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Croaking Science: Portuguese man-o-war and leatherback turtle wash up in Cornwall

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As things calm down after the recent storms, Cornwall has had a number of interesting strandings: in mid-October there were reports of Portuguese Man-O-war jellyfish stranded on beaches near Porthleven. 

Froglife Conservation & Science Manager, Andrew Smart, came across them on nearby Loe Bar.  “I think we probably saw about 10 gas floats along about 500m of beach; they were still really pink so they’d not been on the shore long; you could see the blue base where the trailing tentacles would have attached.” 

The Portuguese man-o-war isn’t actually a jellyfish, but a colonial organism made up of individual ‘zooids’ that have different functions in the colony, such as digestion and stinging tentacles. Recently there have been more regular strandings along the coasts of Cornwall. The gas filled floats are caught by the wind and the man-o-war floats across the ocean following winds and currents, trailing its stinging tentacles behind it.

The oceanic predators of Portuguese man-o-war include the sunfish, which also occurs off Cornwall, and sea turtles, including the UK’s largest reptile, the leatherback turtle. The leatherback is a member of the British Fauna and a UK protected species, with animals in UK waters migrating between the UK and the Caribbean Sea where they nest (see our Croaking Science article here for more information)

Last week, a dead leatherback turtle washed up on Perranporth beach in North Cornwall It was quite badly decomposed and was collected by the Marine Strandings Network and taken away for further analysis. Often leatherbacks die because they swallow floating plastic bags they mistake as jellyfish and investigation of the dead animal will help us understand the extent of this problem. 

This animal measured 1.66m, which sounds large, but the largest recorded, washed up in Wales in the 1908s, was measured at 2.9m (9½ ft) long!  Its quite late in the year for a leatherback stranding but the sea temperature this year off Cornwall is around 13-15  ͦC, higher than the average for November (11- 12  ͦC) and warmer temperatures may have resulted in more jellyfish and salps for leatherbacks to feed on.

At the start of 2023 there were a high number of strandings of other species, small loggerheads and Kemps ridley’s. Sadly, many of these small turtles die from cold shock because they are not adapted to our colder waters like the leatherback, but if they are found quickly enough they can be nursed back to health and released.

Leatherback turtles are stranded and sighted off our coasts every year:

Year

Strandings & sightings

2018

17

2019

15

2020

9

2021

17

2022

9

Data from Marine Environmental Monitoring Turtle Strandings & Sightings Annual Reports 2018-2022

If you’re out walking our western beaches keep an eye out for stranded turtles. They can be stranded anywhere although mainly on the west and south coasts, as can be seen from this diagram of strandings and sightings over the last 10 years, recorded by R.S. Penrose and M.J.B. Westfield in the May 2023 Marine Environmental Monitoring British & Irish Marine Turtle Strandings & Sightings Annual Report 2022

Reptiles Rule!

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Written by Chloe Davey, Transforming Lives Trainee.

In the UK we have a small, but lovely, variety of reptiles, there are seven species to be precise. Three snakes (smooth, grass and adder), three lizards (common, sand and slow-worm) and even the big, darling leatherback turtle.

Each of these reptiles are unique, yet similar to one another. They all start life as eggs, some are laid in a safe area to hatch and others are incubated inside their mother’s body and are born live. I would love to spotlight these truly amazing reptiles and share my passion for them. After all, there are more to reptiles than people think!

It can be quite difficult to spot reptiles when you are on a walk as they like quiet, secluded areas away from people. Warm mornings are your best chance at seeing these amazing creatures as they bask. Being cold-blooded, they rely on the sun’s rays to warm their bodies and give them energy for the day ahead. Some will hunt on land while others will hunt in the water. For example, grass snakes are excellent swimmers and hunt for amphibians, small fish and small mammals. That is why they are often seen in gardens, usually those with ponds.

Leatherback turtles, despite not breeding in the UK, are considered native and spend most of their lives in the water. Males will never leave the water once they enter, they breed in the water and the females will emerge on land to lay eggs.

