The Imitation Game -- Cuckoo Style

The Common Cuckoo (Cuculus canorus) might be known for its iconic call, but its true hallmark is the tactics it uses to shirk parenting duties

During the European spring and summer, the cuckoo's familiar call echoes through the air. 

'CUCK-oo' 'CUCK-oo' 'CUCK-oo'  

Like many birds, common cuckoos journey from their southern wintering grounds to exploit the bounty that comes with warm weather further north. And like many birds, spring heralds that time of year when 'the birds and the bees' are in full flight and buzz. But while caring parents like wrens, robins, and common redstarts busily build their nests, female cuckoos sit and wait. 

And wait.

And wait.

All the meanwhile stuffing themselves on the myriad bugs. Until finally, a nest is complete -- someone else's nest, that is. 

A small Eurasian reed warbler that also made a long northern migration set up camp in the marsh reeds. Strung among a handful of reeds sits its conical nest with three or four cream-colored eggs with brown splotches. As she sits, diligently warming her eggs, a sudden call pierces the air.

'kwik-kwik-kwik-kwik-kwik...'

The warbler lifts her head, alarmed by the threat.

'kwik-kwik-kwik-kwik-kwik...' the call repeats

A Eurasian sparrowhawk -- a predator and close by.

She hurriedly scans the area. The call picks up again. 

'kwik-kwik-kwik-kwik-kwik...'

Not wanting to take any chances for her own life, the warbler hops out of her nest, heading away to other reeds where she can scan the sky better. While away, though, a female cuckoo swoops right near the nest. Forcing herself into the nest, the cuckoo settles for a minute. At twice the size of the warbler's nest, the cuckoo looks like an adult person stuck in a toddler swing. She lays an egg -- cream-colored with brown splotches, just like the warblers. And just before vacating the nest, she lets out a final call.

'kwik-kwik-kwik-kwik-kwik...'

And takes off. 

Clever Cuckoos

As all parents know, it takes a lot of time and energy to raise kids. For animals, the challenge is even greater -- find food, catch the food, feed the kids, avoid the predators, stay alive. It's tough. So for animals, they score big if they can avoid parenthood and hand the reins to someone else. And cuckoos epitomize this technique.

About 58 species of cuckoo have mastered some degree of stealth, cleverness, and imitation in order to fool other birds into raising their young. Its a technique biologists call brood parasitism. It's where cuckoos lay one of their eggs in another bird's nest -- possibly at the expense of one of the eggs already in there -- and let the other bird raise the chick for them. If the cuckoo egg is accepted, the cuckoo flies free, and the other bird ends up raising a chick about twice the size of itself.

A reed warbler feeds a cuckoo chick -- the warbler's chicks having died because the cuckoo chick rolled them out of the nest before they hatched

But why does the other bird tolerate this? And even more so, how does it not recognize the chick looks nothing like it? Cuckoos have a way of coercing other birds to care for their chicks. Sometimes they ransack the nest or harass the bird until it agrees -- or loses all of its own eggs because the cuckoo destroys the nest. But sometimes, the bird won't even recognize that an imposter egg has been placed, and even when born, the warbler responds mostly to the chick's behavior, the size difference aside.

This is because cuckoos are masters of imitation, and for some, the degree of imitation reaches incredible proportions -- eggs that look identical to their host's; feathers that nearly match those of common predators; and now, a recent study published last week in Nature Ecology and Evolution showed that common cuckoos even imitate the sounds or chuckles of hawks to ward off would be vigilant parents.

"[F]emale [cuckoo] chuckles play an important role in a suite of specialized female traits associated with a brood-parasitic lifestyle," wrote authors Jenny York, an associate research professor at the University of Cambridge, and Nick Davies, a professor of behavioral ecology also at the University of Cambridge. Those traits include stealth skills while sneaking up on a host nest, feather plumage that looks much like a hawk's, and incredible spatial memory skills to recall where their victim's nest is. These chuckles, though, unify their toolbox of deceptive skills because it misdirects the host's attention away from the nest.

