. Ever since he became a playable in character in Mario Kart for SNES, I have always jumped at the chance to play as the King of the Koopas, Bowser–yes, even in Smash Bros where he’s a lower tier fighter. Video games aside, the lore around Koopas is sparse (or least from my knowledge), which leads to some great biological questions about how they function and survive. In particular, how do they reproduce and what mating systems might they have? In this post, I’ll set up some of the species boundaries of Koopas and in successive posts I’ll try to tackle some of the more interesting thought experiments.


First, we need to establish what creatures are Koopas. The classic concept of species is the Biological Species Concept, the idea that if populations can interbreed and yield fertile offspring, they are the same species (e.g. horses and donkeys can make a mule, but the mule is sterile–thus horse and donkey are separate species). This is a useful start point as it might help us demarcate what are and aren’t Koopas. In order to test the Biological Species Concept we need to know if interbreeding occurs. This is a dark-corners-of-the-internet thing, so I’ll just assume if a romance is present between characters then the result of a pairing would be fertile offspring. So a quick look at some known pedigrees in the Mario world and it looks like Koopas are pretty consistent in their appearance. Bowser had his 7 children for the early games (later retconned to be his minions, but some guide books still state they are his children), and one other child, Bowser Jr., who appeared in Mario Sunshine. We also have Koops who is the son of Koopley, and Kolorado who had an unnamed father in Paper Mario. Clearly Koopas beget Koopas.


But what about other denizens of the Mushroom Kingdom? Hammer Bros, Lakitus, and Boom Boom also have turtle-like appearances, so we’ll assume there isn’t any cryptic speciation (i.e., two different species that look similar) going on. It is also stated that Koopas evolved from Shellcreepers on the Mario Wikia. Shellcreepers still occasionally appear and certainly Bowser is as old as Shellcreepers, so they cannot be the ancestor to all Koopas. To be more accurate (although one can only do so much in a video game universe), I’m going to say Shellcreepers share a lineage with the other Koopas but are the most ancestral in form. Yoshis are from Dinosaur Land, which Bowser invaded, so they are geographically separate from Koopas.  The only romantic entanglement we get from Yoshi is Birdo and neither are have the turtle-like appearance, so it seems unlikely Yoshi is a kind of Koopa. There is a strange thing to consider though; while not cannon, the Mario Bros live action movie suggests that most inhabitants are related despite their drastically different appearances (I would lose so much time nitpicking its evolution statements–so I’ll ignore them). It also shows Princess Daisy as being hatched from an egg–implying the human-like characters might be reptile-like as are the Koopas. We know Bowser is infatuated with Princess Peach, and his claim that Bowser Jr. was her son was believed, though not true. The fact that such a lie wasn’t immediately rejected suggests some plausibility. So it may be that the human-looking characters of the Mushroom Kingdom are also Koopas.


We can already see an issue with the Biological Species Concept. Are the princesses and Toads actually Koopas? What about Mario and Luigi? They’re from a different land and stated to be human, but have romantic interests in the two princesses. Clearly defining species is an interesting challenge in evolutionary biology. There are many ways to define a species in biology, though Biological is often a good start point for determining speciation in most sexually reproducing species. Scientists are uncovering more and more that hybridizations occur across lineages that are distinct enough to be considered separate species without sterilizing the offspring. In fact, humans have interbred with Neanderthals in the past despite the two being considered separate species. Evidence of this lies in sections of human genomes where parts of the sequence more closely match Neanderthal than the more ancient African lineage.  Thus, those who can trace their ancestry to regions where Neanderthals co-occurred with humans have ~3-5% of their DNA inherited from Neanderthals. So even though it is hinted at the intermingling of the human-like members of Mushroom Kingdom and Koopas, it is still possible to consider them different species; therefore, given the drastically different morphologies and their reluctance to even cohabitate, I will treat the human-looking members of the Mushroom Kingdom as a separate species, but one capable of interbreeding with Koopas.


