Sagittal section of a Short-Tailed Opossum (Monodelphis domestica) at 14 days after birth. One of the final steps in the visualizing process we started here and here.  Fluorescent immunostaining for muscle (red), nerve tissue (green), and cell nuclei to visualize the outlines of the animal (blue). Check out those whisker pits!  Image credit: Curious Sengi.



Remember those Short-Tailed Opossum (Monodelphis domestica) neonates from last week?  After some chemical treatments, the specimens were frozen into a cutting compound, and then sectioned into 300 micron (0.01 inch) thick slices on the cryostat.  Here are three slices from the same animal.  The sagittal sections (parallel to the midline that divides the body into right and left halves) show all sorts of wonderful details such as the chain of the vertebral column in white, developing bones of the skull, liver, gut, eye, and tongue.  These sections look amazing, but they are not done yet!  Come back to find out what happens next.  Image credit: Curious Sengi.


A pair of Short-Tailed Opossum (Monodelphis domestica), 14 days postnatal.  Note the presence of hairs and whiskers on the face as well as the bones ossifying in the fore and hindfeet.  Image credit: Curious Sengi.

Greetings, fellow Snurflers!

We have now entered that season during the academic year when many of us are facing great hurdles:  qualifying exams and dissertation defenses.  I will be presenting my research proposal at quals in a few weeks and I wanted to share some of the images from my work with you.

My research on the evolutionary origin and subsequent modifications of facial muscles in mammals involves a lot of comparative morphology:  looking at a wide range of animals (including non-mammals) to piece together a picture of what is old and what is novel, ancestral and derived, conserved and innovative, and what is just plain weird.  I am using techniques and ideas from developmental biology to show the spatiotemporal sequence of how facial muscles grow and differentiate in different embryos, but to also shed light on some key processes behind the question of why muscles grow over the faces of mammals, but not in animals like reptiles.

At the moment, I am working a lot with the embryos and neonates of the Brazilian Short-Tailed Opossum (Monodelphis domestica), a marsupial that is increasingly being used as a model organism in laboratories.  The images here are of young opossums, collected 14 days after birth.  Like all other marsupials, gestation time is short and the babies are born in an extremely underdeveloped state where they are essentially all just forelimbs and a mouth.  Now a few weeks after birth, these little guys are starting to look much more like recognizable animals.

These photographs were taken after the specimens were bleached into a ghostly white.  There are more steps ahead before we can visualize the development of facial muscles.  See what happens next in our following posts!

Image credit: Curious Sengi.

Bunny Surprises

Bunnies are hiding more than just Easter eggs. . . . . . Image credit: Wikimedia Commons.

The ubiquitous presence of rabbits in North America and Europe has percolated this cute, fluffy animal into the Western popular imagination.  Rabbits are carrot-nibbling vegetarians.  They are timid.  And bunnies must be boffing continuously, given their prolific fecundity.

Let’s take a closer look through three vignettes that shed slightly different light on these animals.

Not a bad mom, just a bad day

Rabbits do, in fact, breed like rabbits.  The Common Rabbit (Oryctolagus cuniculus) gives birth to an average of 5 to 6 young after a gestation of about a month.  Though the young are altricial — born with eyes closed, pink, and in need of intensive parental care — the female is ready to mate again a few hours after giving birth.  Under ideal conditions, a female can potentially have anywhere from 5 to 7 litters per year.  However, conditions are rarely ideal.  It is estimated that at least 60% of all O. cuniculus pregnancies are aborted, with the embryos simply resorbing back into the mother’s body (Macmillan Illustrated Animal Encyclopedia 1984; Nowak 1999).

Recently-born rabbits, or kits, peeking out from their fur-lined nest.  Image credit: Ruth : ) via Flickr.

Even under optimal conditions in captivity where there is ready access to nutritious food, water, and nesting material, things do not always go according to plan.  In a study of wild O. cuniculus kept in enclosures, researchers noted that about 13% of all litters experienced neonatal cannibalism, where the mother purposefully killed and consumed her newborns.  (It should be noted that neonatal cannibalism not uncommon amongst mammals.)