Various cuckoo species have evolved astounding imitations, such as egg colors that almost perfectly match their host's

(Source: http://fsc.fernbank.edu/Birding/parasitism.htm)

"Female cuckoos rarely call," wrote the authors, a point that differs diametrically from male cuckoos that announce their position with the famous 'CUCK-oo' call. But oddly enough, the time when female cuckoos do call is while monitoring a host's nest and immediately after parasitizing the nest, both of which open a slew of possible problems for the cuckoo.

"Previous studies have shown that hosts of the common cuckoo...defend against parasitism by mobbing the adult cuckoos," noted the authors, "and by rejecting eggs that differ from their own." Cuckoos compensate for some of this through egg mimicry and stealthy operation. But announcing their position just as they plan to swoop in on the nest still seems odd.

The clue that tipped York and Davies came from the similarity between a cuckoo's chuckle and a Eurasian sparrowhawk's call, the latter a common predator of the area.

"[The cuckoo's] chuckle call -- a rapidly repeated 'kwik-kwik-kwik-...' -- is similar in fundamental frequency and rate to the 'kii-kii-kii...' of Accipiter hawks..." On top of it, common cuckoo's can look a lot like the sparrowhawk.

The similar feather plumage between a common cuckoo (left) and a Eurasian sparrowhawk (right) is uncanny but serves an important purpose in helping the cuckoo fool others to raise its chicks.
(Sources: http://www.birdforum.net/opus/images/thumb/0/0c/Common_Hawk_Cuckoo.jpg/500px-Common_Hawk_Cuckoo.jpg
and
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizdI6bq7ukOMVWS12ETwK7_ZEkAwiZ7c8d99hvn57TGgHAeis7f8zzNkt677zqDLGd1aQIofDiu0Ubq6RIMDHgV_HIAtKQepUQlRSLkJszyYjpYuQNAZI71KESvve9jL12x00Qjf1ZdAgF/s1600/Sparrowhawk.jpg)

York and Davies put their idea to the test by playing multiple bird sounds through speakers placed near reed warbler nests. Some bird sounds were from harmless neighbors, like the coos of collared doves and 'cuck-oo' of male cuckoos, whereas the others came from potential threats, like the Eurasian sparrowhawk call and the female cuckoo's chuckle.

They found that reed warblers increased their vigilance and scanning when they heard either the sparrowhawk call or cuckoo's chuckle but remained imperturbed by the other calls. This was true even when they tested calls on great tits and blue tits, birds in the area not parasitized by cuckoos.

But even if cuckoos distract the warblers, that doesn't mean the cuckoo egg laid in the nest will be accepted. So to really nail how effective this tactic is, York and Davies manipulated the nests with dummy eggs, painting a warbler's egg brown so it looked like the nest had been parasitized by a cuckoo and then stuck a wooden cuckoo next to the nest with a speaker behind it.

Though warblers invariably attacked the models, it turned out that the probability of warblers accepting the "parasitic" egg after a female cuckoo chuckle was nearly 80 percent. This seems to make sense to York and Davies.

"If hosts respond to a cuckoo call as if it were a hawk, they are less likely to reject a cuckoo egg, but if they fail to respond to a hawk call they may lose their life."

The trade-off for the warbler is clear: Protect your young or protect your life. It's a stunning discovery that shows how female cuckoos increase their chances of success and adds yet one more skill to the cuckoo's ever-growing resume of manipulation and imitation.

"[T]he female cuckoo might have 'the last laugh' in this particular battle between host defense and parasite trickery."

Check out the original article here in Nature Ecology and Evolution and listen to the chuckle call of the female common cuckoo!

Ideas in Conservation: The Case of Declining Bumblebee Populations

News has been all abuzz about declining global bee populations for several years now. From talk of cell phone radiation disorienting bees to overexposure to pesticides and, particularly, insecticides threatening populations, bees have made headlines for their growing threatened status. But while bee populations, in general, are on the decline, a new study published this past week in Proceedings of the Royal Society, B delved a bit deeper into the story using bumblebee species and found nearly one-third of species examined were declining--and all of them closely related.

