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Posts Tagged reptiles

The weird ways of reptile reproduction Hilary Miller Nov 04

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Two studies on reproduction in reptiles have made me go “wow, thats cool” this week.

Firstly, the report of a boa constrictor giving birth to two litters of offspring without the need for a father.  This sort of “virgin birth” is called parthenogenesis, and is not that uncommon in itself, having been previously observed in a few other reptile and fish species, and numerous invertebrates.  What separates this latest report from the others is the unusual complement of sex chromosomes observed in the offspring.

When vertebrates reproduce by parthenogenesis, the offspring are usually “half clones” of the mother.  Chromosomes come in pairs, and normally you get one half of the pair from your father and the other half from your mother.  But in parthenogenesis, both halves come from the mother – that is, two copies of one half of the mother’s chromosomes are inherited.   This means that when it comes to the sex chromosomes, the offspring end up with two of the copies of the same chromosome.  In all other recorded instances of pathenogenesis in vertebrates, species like snakes with the ZW sex chromosome system (where males have ZZ and females have ZW chromosomes) produce only male offspring, and species with XY chromosomes (males XY and females XX)  produce only female  offspring.   In other words, only the sex where the two sex chromosomes are the same is produced and the opposite scenario (WW females or YY males) was thought to result in non-viable offspring.  Until this boa constrictor came along, that is –  yep, her litters are made up entirely of WW female snakes, a finding which “up-ends decades of scientific theory on reptile reproduction”.

This study was published online this week in Biology Letters, and the BBC news has more on the story here

The second study, published online in Nature this week, is an great example of just how malleable sex determining systems can be.  For many reptiles, sex is determined not by sex chromosomes, but by what temperature the egg is incubated at.  In tuatara for example, incubating eggs at high temperatures produces males, while low temperatures produce females.  You might think that chromosomal and temperature-dependent sex determination are two fundamentally different, mutually exclusive ways of determining sex.  However, in some species the line between these two systems is blurred, suggesting that switching between systems is easier than you would think.  This study found that the Tasmanian snow skink (Niveoscincus ocellatus) has both types of sex determination, and all it has taken to switch between the two is a shift in climate. 

The snow skink lives in both the warm lowlands and cool highlands of Tasmania.  The study found that in the highlands, the skink uses chromosomal sex determination, producing an equal ratio of male and female offspring regardless of the temperature.  However, in the lowlands its sex determination is temperature-dependent – cool cloudy days produce males, and warm, sunny days produce females.  The researchers speculate that the divergence in sex determination mechanisms was caused by temperature differences, enabling the lizards to maximise their reproductive output in the differing climates. 

The paper is here, and ABC news has a report on this here.

Rare giant gecko turns up (dead) in mainland sanctuary Hilary Miller Apr 22

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Here’s one from the good news but bad news file:  The good news is that a Duvaucel’s gecko (Hoplodactylus duvaucelii) has been found on the New Zealand mainland for the first time in nearly 100 years.  The bad news is that it was found dead in a mouse trap.

Duvaucels geckos are the largest of our native geckos, and one of the biggest geckos in the world, growing to up to 30cm in total length.  They are found on a number of offshore islands off the north-east of the North Island and in Cook Strait, and were thought to be extinct on mainland New Zealand.  The last recorded sighting of this species on the mainland was near Thames in the 1920s, but subfossil remains have been found on both the North and South Islands, suggesting it was once widespread across the country. 

The dead gecko was found at Maungatautari, in the Waikato.  Maungatautari is a 3400 ha nature reserve ringed with a predator-proof fence, making it the largest pest-free area on the mainland.  Many rare species have been released into the sanctuary, including kiwi, kaka, takahe and hihi, and reintroductions of many more species are planned.  The Duvaucel’s gecko find suggests that there is a remnant nautral population of the species in the sanctuary, which somehow survived the years when the area was overrun with introduced predators. The hunt is now underway for more of the geckos (which will hopefully be found alive).

This discovery shows that you never know what you might find when you protect an area instead of mining it.

More on the discovery on Stuff.

New species of giant lizard ’discovered’ in the Phillipines Hilary Miller Apr 08

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In Biology Letters this week is the report of a new giant monitor lizard discovered in the Phillipines.  Varanus bitatawa is 2m long, brightly coloured and has a double penis, and lives high up in the trees on the island of Luzon. 

It always amazes me when new species of large vertebrates are discovered in this day and age, when you would think that the majority of the world has been given a thorough going-over, and that 2m long lizards would have been noticed.  Actual new discoveries – as in “thats the first time we’ve seen THAT animal”, as opposed to an organism thats been known about for years but only been named as a separate species on the basis of DNA analysis – are pretty rare these days.  So I was slightly disappointed to find out that Varanus bitatawa is only a new discovery from a western scientific perspective - Filipino tribal hunters have of course known about it for years. 

National geographic has more on the discovery here.

Varabus bitatawa (photo by Joseph Brown)

Origins of NZ skinks revealed Hilary Miller Oct 18

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ResearchBlogging.org Most New Zealanders can name at least a dozen or so species of native bird, but how many can do the same for our native reptiles?  If you starting counting and only got as far as 1. tuatara, you’re probably not alone.  Although we are missing some of the major groups of reptiles (like snakes and alligators), we do have a diverse array of lizards.  In fact New Zealand has around 80 different lizard species in two major groups – geckos and skinks (tuatara are not lizards, they are Sphenodontids). 

