National Geographic

Of Men, Navigation, and Zits

How, you may be asking yourself, is a good sense of direction like a bad case of acne?

Over many decades, psychologists have measured the minds of men and women, looking for similarities and differences. Reliable results are  notoriously hard to come by, because it can be very easy to find differences where none really exist. If you decided in 1970 to look at the fraction of scientific and medical Ph.D. awarded to women–under 5 percent–you might conclude that women’s brains just weren’t suited to the task. Today, that figure has reached about 50 percent. Women’s brains haven’t evolved over the past 40 years. Their social environment has.

Yet a few differences between the sexes do seem to hold up to scrutiny. One is spatial abilities. If men look at an object, for example, they are slightly faster at guessing what it would look like if it were rotated 180 degrees. There are plenty of women who do better than individual men. But overall there’s a statistically significant difference in their average performance. This kind of difference carries over from one culture to another. It’s even detectable in babies.

What accounts for the difference? Some scientists argue that it is an adaptation. Obviously, the evolution of the human race hasn’t hinged on being able to turn a stack of blocks around in our heads. But spatial abilities can have some far more practical benefits. If you can picture a landscape clearly in your head, you are less likely to get lost in it. People who score high on spatial ability tests also tend to do well on navigation tests. And in some studies (but by no means all), men do better at finding their way through a new place–be it a university building or a forest.

But why should men be better at women at navigation? Some researchers argue that we have to look at the different roles of men and women over course of our evolutionary history. Women spent most of their time in small ranges, either caring for children or gathering food. Men, on the other hand, had to rove over much bigger ranges to hunt game. They benefited from a better sense of navigation because they were at a greater risk of getting lost or failing to find game.

Whenever we reflect on human evolution, it pays to compare our species to other animals. And in the case of spatial abilities, the comparison is fascinating. Almost a century ago, the psychologist Helen Hubbet found that male rats could get through a maze faster than females. The difference can also be found in a number of other species.

If the “home range” explanation accounts for the difference, then you’d expect that species with a big difference between male and female home ranges should have a big difference in their spatial abilities. Some studies seem to bear that prediction out. Scientists have long been intrigued by meadow voles and their close relatives, the pine voles. Male meadow voles mate with lots of females, and have to defend a large home range against other males. Pine voles, on the other hand, are monogamous, and the males unsurprisingly have home ranges no bigger than their mates. When scientists look at their spatial abilities, they find that male meadow voles–the ones with big ranges–score much better than females. In pine voles, there’s very little difference.

QED? Not quite.

Two species are not enough to make a meaningful comparison, and so Edward Clint, a psychologist at the University of Illinois, and his colleagues recently made a larger comparison. They gathered data from studies on 11 species, including humans, voles, rats, and even horses and cuttlefish. They compared the differences in home ranges and spatial abilities. In the latest issue of Quarterly Review of Biology, they published their results, which can be summed up with this graph.

If the home range explanation was correct, you’d expect the points to fall along a line going from the lower left corner to the upper right. If you only look at the pine vole and the meadow vole, that indeed is what you find. But if you look at all 11 species, they’re all over the map. There is no correlation whatsoever.

Clint and his colleagues propose a different explanation: male spatial ability is not an adaptation so much as a side effect. Males produce testosterone as they develop, and the hormone has a clear benefit in terms of reproduction, increasing male fertility. But testosterone also happens to produce a lot of side effects, including male pattern baldness and an increased chance of developing acne. It would be absurd to say acne was an adaptation favored by natural selection. The same goes for the male edge in spatial ability, Clint and his colleagues argue. They note that when male rats are castrated, they do worse at navigating a maze; when they are given shots of testosterone, they regain their skill.

There are many sorts of behavior that have been shaped by natural selection. But it’s always important to bear in mind that what looks like an adaptation may be nothing of the sort.

 

[Image: Reinhard Dietrich, Wikipedia]

There are 16 Comments. Add Yours.