 Results from this new study show that individuals from the older Thoracobombus (top) subgenus are significantly more susceptible to extinction than those from Pyrobombus (bottom), which currently shows almost no population decline.
Photo sources: https://www.flickr.com/photos/giaa/5073361366/ and https://www.flickr.com/photos/29697818@N03/7304148034/

Bumblebees are fuzzy, flying juggernauts of insects, the largest of which lives in Chile and reaches a body length of 1.6 inches - "a monstrous, fluffy, ginger beast," as British scientist David Goulson described it in his book A Sting in the Tale. With 260 species, bumblebees populate almost every continent around the globe (Antarctica and Australia being the exceptions), preferring higher altitudes and latitudes where temperatures are cooler, which raises concern. As the global climate continues to rise, this penchant for cooler climate may have dire consequences for many bumblebee species and possibly explain their declining populations. But that might be only one factor. Habitat loss and fragmentation of land repurposed for agriculture, increased use of pesticides and insecticides, and transmission of pathogens between species all may play a part, but nobody is certain.

"[T]he global picture of bumblebee decline is still fragmentary," wrote Marina Arbetman and colleagues, the authors of the study. "[N]o previous study has evaluated worldwide patterns of bumblebee decline as well as their potential predictors within a phylogenetic framework." 

Traditional methods of determining conservation status stem from collecting information pertaining to an organism's biology: where it lives; how its populations are distributed across a town, a country, a continent; what habitat it prefers; what the major threats are to that habitat; what its ecological role is in that habitat. From this information, a conservation status is determined: least concern, near threatened, endangered, critically endangered. 

That's exactly what the International Union for the Conservation of Nature (IUCN) did for bees in Europe back in 2014. They found about eight percent of nearly 2000 bee species in Europe had declining populations, which seems small, but that's because 79 percent of species had insufficient population data to draw any conclusion -- they were only seeing the tip of the iceberg. Even so, bumblebees, making up less than four percent of bee species in Europe, were almost entirely accounted for and over a quarter were threatened or near-threatened.

Traditional conservation status methods, however, exclude what some argue is an important puzzle piece: the phylogenetic diversity of a group. Phylogenies--the evolutionary relations of organisms--act as an excellent proxy for the functional diversity of organisms, which in turn helps scientists grasp what role those species play in an ecosystem and what services they perform for both the ecosystem and for humans. 

"[T]he conservation of higher phylogenetic diversity is likely to safeguard the evolutionary legacy of bumblebees," wrote Nicolas Vereecken, an assistant professor at the Université Libre de Bruxelles in Belgium, earlier this year and who was not an author on this paper. He believes that the conservation of phylogenetic diversity of bumblebees may protect their potential to face environmental change and ultimately play a pivotal role in the maintenance of ecosystem processes and services. "The time is now ripe for the incorporation of phylogenetic diversity as an alternative biodiversity metric into conservation planning to avoid worst-case losses of long branches from the bumblebee tree of life."

Using a New Tool

Arbetman and colleagues looked into the global phylogenetic diversity of bumblebees, asking if the species that were declining fell into specific lineages that play critical ecological roles as well as what factors may contribute to their declining populations, such as range size, presence of certain parasites, and even tongue length, a characteristic that can indicate the bee's specialization to feeding on specific flowers.

The study found that roughly one-third of the bumblebee species evaluated were declining in populations, and the highest proportion of these fell within three subgenera of bumblebees rather than being scattered randomly across the bumblebee tree. One of those subgenera, the Thoracobombus, is the second largest bumblebee subgenus and also one of the oldest.

"[Twenty-two] of the 52 recognized species in [Thoracobombus]...have a well-established extinction risk status," wrote the authors. "From them, almost two-thirds of these 22 species...are declining." The authors also concluded that the loss of this as well as the subgenus Cullumanobombus, another of the subgenera severely threatened, would result in higher losses of phylogenetic diversity than would be predicted from random extinctions of bumblebees. 

"Thus, from a phylogenetic perspective, species of these subgenera deserve the highest conservation priority," concluded the authors.

In striking contrast, bumblebees from the largest subgenus Pyrobombus showed almost no indication of population decline or threatened status. "Out of the 50 species belonging to this subgenus, 32 species (64%) had well-established extinction risk status...with only two species...being threatened," wrote the authors. 

