Photo by ricmcarthur.Or maybe it should be Carnival of Evolution XXX? Anyway, it’s online at This Scientific Life, and full of good posts from all over the evolution-inclined science blogosphere. Go check it out!
In natural communities, each species is embedded in a web of interactions with other species—predators, prey, competitors, mutualists, and parasites. The effects of all these other species combine in complex, unpredictable ways. I recently discussed a study of protozoans living inside pitcher plants that found predators and competitors can cancel out each others’ evolutionary effects. Now another study finds that parasites and predators can interact to make desert-living gerbils adopt less effective foraging strategies [$a].
Allenby’s gerbil is a small desert rodent native to Israel’s Negev Desert. They make a living foraging for seeds, which might seem simple enough—but for small desert mammals, it’s a constant balancing act. Foraging requires continuously judging how profitable it is to continue gathering seeds in one spot compared to looking for another, maybe better, spot; and all the while watching out for predators.
The red fox—a major threat if you’re a tiny rodent, but hard to watch for when you’re scratching fleas all the time. Photo by HyperViper.For small mammals, parasites like fleas can impose a real physiological cost—but they might also cause irritation that interfere with effective foraging. This idea led a group of Israeli reserachers to experimentally infest captive gerbils with fleas, and release them into an enclosure with a red fox.
It’s okay—the fox was muzzled! The research group was interested in how effectively the gerbils foraged in standardized patches of resources (trays of seed mixed with sand) in the presence of predators, and how being flea-ridden changed that foraging behavior. As metrics of foraging efficiency, they recorded how rapidly the gerbils gave up foraging in a single tray before moving on to another, which approximates how many seeds they left behind.
With no fleas, gerbils spent slightly—but not significantly—less time foraging in a single tray when a fox was in the enclosure with them. But gerbils infested with fleas moved on to a new tray substantially faster in the presence of a fox, leaving behind more seeds in the process. The study’s authors suggest that this is because the irritation caused by fleas distracted the gerbils too much to keep watch for a predator and forage at the same time—so flea-ridden gerbils made up for being less watchful by moving between patches of resources more rapidly.
So for gerbils, the presence of a second, different kind of antagonist amplifies the effects of a nearby predator. Fleas and foxes aren’t just a double whammy—the effects of both together are worse than the sum of each individually.
Reference
Raveh, A., Kotler, B., Abramsky, Z., & Krasnov, B. (2010). Driven to distraction: detecting the hidden costs of flea parasitism through foraging behaviour in gerbils. Ecology Letters DOI: 10.1111/j.1461-0248.2010.01549.x
There may be more going on in those tiny heads than you think. Photo by shadarington.And finally, Robert Krulwich narrates a beautifully animated short film about an enduring mystery of human behavior: our inability to walk in a straight line without help from visual cues.
My first marathon was last year’s Portland Marathon. Prior to 2009, I’d never run a race longer than five miles, but then that spring I let friends talk me into a half-marathon, and after running more than 13 miles, 26.2 suddenly didn’t seem quite so insane. Even so, training up for Portland was more than enough to make me realize that running what was (for me) a 3 hour-45 minute course is not really the same thing as running eight or nine 5k’s in a row.
I can make it through even a half-marathon on a good breakfast and carefully-judged pre-race hydration, but to go much longer I need more food (and water) mid-run. The long-term exercise involved in a long race is fueled by a combination of fat reserves and glycogen stored in the liver and muscle tissue. Glycogen is the more efficient fuel, so as exercise intensity increases, muscles draw on it more heavily.
If his muscles runs out of glycogen, a runner “hits the wall,” and may be forced to stop running altogether. I’ve done this a few times on long training runs, and it’s not pleasant—I’d end up all but walking the last couple painful miles. How long I can go before I hit the wall depends on my glycogen reserves, which in turn depend on the muscle mass in my legs—but it also depends on how fast I’m running, since glycogen use increases with effort. A computational study of the interactions between exercise intensity and glycogen consumption suggests that my first marathon time, 3:45, was close to the upper limit of glycogen consumption for a “trained endurance athlete”—and I probably don’t really qualify as “trained,” in the sense the study uses. So to survive a marathon, I have to take on supplementary energy mid-race, for which I will carry tubes of disgusting sugar syrup.
Continue readingBased on a careful analysis of D&T visitors over the past month, I conclude that orgasms are a popular topic.
Absolute unique visitors per day, tabulated by Google Analytics..So perhaps you folks would be interested in an entire blog carnival about orgasms? I think this is very likely. Fortunately for you, orgasm is the theme of this month’s Carnal Carnival, hosted with great enthusiasm by Scicurious. Enjoy!
