Tag Archives: Research Blogging

Cost of killing nest-mates offset by benefits of killing nest-mates

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ResearchBlogging.orgAmong birds, brood parasites are the ultimate freeloaders — species like the common cuckoo and the brown-headed cowbird lay their eggs in other birds’ nests, leaving the host to raise the parasite chicks at the expense of its own. But while brood parasitism is easy on the parents, it isn’t so easy on their chicks, as a study recently published in PLoS ONE suggests.


A reed warbler feeds a common cuckoo chick. Photo from WikiMedia Commons.

A brood parasitic chick faces two challenges. The first is to avoid being recognized by its adoptive parents and ejected from the nest; the second is to win parental attention in competition with their adoptive nest-mates. The first challenge may be partially met by the evolution of eggshells that match host eggshells; and brood parasite parents may also help by keeping watch on the host nest so they can punsish hosts who eject introduced eggs. (This punishment behavior has been described as an “avian mafia [$-a].”)

In competition with their adoptive nest-mates, though, parasitic chicks are on their own. If the host’s own eggs hatch, the host has more mouths to feed and less time to spend on the parasitic chick. On the other hand, a brood parasitic mother can’t kick out the host’s eggs at the time she leaves her own egg with the host, because the host may abandon a nest that contains only a single unfamiliar-looking egg. This leaves it to freshly-hatched brood parasite chicks to do the heavy lifting involved in ejecting their host’s eggs themselves.


A common cuckoo chick pushes one of its host’s eggs out of the nest. Detail of figure 1 from Anderson et al. (2009).

Egg eviction looks like hard work — the chicks attempt it while they’re not much bigger than the eggs. Anderson et al. investigated the cost of all this adoptive-siblicidal effort by manipulating reed warbler nests that had been parasitized by common cuckoos,* taking away the hosts’ eggs in experimental nests, and comparing the growth of cuckoo chicks in those nests to that of chicks in unmanipulated nests, who had to do the evicting themselves.

They found that there is a cost to eviction effort: during the period of development when they would be doing all they could to push eggs out of the nest, cuckoo chicks grew faster when they didn’t have eggs to push. But they didn’t grow much faster, and by the time they were ready to leave the nest, the advantage had disappeared. Anderson et al. take this to mean that the cost of eviction is “recoverable” through the benefits of increased parental attention later on. I would add that it points out how important your choice of time frame can be when investigating how traits or behaviors affect organisms’ evolutionary fitness — sometimes a cost paid at one point in development is an investment toward later benefits.

——–
*The common cuckoo is the species first known to parasitize other birds’ nests, and its name is the linguistic source of the term “cuckold.”

References

Anderson, M., Moskát, C., Bán, M., Grim, T., Cassey, P., & Hauber, M. (2009). Egg eviction imposes a recoverable cost of virulence in chicks of a brood parasite. PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007725

Hoover, J., & Robinson, S. (2007). Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proc. Nat. Acad. Sci. USA, 104 (11), 4479-83 DOI: 10.1073/pnas.0609710104

Lahti, D. (2005). Evolution of bird eggs in the absence of cuckoo parasitism. Proceedings of the National Academy of Sciences, 102 (50), 18057-62 DOI: 10.1073/pnas.0508930102

Soler, M., Soler, J., Martinez, J., & Moller, A. (1995). Magpie host manipulation by great spotted cuckoos: Evidence for an avian mafia? Evolution, 49 (4), 770-5 DOI: 10.2307/2410329

Pollination before flowers

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ResearchBlogging.orgWhich came first, the pollinator or the pollinated? An article in this week’s Science suggests that a diverse group of insects may have been drinking nectar and pollinating plants millions of years before the appearance of modern flowering plants [$-a].



Panorpis communis, a modern scorpionfly species, and a sketch of ancient, pollinating scorpionflies. Photo by JR Guillaumin; sketch from Ollerton and Coulthard (2009).

