Category Archives: science

Aphid-tending ants cull the sick from the herd

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ResearchBlogging.orgJust released online at Biology Letters: aphid-tending ants have been observed to selectively remove sick members of their “herd” [$-a].

Most aphid species produce some sort of sweet honeydew as waste while feeding on their host plants; ant-attended aphid species use this honeydew to attract ants. In many cases, the ants “milk” the aphids by stroking them to prompt release of the honeydew. While exploiting a colony of aphids, ants defend it as a food resource, protecting the aphids from predators. Aphid species that commonly rely on ant protection often lack defensive adaptations [$-a] found on species that don’t interact with ants.


Ants tend aphids on a milkweed plant. Photo by dmills727.

Niesen et al. report the results of experiments performed ants attending colonies of milkweed aphids, Aphis asclepiadis, which are susceptible to a fungal pathogen that can wipe out aphid colonies in a matter of days. In two experiments, they introduced aphids into the ant-attended colonies, and tracked what the ants did to them. They found that

  • Ants were more likely to remove the corpses of fungus-killed aphids than either the corpses of aphids killed by freezing or introduced live aphids; and
  • Ants were more likely to remove live aphids contaminated with fungal spores (conidia) than live aphids without spores.

The authors speculate that this behavior is a re-application of ants’ treatment of their own sick and dead within the colony. It seems clear that it should have benefits to both ants and aphids in this new context, slowing or preventing the spread of the fungus within an aphid colony. This benefit isn’t directly tested by Nielsen et al., but such an experiment is a logical next step.

Reference

Nielsen, C., Agrawal, A., & Hajek, A. (2009). Ants defend aphids against lethal disease Biology Letters DOI: 10.1098/rsbl.2009.0743

Way, M. (1963). Mutualism between ants and honeydew-producing Homoptera. Ann. Rev. Entomology, 8 (1), 307-44 DOI: 10.1146/annurev.en.08.010163.001515

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

Hey, PNAS?

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Hey, Proceedings of the National Academy of Sciences? (Can I call you PNAS? Thanks.) You know what really drives me crazy, as a scientist and a blogger, PNAS? Internationally-recognized journals that release scientific studies to the press before they make them available online.

Why does this make me crazy? I’m glad you asked, PNAS. It makes me crazy because sometimes I read something as batshit absurd as this gem from Wired Science:

No exact rule exists for deciding when a group of animals constitutes a separate species. That question “is rarely if ever asked,” as speciation isn’t something that scientists have been fortunate enough to watch at the precise moment of divergence … [emphasis mine]

(Which statement is like claiming that physicists rarely ask about gravity.) And when I read something like this I’d really like to be able to go and see whether that’s actually in the peer-reviewed article in question, or if it just materialized in the course of the sausage-making that is popular science journalism.

But, apparently, PNAS, you’re more interested in having your results butchered by people who think biologists don’t ask questions about speciation than you are in having them read by, you know, biologists. You’ll forgive me, I hope, if I take that a little bit personally.

Joshua trees at LiveScience

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Over at LiveScience, my collaborator Chris Smith describes the research we’ve done so far on the interaction between Joshua trees and their pollinators:

First, the match between the Joshua tree flowers and the moths’ ovipositors suggested that coevolution might have molded the relationship between the plant and the pollinator. Second, because the plants are completely dependent on the moths for reproduction, the differences in the flowers might have caused Joshua trees to split into two different species.


Yucca brevifolia in Tikaboo Valley, Nevada. Photo by jby.

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

Carnival of Evolution #17 at Adaptive Complexity

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Over at Adaptive Complexity, Michael White has just compiled the 17th monthly Carnival of Evolution. Marking the 150th anniversary of the publication of The Origin of Species (November, 1859), Michael structures submitted posts into a “virtual voyage of the Beagle.” Topics range from the sexual habits of our ape-like ancestors* to a highly optimistic study predicting that the frequency of the creationist meme in the United States will drop to 0% by 2050.

* Which habits might, I think, explain why we never invite them round to tea.

I want to teach this course

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Via io9: BIOL 103, “Biology in Science Fiction”, at Kenyon College. Sure, the lesson on tribbles is probably just about density-dependent population growth. But it’s a lesson on tribbles.

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

Someone’s on the ball

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The website for Evolution2010 is already online! I really like the logo. The site design looks familiar, and good for the organizers for not wasting time to re-evolve the camera eye, say I. Portland is going to be a great location, I think — my experience is that Evolution meetings are largely remembered by the quality of beer available, which should be hard to get wrong in that town.


Image from Evolution2010.

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