Later in the year as the weather gets colder, reptiles will settle down to brumate, which is like a hibernation for reptiles. Instead of sleeping all the way through the winter, reptiles can still awaken every so often to feed and even roam about. Without the sun’s warmth they will become more tired and slow.

It is so incredibly fascinating to know that there are currently more than 10,000 species of reptiles in the world and an additional 2,700 subspecies. It is so important to protect the native species we have here.

Hopefully together we can help increase the population and protect our native species!

Please download our free Dragon Finder app to record your sightings on reptiles in the UK.

Cultural Artistic Representations of Reptiles

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Written by Kaitlin Oliver, Transforming Lives Trainee

Throughout history, reptiles have found their way into many different forms of art. All cultures have represented these animals through arts such as paintings, pottery and tattoos. Each culture have their own beliefs on what reptiles mean to them.

In aboriginal art, lizards are often painted in dotted mosaics. Belief says that lizards are part of the Dreaming Stories that relate to the creation of the natural world and to the role of Ancestors and humans in that world.

Dragons are seen in many cultures on all forms of art. In Chinese culture, the dragon is a mythical creature often depicted as a serpent-like being with the ability to fly. It is seen as symbol of power, strength, and good luck. It’s a central figure in Chinese art, especially in traditional paintings and sculptures.

A large petroglyph resembling a lizard at Dinosaur National Monument, Utah, USA

Many Native American tribes created rock carvings (known as petroglyphs) featuring reptiles, such as snakes and turtles. These images often held spiritual and symbolic significance, connecting the people to their natural surroundings.

In the modern tattoo word, reptiles continue to be a source of inspiration. Reptilian motifs, such as snakes and dragons, are popular choices in tattoo art across various cultures. They can symbolize protection, transformation, or personal strength.

Showcasing the diversity of cultural artistic representations of reptiles, you can see that reptiles have held a complex and multifaceted place in human culture, from ancient mythologies to contemporary expression. Reptiles often symbolize both awe and fear, power and danger, and the natural world’s intricate beauty.

 

Reptile Awareness Day

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Written by Jade Walton, Transforming Lives Trainee

The 21st October marks Reptile Awareness Day, and what better way to celebrate than to learn some interesting facts about our six native reptile species! 

In the United Kingdom we have six native reptiles, three snakes and three lizards.

Our snakes are:

Grass snakes

Smooth snakes

and adders

Our lizards are:

Common lizards

Sand lizards

and slow-worm

Each of these reptiles have interesting facts about them that easily set them aside from their other reptile counterparts. 

Firstly, lets look to our snakes. Adders are our only venomous species, though not particularly dangerous to most humans, it is best to keep your dog on a lead when walking in areas inhabited by adders. Adders are surprisingly small, usually measuring between 60cm and 80cm when fully grown. Females birth live young that are less than 20cm long. 

Next, we come to the smooth snake. Just like their adder relatives, smooth snakes are quite short in length, usually growing to around 60-70cm. These are the rarest of our UK reptile species, only to be found in specific heathland sites. Smooth snakes are also the only snake in the UK to constrict their prey. 

Our final snake is the grass snake. Easily the UK’s longest snake growing to around 150cm, they are also the only native snake to lay eggs rather than birth live young. This is a contributing factor as to why they are not found in Scotland as they need warm enough temperatures through the summer months for their eggs to incubate. 

Adder
Smooth snake
Grass snake

Our other three reptile species are types of lizard. Sand lizards are another rare UK reptile as their habitats are greatly limited to certain sandy heathlands. The sand lizard is our only lizard in the UK to lay eggs, which they bury under sand in sunny areas to allow for them to incubate. Like our other lizards, they can drop their tails as a defence mechanism against predators. 

Common lizards are widespread throughout the UK, though their numbers are thought to be declining due to habitat loss. They are fairly often mistaken for newts as they are quite similar in appearance, the best way to tell the difference is to see the scaly skin of the lizard versus the smooth skin of the newt, they also move a lot faster. 

Our final, and perhaps most confusing reptile is the slow-worm. Many could mistake a slow worm for a snake, given they have no legs, but they are, in fact, a lizard. They are distinguished from snakes by their ability to blink and drop their tails when threatened. ​

Sand lizard (female) (c) Tracy Farrer
Common lizard (c) David Palmer
Slow-worm

 

Turtles in UK waters

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By Andrew Smart, Conservation and Science Senior Manager. 