The difference may stem from the other correlates that they found: bumblebees with small ranges, long tongues, and almost no pathogens--all indications of specialization to some degree--showed higher susceptibility to extinction. This becomes even more worrying considering that global commercial exchange and transportation of bee colonies has increased over the past decade, giving prime opportunity for species to invade and potentially out-compete these vulnerable bumblebee lines; the process that has already begun in Chile, where Bombus dahlbomii -- the largest bumblebee species in the world and a member of the declining Thoracobombus subgenus -- is threatened by commercial bees with pathogens previously unknown to the area.

Broader Questions

Overall, this study provides a precise method of determining organisms to prioritize in conservation efforts as well as a key to determining the species crucial to an ecosystem or the preservation of a lineage, an otherwise daunting challenge considering that many inhabitants are declining simultaneously. 

But why should this matter to us?

"Bumblebees are an essential component in our agroecosystem," wrote Vereecken. "[T]heir decline represents a major threat for the sexual reproduction -- and hence survival -- of wild flowers and several pollinator-dependent crops alike." 

Bumblebees play not only a crucial role in natural ecosystems but also our own crop productions through what conservationists call "ecosystem services." We ultimately depend on pollinators like bumblebees, bats, hummingbirds, and various other insects to help produce our food from year-to-year, which in turn save companies, farms, and consumers millions of dollars. This dramatic impact has led some conservation organizations to shift their focus from traditional "preservation of biodiversity" conservation campaigns to ecosystem service campaigns when discussing the need for conservation. People seem to understand money better. 

But Arbetman and colleagues employ a method and present results that hinge on the principle of preserving diversity. From a market's view, though, only a species that optimally performs the desired service is necessary--the others are theoretically expendable. This raises concerns for some conservationists: Focus on a species' ecosystem service neglects the complete picture of natural interactions. After all, nothing in nature is actually independent of the other parts. 

So, is there a way we discuss both economic benefits to a market and people and still emphasize biodiversity preservation? A group of environmental scientists from the United States and France believes they have found a way.

I'll talk about that in the next post.

Ex uno multa

My friend Som and I love giraffes. Standing at the Southwick Zoo in Massachusetts, we watched as four adult giraffes and a calf meandered around their enclosure. We both were entranced by the odd beauty of these animals. Of course, then, the recent news that the giraffe we know is not one but actually four species was exciting, but came as a surprise to me, Som, and many others.

Four species of giraffe determined through genetic work (Image from NYTimes --see link above)

"It seems surprising that we didn't already know that," Som commented to me. Many others express the same incredulity that such a detail could be overlooked for so long. But here's a secret: many things you assume we know and understand we actually don't, especially if it connects to organismal relationships.

Take, for instance, a conversation I recently had with a professor in my department at Brown University. Her student (who also is a friend of mine) is interested in the breathing mechanics of snakes. Why? Because we have almost no clue how snakes breathe.

Shocking? Possibly. Unbelievable? Absolutely not.

To further digress on the subject, reptiles have an assortment of mechanisms to inflate their lungs--many lizards use muscles between their ribs called the intercostal muscles to pivot the rib back-and-forth. This expands the thoracic cavity, i.e. the rib cage, and increases the volume inside, enabling the lungs to fill up more space with air (see video below, on top). Thus air is drawn into the lungs until the intercostal muscles relax, closing the rib cage, and causing exhalation. Crocodilians have a special muscular system that attaches to their livers, acting much like a piston. This concomitantly operates with a few leg muscles to act exactly as lizard intercostals--contraction of the muscles expand the thoracic cavity, increasing volume and inhaling air (video below, on bottom). Snakes, on the other hand, pose an odd problem. Snakes "walk" on their ribs, but likely breathe by a similar mechanism as lizards (because snakes and lizards are related to one another--they're Squamates). How can you walk on your ribs and breathe simultaneously with them? We have a good idea that snakes actually don't breathe when they slither along, but they need to breathe at some point. So when sitting stagnant, how do they do it?

X-ray videos of an iguana and American alligator breathing. Videos taken using XROMM (X-Ray Reconstruction of Moving Morphology) at Brown University

This raises a point about biology--even the seemingly most straightforward topics present an enigmatic problem that we operate with under assumption for years, even centuries. Somewhat, that's the beauty of science, although eventually all assumptions must be justified.

In the case of giraffes, we assumed that all giraffes belong to a single species, broken down into multiple subspecies. In fact, the estimate of giraffe subspecies was originally 7-9, depending on the study. Now biologists have cut it down to five subspecies, all belonging to two of the new four species of giraffe.