Now that we have determined what is and isn’t a Koopa, we can now move into the behaviors and morphologies that are found within the species.  Stay tuned for the next sections of this blog which will discuss the mating system and brood-care strategies of Koopas…

What’s Good for the Goose is NOT Always Good for the Gander

by Andrew Anderson  (@AndersonEvolve)

To start this post, I feel the need to point out that I detest Naturalistic Fallacies (i.e., what’s natural is morally good), and the situations I will describe can be sensitive to some readers.  These situations occur in nature and in no way do I think (and no one should think) that they justify disgusting attitudes and behaviors seen in our society. If you do not wish to read those descriptions, they will be in blue text so you can skip them.

Four-line Cardinalfish–A mouthbrooder that will engage in filial cannibalism.

One of the more interesting aspects of sex-roles and sexual selection is the concept of sexual conflict.  Put simply, sexual conflict is whenever a trait or action benefits one sex over the other. There are two kinds of conflict, the first is called intralocus (within location) conflict.  A rather humorous example is the thought,“why do males have nipples?” The genetic architecture for nipples, and milk delivery in general, is vital to the survival of mammals. A female without nipples would not be able to deliver milk to her offspring and would not have any successful offspring (since they didn’t make it past birth).  Disentangling male and female expression of traits can take some evolutionary time, but more importantly, a mutation that stops nipple formation is very costly unless it occurs in males only.. Further, as the presence of nipples doesn’t impose a much of cost (survival or energetic) to males, they’re just there. However, milk delivery is more costly, so evolution can act with a little more intensity to separate the sexes, so males don’t have those as fully developed.  The mechanism for the male/female expression is hormonal control, thus a male given a female hormone will start to express those traits. Another example is secondary sex traits. Bright, large feathers in peacocks are energetically costly and make them more vulnerable to predators, peahens would do well not to express those traits. Thus the architecture for the feathers is different for males and females, again this mostly done by hormonal control.

The second kind of conflict is interlocus (between location) conflict.  This is usually when males or females engage in behaviors that are self-beneficial but potentially harmful to the other sex. 

Examples of interlocus conflict can be found in insects, some species have evolved brushes to scrape out rival males’ sperm or plugs to prevent other males from mating.  Clearly, the benefit to the male is the ability to reduce his mating competition without guarding the female, but these mechanisms can damage the female and prevent her from mating with a male she might deem more suitable or prevent her from being ever able to mate again even after she lays her eggs.  Male water striders have evolved hooks to grab females and hold themselves in place to complete copulation. Ducks will actually force copulation with a female in a bid to put their genes into the next generation. While biologists focused on the male behaviors at first (they’re more visible and there has been a bias on male traits since historically science is male dominated), we’re learning more and more that females are engaged in an arms race themselves.  Females can chemically “help” certain males’ sperm and have evolved ways to resist damage (insects), become bigger and stronger to shake males (water striders), or evolve complex vaginas to shunt away a male’s sperm who forces himself on her (ducks).

Okay, okay.  You know what’s coming next if you’ve been keeping up with me… FISHES!  Again, I’m going to focus on brood care. In my lab, others have shown that male pipefish (check my first post about them here) actually will provide more care to some female eggs over others.  Pipefish males actually exchange resources with the eggs they are brooding (similar to mammalian pregnancy) and it has been shown that some eggs are reduced during pregnancy.  This means that males can actively take resources from the eggs the female provided. Larger females produce more eggs and can completely fill a male pouch making a good return on investment for the male.  Males tend to prefer larger females and will reduce a larger portion of eggs from small females, presumably to gain energy to invest in future pregnancies with large females. Thus, a small female has spent relatively more resources on reproduction only to have a low amount actually born.

  One of my favorite examples outside of pipefish are the cardinalfishes (Apogonidae), which actually engage in filial cannibalism.  Cardinalfishes engage in male mouthbrooding, a process which is costly to males. They cannot eat until the eggs are hatched. Since they are investing in a brood they want to maximize that investment.  If a female produces a small egg mass, the male may actually consume the entire brood in one gulp, especially if he encounters another gravid female and he has not been brooding long. While this is an amazing feature, what’s even more mind-blowing is some cardinalfishes the females have evolved a response.  They don’t fully develop some eggs (saving energy), but include those eggs in the masses they give to the male, so the mass seems like it has more eggs and the male would be less likely to consume it.

