I had planned merely to mention a few bat penis bones in passing, as a sideline – this is supposed to be the Doctor Moreau oriented blog, and I should be poke-poking you-all to buy my book as the big goal. Yet here we are on part 6 of rooting about in my twenty-plus year old research. There are some more things in that pile too, but for now, this post is probably going to cap it for the present.
The background for the present point is the traditional comparative method in biology, especially when it’s employed for evolutionary inquiries. Basically, measuring something-or-other across a bunch of different species and throwing it or them onto a big chart to see what’s up. It had produced some triumphs by the late 1980s: especially in behavioral ecology, which was very well-represented at the U of Florida, and in metabolic studies; regarding the latter, you might not be surprised that by the latter part of my grad career there, Brian McNab and I were sparks-striking, argumentative friends.
I was the squeaky-wheel grad student there who said the traditional method was full of problems and we should all be making and drawing phylogenies before doing mathematical about evolution. I went over the statistical reason why in Count Bacula, part 5: Indescribable geometries, the part about independent contrasts. Now I’m talking about the rather more pointed and not always welcome side of that: comparative biologists, everyone, needed to enhance the power of prior plausibility in our experimental designs.
I’ll use one of McNab’s classic comparisons for a teaching example. Here’s White & Seymour‘s graph of body weight vs. metabolic rate using McNab’s methods (OK, logarithms and cube roots are involved, never mind why) for mammals. See how the line is strong, but there are some identifiable high-residual groups? The logic is, species which fall on or close to the line are “just mammals” and require no special attention to their metabolisms, whereas high-residual species have especially low or high metabolisms and should be investigated in terms of special adaptive circumstances.
I stress that if pure physics and physiology are the concern, without hypotheses or baseline assumptions about evolutionary history being involved, then a species is a species and as descriptive work, this is no big deal (M.A.R. Koehl published an influential paper about that issue). But if one is either relying upon or questioning a particular “this happened then that happened, and how many times” or similarly, “this was selected for because of that contextual variable” evolutionary history, then dumping in per-species data points is not only statistically ass (the technical term is pseudoreplication), but identifying other variables as causal, especially in terms of selection, is very very uncertain. To pose causal explanations, you need a genuine history of which features evolved in which groups and in what order.
Primatologists were and are huge fans of the traditional method, and as far as living species are concerned, many groups they’re most interested in have few species per genus so pseudoreplication is a minor problem. Plus, there’s a big win for them which pretty much set the gold standard for how to do it, professsionally/publication wise anyway: Harcourt’s 1981 analysis of body size and testis size, relative to social mating system. Briefly, clusters or distinctive regression lines for the anatomy are fireworks-signalling for social mating system selection, as far as this subdiscipline is concerned.
Over in my zone pf interest, Alan Dixson published two closely-related papers in 1987 about the primate baculum which provided about as good a case study of the problem as one could imagine. When I decided to follow up on it by applying both independent contrasts and character mapping, I wrote to him and to his credit he was very supportive and shared a lot of data for me to use. Here’s my graph of his analysis using slightly more complete data than his papers:
His suggestion was that the species represented by the filled-in circles (Eoticus elegantulus, several species of Galago, and Macaca arctoides) comprise a distinctive baculum/body-size relationship that’s tied to the unusual copulatory behaviors in each species. (Mathematically, that they comprise a line of their own and don’t belong to the line formed, loosely, by the other circles.)
To get gaudy, you can tell by the graph that each has a big baculum relative to body size, and the prosimians (Galago and Eoticus) spend a lot of time, uh, inserted during copulation, and the bear macaque does this thing called a “post-ejaculatory sit,” meaning, staying there for a while after. Dixson suggested the big baculum has something to do with these behaviors.
Now I am all about making sense of genital anatomy, distinctive copulatory behavior, and evolutionary history, but I am of the sabertoothed school of evo thought, and I saw that this needed a different approach.
Mathematically, here are the problems. The Galago and related species’ data points could be one evolutionary instance rather than several, therefore we need independent contrasts; and in Macaca, we don’t know whether baculum size and body size evolve in tandem throughout the genus, meaning, whether the M. arctoides datapoint is “weird” or not. Nor do we know when and how the copulatory behaviors evolved relative to each known species having an off-brand version now.
I was also irked (and more so, actually, as Dixson’s work could hardly be expected to have incorporated Felsenstein’s at that date) by flaws of overly linear contrasts analysis, especially in the book which everyone seemed to use as the Bible for the technique, Paul Harvey and Mark Pagel’s The Comparative Method in Evolutionary Biology (1991). I never liked its obsession with linear regressions on contrasts and much preferred to rely on Felsenstein’s original paper and follow-up work by others, which specifically treated “how to interpret it” as a developing art rather than a cookie-cutter.
So what’d I get by running contrasts? Remember, in these graphs, each point isn’t a species, but a node (split) in the phylogeny, regardless of whether that node is taxonomically named. The higher an X or Y value, the more the body mass or baculum, respectively, changes at that node. If it looked like a tight line, that’d mean that with every shift in body size, the baculum size would shift accordingly. Obviously, it doesn’t. (the graph on the left is a close-up of the lower left corner of the graph on the right)
The first conclusion is simply to abandon the original linear regressions as relevant to evolutionary questions – the contrasts spray all over and no credible “line” is evident. What’s revealed beyond that, though, are the two really interesting moments: the origin of Macaca , in which the baculum increases dramatically with no corresponding shift in body mass; and also of our own nifty gorilla-chimp-human clade, which begins with a baculum and body mass shift, and then follows with a big body mass shift with no baculum change. I’ve held forth about the latter in several posts, including Ape, man and Count Bacula, part 3: the case of the missing baculum, and will say here only that these results validate the notion that it’s the two earlier developments in the humans-and-pals clade, not isolated human baculum loss, which are evolutionarily important. I’ll now turn to give some cred to the wonderful and fascinating former, the genus of 22 monkey species which is spread over more geography and climate than any other – basically, what humans do for apes (go everywhere, root in anything), macaques do almost as much for monkeys.
Credit goes too to Jack Fooden, the taxonomist I knew well from my previous employment at the Field Museum who specialized in macaque evolution and ecology, hands-down one of the most decent, nicest biologists of the whole culture, and who would later help me considerably in discussing this study.
Here’s the thing: species by species, macaques display a wide range of social mating systems, a variety of nifty penis anatomies, differing naughty copulatory positions and methods, and as mentioned, a panoply of interesting bacula (for primates anyway, whose bacula are generally a bit boring). They would hypothetically offer a prime subject for linking these things into highly-specialized, highly-identifiable clusters.
However, what I’m showing is that the lovely sort of relationship between social mating system and testis size doesn’t apply in this 1:1 consistent way for the baculum. We have instead the ancient question of how the whole lineage, prior to its modern species diversity, came to have a surprising big baculum for a primate, and the species by species, highly individualized question of what, if anything, distinctive penis anatomy has to do with distinctive penis, uh, usage. A question which I showed cannot be quickly and easily passed off with a “for this” generalization.
When apprised of my … interesting research topic back then, people typically ask what I “found out.” And I found out some neat things. But to my mind, what really mattered were the things that were known, or assumed to be obviously a certain way, that I blowtorched. This was one of them.
Next: Behind any eyes