Since it is National Marine Week (22 July to 6 August, organised by the Wildlife Trusts), it is appropriate to return to our only marine reptile species, the marine turtles, last covered by Croaking Science in April 2021: Should we count marine turtles as members of the British fauna? One species, the leatherback turtle (Dermochelys coriacea), is considered to be a member of the British fauna, while the other species, perhaps best described as hard shelled turtles, are vagrants who have moved away from their normal migration paths and, one way or another, end up stranded on UK beaches Turtles stranded on UK beaches after storms send them off course.

These strandings are generally of loggerhead turtles (Caretta caretta) and more recently, Kemp’s ridley, (Lepidochelys kempii). Occasionally green turtle (Chelonia mydas) and very rarely, hawksbill (Eretmochelys imbricata) and olive ridley turtles (Lepidochelys olivacea) (both single records) are washed up on our beaches1. Loggerheads and Kemp’s ridley originate from the Caribbean, most likely from nesting beaches on the Florida coastline and Gulf of Mexico, from where hatchlings travel across the Atlantic.  The hatchlings spend their early years, often called the ‘lost years’, in the North American basin and many move into the North Atlantic gyre2, travelling across the ocean from east to west, then follow the current south past the Azores and Canary Islands before travelling westward back towards their nesting beaches as juveniles3.

Loggerheads are sometimes killed after interactions with boats, caught in longline fisheries4, tangled in pot lines or in ‘ghost nets’, but many animals suffer from ‘cold stunning’ when they move out of warm water into colder temperatures (10 – 15oC) that cause them to effectively ‘shut down’5. Turtle hatchlings will die at temperatures below 10oC6. Animals that have been ‘cold stunned’ or died are stranded along European coasts and are collected and examined. Any UK strandings of turtles (or any other marine animal) can be reported to the Marine Strandings Scheme (or in Scotland SMASS). Many stranded dead turtles have been found to have frequently ingested plastics (69% of individuals examined) which they may mistake for food 7, 8; jellyfish and salps. Animals stranded in the UK tend to be juveniles, with a carapace measurement of less than 40 cm, and tend to be stranded in the winter months1. In the last three decades numbers of loggerhead and Kemp’s ridley turtles have increased, with more than twice the number of animals stranded when compared to the 1980s1.  

Genetic analysis of loggerheads stranded in France indicate that as well as animals from Florida and the Caribbean, some also come from the nesting beach on the Cape Verde Islands,  from where animals are occasionally blown north by storms along the Portuguese and Spanish coasts9 and into the Bay of Biscay.  Loggerheads nesting in the Mediterranean move in and out of the Straits of Gibraltar along with animals from the Caribbean10, which move inwards through the ‘Straits’ and are sometimes recorded off the coast of Algeria. Mediterranean turtles would be unlikely to move northwards into colder water around the UK. However, recent work suggests that there may be a population of juvenile loggerheads using the Bay of Biscay as feeding grounds during warmer periods11.  

Kemp’s ridleys also come from the Gulf of Mexico and are being recorded more frequently as UK strandings in the last three decades. Very occasionally there are strandings of olive ridley, hawksbill and green turtle1.  Green turtles stranded on French coasts have been from African populations not from the Mediterranean12, where the nesting green turtle populations in the east generally travel along the North African coast to feed before returning to nest in Turkey and Syria13.

The leatherback turtle is the largest turtle species; the biggest recorded washed up on a Welsh beach in the 1980s, weighed over 900 kg and was 2.9 m (9 ½ ft) long14. Leatherbacks feed mainly on jellyfish and other gelatinous plankton, such as salps, and spend a lot of their time feeding because this prey is not very nutritious. Strandings are often linked to plastic ingestion because of the similarity to jellyfish, their main prey species. The animal has a remarkable throat structure with backwards pointing barbs that holds jellyfish in place until it’s swallowed14.