Prior to the origins of modern flowering plants, or angiosperms, in the early-middle Cretaceous period, most of the diversity of land plants were gymnosperms. These plants are characterized by “naked seeds” — reproductive organs exposed to the air, where the wind can catch pollen and carry it from one plant to fertilize the ovules of another. In a world dominated by gymnosperms, the thinking used to be, animal pollinators were mostly unnecessary.

The new paper by Ren et al. challenges this idea with the description of a set of fossilized scorpionflies, all of which have strikingly long probosces that are clearly suited to sucking up liquid. The earliest of these fossils are from the Jurassic, tens of millions of years before the flowering plants began to diversify. In modern insects, sucking mouthparts like the ones described are associated with two kinds of feeding: drinking pollen, and drinking blood. To determine which was most likely in this case, Ren et al. performed energy-dispersive spectroscopy on the best-preserved fossil, and found no sign of the elevated levels of iron in the proboscis that would result from the residue of blood meals. This suggests that the scorpionflies were drinking nectar, or something like it.

Nectar has one major function in plants: to attract insects. Ant-protected plants reward their ants with nectar, and flowering plants use nectar to lure animal pollinators close enough to pick up or drop off pollen. If these ancient scorpionflies were, in fact, living on nectar, Ren et al. reason they were probably pollinating contemporary plants, which were all gymnosperms. The authors identify a diverse list of candidate host plants, including seed ferns and a relative of the modern ginkgo, whose reproductive structures were (1) too well-sheltered for efficient wind pollination or (2) included tubular structures similar to those that modern plants use to guide nectar-feeding pollinators. Finally, the authors point out, many modern gymnosperms produce “ovular secretions” that are very similar to the nectar produced by angiosperms.

As a neontologist, I’m often amazed how much can be told from million-years-old fossils — who knew there was a way to test for residual blood in a fossilized proboscis? At the same time, Ren et al. connect some mighty scattered dots to build their hypothesis. The real clincher is that it seems mighty unlikely that animal pollination would be rare in a world that already has both flying insects and pollen-producing plants. Animal pollination is much more efficient than wind pollination, and if there’s one constant in evolutionary history, it’s that living things rarely miss an opportunity like that.

References

Ollerton, J., & Coulthard, E. (2009). Evolution of animal pollination. Science, 326 (5954), 808-9 DOI: 10.1126/science.1181154

Ren, D., Labandeira, C., Santiago-Blay, J., Rasnitsyn, A., Shih, C., Bashkuev, A., Logan, M., Hotton, C., & Dilcher, D. (2009). A probable pollination mode before angiosperms: Eurasian, long-proboscid scorpionflies. Science, 326 (5954), 840-7 DOI: 10.1126/science.1178338

Endless forms: Oral sex by fruit bats

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ResearchBlogging.orgOne of those scientific papers that seems to have been written with the blogosphere in mind: biologists have just published records of fellatio by the fruit bat Cynopterus sphinx. Apparently C. sphinx females are pretty flexible — they lick their mate’s penis during copulation, which evidently induces him to stay in longer (see the graph below, with drawing). The authors offer a handful of non-mutually-exclusive hypotheses for the adaptive benefit of the behavior, ranging from lubrication to increased fertilization efficiency. The full text is available for free at PLoS ONE, if you’re up for some hot-and-heavy behavioral observations.


Graph from Tan et al. (2009), Figure 3.

Update: In a more in-depth post over at Boing-Boing, Maggie Koerth-Baker wonders why there needs to be an adaptive purpose for a pleasurable behavior (there doesn’t, as far as I’m concerned), and points out that there’s also a video in the supporting information. Which video has some totally unscientific background music.

Reference

Tan, M., Jones, G., Zhu, G., Ye, J., Hong, T., Zhou, S., Zhang, S., & Zhang, L. (2009). Fellatio by fruit bats prolongs copulation time PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007595

How to synchronize flowering without really trying

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This post was chosen as an Editor's Selection for ResearchBlogging.orgOne way plants can gain an advantage in their dealings with pollinators, seed dispersers, or herbivores is to act collectively. For instance, when oak trees husband their resources for an extra-big crop of acorns every few years instead of spreading them out, acorn-eating rodents are overwhelmed by the bumper crop, and more likely to miss some, or even forget some of the nuts they cache. These benefits of synchronized mass seed production, or “masting,” are straightforward, but how it happens is less clear. A paper in the latest issue of Ecology Letters has an answer — synchronization happens accidentally [$-a].


