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The origin of tiger snakes on Carnac Island

Authors:
  • Animal Plant Mineral Pty Ltd

Abstract

Despite considerable research on the origin of the tiger snakes (Notechis scutatus) on the small island of Carnac off the coast of Perth in Western Australia, their source remains a mystery. Small morphological and ecological differences between the Carnac snakes and tiger snakes in other parts of Australia have been taken to suggest an unique lineage, but this is confounded by the possibility of colonisation of the island during the last marine transgression some 7000 years ago. Tiger snakes are also present on nearby Garden Island and elapid snakes are excellent swimmers. On the other hand, anecdotal reports suggest that the snakes were deliberately released on the island in 1930 by a snake handler. Information from the National Archives shows, with high probability, that this island population was established from a small number of snakes released in the early decades of the 20 th century and that the scenario of isolation due to marine transgression does not apply. Now, interpretations of population-specific phenotypic and genotypic variations in the ecology, ecophysiology, demographics of the Carnac Island tiger snakes can be made with more confidence, based on the resolution that the population is less than 100 years old.
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Journal of the Royal Society of Western Australia, 103: 39–42, 2020
© Royal Society of Western Australia 2020
The origin of tiger snakes on Carnac Island
MITCH LADYMAN 1, EARLE SEUBERT 2 & DON BRADSHAW 3*
1 School of Biological Science, Edith Cowan University, Joondalup, WA 6027, Australia
2 Friends of Woodman Point Quarantine, 74 O’Kane Court, Munster, WA 6166, Australia (deceased)
3 School of Biological Sciences, The University of Western Australia, Crawley, WA 6009, Australia
* Corresponding author: don.bradshaw@uwa.edu.au
Abstract
Despite considerable research on the origin of the tiger snakes (Notechis scutatus) on the small
   

of Australia have been taken to suggest an unique lineage, but this is confounded by the possibility

  

            
              
of the 20th      
          


Keywords: phenotypic plasticity, adaptive evolution, island, evolution, ectotherm, reptile, snake,
Notechis scutatus
Revised manuscript received 9 March 2020, accepted 26 March 2020
most proximate island and mainland populations are, in
 et al.
         
of Western Australia and 10 km from the port city of
       
   et al.     
   
      
        
          
       
the last decade and a half (Bonnet et al. 2002; Ladyman
    et al. 2004a,b, 2006; Bonnet
et al. 2005; Ladyman et al. 2006; Aubret & Shine 2007,
         
  
     
       
the mainland and Carnac Island rather than by genetic
     
      
  
        
 et al.

More recently, studies on the Carnac Island snakes
       
drivers for polymorphism: phenotypic plasticity and
       
a basis, Aubret & Shine (2009) concluded that phenotypic
        
INTRODUCTION

       
on islands along the southern coast of Australia and on
the adjacent mainland in contrasting conditions (Shine
        et
al.
        et al.

       

 et al.   
       
     
 

 
      
novel opportunities to investigate adaptive evolution and
exaptation (Gould & Vrba 1982), primarily because of the
      
      
populations have been separated for 5000–7000 years,

          
populations on islands are mainland tiger snakes and the
40
Journal of the Royal Society of Western Australia, 103, 2020
Bathymetry
10 km
Rottnest Island
Garden
Island
WA
500 m
Perth
Fremantle
Rockingham
Cockburn
Sound
Gage Roads
a)
b)
Figure 1b
INDIAN OCEAN
115°32’54”E
32°7’15”S
32°15’S
32°S
115°45’E
115°15’E
Carnac
Island
Figure 1
 
Island and Fremantle, Western
Australia (bathymetric image
  
of Mines, Industry Regulations
and Safety using data from the
    
   

colonise these novel island environments but, over time,
        
         

      
       
       
phase, regardless of their capacity to do so (Aubret &

For the majority of studies referenced herein, the
        
 
      
      
       
their respective habitats and in the absence of genetic
       
      

But for the studies that seek to determine the
role of phenotypic plasticity in novel environments
versus genetic divergence in older populations, there
         
   
although the adaptive advantages of phenotypic
plasticity versus genetic assimilation are obvious, the



snake population have been based on the premise that the
population is less than 100 years old and test individuals
       
population of some 40 snakes released on the island by
       

  
of the snakes and that the population did not arise as an
isolated relic about 5000–10 000 years ago as a result of

required level of certainty about the origin of the Carnac

FEASABILITY OF A NATURALLY-
OCCURRING POPULATION ON THE
ISLAND, 1829–1930
      
of tiger snakes on adjacent Garden Island, and that
       
shore islands, it is perfectly reasonable to expect that
        
         
         