Hey, there! Photo by hankplank.“Sociable tortoises” would make a pretty good name for a band. I assume they’d be somewhere in the genre phenotype space between Vampire Weekend and The Decemberists.
Apparently trying to top the transcendent union of “Star Trek” and Monty Python, the Internet presents Harry Potter singing Tom Lehrer. I’ll admit, this upgraded my opinion of Daniel Radcliffe from “Hollywood nerd” to “nerd.”
Correction, 22 December 2010: Vincent Lynch, author of the second paper discussed in this post, notes in the comments that he didn’t actually conclude that female orgasm was an adaptation. I’ve corrected the post accordingly.
Whether or not a trait is an adaptation, shaped by natural selection for a specific function, can be a surprisingly contentious question in evolutionary biology. When the trait in question belongs to human beings, though, “contentious” reaches a whole new level—because when evolutionary biologists consider humans, their conclusions get personal.
Erotic sculpture on temple wall, Khajuraho, India. Photo by Abhishek Singh aka Bailoo.Among the myriad traits and behaviors of Homo sapiens evolutionary biologists might choose to study, few can be as personal as the female orgasm. The adaptive function of male orgasm is about as clear-cut as possible—it’s a mechanistic necessity for uniting a sperm with an egg. But while female orgasm is enjoyable (or so I am told; this is as lousy a point as any to admit that my expertise in this phenomenon is purely academic), it isn’t necessary for fertilization. No man can be a father without having had at least one orgasm, but a woman could conceivably give birth to a huge family without having any.
To explain the existence of female orgasm in an evolutionary context, then, biologists have two options: (1) discover a way in which female orgasm shapes reproductive success indirectly, or (2) conclude that female orgasm isn’t an adaptation. Possibilities advanced for the first option range from the benefits of closer bonding with a mate—sex is, after all, about more than mere reproduction—to suppositions that the contractions associated with orgasm help draw semen into a woman’s reproductive tract.
The argument in support of non-adaptive female orgasm takes a developmental perspective: that female orgasm is really male orgasm, as experienced in a female developmental context. That is, women have orgasms for the same reason men have nipples—because the anatomies of both sexes are constrained by their origins in the same underlying developmental program. If this is the case, natural selection would work to optimize male orgasm, without necessarily affecting female orgasm—and that suggests a way to test whether female orgasm is an adaptation.
Natural selection removes less-fit versions of traits from a population—making that trait less variable within the population under selection. Traits that don’t affect survival or reproductive success, on the other hand, are free to accumulate variation via mutation. So non-adaptive traits can be identified by comparing their variation to traits with known adaptive functions.
Who cares what natural selection thinks, anyway? Photo by JorgeMiente.es.Psychologist Kim Wallen and philosopher of science Elisabeth Lloyd (who had advanced the hypothesis that female orgasm is non-adaptive in a 2005 book) made just such a comparison in a 2008 study. Variation in female orgasm would be challenging to measure, so they used the clitoris as an anatomic proxy. This let them use the penis—which shares a developmental origin with the clitoris and is presumably under natural selection associated with male sexual function—as an adaptive standard for comparison. In comparison to (flaccid) penis length, Wallen and Lloyd found that clitoris length was indeed more variable [$a]. As a second control, the authors also compared variation in clitoris and penis length to variation in the length of women’s vaginas, understanding that this trait, unlike the clitoris, is important for female reproductive success. Vaginal length turned out to be about as variable as penis length, and much less so than clitoris length.
There are several objections to be made to Wallen and Lloyd’s analysis, and many were made in a response [$a] by evolutionary biologist Vincent Lynch. Lynch objected to the use of length as the focal measure for the size of the clitoris, and showed that clitoral volume was about as variable as penile volume. (I would add that the study of social insects Wallen and Lloyd cite as a precedent for their analysis isn’t actually focused on variation, but on the symmetry of traits under consideration, which is not quite the same thing.) More critically, though, Lynch points out that there isn’t any known relationship between clitoral size and ability to achieve orgasm—so the data don’t have the bearing on the question that Wallen and Lloyd assigned in the first place. Lynch concluded that female orgasm is an adaptation after all—a more conservative interpretation of his result is that we can’t answer the question by measuring clitorises.
Understanding the evolution of human sexual behaviors can help us to figure out how best to navigate the tricky business of a sexual relationship with another person—an approach most recently exemplified in the book Sex at Dawn. But we also tend to view evidence that natural selection favors a particular trait or behavior as a kind of approval, or as evidence of what is “natural.” That’s silly. Whether or not they help to make more babies, orgasms are fun, and they’re a wonderful part of our most intimate expression of affection and love. In some respects, that’s all we need to know.