The authors of the study concluded that this behavior was most likely triggered by stressful conditions, in this case, placing wild animals into a confined space.  Stress during the time leading up to the birth of the young causes something along the hormonal-neuronal pathway to be disrupted, thus failing to trigger the onset of maternal behavior.  The warning signs were a failure of the mother to construct a proper nest from grasses and especially the lack of nest lining made from the mother’s own belly fur.  Neonatal cannibalism seems to be dependent on specific circumstances.  This behavior destroys an entire litter, but when conditions improve, cannibalistic mothers are perfectly capable of populating the world with their progeny (Gonzalez-Redondo & Zamora-Lozano 2008).

Attack of the “Killer Rabbit”

Back in 1979, the United States was facing another sort of crisis of confidence in its president, Jimmy Carter.  It was a tumultuous time for many people and as the nation looked towards their elected leader for hope, they were hit with a different vision entirely:  the president sitting in a fishing boat on a quiet Georgia lake, flailing an oar at a crazed rabbit swimming towards him (O’Grady 2014).

President v. rabbit, with the President finally gaining the upper hand.  Image credit: Jimmy Carter Library and Museum via WNYC.

It took no time for the public and Carter’s political detractors to latch onto this story of the “killer rabbit”, pointing out that a leader nearly bested by a little bunny had no place handling serious affairs on the world stage (O’Grady 2014).  But let’s step back for a moment and focus on the biology here.  Carter was most likely accosted by a Swamp Rabbit (Sylvilagus aquaticus), which is found inhabiting marshes and wetlands of the southern U.S.  S. aquaticus is particularly adept at swimming and diving, often taking to the water when pursued by predators or traveling to new feeding areas.  This species is amongst the largest of the cottontail genus, robustly-built and “large-headed” (Macmillan Illustrated Animal Encyclopedia 1984).  The rabbit is generally docile, but it is a territorial animal (Nowak 1999) and confrontation between males can result in vicious fights that leave serious wounds (Macmillan Illustrated Animal Encyclopedia 1984).  Though it is highly unlikely the creature was intentionally going after the President, a swimming rabbit making a chance encounter with one of the most powerful people in the world was just too good to resist.

Expectation:  the killer Rabbit of Caerbannog from “Monty Python and the Holy Grail”.  Reality (at best):  a chance encounter between the Commander-in-Chief and a local Swamp Rabbit doing what Swamp Rabbits do, which is swim and maybe feel a little bit territorial.  Image credit: Wikipedia.

Back to blood

“Are carnivorous rabbits possible, anywhere?  No, this is a theoretical absurdity.”  

(Seddon 1972, quoted in Clauss et al. 2016)

How it’s supposed to be. . . .right?  Image credit: via Pexels.

Rabbits are iconic vegetarians.  It had long been assumed that anything other than herbivory was physiologically impossible.  The lack of carnivorous teeth and a digestive system unadapted to processing meat makes this seem obvious (Clauss et al. 2016).  But we did see instances of neonatal cannibalism in our first vignette.

Clauss et al. 2016 report on two domestic dwarf rabbits that were kept in a mixed-species enclosure at a Swiss raptor rehabilitation center.  Over a 9 month period, these two rabbits shared their home with a variety of kestrels, kites, buzzards, and other birds of prey. . . . as well as a shared taste for meat.  While the rabbits were amply provided with fresh vegetarian fare, the raptors were offered whole mice, rats, and day-old chicks.  The rabbits did not neglect their own food, but were drawn to the meaty offering, even chasing birds away from the food.  The authors noted that the rabbits

. . . used a gnawing ingestion style with the extremities of day-old chicks and rodent tails.  When the caretaker brought the daily prey ration for the raptors, the rabbits immediately ran towards the person and followed him even when the dish was placed on an upper perch, where they had to climb a ladder-like staircase (Clauss et al. 2016).

It is like some kind of Easter gathering gone horribly wrong.  These domestic carnivorous rabbits reportedly had neither a favorite prey item nor seemed to suffer any ill-effects from consuming meat.  Image credit: Clauss et al. 2016.

The question is whether this represents some kind of pathological behavior.  An increasing body of observations of presumably exclusive herbivores consuming carcasses and small animals seems to indicate that this is within the realm of natural activity.  If anything, these instances of carnivory were driven by opportunity.  Under usual circumstances, it is unlikely that a rabbit would remain out in the open, gnawing away at a carcass when it was in danger of meeting the same fate from predators.  But in the safe environment of the enclosure (incidentally, the rehabilitating raptors did not pose a threat) the rabbits were at leisure to engage in some casual carnivory (Clauss et al. 2016).