       
  
         
       
       
         
by naturalists also do not document the presence of tiger

THE LACK OF EVIDENCE OF SNAKES ON
THE ISLAND 1829–1930
       

    


         
      
Carnac Island as a delightful and convenient resort only
         
         
41
et al
    

         
       
  
    
       
 1
        
for many years prior to the turn of 20th
          
       
        
      
        
        
        
and tribulations endured, but none of these articles
   
  
  
the presence of a large and highly venomous elapid to
feature prominently in any communications about life
    
        
        
           
the reptile database of the Western Australian Museum
found that the earliest records of tiger snakes from
 

        

EVIDENCE FOR THE RELEASE OF
SNAKES BY ROCKY VANE
       
         
        
         
          
         
further validate our conclusions requires an explanation
     
releasing some 40 adult tiger snakes on Carnac Island
       

     
  
   
      
        
        
    
         

       
       
        
          
        

We collected many articles documenting the almost
       

        
         

  
   
          
      
  
       


       


       
1931), he could easily have replenished his collection in

        

      
         
        
Aubret et al. (2004b) cite a personal communication from
      
         
         
           
not snakes existed on the island prior to their supposed

       
        
validate or verify the story concerning the release of
       
       
     

         

  
and having only been in Western Australia for a very
         
        

     
          
snakes, small enough for him to retrieve them and easy

         
 

         
  


1 
 
 

42
Journal of the Royal Society of Western Australia, 103, 2020
       

CONCLUSION
       
origin of the Carnac Island tiger snakes and give some
certainty as to the question of the age of the population

 


island and mainland populations must have developed in
the intervening 90 years, rather than the many thousands
         
our interpretations are subjective, this suite of archival
         
conclusions of Aubret & Shine (2009) thereby supporting
the notion that phenotypic plasticity is most valuable and
prevalent in novel environments recently colonised by

REFERENCES
     
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        The Western
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         
   The West Australian, Saturday February
20th
Western Mail: Friday
March 20th
     The West Australian Times: Court

      The Daily News (Perth,
WA) th
        The
Daily News, January 15th
 F,  X,  S,  S D & 
          
 Notechis scutatus
Amphibia-Reptilia 25,
 F,  X,  S,  G & 
        
   Notechis scutatus Behavioural Ecology 17,

 F &         
     Journal of
Experimental Biology 213,
 F &       
      
Current Biology 19,
 F & 
in an isolated snake population, Notechis scutatus
Austral Ecology 32,
 F,  R &  
Nature 431,
 X,  F,    M,  D &
        
Ethology 111,
 X & 
Landscope 22,
 X,  D,  M,    
         
  Notechis scutatus) on Carnac Island, Western
Austral Ecology 27,
    Ecophysiology of Desert Reptiles, Sydney,

 S J &   
Paleobiology 8,
           
Australian Heritage Autumn 2007
          
 Biological Journal
of the Linnean Society 63,
 J S,  I A W & 

Evolution 59,
 M & 
        
Notechis scutatus Journal of Comparative Physiology, B. 173,

 S D & 
        
snake, Notechis scutatusJournal of Comparative Physiology, B.
176,
         
10–15 In:Research on Ronest Island.
 

         Notechis
ater:       
Journal of Herpetology 25,
           
    Journal of
Herpetology 24,
 I A W,  J S,  J S & 
      
Notechis
in the closely related genus Hoplocephalus Molecular Ecology
Notes 1,
        
populations of Australian tigersnakes (Notechis 
Herpetologica 43,
... We also collected nine samples from Carnac Island, approximately 7 km off-shore (Fig 1). Carnac Island is a small freshwaterdevoid island (19 ha) with the tiger snake population thought to originate from human introduction approximately 90 years ago, with the suspected source population coming from the nearby mainland [24,36]. Kogolup Lake, Black Swan Lake and Carnac Island were surveyed less than the other sites (a few days compared with several weeks), which resulted in lower sample sizes for these sites. ...
... Considering the small size of Carnac Island (19 ha), we expected the tiger snake population to show low genomic diversity as island populations are renowned for having lower genetic diversity compared to adjacent mainland populations [92][93][94], even when the island introduction is less than 100 years [76]. A large founding population could have resulted in high heterozygosity [92,95], however just~40 adult snakes [36] were speculated to have been released on Carnac Island. Maintaining a large population size over time would also be necessary, as extended bottlenecks in the population size would have led to reductions in genetic diversity [95,96]. ...
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