Leatherbacks are regularly seen off the UK and Irish coasts and these North Atlantic animals have migrated from their nesting sites along the coast of northern South America and some Caribbean islands15. Females nest at these rookeries, often on a 2 yearly cycle, and animals follow one of three different migration paths based on directions taken by tracked animals. Animals from the North Atlantic population migrate northwards up to the coast of Nova Scotia in Canada, eastwards across the Atlantic towards the Azores and the coast of Africa or north eastwards up to the Atlantic coast of the island of Ireland, the Irish Sea and the Celtic Sea16, 17, where they find jellyfish swarms on which to feed18. They have been recorded as far north as Norway14 and can survive in these cold waters because they are able to generate their own body heat and being large, once warm they take longer to cool down.  They also have heat conservation and heat exchange mechanisms that enable them to feed actively in waters below 15oC, temperatures that would cause ‘cold stunning’ to other turtle species. Leatherbacks that are found further north tend to be larger than those off the Spanish and French coasts19, 20.

The continental shelf around Europe and off Canada are ideal sites for their main food, the barrel jellyfish, Rhizostoma pulmo, but they will feed on other gelatinous or jelly-like plankton. Barrel jellyfish can reach 90 cm in diameter and ‘blooms’, large congregations, of jellyfish can occur when conditions are ideal and food (plankton) is plentiful21.  A leatherback filmed off the Canadian coast22 was found to eat over 50% of their entire annual energy requirement in the 90 days that they spent in northern waters and this is likely to be the case off the UK in the Celtic and Irish seas. Leatherbacks are probably moving into high quality feeding areas where the location of a feeding ‘patch’ will provide them with lots of energy at low cost 23,  24. Unfortunately, some of their foraging areas in the Atlantic align with existing long line fisheries and this has an impact on the population, with estimates in the year 2000, of a leatherback bycatch of between 30,000 and 60,000 animals from both north and south Atlantic populations combined4, 25.

After feeding through the summer the leatherbacks head back south-westerly and generally follow a straight line migration path back to their nesting beaches26. Scientists have been able to track animals leaving nesting beaches and also animals that have been caught in fishing gear off the UK coast and found that when they’re feeding in northern waters they tend to stay close to the surface, feeding in localised patches, suggesting feeding in jellyfish swarms in the surface waters21, 27, 28, 29.  Tracking results indicate that when they travel to and from their nesting beaches they swim much faster and tend to be more prepared to undertake deeper dives to find food29, 30.

How has this leatherback migration pattern developed? One suggestion is that leatherback hatchlings drift out of the gyre, get caught up in the North Atlantic current and drift northwards during the so-called hatchling/juvenile ‘lost years’, finding areas where foraging is high-quality17. Having returned to nesting grounds, they are able to find these sites again as adults, using some form of navigation system15. Whatever the process they use, as long as we have our existing ocean currents, and large jellyfish blooms, we are lucky enough to provide an important feeding area for these migratory turtles, the most widely distributed reptile species in the world.

 

Published work referred to in this article:

1 Botterell ZLR, Penrose R, Witt MJ, Godley BJ (2020) Longterm insights into marine turtle sightings, strandings and captures around the UK and Ireland (1910-2018). Journal of the Marine Biological Association of the United Kingdom 100:869-877

 2 Putman NF, Seney EE, Verley P, Shaver DJ, López-Castro MC, Cook M, Guzmán V and others (2020) Predicted distributions and abundances of the sea turtle ‘lost years’ in the western North Atlantic Ocean. Ecography 43:506-517

3 Monzo´n-Argu¨ello C., Dell’Amico F,. Morinie`re P,. Marco, A Lo´pez-Jurado L. F.,. Hays G C, Scott R, Marsh R and Lee P, (2012) Lost at sea: genetic, oceanographic and meteorological evidence for storm-forced dispersal Journal of the Royal Society Interface 9, 1725–1732

4 Lewison RL,. Freeman S A and. Crowder  L B (2004) Quantifying the effects of fisheries on threatened species: the impact of pelagic longlines on loggerhead and leatherback sea turtles Ecology Letters, 7: 221–231

5 Bellido J, Castillo J, Pinto F, Marti’n J, Mons J, Ba’ez J and Real R (2010) Differential geographical trends for loggerhead turtles stranding dead or alive along the Andalusian coast, southern Spain Journal of the Marine Biological Association of the United Kingdom, 90(2), 225–231.