There is so much more to say about rabbits and their relatives than I have time for right now.  But I hope this little trio of information has sparked your interest in these unassuming, yet utterly surprising animals!


Clauss, Marcus, Andreas Lischke, Heike Botha, & Jean-Michel Hatt.  2016.  “Carcass consumption by domestic rabbits (Oryctolagus cuniculus).”  European Journal of Wildlife Research 62:  143 – 145.

González-Redondo, P. & M. Zamora-Lozano.  2008.  “Neonatal cannibalism in cage-bred wild rabbits (Oryctolagus cuniculus).”  Archivos de Medicina Veterinaria 40:  281 – 287.

Macmillan Illustrated Animal Encyclopedia.  1984.  Philip Whitfield, ed.  New York, NY:  Macmillan Publishing Company.

Nowak, Ronald M.  1999.  Walker’s Mammals of the World, Vol. II.  6th edition.  Baltimore, MD:  The Johns Hopkins University Press.

O’Grady, Jim.  “How Jimmy Carter’s Face-Off with a Rabbit Changed the Presidency.”  WNYC .  New York Public Radio, 17 February 2014.  Web.  Accessed 15 April 2017.

One Year of Curious Sengi

Image credit: Black and Rufous Sengi (Rhynchocyon petersi).  Philadelphia Zoo / Curious Sengi.

Thanks for reading Curious Sengi!

Let’s keep snurfling and discovering all sorts of wonderful things about the living world around us.  I look forward to sharing what I find with you!

Sloths Wearing of the Green

Three-toed sloth festively sporting green fur.  In case the common names get confusing, remember that all sloths have three toes.  The distinguishing number of digits is in the hand.  Image credit:  Azulia via Pixabay.

Sloths are pretty darned slow.  So slow that they seem overtaken by the ever-creeping growth of the rainforest.

There are six living species of sloth distributed across Central and South America and each of these species have individuals observed with striking green fur.  This green comes from algae, primarily Trichophilus welckeri (Suutari et al. 2010).  But it is not just green algae that inhabit sloth fur.  The hairy pelage is a practical ecosystem supporting a community of microbes, fungi, roundworms, beetles, cockroaches, and moths (Suutari et al. 2010).

You might want to think about that the next time you are taken with the impulse to pluck a sloth out of a tree and hug it.

Modern research has shed some interesting light upon the relationship between sloths and the green algae in their fur.  Suutari et al. (2010) undertook an analysis of hair samples taken from all six living sloth species.  Of the 71 total samples, 73% hosted green algae, with Trichophilus being the most frequently found and abundant.  But genetic signatures from rRNA retrieved from the hairs showed a diverse assemblage of eukaryotes most likely acquired from the environment.

Microstructure of three-toed sloth, Bradypus hair.  Note the irregular cracks.  Image taken with scanning electron microscopy (SEM).  Image credit:  Wujek & Cocuzza (1986).

SEM of two-toed sloth, Choloepus hair.  The microstructure of this hair shows longitudinal grooves.  In both the three- and two-toed sloth, these grooves and cracks are home to green algae, cyanobacteria, diatoms, and other microbes (Suutari et al. 2010).  Image credit: Wujek & Cocuzza (1986).

The appearance of green algae was most commonly found in the three-toed sloths (within the genus Bradypus).  But what was even more interesting is that the various strains of Trichophilus algae found on all the different species of Bradypus throughout Central and South America are all closely related to each other.  In fact, the Trichophilus algae found on a given species of Bradypus would be more genetically related to the algae on a geographically distant Bradypus species than with the Trichophilus algae found in the fur of a local two-toed sloth (Choloepus spp.).  This suggests a long history of co-evolution between Bradypus and Trichophilus, one that might be an echo of the divergence between three- and two-toed sloths approximately 20 million years ago (Suutari et al. 2010).

Green-furred two-toed sloth (Choloepus spp.) crossing a road in Costa Rica.  Image credit: Chirriposa Retreats via Vimeo.

It had long been assumed that the presence of green algae was part of a symbiotic relationship where the sloth provided shelter for algal growth and the algae in turn provided some kind of camouflage for sloths as they whiled away in the verdant canopy, protecting them against aerial predators (Pauli et al. 2014).  There are other hypotheses out there as well.  And some of these are a bit “out there.”  It has been proposed that algae provide a nutritional boost for sloths, who are burdened with a low-energy leaf diet.  Someone has suggested that sloths receive nutrients that diffuse into the hairs and absorb into the skin (Suutari et al. 2010).  But that seems a bit silly.