Bumper acorn crops ensure that squirrels miss a few. Photo by douglas.earl.

When Dan Janzen first described masting as an adaptation in plants’ coevolution with seed predators, he proposed that “an internal physiological system” [$-a] acted as a timer between masting events, with masting ultimately triggered by weather conditions. However, mathematical models have suggested a different possibility, the “resource-budget hypothesis:” that masting synchronization arises through an interaction of resource and pollen limitation [$-a].

Resource limitation works in concert with pollen limitation by catching plants at two stages of the seed-production process. First, if the resources required for seed production are more than can be accumulated in a single year, or if the availability of resources varies from year to year, then some years will be spent building up reserves instead of producing flowers. When reserves are built up, seed production is limited by the availability of pollen to fertilize flowers. Plants that flower when most of the rest of the population doesn’t will fail to set much seed, so they’ll have reserves to make seeds in the next year. This doesn’t require Janzen’s “internal physiological system” for the plants to synchronize, although such a system might evolve to reduce the likelihood of wasting resources by flowering out of synch.

The new paper tests this model in populations of a western U.S. wildflower, Astralagus scaphoides, which flowers at high frequency every alternate year. The authors prevented seed production in the plants by removing their flowers, either in a “press” of three years in a row or in a single “pulse” during one high-flowering year. The plants’ response to these treatments would reveal the role of resource and pollen limitation in synchronizing seed production.

If resource depletion after fruit set prevents reproduction in successive years, we predicted that ‘press’ plants would flower more than control plants every year, as they were never allowed to set fruit. We predicted that ‘pulse’ plants would flower again in 2006, but not set fruit due to density-dependent pollen limitation in a low-flowering year.

The authors also measured the sugars stored in the roots of plants collected before and after flowering in a high-flowering year.


Seed predator in action. Photo by tombream07.

The resource-budget hypothesis worked. Plants prevented from setting seed were forced out of synch with the rest of the population. “Pulse” plants flowered the year after treatment, but because few other plants did, they received little pollen and set little seed. They then had resources to flower yet another year, with the rest of the population this time, and set much more seed, depleting their reserves and bringing them back into synch. “Press” plants continued to flower at high rates each year, as long as they were prevented from setting any seed. Sugar levels built up in the tested roots during non-flowering years, and dropped after high-flowering years.

So masting arises as an emergent result of two limitations acting on plants — the resources needed to make seed, and good access to pollen. A couple of simple rules lead, undirected, to an ordered system that affects entire natural communities.

References

Crone, E., Miller, E., & Sala, A. (2009). How do plants know when other plants are flowering? Resource depletion, pollen limitation and mast-seeding in a perennial wildflower. Ecology Letters, 12 (11), 1119-26 DOI: 10.1111/j.1461-0248.2009.01365.x

Janzen, D. (1971). Seed predation by animals Ann. Rev. Ecol. Syst., 2 (1), 465-92 DOI: 10.1146/annurev.es.02.110171.002341

Janzen, D. (1976). Why bamboos wait so long to flower Ann. Rev. Ecol. Syst., 7 (1), 347-91 DOI: 10.1146/annurev.es.07.110176.002023

Satake, A., & Iwasa, Y. (2000). Pollen coupling of forest trees: Forming synchronized and periodic reproduction out of chaos. J. Theoretical Biol., 203 (2), 63-84 DOI: 10.1006/jtbi.1999.1066

Social termites team up with non-relatives

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This post was chosen as an Editor's Selection for ResearchBlogging.orgIn social insects, colonies of hundreds or thousands of workers and soldiers forgo reproduction to support one or a few “reproductives” — drones and a queen. In most cases, this isn’t as selfless as it might seem. Because the workers in a colony are all offspring of the queen, they’re really reproducing through her — because the queen shares genes with the workers, when she reproduces it contributes to their evolutionary fitness.