 The world of sexual selection and fishes is wild and weird and there’s nothing quite like it!        


Fish Also Dig the Dad Bod

by Andrew Anderson (@AndersonEvolve)

Imagine you’re on a dating site looking for a potential partner.  You browse through a couple of individuals and find a few that merit a further look.  This particular site allows feedback from individuals who have contacted or dated a person to be viewed on that person’s page.  Would you read the comments? Would you weigh the comments in your decision to engage in further conversations/dates with the person?  If so, you have employed a mate choice strategy called mate copying.

Clunky hypotheticals aside (AOL Instant Messenger was the social media of choice when I last dated), mate copying has been observed in mammals (yes, possibly humans), birds, fishes, and even insects. Most often, these are confirmed by testing if a female’s interest in a male is altered if she observes him with another female.  Personally, I would love to see if this occurs in role-reversed systems, but the research that have studied this pattern generally find females to be the choosier sex. So why would a female rely on another female’s choice? There are some hypotheses that have been proposed, such as: searching for a mate is costly (i.e. lose energy/time or become a target to predators inspecting each potential mate), a female may not have enough experience to determine male quality, or distinguishing between quality males is  challenging. Females do not have to directly observe males being successful with other females; they can use other, more subtle signals to indicate the desirableness of a male. In rats, there is some evidence that the smell of a male who has recently copulated is a potential driver of female choice.

Mate copying is something that occurs across taxa; but, in my opinion, fishes have the most interesting behaviors associated with it.  As a reminder, the dads are more likely to take care of the young in fishes that engage in brood care. Even though dads care for the young,males are still more likely to engage in competition for mates rather than have females compete for them (although I study a few awesome exceptions).  One possible reason for this is that some species of males can tend nests larger than one female can fill with eggs. Some males will have eggs from many females and others have no eggs to care for (unequal mating success is an impetus for sexual selection). As you might be piecing together, females can use the amount of eggs already present in a male’s nest as an indicator of how “sexy” other females have found him.  In fact, in some species of fishes females prefer males who have eggs already in the nest. This has been tested in several species by adding or removing eggs from males’ nests and observing the resulting female choice.

Now the evolutionary mayhem begins.

Egg stealing males.  Left: Three-spined stickleback.  Right: River Bullhead. Bottom: Striped Darter

In three-spined sticklebacks, the males engage in a hurly-burly of activity centered around mating.  Before deciding to mate with a male the female will inspect the male’s nest, his bright colors, his swimming behaviors, and if his nest has eggs in it.  If she is satisfied he has met her criteria, she will lay eggs in his nest. While she’s doing so, other males will try to “sneak” a mating in by releasing sperm next to her.  Such sneak behavior is fairly common among fishes, but some males will also steal eggs from the nest and bring them back to their own. These eggs are not their own and therefore that male has no paternity, but he will care for them and raise them as if he did.  Since females use the presence of eggs in a nest to judge a potential mate’s quality, such behavior may end up actually increasing the total number of offspring they father.

Other species,such as the river bullhead, don’t even bother stealing eggs.  Males nest in close proximity to each other, and females choose which nest to lay eggs in –again with consideration for the presence/absence of eggs.  Instead of stealing a few eggs, males who haven’t mated will attempt to evict egged males from their nest and take over the entire clutch. The expectation, again, is that males who engage in that behavior might do better in overall reproduction than those that don’t, even though some of the eggs they invest in aren’t their own.

There is another example that is rather bizarre.  Darters are small fishes found in creeks that sometimes engage in egg-raiding.  One species, the striped darter, has evolved a unique coloration on its fins. This coloration creates a design that could be considered a facsimile of eggs.  During courtship, the male will display these markings, and there is a correlation between mating success and number of egg-spots on their fins. The hypothesis is that these “egg spots” stimulate the female the same way that seeing eggs might, making her more likely to mate.