Pauli et al. (2014) proposed a more complex symbiotic relationship based on the observation that three-toed sloths (Bradypus spp.) make regular descents down from the trees to poop on the ground.  Why bother with this dangerous and exhausting journey to do your business?  Clambering down a tree once a week is estimated to use up about 8% of a sloth’s daily energetic budget, in addition to the statistics that more than half of adult sloth mortalities occur as a result of predation events on or near the ground.  The authors of this study put forth that these weekly visits to established latrine sites “sustains an ecosystem in the fur of sloths, which confers cryptic nutritional benefits.”

Are you ready for this?

Remember the moths that also live in sloth fur?  They lay their eggs in sloth poop.  Revisiting these latrine sites provides an opportunity for female moths to lay eggs and for the next generation of moths to colonize the sloth’s fur.  An increase in the moth population also increases the amount of nitrogen, which feeds the growth of algae.  Finally, the sloth consumes this “algae-garden”, which is supposedly highly digestible and rich in fats, as a nutritional supplement to their otherwise poor diet (Pauli et al 2014).

Image credit: Pauli et al. (2014).

While this is certainly a novel idea that could benefit from further investigation, the researchers have yet to provide conclusive evidence for this symbiosis.  While algal cells were found in the stomach of sloths, it is difficult to say if there is enough being consumed to provide significant nutritional benefits.  Likewise, this hypothesis could be supported by behavioral studies showing licking of the fur that would remove algae for consumption.  Also, it does not address the observation that the presence of green algae is common, but not universal, amongst three-toed sloths.

Green sloths remain a mystery.  But in the world of mammals, green is a very rare color to be found in the hair or skin.  So let’s raise our glasses to the sloths, who are so lucky to have this unusual color!

This Bradypus is keepin’ it green, keepin’ it real.  Image credit: Justin Lindsay via Flickr.


Fountain, Emily D. et al.  2017.  “Cophylogenetics and biogeography reveal a coevolved relationship between sloths and their symbiont algae.”  Molecular Phylogenetics and Evolution 110:  73 – 80.

Pauli, Jonathan N. et al. 2014.  “A syndrome of mutualism reinforces the lifestyle of a sloth.”  Proceedings of the Royal Society B 281 (1778):  20133006.

Suutari, Milla et al.  2010.  “Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae).”  BMC Evolutionary Biology 10 (1):  86.

Wujek, Daniel E. & Joan M. Cocuzza.  1986.  “Morphology of hair of two- and three-toed sloths (Edentata:  Bradypodidae).”  Revista de Biologia Tropical 34 (2):  243 – 246.

Got My Valentine Right Here


A pair of aardvarks (Orycteropus afar) dozing together under an observational red light at the Philadelphia Zoo.  Aardvarks are sexually monomorphic, making them particularly difficult to sex (Parys 2012); however, the pair seen here are Sunshine (female) and AJ (male)*.  Image credit: Philadelphia Zoo / Curious Sengi.

To be honest, there is not much known about the love-life of aardvarks.

These animals are solitary for most of the year, until the rainy season floods out their haunts in the grasslands and forces them to retreat to higher ground.  This concentration of the population into a smaller area might initiate some aardvark love, as observers have noted male-female pairs “gambolling” and entering burrows together during this time.  However it may work, baby aardvarks appear about seven months later.

Aardvarks dig out sleeping holes with their powerful claws.  These chambers are usually just a little larger than the size of the body and the animals sleep curled up, snout covered by tail and hindfeet (Kingdon 1971).  Squeezing in two aardvarks into a single sleeping hole is a bit tight, but their predilection for snoozing snoot to foot is pretty darn cute!


Image credit: Sebastien Millon via DeviantArt.

* A sad note:  AJ the aardvark recently died at the Philadelphia Zoo in January 2017.  Read about it here.


Kingdon, Jonathan.  1971.  East African Mammals:  An Atlas of Evolution in Africa, Volume I.  London:  Academic Press.

Parys, Astrid et al.  2012.  “Newcomers enrich the European zoo aardvark population.”  Afrotherian Conservation 9:  2 – 5.

