Secret Science Club Post-Lecture Recap: Caves and Crocodiles

Last night, I headed down to the beautiful Bell House, in the Gowanus section of Brooklyn, for this month's Secret Science Club lecture. Last night's lecture marked the triumphant return of Dr Evon Hekkala, evolutionary biologist at the American Museum of Natural History and biology professor at Fordham University. Dr Hekkala lectured back in 2013, so this wasn't her first time at the Secret Science rodeo.

Dr Hekkala began her lecture with a simple declaration: Surprises should be part of science. She noted that sometimes things will be what you don't expect, citing the unexpected discoveries of Rosalind Franklin, Luis and Walter Alvarez, Raymond Dart, scientists whose discoveries upended prevailing conventional wisdom. She noted that life is full of good accidents, citing her own PhD woes, when she was unable to study lemurs, and settling on crocodilians as a field of study instead. She noted that ideas come from everyone and everywhere because people start out with curiosity... she joked that a child who overturns a container of milk is performing an experiment. In her case, she started out studying art history and anthropology.

She characterized art as depicting hypotheses, specifically citing the dioramas at AMNH, each of which depicts a particular moment in space and time. Museum collections are archives of the natural world. Museums started off as curiosity cabinets, collections of wonders from around the world. Most museum specimens are not on display- there are vast collections of preserved specimens behind the scenes. Archives are specimens, Dr Hekkala cited a 1930s-era letter in the AMNH archives concerning the danger of extinction faced by the indri, a letter she described as 'a smoking gun for a changing planet'. Dr Hekkala also noted that museums besides natural history museums have important scientific evidence- for example, the Metropolitan Museum of Art has a crocodile mummy.

Dr Hekkala's particular specialty is the study of crocodiles. Two decades ago, it was thought that Nile crocodiles (Crocodylus niloticus) were well known, but people were wrong about a lot of things, especially by thinking that crocodiles were 'living fossils'. One mystery about the crocodiles of Madagascar is whether there are separate species on the island. Back in the 1970s, African crocodiles were in danger of extinction, but were brought back by the introduction of sustainable crocodile leather production- economic value was an important factor in conservation. Alligators are approximately as closely related to crocodiles as humans are to bats, having split from each other approximately sixty-five million years ago- they have a similar form because this particular submarine predator physique works well. The extant crocodile populations of African can be divided into two species, with Crocodylus suchus inhabiting West Africa and Crocodylus niloticus inhabiting East Africa. The crocodiles of the New World are more closely related to the East African crocodiles than to the West African ones, with the lineages splitting between three and six million years ago. C. suchus and C. niloticus were long considered the same species, until Dr Hekkala ran DNA tests on samples of Egyptian crocodile mummies, finding them to be a distinct species, C. suchus. She joked that she was 'scooped' by a couple of centuries by French naturalist Étienne Geoffroy Saint-Hilaire who noted differences between the two species. Saint-Hilaire was inspired by the works of Herodotus, which described two separate, distinct populations of African crocodiles. Dr Hekkala is at the forefront of a new era, extracting ancient DNA from old specimens... she has been responsible for mummy genomic projects, extracting DNA from temples and museums full of crocodile mummies. She noted that the Egyptians could distinguish between niloticus and suchus, even without the benefit of DNA sequencing. Crocodile mummies are also being scanned, with one mummy actually containing almost fifty crocodile hatchlings and two larger specimens.

Dr Hekkala noted that crocodilians were considered important by many cultures that were in contact with them throughout their range- Egypt, Australia, Mexico, Madagascar. People observe the animals they live with, and crocodiles were considered to have various powers. The apothecary alligator crops up in art throughout the ages, and crocodilians were considered to have healing powers. Crocodilians' blood has antibiotic properties.

Dr Hekkala then narrowed her focus to the crocodiles of Madagascar. Once considered a separate species, Crocodylus madagascariensis, the Malagasy crocodile is now considered a subspecies of Crocodylys niloticus. Prior to the arrival of C. niloticus to the island, there was an endemic crocodilian species, Crocodylus robustus or Voay robustus (also known as the horned crocodile), which is now extinct, along with other Malagasy megafauna such as gorilla sized lemurs, dwarf hippopotomuses, and elephant birds. The extinction of Madagascar's megafauna is a big whodunit- was it people or pathogens or environmental change? What happened to the endemic horned crocodile of Madagascar? The Nile crocodile only arrived on the island about two thousand years ago. Humans started sporadically colonizing the island about twelve thousand years ago, with a sustained human habitation being achieved about two thousand years ago. Did the two crocodile species coexist? Up until the 1860s, explorers described two types of crocodiles. Did the horned crocodile succumb to human overexploitation? The horned crocodile is allied with the dwarf crocodiles, but only seven complete specimens are known. Did the Nile crocodile outcompete the horned crocodile? Did it 'swamp out' the horned crocodile through hybridization (if they were closely related enough). This being a whodunit, she went to the archives for clues, perusing the account of shipwrecked sailor Robert Drury for descriptions of wildlife.