This is called kin selection, and in many cases it’s a good explanation for the way the interests and behavior of individual workers are overridden by the interests of the colony. There are, however, exceptions — and an open-access paper in the latest issue of PNAS describes what looks like a good case: mergers between unrelated colonies of termites.


Zootermopsis nevadensis, a social insect inclined to negotiated settlements. Photo by BugGuide/ Will Chatfield-Taylor.

The termite Zootermopsis nevadensis lives in small, socially-stratified colonies that tunnel through rotting logs. Each colony has a pair of reproductive individuals, a king and queen, served by sterile workers and soldiers. Multiple unrelated colonies usually nest in a single log, and when they encroach on each other’s territory, something interesting happens — they merge.

In what the authors refer to obliquely as the “interaction” that precedes a merger, the king and queen of one or both colonies may die. Mergers occur in the aftermath, as workers from the two colonies began to work in concert, and one or a few of them become replacement reproductives. This ability of sterile workers to start reproducing in the absence of a king and queen is unique to termites. DNA analysis shows what happened after mergers — new reproductives could arise come from either or both colonies, and that in some cases they interbred.

It’s this possibility to become genetically invested in the newly merged colony, the authors say, that motivates workers from two unrelated colonies to work together. If this is the case, it means that kin selection is not what keeps merged colonies together. Group selection might be a better explanation. Kin selection is often contrasted with group selection, in which unrelated individuals sacrifice their own interests to those of a larger group, so that their colony can better compete against rival colonies. In a classic 1964 Nature paper [$-a], John Maynard Smith discussed the conditions under which kin selection operates well:

By kin selection I mean the evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in population breeding structure.

And contrasts them to conditions necessary for group selection to work:

[Under group selection] … If all members of a group acquire some characteristic which, although individually diadvantageous, increases the fitness of the group, then that group is more likely to split into two, and in this way bring about an increase in the proportion of individuals in the whole population with the characteristic in question. The unit on which selection is operating is the group and not the individual.

The ecology of Zootermopsis nevadensis may set the stage for group selection to overpower kin selection. With many small colonies competing for a single rotting log, the benefits of possibly contributing to the reproduction of a larger, more competitive colony make mergers worthwhile. Something similar has been documented in ants, which can form supercolonies of unrelated colonies if there is some external threat (another ant species) to force them to band together — you can find discussion of a recent paper on this case over at Primate Diaries.

References

SMITH, J. (1964). Group selection and kin selection Nature, 201 (4924), 1145-1147 DOI: 10.1038/2011145a0

Johns, P., Howard, K., Breisch, N., Rivera, A., & Thorne, B. (2009). Nonrelatives inherit colony resources in a primitive termite Proc. Nat. Acad. Sci. USA, 106 (41), 17452-6 DOI: 10.1073/pnas.0907961106

Video of yucca pollination

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ResearchBlogging.orgWith permission from my doctoral advisor, Olle Pellmyr, I’ve just uploaded a unique video to Vimeo: a yucca moth laying eggs in, then pollinating, a yucca flower. I don’t know why I didn’t think of this earlier — it’s great footage, and deserves to be seen more widely.

A female yucca moth mates, then collects pollen from a yucca flower in specialized mouthparts. She carries it to another flower where, as shown in the video, she drills into the floral pistil with her ovipositor and lays eggs inside, then climbs to the tip of the pistil and applies pollen to fertilize the flower. When the flower develops into a fruit, the eggs hatch and the caterpillars eat some of the seeds inside.

Yuccas and yucca moths are completely dependent on each other [PDF] — nothing else pollinates yuccas, and the moths have no other source of food (they don’t eat as adults). Recently, the Pellmyr lab has shown that this interaction may be leading to speciation in one yucca species, the Joshua tree — Joshua trees pollinated by two different species of yucca moths have differently-shaped flowers [PDF], but these two tree types may not be totally genetically isolated [PDF]. I’ve written about this work before — for more information about the interaction, check out Olle’s publication page.