6 Kettemer L, Biastoch A, Wagner P,. Coombs E J, Penrose R, and Scott R, (2022) Oceanic drivers of juvenile sea turtle strandings in the UK Endangered Species Research 48: 15–29

7 Darmon, G.; Schulz, M.; Matiddi, M.; Liria Loza, A.; Tom, J.; Camedda, A.; Chaieb, O.; El Hili, H.A.; Bradai, M.N.; Bray, L.; et al. (2022) Drivers of litter ingestion by sea turtles: Three decades of empirical data collected in Atlantic Europe and the Mediterranean. Marine Pollution Bulletin , 185, 114364

8 Sala B, Balasch, A, Eljarrat E, Cardona L (2021) First study on the presence of plastic additives in loggerhead sea turtles (Caretta caretta) from the Mediterranean Sea Environmental Pollution 283 117108

9 Nicolau, L., Ferreira, M., Santos, J. et al. (2016). Sea turtle strandings along the Portuguese mainland coast: spatio-temporal occurrence and main threats. Marine Biology 163, 21 https://doi.org/10.1007/s00227-015-2783-9

10 Garofalo L., Mastrogiacomo A., Casale P., Carlini R., Eleni C., Freggi D., Gelli D. Et al.  (2013). Genetic characterization of central Mediterranean stocks of the loggerhead turtle (Caretta caretta) using mitochondrial and nuclear markers, and conservation implications. Aquatic Conservation: Marine and Freshwater Ecosystems, 23: 868–884.

11 Chambault P, Gaspar P and Dell’Amico F (2021) Ecological Trap or Favorable Habitat? First Evidence That Immature Sea Turtles May Survive at Their Range-Limits in the North-East Atlantic. Frontiers in Marine Science. 8:736604. doi: 10.3389/fmars.2021.736604

12 Avens L and Amico FD (2018) Evaluating viability of sea turtle foraging populations at high latitudes: age and growth of juveniles along the French Atlantic coast. Endangered Species Research 37, 25–36

13 Clusa, M., Carreras, C., Pascual, M. et al. (2014). Fine-scale distribution of juvenile Atlantic and Mediterranean loggerhead turtles (Caretta caretta) in the Mediterranean Sea. Marine Biology 161, 509–519 https://doi.org/10.1007/s00227-013-2353-y

14 Beebee, T.J.C. & Griffiths, R.A. (2000) Amphibians and Reptiles. A Natural History of the British Herpetofauna. Harper Collins Publishers, London, UK

15 Dodge KL, Galuardi B, Lutcavage ME. (2015) Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre. Proceedings of the Royal Society B 282: 20143129. http://dx.doi.org/10.1098/rspb.2014.3129

16 Fossette S, Hobson V J.,  Girard C, Calmettes B , Gaspar P, Georges J-Y ,. Hays G C (2010) Spatio-temporal foraging patterns of a giant zooplanktivore, the leatherback turtle Journal of Marine Systems 81 225–234

17 Fossette S, Girard C, Lo´pez-Mendilaharsu M, Miller P, Domingo A, et al. (2010) Atlantic Leatherback Migratory Paths and Temporary Residence Areas. PLoS ONE 5(11): e13908. doi:10.1371/journal.pone.0013908

18 Bailey H, Fossette S, Bograd SJ, Shillinger GL, Swithenbank AM, et al. (2012) Movement Patterns for a Critically Endangered Species, the Leatherback Turtle (Dermochelys coriacea), Linked to Foraging Success and Population Status. PLoS ONE 7(5): e36401. doi:10.1371/journal.pone.0036401

19 Eckert S A. 2002 Distribution of juvenile leatherback sea turtle Dermochelys coriacea sightings Marine Ecology Progress Series 230: 289–293,

20 Witt MJ, Penrose R, Godley BJ (2006) Spatio-temporal patterns of juvenile marine turtle occurrence in waters of the European continental shelf. Marine Biology 151:873-885