In a stroke of luck, while watching a video of cave divers collecting lemur bones in a flooded Madagascar cave, Dr Hekkala noted a crocodile skull:





It turned out that PBS wanted to produce a program on the crocodiles of Madagascar, which led to what Dr Hekkala comically referred to as 'In Search of Lost Crocodile Lineages of Madagascar, or What I Did on my Summer Vacation'. I have to note that it just wouldn't be a Dr Evon Hekkala lecture without a generous dose of globetrotting adventure. She had twelve days to head to Madagascar with a film crew which would visit four locations on the island. She joked that it seemed impossible for this expedition to be pulled off in such short time, but a local 'fixer' was able to sort things out for them. They started off at the Croc Farm, then visited some sacred crocodile lakes (in local legend, crocodiles are ancestors of the populace- they were inhospitable people transformed into crocodiles because of their nastiness). While she and the crew weren't able to touch the crocodiles in the sacred lakes, where zebu are sacrificed to the crocs, they were able to collect DNA from traces in the water. They then traveled via boat and zebu cart (Dr Hekkala quipped 'planes, trains, and automobiles') to the spiky karst landscape of Ankarana Special Reserve. All the while, she was obtaining evidence- DNA from footprints, signs of 'robustus' among the crocodiles at Croc Farm. She kept the audience enthralled with pictures of lemurs, of giant baobabs, of rugged landscapes. The cave which the team visited was accessible by a forty foot ladder- originally, spelunkers would have to climb down a banyan root into the cave. There are numerous birds in the cave- raptors, kingfishers. The water in the cave is about 80F. The expedition was headed by Dr Laurie Godfrey, who Dr Hekkala photographed holding a lemur femur.

The cave was a trove of bones- four extinct lemur species could be found in one meter of sediment, and the bone collectors hit their specimen limit in four hours. The Madagascar Cave Diving Association provided the divers for the expedition. Dr Hekkala related a funny anecdote about falling in the mud in the cave, whereupon it took twenty minutes to extract her. To make matters more unpleasant, in order to maintain 'continuity' for the filming, she had to wear the muddy clothes the following day.

The cave dive was productive, a small 'robustus' skull was quickly found. The divers eventually found thirty crocodile skulls 'grinning' from the cave walls, though only two skulls were removed from the cave. Unfortunately, the team only had permits to collect lemur bones, not crocodile skulls, so Dr Hekkala took a tooth from one skull in order to obtain DNA samples. DNA testing will resolve the question of whether these crocodiles were a relic population with low genetic diversity or if they hybridized with the newly arrived Nile crocodiles. The two skulls examined by Dr Hekkala increased the diversity of known 'robustus' specimens by almost thirty percent. The twelve day trip was an accident, a happy one, and left Dr Hekkala resolved to obtain cave diving certification (as if she's not 'Indiana Jones' enough as it is!).

Dr Hekkala wrapped up her lecture by noting that, in order to understand the future, sometimes we have to visit the past. The study of biodiversity is crucial to our human survival. Also, outreach to children is crucial to the future success of humanity. so she is dedicated to teaching little kids as well as graduate students and, on occasion, barflies.

The lecture was followed with a Q&A session. Regarding the relationship between New World crocodiles and East African crocodiles, the divergence took place between the two lineages took place three to six million years ago. In a somewhat macabre aside, Dr Hekkala ruefully noted that the crocodiles probably drifted along the same currents that slave traders ended up using. The Nile crocodiles of Madagascar probably originated from the Zambezi River. Saltwater crocodiles have been encountered seven hundred miles from shore, so crocodiles are capable of long sea voyages. Since the construction of the Aswan Dam, crocodiles have gone extinct in Egypt, so no DNA is available for comparison.

Another question involved the cave ecology- pollen trapped in speleothems can provide clues about conditions in the cave, whether it was wetter or drier at various times. The crocodile remains are intact and articulated, so the crocs died in situ. Banyan trees near the cave entrance tend to attract animals, but perhaps the crocs came in through another entrance.

Another question involved the diversity of crocodilians- along with the birds, crocodilians are one of the great surviving archosaur lineages. There are two alligator species, several true crocodile species, dwarf crocodiles, caimans, and slender snouted gharials. These lineages diverged about sixty-five million years ago.

Another question regarded the suchus/niloticus divide in Africa- the suchus crocodiles are smaller and less aggressive than their eastern counterparts. It's possible that some hybridization occurs, but the extent is unknown.

Another question involved the antibiotic properties of crocodilian blood. Croc blood plasma was effective in killing staphlococcus, E. coli, and streptococcus bacteria. Additional testing is needed to gauge the extent of croc plasma antibiotic properties.

Some bastard in the audience asked Dr Hekkala to speculate why New World crocodiles haven't penetrated into the territory of the American alligator while the Chinese alligator has a tiny geographic distribution... could alligators have better cold tolerance? Dr Hekkala noted that there are crocodile populations at high elevations in Madagascar where the crocodiles are exposed to cool temperatures.

One again, Dr Hekkala knocked it out of the park... she hit what I call the 'Secret Science sweet spot'- a heavy dose of hard science with revelation of new discoveries, combined with adventure narrative and science/conservation advocacy, the whole shebang leavened with humor. Kudos to Dr Hekkala, Dorian and Margaret, and the staff of the beautiful Bell House. Now, here's a video of Dr Hekkala giving an overview of the 'Crocs' exhibit at AMNH:





Pour yourself a nice beverage and soak in that SCIENCE!!!