References

Godsoe, W., Yoder, J., Smith, C., & Pellmyr, O. (2008). Coevolution and Divergence in the Joshua Tree/Yucca Moth Mutualism. The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

Pellmyr, O. (2003). Yuccas, yucca moths, and coevolution: A review. Annals of the Missouri Botanical Garden, 90 (1) DOI: 10.2307/3298524

Smith, C., Godsoe, W., Tank, S., Yoder, J., & Pellmyr, O. (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: Asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

First step to mutualism doesn’t look so friendly

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This post was chosen as an Editor's Selection for ResearchBlogging.orgAnt-plant protection mutualism is a widespread and elegant species interaction. How do species strike bargain like this, requiring specialized behaviors and structures in each partner, in the first place? A new report in The American Naturalist suggests an answer: maybe ants took the initiative [$-a].

In exchange for protection from herbivores and competitors [big PDF], “myrmecophytic” host plants grow hollow structures called domatia and often produce nectar to shelter and feed a colony of ants. This mutualism is really a sort of negotiated settlement between the partners; both ants and plants do what they can to get the most out of the interaction. We have evidence in some cases that host plants cut back support for ants if there aren’t any herbivores around; and, in other cases, that ants prune their host plants to prompt the growth of more domatia.



Domatium diversity: Ant domatia on Acacia (above) and Cordia nodosa (below). Photos by Alastair Rae and Russian_in_Brazil.

So it isn’t entirely surprising that there might be cases where that bargain hasn’t been established yet, and that’s what the new paper reports. The observation turned up in connection with one of the most interesting forms of the ant-plant mutualism: the “devil’s gardens” of the Amazonian rainforest. Devil’s gardens are created by colonies of the ant Myrmelachista schumanni, which attacks possible competitors to its preferred host [$-a], Duroia hirsuta, leaving patches where nothing but D. hirsuta grows.

Clued in by native research assistants, the group studying the devil’s garden interaction discovered that trees growing at the edge of a garden are often afflicted with swollen, distorted trunks. Cutting into the swellings, they found them riddled with passages and populated by M. schumanni. The trees in question are not known as myrmecophytes, and it’s not clear that they receive any benefit from hosting ants. In fact, the authors report that ant-occupied trunks are weakened, and prone to breakage under their own weight or under heavy wind.

The paper doesn’t present direct evidence that the ants create the galls, but as the authors explain, this seems likely — M. schumanni kills its hosts’ competition by injecting them with formic acid, which parallels the irritants other gall-making insects inject into their host plants. It make sense that gall-making might have started as ants’ attempts to kill off trees that are too big to succumb to formic acid outright, but respond to it by growing galls like scar tissue. Furthermore — and this is pure speculation, of course — this looks like a first evolutionary step toward true ant-plant mutualism. Domatia may have originally evolved to redirect ants from more damaging gall-making, and since ants are naturally territorial about their nests, it might not take much behavioral change before they end up protecting their host.

References

Edwards, D., Frederickson, M., Shepard, G., & Yu, D. (2009). A plant needs ants like a dog needs fleas: Myrmelachista schumanni ants gall many tree species to create housing. The American Naturalist, 174 (5), 734-40 DOI: 10.1086/606022

Frederickson, M., Greene, M., & Gordon, D. (2005). “Devil’s gardens” bedevilled by ants Nature, 437 (7058), 495-6 DOI: 10.1038/437495a

Janzen, D. (1966). Coevolution of mutualism between ants and acacias in Central America Evolution, 20 (3), 249-75 DOI: 10.2307/2406628

Aiming at a moving target with a shaky pistol: Evolution in a random, changing world

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ResearchBlogging.orgBiologists can become distinctly cranky when we hear evolution described as “random.” This is because evolution isn’t random — it’s undirected. Although it acts on mutations that turn up randomly, natural selection is highly nonrandom, in that (all else being equal) traits that help their owners make more babies are always the ones that spread through a population.