21 Houghton J.D.R., Doyle T.K., Wilson M.W., Davenport J., Hays G.C. (2006) Jellyfish Aggregations and Leatherback Turtle Foraging Patterns in a Temperate Coastal Environment. Ecology.;87:1967–1972. doi: 10.1890/0012-9658(2006)87[1967:JAALTF]2.0.CO;2. – DOI – PubMed

22 Wallace, B.P., Zolkewitz, M. & James, M.C. (2018) Discrete, high-latitude foraging areas are important to energy budgets and population dynamics of migratory leatherback turtles. Science Reports 8, 11017. https://doi.org/10.1038/s41598-018-29106-1

23 Eckert Scott A. (2006) High-use oceanic areas for Atlantic leatherback sea turtles (Dermochelys coriacea) as identified using satellite telemetered location and dive information Marine Biology 149: 1257–1267

24 Chambault P, Roquet F, Benhamou S, Baudena A, Pauthenet E, de Thoisy B, et al. (2017). The Gulf Stream frontal system: A key oceanographic feature in the habitat selection of the leatherback turtle? Deep Sea Research Part I: Oceanographic Research Papers, 123: 35–47.

25 Fossette S, Witt, M. J.  et al. (2014) Pan-Atlantic analysis of the overlap of a highly migratory species, the leatherback turtle, with pelagic longline fisheries. Proceedings of the Royal Society B 281: 20133065. http://dx.doi.org/10.1098/rspb.2013.3065

26 Dodge KL, Galuardi B, Lutcavage ME. (2015) Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre. Proceedings of the Royal Society B 282: 20143129. http://dx.doi.org/10.1098/rspb.2014.3129

27 Doyle TK, Houghton, JD O’Súilleabháin, PF Hobson, VJ Marnell, F Davenport, J Hays, GC and Hobson V (2008) Leatherback turtles satellite-tagged in European waters Endangered Species Research: 4, Pages: 23 – 31,

28 Hays GC Farquhar MR, Luschi P, Steven L.H., Tierney T,. Thys M (2009) Vertical niche overlap by two ocean giants with similar diets: Ocean sunfish and leatherback turtles Journal of Experimental Marine Biology and Ecology 370 134–143

29 Dodge KL, Galuardi B, Miller TJ, Lutcavage ME (2014) Leatherback Turtle Movements, Dive Behavior, and Habitat Characteristics in Ecoregions of the Northwest Atlantic Ocean. PLoS ONE 9(3): e91726. doi:10.1371/journal.pone.0091726

30 Hays, G.C. ; Houghton, J; Myers, A.E. (2004) Endanqered species – Pan-Atlantic leatherback turtle movements. Nature. 429(6991). pp. 522-522

 

Reproduction without sex: some mother reptiles can do it on their own.

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Roger Downie, Froglife and University of Glasgow

Recently, media outlets summarised the findings of a paper published in Biology Letters (Booth et al., 2023) that reported the first known case of ‘virgin birth’ in a crocodile. Here, I describe the case and put it into the context of other known examples of reproduction without sex in reptiles.

An 18 year old American crocodile (Crocodylus acutus) had been in captivity, in isolation from other crocodiles, in a Costa Rican reptile park since the age of two. To the keepers’ surprise, it produced a clutch of 18 eggs. These were incubated artificially for 3 months, but failed to hatch. They were then opened up: only one contained recognizable contents, a well-developed baby crocodile, sadly dead. Its gonads showed it to be a female, and genetic analysis demonstrated that it was the offspring only of its mother, with no male input.

The technical term for virgin birth, where a female produces offspring without input from a male is ‘parthenogenesis’, and it can be of two general kinds: a) obligate, where it is the only kind of reproduction occurring in that species; b) facultative, where both sexual and non-sexual reproduction can occur.