  1. Ross Marks
    December 28, 2012

    Can you confirm the photo that illustrates this article was taken in New Zealand?

    [CZ: Indeed. Check out the Wikipedia link for details.]

  2. John Kubie
    December 28, 2012

    I don’t completely follow the final argument. Why is the relation of navigational ability to testosterone a spandrel? Seems more like a potentially useful adaptation tagged onto a single mechanism. Similar to the number of independent but useful adaptations connected to breathing (speech, temp, O2-CO2 exchange).

    [CZ: The question is not about navigation per se, but the greater spatial ability of men compared to women. Is there something adaptive, in other words, about males being particularly good with navigation? To address that question, scientists have to come up with a hypothesis about males in particular that can be falsified. A home range hypothesis is one such explanation. And it is falsified in the new study.]

  3. Jim Castagneri
    December 28, 2012

    Do you suppose there is a link between a mans’ ability to navigate the world and his refusal to ask for directions?

  4. gaddeswarup
    December 28, 2012

    Coincidentally, just scrolling through my science blogs, I noticed this
    http://micromath.wordpress.com/2012/07/22/women-in-the-violent-world-of-mathematics-6/#comments

  5. Typo
    December 29, 2012

    “There are plenty of women who do better than individual men. But overall there’s a stasticially significant difference in their average performance.”

  6. John Roth
    December 29, 2012

    Two comments. First, the study is subject to the usual criticism about selection bias – not that I think it applies in this case, but that’s still an awfully small number of species compared to the number of species out there.

    Second, I think there’s a better explanation – situational awareness. In close combat it’s useful to keep a good mental map of your surroundings and where everyone on the field is at, and to do it in real time, without having to devote conscious awareness to it.

    [CZ: Thanks for your comment, John. As the scientists explain in their paper, they searched for all the studies they could find that measured the variables they were looking for. They ended up using 35 studies on 11 species. That doesn't sound like selection bias to me. The authors themselves note that 11 species is not a big number and they look forward to bigger data sets on which the hypotheses can be tested. But these data don't come easy--scientists have to spend a lot of time following animals around to estimate their ranges, and take the extra effort to note their sexes. As for your explanation, that's a testable hypothesis too. I could see testing it by seeing whether species in which males fight a lot with each other have a bigger difference in spatial abilities than species where males leave each other alone.]

  7. Chris Crawford
    December 30, 2012

    I don’t think that this study of 11 species provides much in the way of falsification. Leave out a handful of species and suddenly it supports the hypothesis. Small-number statistics are always dangerous. Did they calculate the variance of the correlation coefficient for this data? If so, I’d expect it to be so large as to render the study unconvincing.

    A factor we should not overlook here is the possible existence of supporting phenomena in the case of human male navigational skills. Let’s take the folk wisdom that men never ask for directions. If this were true, then it would tend to support the evolutionary hypothesis based on the presumption that males out of their territory were summarily killed, while females were adopted. Do we have any actual data on that possible behavior?

    There’s definitely something real going on here, and I find the testosterone hypothesis implausible, because testosterone doesn’t affect human cognition at so fine a level. The behaviors that we know it to affect are pretty much basal-level behaviors. Attributing a complex cognitive behavior like spatial navigation to testosterone strikes me as far-fetched. However, there is good evidence supporting it in the form of that rat study. I’m going to see if I can’t find it.

  8. Tony Mach
    January 2, 2013

    Just wanted to add that I traced my acne to dairy from pasteurized milk, and to a lesser degree to eggs. Haven’t done a blinded test (but want to do), but unless someone proposes a convincing placebo effect that can cause acne, I think my results will hold up when repeated blinded.

    So I wanted to say that I would be surprised if I were the only person in the world that has acne, and whose acne is caused by dairy and eggs. Other peoples milage may vary though (and I wouldn’t be surprised if other people find other causes for their acne).