However, even if natural selection predictably aims for the same target, that target is not necessarily fixed. The most obvious case of this is in the coevolution of interacting species, where adaptation by one forces adaptation in the other. This is a field of study in its own right; but one recent innovation is a theory paper by Gandon and Day, which tracks changes in the “fitness landscape” resulting from adaptations and counter-adaptations [$-a]. (For more detail on the paper, see Coevolvers.)


Ground finches (Geospiza fortis) with big beaks might be favored this year, but what about next? Photo by kookr.

Empirical studies have shown that selection’s target can move in unexpected ways, too. One of the best examples of this turned up in the course of the ongoing, decades-long study of finches on the Galapagos Island Daphne Major. As rainfall on the island fluctuated from year to year, the mix of available seeds changed as well, and the finches’ beaks — the size of which determines what seeds are easily cracked and eaten — evolved to keep up [$-a]. The resulting evolutionary path looks like a drunkard’s walk, and the study’s authors, Peter and Rosemary Grant, put the word unpredictable right in the title.

Making things still more complicated, there is actually a random component to the effects of natural selection. That is, in the real world, advantageous traits may not automatically result in greater fitness — they result in greater expected fitness. Last year, Sean Rice published a mathematical model of evolution in which fitness is a random variable. He found that greater variation around the expected fitness can increase the strength of natural selection; that is, more uncertainty about the relationship between fitness and a given trait may actually make that trait adapt more rapidly. In a just-published extension of this work, Rice and Anthony Papadopolous examined the effect of random migration among different populations on adaptive evolution in each population, and found that greater variation in migration rates can reduce the effect of migration on local evolution.

Introducing all this randomness into our view of evolution doesn’t necessarily make evolution unpredictable. As an excellent recent Radiolab episode discusses, there are patterns to be extracted from randomness. It takes more work — larger sample sizes, longer-term studies — for these patterns to become apparent. Yet it’s clear that this is work we’ll have to do in order to understand biological systems.

References

Gandon, S., & Day, T. (2009). Evolutionary epidemiology and the dynamics of adaptation Evolution, 63 (4), 826-38 DOI: 10.1111/j.1558-5646.2009.00609.x

Grant, P., & Grant, R. (2002). Unpredictable evolution in a 30-year study of Darwin’s finches Science, 296 (5568), 707-11 DOI: 10.1126/science.1070315

Rice, S. (2008). A stochastic version of the Price equation reveals the interplay of deterministic and stochastic processes in evolution BMC Evolutionary Biology, 8 (1) DOI: 10.1186/1471-2148-8-262

Rice, S., & Papadopoulos, A. (2009). Evolution with stochastic fitness and stochastic migration PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007130

Empirical pacifism?

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ResearchBlogging.orgSlogger Charles Mudede points to a new epidemiological study on the effectiveness of carrying a gun for self defense [$-a]. Not only does packing heat fail to help in the event of an armed robbery,

… individuals in possession of a gun were 4.46 (P < 0.05) times more likely to be shot in an assault than those not in possession. Among gun assaults where the victim had at least some chance to resist, this adjusted odds ratio increased to 5.45 (P < 0.05).

That’s right, carrying a gun increases the odds that you’ll be shot by an armed assailant. It also increases the odds that you’ll be shot fatally, by about 4.23 times. The authors interviewed 677 gun assault victims in Philadelphia, from between 2003 and 2006, with 648 interviews drawn from the general population in the same period as a control. (If you can’t get to the paper on the journal website, Mudede links to a ScienceDaily article about the result that gives more detail.)

Here’s empirical evidence that returning violence with violence (or having the ability to do so) doesn’t lead to better outcomes — unless, of course, you’re of the school of thought that it’s better to be shot than to lose your wallet or your pride. I doubt this will have much impact on the U.S. political conversation about guns and gun control, because as I’ve noted before, this is not a subject about which people think rationally. Nevertheless, it’s a statistic I intend to remember for the next time I’m asked to defend the ethics of nonresistance.