But how do they know? Why break the assumption now?

To say that all assumptions are broken is slightly inaccurate, but this case relies heavily on collective evidence and firm statistical analysis. According to the original article, the authors suggest the presence of four species based upon "multi-locus nuclear gene analyses, morphological data, mtDNA sequences, and microsatellites." That spew of words translates to consistent and repeatable DNA information coming from both the nucleus and mitochondria--the powerhouse of the cell, which also happens to have its own DNA--as well as physical characteristics of giraffes. Add the statistical analysis and voila they know there are four species rather than one. (Well, it's actually not that simple, but close enough).

The thing is that much of the information was collected previously--these authors punctuated the former with unequivocal evidence in the form of nuclear DNA from the most inclusive sampling set and produced the clearest statistics yet. It's the combined efforts of multiple studies that gives power to this conclusion. Similar to a court case where substantial evidence validates a judgment, so it is in science. It was not only the work done by these authors that substantiates the claim of four giraffe species, but a combination of information together validates the claim.

But why was it overlooked for so long?

That in itself assumes that it was overlooked, which likely is not true. Surprisingly little is known about giraffes, as a short conversation with a giraffe keeper and quick study of giraffe literature will attest to. This might not be due to lack of interest, but more practical reasons such as lack of funding, ease of experimentation (always a challenge with large animals), value to our scientific knowledge, and exigency of knowing the facts. Unfortunately people cannot pick up their bags, fly to Africa, and collect information haphazardly about whatever he/she wants. Science costs money, and money comes from funding agencies that want to subsidize research that is justified and important.

In the case of giraffes, there is reason to be concerned and study them, but it was only appreciated in the past few decades. Giraffes, as with many organisms, suffer from habitat destruction, cornering the remaining populations into small segments of Africa (see image below).

Historical and contemporary geographical range of giraffes (Source: http://galleryhip.com/where-do-giraffes-live-map.html)

With newly ascertained knowledge that four species of giraffes exist, and that two of them are endangered, the value of this study suddenly escalates. Compounding this effect is the fact that biologists originally thought the giraffe population housed a highly diverse gene pool. Having four species, however, cuts genetic diversity in fractions. Low genetic diversity means greater chance of inbreeding, which often is detrimental to populations--case in point are cheetahs, which went through a "bottleneck" in genetic diversity not too long ago, leaving little diversity for evolution to operate with. This study, then, comes at a point when conservation management can still rescue decreasing giraffe populations.

So here's the takeaway: "simple things" in science are rarely simple and often unanswered; science operates through accordance of multiple studies to draw a conclusion; and some topics are left unstudied not for lack of interest, but lack of exigency or funding.

Overall, it's pretty amazing that we have four giraffe species living in Africa. That's exciting! A family of animals thought to only have two living species in it--the giraffe and okapi--has now more than doubled.

In the spirit of "scientific assumptions," though, I would feel remiss if I circumvented one topic that still has yet to be answered: What is a species? 

Let's leave that for a later discussion.



Want to help the conservation of giraffes? Interested in learning more about these superlative and majestic creatures? Check out the Giraffe Conservation Foundation to learn more about how you can help.

"For Science!"

Lest anyone ever be deceived by the convincing and omnipotent power of textbooks, science is a perpetually changing field, both in that new branches of study emerge from the main "trunk" of scientific disciplines and that our previous hypotheses and theories can be (and often are) eliminated by new findings.

I recall sitting as a child, peering through my dad's old dinosaur books that depicted Brachiosaurus feeding while mostly submerged in water.

Heck, even Disney's Fantasia depicted Diplodicus feeding in quagmires, the illustrators only depicting what scientists believed to be true at the time.