    BTW, my testosterone seems high according to the usual signs, but it seems it was much higher when I was consuming much dairy (and much grains) – so I suspect a correlation there. But more like acne and high testosterone are caused by dairy from pasteurized milk.

  9. David Chase
    January 2, 2013

    Note that if “asking for directions” is one of those things that would indicate a low testosterone level, it is not something that a mate+reproduction-oriented man would care to signal, even if he were a little bit lost. Because after all, priorities.

  10. mk
    January 2, 2013

    Let’s hope none of those rabid Pharyngulites read this.

  11. peeter_t
    January 3, 2013

    An interesting read, and I enjoyed the general background, but I have to agree, the main argument seems rather dodgy. 11 instead of 2 species seems naturally less anecdotal, but even with a 100 species I can’t see the lack of correlation point to anything else, but the inadequacy of the method of cross-species comparison to evaluate whether a particular trait was an adaptation or a spandrel (perhaps with a full explanation of the involved proximal mechanisms supposed in each of the species as to the differential explanatory value of each data point it might be improved). From very basic statistics – the lack of evidence is not the same as evidence of a lack. My biology background is rather very limited, and I would be keen to hear someone explain this method of cross-species comparison for evaluating adaptive/non-adaptive explanations for a trait on this blog or elsewhere. I have created an ad hoc space for the topic here – http://peeter-t.livejournal.com/637.html – and I would invite anyone who could elaborate on the matter further to clarify it a bit. Thanks!

    [CZ: Peeter, one of the co-authors of the new paper, Ted Garland, has written a number of introductory pieces on this general question. Here's one pdf from his site.]

  12. Dabbe
    January 3, 2013

    “It would be absurd to say acne was an adaptation favored by natural selection.”
    But it wouldn’t be so absurd to hypothesize it favored by sexual selection..

  13. Dabbe
    January 3, 2013

    @Tony; Dairy and eggs are a great source of cholesterol, which is the natural precursor for steroid hormones.

  14. Tony Mach
    January 4, 2013

    @Dabbe: Yes, but then all steroid-hormones would be too high – some of mine are too low, some are normal. Besides, there is a rate-limiting step in the conversion from cholesterol to DHEA, as far as I can remember.

    And it is now consensus in medical sciences, AFAIK, that dietary cholesterol does not influence blood cholesterol levels.

    (And as I said, I traced my acne problem to pasteurized milk, so it must be something changed in the milk due to pasteurization, not due to the content of cholesterol in milk – given this suspected link between acne and testosterone.)

    I suspect that it is not that simple and I would not be surprised if some other effects are responsible. With the molecular-biology toolset with is now emerging, I think we should be able to see a solution to this riddle within our lifetime – if researchers ask the right questions, that is.

  15. Mykeljon
    January 5, 2013

    I guess I lack testosterone or something. My wife and both of my daughters are excellent navigators. They also have excellent spatial recognition skills. As for me, I am happy to have any of them tell me where to go. :)

  16. Edward Clint
    February 22, 2013

    Hello Mr. Zimmer, thank you for the thoughtful write-up. You’ve fielded the questions here quite well. I would add to a few of them…

    re: Cross species comparative analysis

    This is a well-honed tool of biologists based on the logic that if an ecological factor (x) induces selective pressure for trait (y) then we should see them correlated. For example, having a white coloration if you are a predator or a prey animal living in a snowy environment.

    This can be a very powerful tool because if you only studied one species or a small number, you get similarities based on relatedness/phylogenetic inertia instead of selection per se. For example, many mammals have 5 digits on their fore limbs because they have a common ancestor, not because each one has faced a continued selective pressure independently.

    re: 11 species
    Mr. Zimmer is quite right that the 4 pieces of needed data are hard to come by. (Male/female range, male/female spatial ability). I sifted through thousands of papers to find the data. Like all studies, this one is conclusive of little until replication, and I would love to see further attempts at replication and new data collected on new species. In the paper, I went so far as to suggest good candidate species for study including cowbirds and blenniid fishes.

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