How common is parthenogenesis? Obligate parthenogernesis is known in about 80 unisexual species of vertebrates, most of them lizards (Neaves and Baumann, 2011). The best-known examples are the whiptail lizards of Mexico and the southern states of USA, with 12 out of over 40 described species of Aspidoscelis (formerly Cnemidophorus) unisexual; and the rock lizards of the Caucasus mountains, with 7 out of 30 Darevskia species unisexual (Spangenburg et al., 2020). The unisexual lizards are nearly all hybrid clones resulting from matings between related species. The unisexual species have either three (triploid) or four (tetraploid) sets of chromosomes which can be identified in terms of the parental species contributing to the cross. It is believed that the unisexual species overcome any disadvantage from the lack of sexual reproduction through the hybrid vigour derived for their extra sets of genes. In Aspidoscelis, Crews et al. (1986) found that the females of unisexual species show female-female courtship behaviour and pseudo-copulation which enhances ovulation.

Obligate parthenogenesis is relatively easy to detect from the complete lack of males in populations of these unisexual species. Facultative parthenogenesis (FP) is more problematic, partly because many species have long-lived sperm which females can retain in their reproductive tracts after mating. The best evidence therefore comes from captive females kept in the absence of males, as in the crocodile case above. It is the first reported example of FP in any crocodilian, and also the first case from a species with temperature-dependent sex determination. In crocodiles, eggs incubated at low and high temperatures develop as females, with males appearing from incubation at intermediate temperatures: the Costa Rican crocodile’s eggs were incubated at 29-300 C, in the female-determining range. FP has been reported from two captive Komodo dragons (Varanus komodoensis) in Chester and London zoos respectively (Watts et al., 2006). The authors note that Komodo dragons kept for the captive breeding of this endangered species are usually housed as single females, with males brought in when the females are in good condition. Their discovery of eggs laid by unmated females (25 eggs, 8 viable in one case; 4 eggs, one viable in the other) may get in the way of the breeding programme. Komodo dragons have ZZ/ZW sex chromosomes, rather than the XX/XY pattern we are more familiar with in mammals. In ZZ/ZW, males are ZZ and females ZW (in XX/XY, males are XY and females XX). Parthenogenetic development from a female Komodo dragon produces ZZ or WW individuals, with ZZ male, and WW probably non-viable. Booth et al. (2012) published the first report of FP in wild snakes: they collected 22 pregnant copperheads (Agkistrodon contortrix) and 37 cottonmouths (A. piscivorus) and examined the litters for signs of FP (few if any male offspring; high proportion developmental failures). Two litters were selected for molecular analysis, to compare the offspring and maternal genomes, and both indicated that they were the products of only the maternal genomes. The authors note that in both cases, one theory for the occurrence of FP did not fit the facts; it has been suggested that females may use FP to overcome a shortage of males in a population, but in neither case was this true. Although unisexuality with obligate parthenogenesis has some advantages over sexual reproduction, it is not clear why FP should occur as an occasional variant on normal sexual reproduction. The finding of FP in a crocodile means that the only reptile taxon where parthenogenesis is unknown is the Chelonia.

The means by which parthenogenesis works as a reproductive mode can be complex. For example, the entry of the sperm into the ripe egg is usually the trigger for development to begin, so how is development triggered in the absence of fertilization? And what happens to the normal chromosomal reduction divisions of meiosis, when no sperm arrives to restore the diploid number? The references to this article will lead you to some of the answers.

You might also ask: does parthenogenesis also occur in amphibians? So far, it has not been reported, but there are unisexual species of amphibians, though they do not reproduce by parthenogenesis: a story for another day.

References

Booth et al. (2012). Facultative parthenogenesis discovered in wild vertebrates. Biology Letters 8, 983-5.

Booth et al. (2023). Discovery of facultative parthenogenesis in a new world crocodile. Biology Letters 19, 20230129.

Crews et al. (1986). Behavioural facilitation of reproduction in sexual and unisexual whiptail lizards. PNAS 83, 9547-9550.

Neaves and Baumann (2011). Unisexual reproduction among vertebrates. Trends in Genetics 27, 81-88.

Spangenburg et al. (2020). Cytogenetic mechanisms of unisexuality in rock lizards. Scientific Reports 10, 8697.

Watts et al. (2006). Parthenogenesis in Komodo dragons. Nature 444, 1021-2.

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