Now I'm not studying paleontology, but I know all of this was built under the premise that sauropods (more colloquially known as "long-necks" thanks to the infinite series The Land Before Time) could not possibly have lived on land. Good heavens, the size of their femur is larger than my 8-year-old nephew! Can you imagine the forces on that bone from gravity? The rationalization is actually not that illogical--the largest animal to ever live on the Earth is the Blue Whale (Balaenoptera musculus), reaching lengths around 100 ft and on average weighing in at an astronomical 190 tons. The only way such an organism can reach such values is by having a buoyancy force to counter a large percentage of the weight. If that's true for the Blue Whale, then is it so irrational to believe that sauropods (deemed as the largest terrestrial organisms to ever live on the Earth--gotta love those superlatives) would have the same limitation? What's an interesting thought about this, however, is the forces acting on a standing Brachiosaurus in the water. If a Brachiosaurus eats vegetation around 30 ft. high, then let's imagine that it's heart is located roughly 12 ft. from the ground. Fluid statics says that the differential pressure changes solely with the change in height in the fluid, i.e. pressure increases only as one changes depth. With the heart at about 17 ft. deep (~5.2 m), the pressure is about 22.1 psi, where the average erect giraffe systolic heart pressure is about 2.3 psi (Citters et al., Comp. Biochem. and Phys., 1968). There is a specific point to make about this. Depending on the circumstance having that pressure could actually be fine, but it could also lead to a potential problem because the heart might not be able to generate the pressure needed to ascend up the neck of the sauropod (near the surface where pressure is lower) or down to the appendages (at the base where the pressure is even greater). So although we have some logical idea about size/mass being a limitation as to where a sauropod can actually live, we now also have to take into account the physiological consequences of this proposal and realize that it may also be problematic for the theory.

 Here's what is irrefutable: there are sauropod tracks preserved in stone that were from terrestrial habitats, usually deduced by the nature of the rock in which the tracks are preserved.

Similarly, the wear and tear on the teeth of sauropods indicates that they fed on foliage that was tough and significantly more challenging to masticate ("chew") than if all they fed on were algae or other aquatic vegetation. So in retrospect, it seems ridiculous that people once thought that sauropods could only live in water. But it's only easy to say that in retrospect. There are similar changes that have occurred with dinosaurs. Look at the transition of our view of dinosaurs as cold-blooded, lethargic animals to possibly warm-blooded, highly active creatures.

1950s depiction of Corythosaurus--notice the dull colors, slouched body, and dragging tail

Contemporary image of Corythosaurus--notice the bright colors, erect tail, and heightened stature

The facts of the matter are that science is perpetually evolving from one year to the next, and it may be evolving in the exact wrong direction for decades or even centuries. If you don't believe me on that, talk to any computer scientist or molecular biologist, both of which are scientific fields where the information from only a year or two ago is considered antiquated. In fact, genetics and molecular biology combined advance so quickly that the federal government can't keep up with the legal and ethical issues that come about with each major discovery or technological advancement. 

As another example, Charles Darwin himself used to believe in particles he called "gemmules" that would permeate throughout the body, but somehow aggregate together in the reproductive organs of organisms. When an egg and sperm merged together, the gemmules inside would merge together, making the offspring a blend of the two parents, although some of those gemmules may remain latent until future generations. Unbeknownst to Darwin, Gregor Mendel was performing his iconic pea experiments, yielding some of the same conclusions as Darwin (albeit more accurate) but building the pathway toward understanding the idea of a "gene" in inheritance. It was assumed that of the four major groups of biomolecules (carbohydrates, lipids, proteins, and nucleic acids) that proteins were the most likely candidates for inheritance. Through a series of ingenious experiments, however, we learned that inheritance was dependent on nucleic acids. And with the discovery of the structure of DNA by Watson and Crick in the 1953, a whole new avenue of understanding inheritance began.

Ultimately the objective here is to make this quite clear: science is not stagnant. Through one of the most powerful techniques ever conceived by the intellect of man (the scientific method), science has made leaps and bounds in its knowledge about the Universe and everything in it. Theories come and go. Questioning and modeling help smooth rough edges. Ingenuity and intrigue pave new paths to follow. In all, science presents one of the most exciting, fascinating and complex disciplines that any person could choose to study. The objective of this blog is to post about recent studies in scientific fields as they are published in the literature. I take an approach that strives to see a scientific study from its undergirding theories, historical foundations, and basic assumptions. Why are these studies useful or interesting? What does it build upon or reveal? How has our opinion on this study changed over the years? With each study posted, hopefully both you and I gain a greater appreciation for the research going on and for the beautiful and magnificent world that surrounds us.