Darwin’s Birthday Albums


Not a birthday album, but an album of family pictures on display at Darwin’s Down House estate.  Image credit: English Heritage Blog.

The celebration of Charles Darwin’s birthday (aka Darwin Day) has become an international event in recent years.  But how did people celebrate Darwin’s birthday during his lifetime?

When Darwin turned sixty-eight years old in 1877, he received two large parcels in the mail.  One was from Germany, a luxuriously-bound album containing the photographs and signatures of 154 German scientists and naturalists.  The second parcel was another photographic album of admirers, this time from the Netherlands (Darwin 1877; Browne 2002).  Darwin wrote back graciously to Professor A. van Bemmelen, who organized the effort:

SIR,—I received yesterday the magnificent present of the album, together with your letter. I hope that you will endeavour to find some means to express to the two hundred and seventeen distinguished observers and lovers of natural science, who have sent me their photographs, my gratitude for their extreme kindness. I feel deeply gratified by this gift, and I do not think that any testimonial more honourable to me could have been imagined. I am well aware that my books could never have been written, and would not have made any impression on the public mind, had not an immense amount of material been collected by a long series of admirable observers; and it is to them that honour is chiefly due. I suppose that every worker at science occasionally feels depressed, and doubts whether what he has published has been worth the labour which it has cost him, but for the few remaining years of my life, whenever I want cheering, I will look at the portraits of my distinguished co-workers in the field of science, and remember their generous sympathy. When I die, the album will be a most precious bequest to my children. I must further express my obligation for the very interesting history contained in your letter of the progress of opinion in the Netherlands, with respect to Evolution, the whole of which is quite new to me. I must again thank all my kind friends, from my heart, for their ever-memorable testimonial, and I remain, Sir,

Your obliged and grateful servant,


At this point in his career, Darwin had already published his major works:  The Voyage of the Beagle (1839), On the Origin of Species (1859), The Descent of Man (1871), and The Expression of the Emotions in Man and Animals (1872).  He was very much in the public eye, whether as a hero of science or as the villain set on destroying the cozy blinders of Victorian religious faith.  As a man who treasured quietude and harmony, the rancor raised by his theory of evolution by natural selection was deeply distressing.  I do not believe that Darwin’s letter to van Bemmelen expresses any sort of false modesty, but a true gratitude, both for kindness and the international effort necessary for advancing science.  Even Darwin’s wife, Emma, wrote of the arrival of the gift from the Netherlands:

. . . .yesterday arrived a most gorgeous purple velvet & silver Dutch album of the same sort with 219 portraits — some of youths, some girls & some fat women, I suppose any one who subscribed.  However it shews a v. different state of feeling about him.  You wd. not get boys & fat women in England to subscribe & send him their photos as a mark of respect (quoted from Browne 2002).

An international community of scientists, naturalists, and enthusiasts have continued to commemorate Darwin’s birthday, even after his death in 1882.  The Darwin Day movement began in the United States in the early 2000’s, quickly blossoming into coordinated events celebrating Darwin, evolution, and science education (Wikipedia 2016).

So let’s pick up our glasses and toast to the man, Charles Darwin!


Image credit: Ainsley Seago at


Browne, Janet.  2002.  Charles Darwin:  The Power of Place.  New York, NY:  Alfred A. Knopf.

Darwin, Charles.  1887.  The life and letters of Charles Darwin, including an autobiographical chapter, Volume 3.  Francis Darwin, ed.  London:  John Murray.

Wikipedia contributors.  “Darwin Day.”  Wikipedia, the Free Encyclopedia.  Wikipedia, The Free Encyclopedia, 26 December 2016.  Accessed 12 February 2017.

Happy New Year (Again)!


Happy Lunar New Year and welcome to the Year of the Rooster!  Here is a chicken embryo nestled into a pillow of yolk and surrounded by a network of blood vessels.  The eye is clearly visible as a dark circle; the developing fore- and hindlimb and tail are also visible as projections from the pink body.  At this stage, the yolk membrane is extremely fragile and will rupture when the egg is cracked.  However, this one landed perfectly intact into a petri dish.  We use a lot of chicken embryos in our line of research, so I would like to take this moment to deeply thank them for their contribution to our science and teaching.  For me, the beauty of these embryos are also a potent source of wonder.  Image credit: Curious Sengi.