Around half of our lizard species are skinks.  These are the most commonly encountered native reptiles, being the species most likely to be spotted disappearing under rocks or into long grass on a hot day, and generally being favoured by the domestic moggy.  Now new research is improving our understanding of the origins and evolution of our skink fauna, with some exciting fossil finds and the publication of a comprehensive genetic study. 

David Chapple (formerly of Victoria University of Wellington, now at Monash University) and colleagues have just published a molecular phylogeny for New Zealand’s skink fauna, which investigates the relationships among 32 of our skink species and their closest relatives from Lord Howe Island, New Caledonia and mainland Australia. 

New Zealand’s skinks were previously grouped into 2 separate generaCyclodina and Oligosoma.  Oligosoma species are found throughout New Zealand, are diurnal and have pointed heads and long limbs and toes.   In contrast, Cyclodina species are nocturnal or crepuscular, have squarer heads and bodies, and relatively shorter limbs and toes.  However, Chapple’s genetic work shows that there are in fact 8 different clades of skinks that likely evolved from a single ancestral species after it colonised New Zealand from New Caledonia.  There is no clear division between Cyclodina and Oligosoma, suggesting that the differences in morphology that separate these two genera have evolved on multiple occasions.  Chapple’s paper thus spells the end for Cyclodina as a recognised genus – all New Zealand skinks (plus their closest relative C. lichenigera from Lord Howe Island) are now under the genus name Oligosoma.

Oligosoma alani, one of nine species renamed from the genus Cyclodina

Oligosoma alani, one of nine species renamed from the genus Cyclodina

Genetic studies like this can also give us a better idea of how long ago a species diverged from its common ancestor.  By employing a molecular clock, calibrated against a couple of known timepoints (e.g. known fossil ages or timing of islands emerging), researchers can relate the number of changes in DNA sequence between two species to evolutionary time.  Chapple and colleagues used this method to estimate that skinks first colonised New Zealand 16-22 million years ago.  This date conflicts with previous studies, which suggested a much more recent arrival less than 8 million years ago, but fits with some recent fossil finds from central Otago.  The St Bathans area of central Otago is proving to be a goldmine for early miocene (16-19 million yrs ago) fossils, and a recent paper by Lee and colleagues at the University of Adelaide, Te Papa and Canterbury Museum documents several fossil lizard finds that indicate an Oligosoma-like species was present in New Zealand by 16 million years ago. 

Both the genetic work and the St Bathans fossils point to skinks colonising New Zealand not longer after the “oligocene drowning”, a period 25-35 million years ago when  much of the present-day New Zealand landmass was underwater.  Chapple suggests that after diverging from their New Caledonian cousins, ancestral NZ skink species may have survived on now-submerged volcanic islands along the Lord Howe rise and Norfolk Ridge before reaching New Zealand.  Of course, this scenario requires skinks to have dispersed across large distances of open water, which you may think would be a problem for a non-flying, terrestrial vertebrate.  But this is not as unlikely as is sounds - many of our skink species live in coastal areas, amongst material that is often swept out to sea during storms.  They have also been observed swimming in rock pools, and can stay underwater for up to 20 mins.  So its not unreasonable to think they could survive a trip across the ocean on a raft of kelp or driftwood.

Two of New Zealand's coastal skink species, O. smithii (left), and O. suteri (right)

Two of NZ's coastal skink species, O. smithii (left), and O. suteri (right)

The work of Chapple and colleagues has also resulted in the revision of a number of individual species names for NZ skinks - check out the papers for yourself if you want the details.

Chapple, D., Ritchie, P., & Daugherty, C. (2009). Origin, diversification, and systematics of the New Zealand skink fauna (Reptilia: Scincidae) Molecular Phylogenetics and Evolution, 52 (2), 470-487 DOI: 10.1016/j.ympev.2009.03.021

Lee, M., Hutchinson, M., Worthy, T., Archer, M., Tennyson, A., Worthy, J., & Scofield, R. (2009). Miocene skinks and geckos reveal long-term conservatism of New Zealand’s lizard fauna Biology Letters DOI: 10.1098/rsbl.2009.0440

For more about the aquatic abilities of skinks see also: K Miller (2007) Taking the plunge. Forest and Bird 326: 20-22.

Pesky reptiles confuse palaeontologists Hilary Miller Sep 17

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This from a letter to Nature, in the latest edition:

Could Nature have been unknowingly publishing papers for the past 80 years about crocodilian gastroliths (stomach stones) instead of stones concluded to be 2.5-million-year-old hominid tools? This possibility could cast doubt, for example, on the nature of the Oldowan specimens described by Michael Haslam and colleagues in their Review of primate archaeology (Nature 460, 339—344; 2009).

…Identification of the Oldowan specimens as tools is based on the fact that the soft relict sands of Olduvai Gorge contain no natural stones of their own, so any stone found there must have been moved from distant river beds by some unknown animal transporter – concluded by high science to be Homo habilis. But crocodiles have the curious habit of swallowing rocks: these account for 1% of their body weight, so for a 1-tonne crocodile that’s 10 kg of stones in its stomach at all times. Surprisingly, science has never even considered the crocodile as transporter.

Read the full version here

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