Reference

Branas, C., Richmond, T., Culhane, D., Ten Have, T., & Wiebe, D. (2009). Investigating the link between gun possession and gun assault American Journal of Public Health DOI: 10.2105/AJPH.2008.143099

Bees follow the crowd: Do whole-hive traits override individuals’ genetics?

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ResearchBlogging.orgSocial insects are often considered prototypes of group selection, in which the evolutionary interests of individual organisms are forced to defer to the needs of their social group. Now, the authors of a new study of honeybees argue that colony-level traits can override the genetic predispositions of individual bees [$-a].


Do the needs of the many outweigh the needs of the one? Photo by Max_xx.

The study’s authors, Linksvayer et al.,
made use of artificially-selected colonies of bees that were first developed for a 1995 study [$-a]. The original selection experiment crossed queen bees with drones to create lines of honeybee colonies that collected and stored more pollen (“high pollen” lines) or less pollen (“low pollen” lines) than un-selected colonies do. The total amount of pollen a colony stores is supposed to be a “group” trait — an emergent property of the individual foraging decisions of every worker bee in the hive. But the genetics underlying that trait is encapsulated within the individual workers.

In the new experiment, Linksvayer et al. placed larvae from “high pollen” lines in “low pollen” colonies, and vice-versa. The larvae developed under the care of workers from the adoptive colony; when transplanted larvae reached adulthood, the team dissected them and measured the size of their ovaries — apparently big-ovaried workers collect lots of pollen. They found that “high pollen” larvae reared by “low pollen” workers had smaller ovaries than than those raised by workers of their own type. “Low pollen” larvae reared by “high pollen” workers didn’t end up with larger ovaries, though; and the “high pollen” larvae had substantially larger ovaries than the “low pollen” larvae regardless of who raised them.

There was a statistically significant effect of rearing environment, even if it was (apparently) entirely driven by the change seen in “high pollen” larvae. The authors conclude that this points to a mechanism whereby a bee colony keeps its workers in line with the colony-wide policy:

Thus, our results show that the network of social interactions that shapes development and expressed phenotypes has changed as a result of the colony-level selection program on pollen hoarding. Just as selection shapes physiological networks within organisms, our study shows that selection also shapes regulatory networks of superorganisms.

So the metaphor, then, is that the authors have observed in the hive something like what happens to a transplanted organ — the new host system incorporating the transplant for its own needs. I’m not sure the observed effect is strong enough to justify the meaning they assign to it; but it is an interesting observation.

As a postscript, I’m not sure social insects are a good model of group selection, because we know that they’re probably also experiencing kin selection, in which each worker’s fitness comes from helping the closely-related queen produce more sisters who share the same genes. Rarely, “anarchic” workers are born fertile and mate with drones [$-a] (there’s an open-access paper on the genetics underlying this trait); but in hives without anarchists, “group fitness” is hard to separate from the fitness of individual workers. A paper published in Nature this June showed that in another classic group selection system (parasites within a single host) kin selection is really the more important process.

References

Linksvayer, T., Fondrk, M., & Page Jr., R. (2009). Honeybee social regulatory networks are shaped by colony-level selection. Am. Nat., 173 (3) DOI: 10.1086/596527

Oldroyd, B., Smolenski, A., Cornuet, J., & Crozler, R. (1994). Anarchy in the beehive. Nature, 371 (6500) DOI: 10.1038/371749a0

Oxley, P., Thompson, G., & Oldroyd, B. (2008). Four quantitative trait loci that influence worker sterility in the honeybee (Apis mellifera). Genetics, 179 (3), 1337-1343 DOI: 10.1534/genetics.108.087270

Page, R., & Fondrk, M. (1995). The effects of colony-level selection on the social organization of honey bee (Apis mellifera L.) colonies: colony-level components of pollen hoarding Behavioral Ecol. & Sociobiol., 36 (2), 135-44 DOI: 10.1007/BF00170718