Tiny Vessels

This Father’s Day give your dad a gift to remember – an understanding of the evolution of his gametes

With Father’s Day quickly approaching, I think it’s time we had a little talk about the evolution of dad’s gametes. Before you get creeped out, don’t think of this as being just about your dad, but rather any dad or male of an organism that reproduces sexually (sorry, bacteria!).

While not all dads are the same across the animal or plant kingdoms, they do all share one common feature – sperm. The production of this gamete type is what defines the male within every species. What defines females is their ability to produce eggs. The most important distinction between these two gamete types is their investment in size; sperm are small, eggs are large. It is their union into a zygote that characterizes sexual reproduction. But how did these two gamete types come to be?


A sperm cell fertilizing an egg cell. <WikiCommons>

It all starts with parasitism. Ah, the romance.

One theory is that before females or males existed, all gametes were of the same type and size. Size is an important trait for gametes because being large translates into more nutrition for a developing zygote, which assures it higher survival. Such survival is highly favored by natural selection and, thus, larger gametes also came to be favored. But might there be a cost if gametes are all under the same selective pressure to become large?

Consider a vast ocean with a single target – a giant ocean liner. Your goal is to reach this easy-to-spot target, which you can achieve through building one of two vessels given a set amount of materials: you can either invest your resources into producing a few more giant ocean liners, or you can take these resources and parse them into a large fleet of torpedo-like mini-submarines ready to divide and conquer. Finding a large vessel with another large vessel will prove a difficult task. For one, they’re less maneuverable. Moreover, they’re less likely to encounter one another since they cover less area. But time is of the essence, and a bunch of smaller vessels all working toward a common goal will more frequently encounter their target and, ultimately, will win the race.

Renegade at Dutch Wikipedia

Underwater attack by frogmen on manned torpedoes. <Renegade at Dutch Wikipedia>

In the gamete world, slow and steady will never win the race either. Large gametes are less efficient at finding their target precisely because their strategy of being large constrains them to be fewer in number. On the other hand, by not investing resources into their size, the smaller gametes are freed up to be more abundant. Occurring in greater numbers means they are much more likely to encounter their larger counterparts – an excellent racing strategy if you’re a gamete.

Now, let’s imagine the potential pairings of these different gamete types from the perspective of natural selection. The important rules of the race are two-fold: the more zygotes the better, and enhanced survival of the zygote is key.

In a remarkable feat, suppose two large gametes finally do find one another.



Sure, the fusion of their genetic material will result in a few mighty-super-survivor zygotes because of their equal investments in being large and nutritious. Unfortunately for them, however, their big investments will come at the expense of being rare, so their union will result in fewer zygotes.

Likewise, though the union of two small gametes would be more probable given their abundance, neither gamete can provide sufficient nutrition for zygote survival.


Consequently, neither of these unions between like-gametes would be favored by selection over time.

It’s these unfortunate pairings that help explain how natural selection additionally came to favor the evolution of a smaller gamete, sperm. What remained and became the evolutionary norm was the most productive and efficient strategy – the fusion of two disparate gamete types – large and small. Now, where there were once only ocean liners, there were also millions of torpedo-like mini-submarines.

And now for the unsettling part. In addition to representing a trade-off between quality and quantity, gamete size also embodies a trade-off in offspring investment. While large gametes evolved for nourishment of the zygote, small gametes evolved to be cheats. They downsized to be better able to find the large gamete, which was then entrapped into providing all of the sustenance for the zygote. So here is the rub that necessitated parasitism. By favoring both egg and sperm, selection actually ensured their unequal contributions to the zygote.

Hence, the very reason sperm likely evolved was to exploit the resources of eggs. And it is ultimately the selection of these two very different investment strategies that led to the origin of females and males, moms and dads.

So this Father’s Day, when you’re having trouble finding the right words to say, keep it simple. Thank him for his parasitic little sperm cell. It made all the difference.


Know You

Here is a nifty little write-up featured yesterday in Cornell’s news that discusses a new review in Behavioral Ecology by associate professor Michael Sheehan in my department.

The topic: Sociality and signal evolution

The gist: There’s a trade-off between social recognition (the ability to learn, memorize, and recognize individuals within a group due to repeated interactions) and elaborate, external quality signals (think peacock feathers or lion manes, which don’t require repeated interactions but still can communicate important info about the individual giving the signal). Social group size may drive selection to favor either social recognition (in smaller groups, in which repeated interactions with individuals are common) or external quality signals (in larger groups, in which interactions with randos are common).

One question that remains is how social network size within a larger social group can affect signal evolution. If social networks within a larger social group are relatively small, can selection on signals parallel that within small social groups? Or is it the quantity of interactions (with either unknown or known members or both) in larger groups that primarily drive signal evolution type?