References
Crespi, B., & Vanderkist, B. (1997). Fluctuating asymmetry in vestigial and functional traits of a haplodiploid insect. Heredity, 79 (6), 624-30 DOI: 10.1038/hdy.1997.208
Lynch, V. (2008). Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm. Evolution & Development, 10 (4), 396-7 DOI: 10.1111/j.1525-142X.2008.00248.x
Wallen K, & Lloyd EA (2008). Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evolution & development, 10 (1), 1-2 DOI: 10.1111/j.1525-142X.2007.00207.x
Photo by ucumari.Species interactions are probably pretty important, in the evolution of life. There are all sorts of studies showing that the fitness and evolutionary history of individual species depends upon interactions with pollinators, symbiotes, food plants, herbivores, parasites, predators, and competitors. Most of these studies focus in on a single interaction—but what living thing interacts with only one other organism? Coevolution, when it happens, happens in a community context.
Adding even a second interaction into the scientific picture can be difficult, but it may also dramatically change the evolutionary outcome, as seen in a new study of evolution in the protozoan communities living in purple pitcher plants. Individually, competitors and predators are significant agents of natural selection—but together, they seem to counterbalance each other [$a].
The purple pitcher plant, Sarracenia purpurea. Photo by petrichor.Carnivorous pitcher plants grow funnel-shaped leaves that collect water to form a pitfall trap for hapless insects, which provide a source of nitrogen in swampy, nutrient-poor habitats. One species’ pitfall is another’s ideal habitat, however, and pitchers also play host to diverse micro-communities [PDF] of protozoans, bacteria, and even mosquito larvae. By recreating—and experimentally manipulating—these communities in the laboratory, the new study’s author, Casey terHorst, was able to disentangle the individual and combined effects of two different kinds of species interaction within pitcher plant pitfalls.
TerHorst focused on a protozoan species in the genus Colpoda, a widespread single-celled critter found in moist soil and standing water. In pitcher plants, Colpoda makes a living feeding on bacteria that break down insects trapped by the pitfall—and they themselves are prey for the larvae of the mosquito Wyeomyia smithii.
An example of genus Colpoda, the group of ciliates studied (but probably not the same species). Photo by PROYECTO AGUA** /** WATER PROJECT.To determine the individual and combined effects of competition and predation on Colpoda, terHorst allowed experimental populations of the protozoan to evolve for 20 days (about 60-120 Colpoda generations) with either (1) no competitors or predators, (2) competition from another bacteria-eating protozoan, (3) predation by mosquito larvae, or (4) competition and predation. At the end of the experimental period, he sampled each evolved Colpoda population and measured a number of traits, including the size of Colpoda cells and their speed. Larger Colpoda cells are thought to be better competitors but more vulnerable to predators; faster ones should be better able to evade predation.
Individually, predators and competitors had significant effects on Colpoda evolution. In the presence of mosquito larvae, Colpoda evolved smaller, faster cells than it did alone. Unexpectedly, competitors also caused Colpoda to evolve smaller cells, though not faster ones. TerHorst suggests that this is because competition also favored more rapid reproduction by Colpoda, which meant that individual cells grew less before dividing.
Most interestingly, though, Colpoda evolving in the presence of both predators and competitors looked quite a lot like Colpoda that evolved alone. This is apparently because the mosquito larvae ate both Colpoda and its competitor—the mosquitoes acted to relieve some competitive pressure on Colpoda at the same time they ate fewer Colpoda because they had two prey species to pursue. In fact, the mosquitoes preferred to eat the competitor species, since it tended to hang out in the open while Colpoda hid among the plastic beads lining the base of the artificial habitat.
Thus the indirect effects of the predator offsetting competition, and of the competitor drawing away predation, canceled out the natural selection each imposed on Colpoda individually. Species interactions in a community context, even a simple one like this, are far from straightforward.
References
Buckley, H., Burns, J., Kneitel, J., Walters, E., Munguia, P., & Miller, E. (2004). Small-scale patterns in community structure of Sarracenia purpurea inquilines. Community Ecology, 5 (2), 181-8 DOI: 10.1556/ComEc.5.2004.2.6
terHorst, C. (2010). Evolution in response to direct and indirect ecological effects in pitcher plant inquiline communities. The American Naturalist, 176 (6), 675-85 DOI: 10.1086/657047
It’s all fun and games until a Republican Senator uses your hilarious fake infographics to prove that climate change is a hoax. But until then, enjoy. (Hat tip to Doc Becca.)
Image via Fake Science.I am still laughing at the one about bees’ social organization.