Tag Archives: biology

1.1 Billion

Posted on January 23, 2014 | 2 comments

That’s the number of heartbeats in every animal’s lifetime*. Don’t believe me?

Let’s consider an extreme comparison. A mouse can live for 3 years, and has a heartbeat of about 670 beats per minute. There are 525600 minutes in a year (365 days/year * 24 hours/day * 60 minutes/hour). So, that’s 3 years/lifetime *525600 minutes per year * 670 beats/minute ≈ 1.1 billion beats/lifetime. What about an elephant? They can live up to 70 years and have a heartbeat of about 30 beats/minute. 70 years/lifetime * 525600 minutes/year * 30 beats/minute ≈ 1.1 billion beats/lifetime. Woah… what?!

Ok, ok… you probably noticed that those “equals” signs are actually squiggly “approximately” signs, and if you did the math (you should!) you would see that they are both a little off from exactly 1.1 billion. But still, they are damn close. What gives? Why would the number of heartbeats be an invariant property of the animal kingdom? Let’s dive a little deeper.

The answer lies in allometric scaling, or how different properties of life scale with the body mass of organisms. It turns out that the power (energy per time, metabolism) required to support a given unit of mass of an organism scales with the mass of that organism to the (-1/4) power- meaning that smaller organisms use energy at a faster rate per unit mass than larger organisms. Other rates, such as breathing rate and heartbeat rate, also scale with bodymass^(-1/4). Lifespan, on the other hand, has been shown to scale with bodymass^(1/4). If you want to find how the lifetime total beats scale, you can multiply those two together (beats/time * time = beats). Bodymass^(-1/4) * Bodymass^(1/4) = Bodymass^0, which is always 1, meaning that the total beats is invariant of bodymass! More on allometry and metabolism in later posts. And maybe I’ll learn how to show equations in wordpress someday.

This paper (which probably takes into account more than the 2 points I used above) cites the total number of heartbeats in an animals lifetime as 1.5 billion.

Now, given this number, can we backtrack and use the relationship to see how long humans are predicted to live? Given a certain heart-rate, how long would it take us to use up our 1.5 billion?

lifetime

R code:
curve(1.5e9/(x*525600), xlim=c(40,100), lwd=5,
ylab=”Lifetime (years)”, xlab=”beats/minute”);
abline(v=60, col=”red”);abline(v=70, col=”red”)

If an animal beats its heart between 60 and 70 times a minute, it would use up its 1.5 billion beats in around 40-45 years. Is this a ballpark estimate of a human’s lifetime in the wild? (Aside: if you take the 1.1 billion heart beats derived from mice and elephants and assume a heart-rate of 70 beats per minute for humans you get 29.9 years!)

Now, don’t worry. Humans have found amazing ways to increase their lifespan, and it’s not like everyone has a set number of heart beats to get through before it’s all over. This is just an interesting result of looking at metabolism and ecology – and what’s even more interesting is looking at the animals that stray from the predictions.

*That’s about the predicted number of heartbeats in an average organism’s lifetime

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Posted in Biology, Uncategorized

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Flocking Science

Posted on January 19, 2014 | Leave a comment

Check out the beautiful video below of a “murmuration” (flock) of starlings acting in hypnotic unison:

Now, if you spend all day thinking about how to model biological systems (who doesn’t?), you might see that video and wonder about the rules each bird must follow to allow such spectacular emergent dynamics. Every individual bird probably gets some simple cues (direction, speed) from its neighbors, who get some from their neighbors (and that first bird), etc etc, and when these simple cues are acted upon and combined together all the birds form a giant complex morphing swarm.

A quick search reveals that the starling dynamics, and swarming behavior in general, have been the focus of a considerable amount of research and modeling. I’ll link this PLoS ONE paper since it’s open access (meaning everyone can view it in its entirety for free) and has some really cool videos showing off the modeling endeavors of the authors. In their simulation, each individual is characterized by parameters like mass, speed, position, and orientation- and these parameters get updated based on interactions with other individuals within a certain neighborhood. Just like in real life, these simple interactions scale up to show a swarm of individuals that behave as a complex, yet unified, group. (check out the videos in the link!)

I’ll also share this PLoS Computational Biology paper (also open access) which explores why individual starlings pay attention and respond to exactly seven of their neighbors (the authors report the number is special because it optimizes the balance between group cohesiveness and individual effort).

Another side effect of thinking about biology all day is always having to ask “Why (and how) did this evolve?” That is, what benefit does this intricate dance give the birds that allowed it to selected for and maintained? Being relatively ignorant of birds and their behaviors, it seems that such a show would turn into a buffet for predators. Well, maybe not. Here is a video of a Peregrine Falcon trying to snatch a starling during the flocking behavior and continually coming up empty handed (clawed?). The Peregrine Falcon is the fastest member of the animal kingdom, reaching diving speeds of over 200mph, so maybe this dizzying behavior is a great way to confuse even the quickest of predators.

