Think like a scientist: levels of analysis

Today’s discussion question is: why do the lovebirds Sam and Jesse spend a lot of time together?

Jesse and Sam pause in their destruction of a picture frame to wonder what business it is of yours how they spend their time.

Jesse and Sam pause in their destruction of a picture frame to wonder what business it is of yours how they spend their time.

“Well,” someone says, “it’s because lovebirds’ brain reward centers are stimulated when they interact with their mate. Happy molecules trigger happy receptors, and the birds get happy. So they seek out that reward.”

“No,” someone else says, “it’s because a lovebird that interacts with its mate more has a stronger pair bond, making it less likely to be cuckolded. Lovebirds who spend more time with their mates have more chicks and so pass more genes on to the next generation.”

See? When Jesse leaves Sam alone, she starts checking out other birds!

See? When Jesse leaves Sam alone, she starts checking out other birds!

Actually,” someone else says, causing the first two to roll their eyes and mouth actually at each other in an exaggeratedly snobby manner, “it’s because, although Sam and Jesse were both initially raised by humans, they subsequently spent enough time with other lovebirds to learn to recognize them as their own species; and since they have been housed in an apartment with only each other, they have perforce taken each other as mates, each being the other’s only lovebird option.”

“What?” exclaims an excitable fourth person. “Obviously it’s because, over a very long time, lovebirds have evolved very strong pair bonds – possibly because chicks can best be raised by pairs of adults, or because the parents must be temperamentally matched in order to best cooperate in chick-rearing, or because only pairs can successfully defend good-quality territories. But, you know. They’ve evolved it, over hundreds of generations.”

And everyone gets a D.



None of the answers are necessarily wrong; they’re all decent hypotheses. (To be clear, I did not just do an exhaustive literature search on lovebird pair bonding, so none of those answers are necessarily right either.) These hypothetical responders get bad grades for thinking that they are arguing with each other.

They are not arguing.

Scientific questions often have several answers, in the same way that the question Why are we in the library? might have the answers Because I need to get a book on generalized additive models and Because we turned right on Pine St. and walked up the library steps and opened the door. The answers aren’t mutually exclusive, they’re complimentary.

Questions involving animal behavior have at least four answers, because in animal behavior we have four levels of analysis: mechanism, ontogeny, adaptive value, and evolutionary origins.

Mechanism questions concern the physiological ways that a behavior or trait arises – the signaling molecules, hormones, gene transcription, etc. Ontogeny questions concern how the behavior or trait develops: does the trait develop in the embryo or not until sexual maturity; is the behavior learned from the parents? Mechanism and ontogeny questions are considered proximate, since they deal with issues of immediate, proximate causality.

The next two deal with causality over a much longer timespan, ultimate causality, and are considered ultimate. Adaptive value asks how the trait or behavior affects the fitness of the individual who possesses it. Evolutionary origins asks how the trait or behavior evolved over many generations, across many lineages and even species.

I can ask, “Why is Sam’s butt blue?” at each of these levels of analysis.

You wish your butt was this pretty.

You wish your butt was this pretty.

Mechanism: What (pigment? structure?) makes the feathers blue? What chemicals within the body signal that these feathers should be blue and the rest of the feathers, not blue?

Ontogeny: When do the blue feathers grow in?

Adaptive value: What fitness benefits does Sam get from her blue butt? Can she raise more babies with her blue butt than she could with a different colored butt?

Evolutionary origins: How did the blue butt evolve in this species? Does it come from a long lineage of blue-butted birds, or has it only recently arisen? Why?

Jesse is totally checking out that blue butt.

Jesse is totally checking out that blue butt.

The people arguing over why Sam and Jesse spend a lot of time together aren’t really arguing – they’ve just each answered the question at a different level of analysis. Scientists getting into heated debate over two hypotheses that are at different levels of analysis – and therefore not mutually exclusive at all – is surprisingly common. When considering an explanation for a behavior, I always first figure out what level of analysis we’re talking about, so that I know which other explanations it may contradict – and which it can’t.

It isn’t only behavioral ecologists who need to consider levels of analysis. A few months ago, the New York Times ran a column that argued that people should stop urging each other to wear bicycle helmets because it made other people get scared of the dangers of bicycling, not bicycle, be unfit, and die of obesity-related illnesses. People will overall be more healthy, it argued, if we all just stop talking about bicycle helmets, and that means that you don’t need to wear your helmet either.

This article had a severe case of levels-of-analysis confusion. As a whole society, perhaps, we might (I’m still dubious) lower the mortality rate by having many people bicycle without helmets, rather than having few people bicycle with helmets. However, any given individual person will still have a lower risk of mortality if they bike while wearing a helmet. The whole-society level of analysis does not nullify the individual level of analysis: whatever is happening with society at large, to be as safe as possible, you should still wear a helmet.

I wonder how many people who read that article and decided to stop wearing a helmet are going to incur brain damage that a helmet would have protected them from? Consider your levels of analysis, or you, too, may someday write a New York Times column that unintentionally causes brain injuries.

You wouldn't need helmets if you could bend your neck sideways and turn your head almost all the way around, like us![Note: the lovebirds clearly do not understand the function of helmets. Ignore them.]

You wouldn’t need helmets if you could bend your neck sideways and turn your head almost all the way around, like us!
[Note: the lovebirds clearly do not understand the function of helmets. Ignore them.]


Alcock J. 2005. Animal Behavior, 8th ed. Sunderland, MA: Sinauer Associates.

Sherman PW. 1988. The levels of analysis. Animal Behaviour 36(2):616-619.

6 thoughts on “Think like a scientist: levels of analysis

    • It was fun to be home for the holidays and visit the lovebirds. We had budgies when I was a kid, and they made much nicer sounds than the lovebirds, but they weren’t nearly so hilariously destructive.

  1. I love the comparison you draw with that NYT article. I saw it and had an annoyed reaction to it that I didn’t bother to analyze. I love the way you took that feeling and delved into it!

    • It definitely took me a while to get a grip on what exactly bothered me about it. At first it was just “This seems wrong!” I’m glad you had a similar reaction – that suggests I’m not totally crazy :-)

  2. I have spent much of my life kicking myself for not studying subjects like psychology and sociology in college…but that didn’t stop me from taking them on as a hobby. Science-y posts often catch my attention, but, this time, you had me with “Tough Little Birds.” I had macaws for 23 years. Love your captions.

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