As you can see, fish have a wide diversity of adaptations to one stimulus:  a preference for eggs in nest. It’s worth pointing out that the explanations of what’s been observed have varying degrees of confirmation through experimentation.  Here I have presented three species as examples; indeed, there are more species that have these behaviors and traits which lends credence to the explanations given here.  That is what’s so awesome about evolutionary biology: when something exists in nature that grabs your attention, you get to work to try to piece together what might have led to those traits.  

Who’s watching the kids?

by Andrew Anderson (@AndersonEvolve)

I am an evolutionary biologist and, as such, I find the diversity of life to be amazing and love pondering how divergences between species/populations occurred.  But I have a confession: fishes are the best, hands down. Sexual selection and parental behavior are often intertwined, and fishes cover a wide breadth of behaviors, traits, and systems, especially when compared to other vertebrates.  I am sure that there are probably a lot of entomologists and other invertebrate biologists shaking their fists at the screen right now. I’ll concede that those taxa are also amazing in their range of adaptations, but fishes are just cooler, so I’ll focus on them.

Throughout my time on this blog, I hope to point out many different features and adaptations of fishes as well as what processes may have caused them.  I hope to touch on everything from males that look like females to sneak mates, to males who steal eggs from rival nests to make it seem like more females have chosen them, to a species that loses 20% of its genome from its somatic cells (that is, cells that won’t make eggs/sperm).  For this post, though, I’ll touch on a fascinating evolutionary outcome that I really hope to delve into more as my career progresses: which sex watches the kids.

Male Gulf pipefish, Syngnathus scovelli, exhibiting male brood care.  He’s ready to pop!

Fishes are unique, because in over half of species that care for their offspring, the male engages in parental behaviors instead of the female.  The lab I entered studies Syngnathids, a.k.a. pipefishes and seahorses, which are known for their male pregnancy. What I found out is the family of fishes that are the closest relatives of Syngnathids, Solenostomids, also have brood care.

The ghost pipefish on the bottom is a female with larger fins on its underside that form a pouch to hold eggs until they develop into juveniles.  The top fish is male.

You can see some similarities between the two families, but in Solenostomids, a.k.a. ghost pipefish, the females have evolved brood care.  So we have two closely related families that evolved a pouch to hold developing offspring, but the sex responsible is flipped. What caused that?  A good start point would be to figure out what sex cared for offspring in the ancestor to both groups. Without a strong fossil record we can only infer what that ancestor might have looked like by comparing what traits the next closest families to Syngnathids and Solenostomids might have had. It turns out the next families are trumpetfish, cornetfish, and shrimpfish, none of which engages in brood care. As a result, there are three possibilities: 1) the ancestor had male brood care, 2) the ancestor had female brood care, or 3) the ancestor had no brood care and Syngnathids and Solenostomids independently evolved it with a different sex.

Samurai gourami, Sphaerichthys vaillanti, with a secondary sex trait of bands.  Female is on the top with two males below

In order to tease apart what might have happened, I needed to know if there were other groups of fishes that changed which sex took care of the offspring.  Sure enough, I was able to study this using a group of gouramis. Most species of gouramis have male brood care, and in one species, the samurai gourami, females evolved a secondary sex trait (a trait that is different between the sexes that isn’t directly involved in reproduction that may be used to attract or compete over mates).  

Chocolate gourami, Sphaerichthys osphromenoides, whose sex is not determined.

The closest relative to samurai gourami is the chocolate gourami, which has female brood care and is monomorphic between the sexes. I have sequenced the genomes of both gourami species and I am working on acquiring the transcriptome (what genes are turned on and how many times is a particular gene activated). My hope is to piece together what happened at the genomic level to cause such a wholesale behavioral change.

While this work is personal and I’m excited to share it, my goal is to show readers of this page some other peculiar results of evolution, especially in that most extraordinary group: the fishes.  Until next time