I’m sure there is more to it than just predator avoidance, so feel free to add your 2 cents below!

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Carolina Wolf Spider

Carolina Wolf Spider

Hogna carolinensis, found in my apartment, Gainesville, FL.

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Posted on October 21, 2013 in Uncategorized

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The Second Most Astounding Fact

Posted on October 12, 2013 | 4 comments

“The Universe is within us”

Alright, I can’t compete with Neil deGrasse Tyson’s fact for #1. But, I do know the second most astounding fact. There is a biological universe within us. Well, more like an ecosystem. A planet. Where interplanetary travel and colonization are possible. I’ll explain.

Lets consider “you” the planet. And by “you” I mean the ~30 trillion cells that you are comprised of that have “your” DNA- the cells that make you “you.” Now, your planet has many inhabitants that aren’t “you.” Entire communities that have colonized different parts of your body and work in harmony (synthesizing vitamins, aiding in digestion, etc.) with the planet they live in. In fact, at any given time, your body has about 10 times more foreign bacterial cells within it than its own cells (bacterial cells are much smaller than your own cells)***. And, on top of this, there is another order of magnitude of viruses that inhabit us (or our bacterial flora) for every bacterium.

***[Update: Other research suggests the number of bacterial cells within our bodies may be of the same order as our cells.]

Sometimes alien species invade our planet and (if our own immune system defenses aren’t fighting well enough) we combat them with antibiotics. However, antibiotics don’t discriminate against friendly and unfriendly bacteria, and can annihilate our friendly gut bacteria– leading to digestion issues, or even the recolonization of the gut with unfriendly species. (I’ll save fecal transplants for another post). One instance of this recolonization that has been receiving some press time recently has been of a homebrewer who had a population of Saccharomyces cerevisiae call his gut home. This single celled fungus, also known as “brewers yeast”, plagued the man by, well, doing what it does normally- metabolizing carbohydrates and producing ethanol as a waste-product. Not surprisingly, the condition is known as Auto-brewery syndrome.

Anyway, I always thought the existence of a microbiome within all of us was a really cool and astounding fact. At every instant of your life, there are hundreds of trillions of individuals living their lives within you. Awesome.

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On atoms and seconds.

Posted on September 6, 2013 | Leave a comment

There are more atoms of gold in your body than seconds that have passed since the beginning of the universe. (6.11 * 1017  vs. 4.354 * 1017 (that’s 611,000,000,000,000,000 vs. 435,400,000,000,000,000)). And what does this mean?

Well, nothing really.

I’m just trying to convey the awesome scale of the universe, both space and time, with some familiar terms. Warning: the links in the last sentence are pretty mind-blowing.

I’ll post soon on the scaling of space and time within the realm of biology, and how the space an organism takes up in space plays a role in the space an organism takes up in time (or, Lifetime vs. Body Size)

p.s. to calculate the atoms of gold from the wiki article: (2e-7 kg) * (1000 g / 1 kg) * (1 mol Au / 196.96 g) * (6.0223 atoms Au / mol Au)     (who said we would never need high school chemistry again?)

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Posted in Biology, Chemistry, Cosmology

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The Saddleback Caterpillar and Lepidoptera memory

Posted on September 1, 2013 | Leave a comment

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I met this fellow in O’leno State Park  (along with a bunch of awesome Orb-weavers that I’ll probably post at some point). It’s a Saddleback Caterpillar (Acharia stimulea.) You can barely notice the ‘saddle’ on its back here, but I’m sure you have no problem noticing the venomous spines (known as Urticating hair) and mock-face (a startle display) on its backside. This larvae will eventually metamorphose into a limacodid moth:

Like many of the caterpillar –> butterfly/moth transformations, the adult bears little resemblance to the larvae in both body and behavior. It’s difficult to believe that the two life stages even belong to the same organism. In fact, what does the adult share with the larvae? Once in the cocoon does the body completely melt down and reform, leading to a rebirth and a fresh life? Or, does the adult have some memory of its youth?

Well, research suggests that the adults do share memories with their earlier selves. The scientists trained caterpillars to associate a specific odor with an electrical shock and found that, after metamorphosis, the adult moth avoided the odor that was associated with the shock earlier in life.

This shocking finding (sorry, I had to) may help scientists understand how Lepidoptera select habitat and, who knows, may have implications with human memory retention as well.

 

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Spiny Orb Weaver

Posted on September 1, 2013 | 2 comments

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We saw some interesting wildlife while hiking through O’leno State Park yesterday.

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