Why care? This research can give us a predictive framework to understand species’ use of signals (be they visual, auditory, or chemical) given what their social structure looks like and vice versa. Also, the framework can have far-reaching implications for understanding social behavior within most animals, including us humans. Because of the inherent trade-off between the two, social recognition may limit the evolution of quality signals, which may explain why we don’t see quality signals in humans (at least non-cultural ones).

The bottom line: Understanding signal use is important for understanding social behavior because it gives us an idea of how individuals interact with one another in a social group and how information about potential rivals, allies, or mates is gathered and used.


Some social humans that I like. Photo Credit: Unknown


Outro with Bees

This video is incredibly cool – 2,500 time-lapsed photos of honeybee development in its comb. From wee to, well, less wee.

Photo Credit: Anand Varma

Photo Credit: Anand Varma

I’m amazed at the ingenuity of this project. Read the article to learn more about how an artist turns experimentalist to capture this incredible footage.

Really makes me want to bust out my camera and make pretty, pretty things with animals.

We Are Scientists

I love this for so many reasons.

I’ll give you one.
This video captures how diverse scientists are.

I’ll bet if you closed your eyes to imagine a scientist, you’d think of this:

Professor Frink from The Simpsons (Copyright 20th Century Fox).

Or this:

File:Beaker (Muppet).jpg

The muppet character Beaker (Disney.com via Wikipedia)

Or, you know, some human variation of these two.

But not all scientists are men. And not all of us work in a lab.

And even if we do, not all of us wear a white lab coat.

All shapes. All sizes. All colors.


From the Mad Scientist photo series by Jorge A. Novoa

Brothers on a Hotel Bed

I’m a few days late on this one, but it’s never the wrong time to learn about the fascinating mating habits of turkeys. They don’t call them wild turkeys for nothin’.

You know that saying “It takes two to tango”? Well, in their case it takes three, four, or five.

File:Male north american turkey supersaturated.jpg

Male north american turkey (Meleagris gallopavo) – Wiki Commons

No, no, no – not in that sense.

Mating systems where males form large groups to attract females are called leks, which means ‘play’ in Swedish . Many bird species form leks, in which the males competitively display their stuff to get female attention. Meanwhile, the female wanders from male to male to judge them before making her final pick.

The thing about leks is that they are competitive. Males of most organisms engage in competition of some kind to win the girl. Which is why something like this rhinoceros beetle is equipped with such fabulous weaponry.

But in turkeys, within the larger, competitive leks, are smaller coalitions of two to four males that cooperate with one another. These unique display partnerships in which males cooperatively court females are found in only a few species of birds.

What’s even weirder is that only one of the males (the dominant) actually gets to mate with the female that the group attracts. The other male or males, the subordinates, get no reproduction at all. Nothing. Zilch. Nada. This is akin to Batman getting all the ladies and Robin helping for no other reason than to help Batman get all the ladies. Rather strange, isn’t it?

So why would these subordinate males engage in such altruism, benefiting another at a cost to itself?

Further investigation by Alan Krakauer (then a PhD student) revealed that these duos or groups are formed by close relatives, often brothers. So while it seems that the subordinates are getting nothing for their efforts, they are actually indirectly benefiting by helping out a close relative.

Both of them gain by cooperating. The dominant does better with the subordinate’s help. Likewise, the subordinate does better as a back-up strutter rather than trying to strut his stuff all by himself. Most important of all is that the benefits for helping outweigh the costs.

This famous saying by J.B.S. Haldane gets at the heart of the matter: “Would I lay down my life to save my brother? No, but I would to save two brothers or eight cousins.”

In other words, you share roughly 50% of your genes with your siblings, so saving two of them is equal to saving yourself, who you are 100% related to. Similarly, you share roughly 12.5% of your genes with your cousins, so eight of them equals 100% of you. All this to say, the number of genes you share with your relatives matters.

And in the case of the turkeys, subordinate males gain no direct benefit (i.e. having offspring of their own) by helping, but they do gain indirect benefits by helping their close relatives have offspring. This is called kin selection, and it provides us with an explanation for such altruistic acts as cooperative male display in turkeys.

Now, if we could just figure out what’s in it for Robin.

You can read the original work on turkey courtship displays here and a popular press story about it here.

Totally Enormous Extinct Dinosaurs

Just as I’m sure watching movies involving *your* profession is annoying because they can never quite get it right in the film biz, it’s excruciating for scientists to watch movies with bad science in it. Case in point: I came across this funny blog advertisement for an entomologist (a person who studies insects) for the new Jurassic World movie set to come out in June of 2015.

And this is completely irrelevant to the science in it, but here is a funny comic of the original Jurassic Park with a slight twist in perspective. Enjoy!