The geographic dispersion of many terrestrial species across large expanses of
the oceans is often diffiGult to envisage, but this has undoubtedly occurred among
the reptiles (DARLINGTON, 1957), a feat that must largely reflect their relative
osmotic independence from their environment. Reptiles are thought to have
originated in the old world and migrated to the new world, with a subsequent
pilgrimage from South America to Australia. DARLINGTON states that these
movements can be explained 'without resorting to special land bridges or
continental drift' presumably by making transoceanic voyages. Fossil representatives of an extinct group of land tortoises (Meiolaniidae) from South America have
been found on Lord Howe Island and Walpole Island in the eastern Pacific Ocean.
It seems likely that these reptiles floated across the many hundreds of kilometers
of ocean to occupy these remote islands . Turtles have a remarkable propensity to
survive such conditions. When at Duke University I placed three turtles, Pseudemys scripta, in a tank of sea-water, two of these animals died within 14 days but
the third was still alive after 31 days. I replaced it in fresh water after this time,
and it proceeded to carryon with its normal existence. Among the snakes, the genus
Natrix has a remarkably wide geographic distribution and is found in Europe, Asia,
Africa, Australia, America and islands such as Cuba and Madagascar. These snakes
are primarily aquatic, but have probably attained their cosmopolitan status by
crossing the seas. PETTUS (1958) has .cornpared the fluid metabolism of two races
of Natrix sipedon living in the southern United States. One of these groups normally lives in fresh water and the other in brackish water; but the former cannot
usually survive transfer to a saline medium. The success of such adaptation seems
to reside solely in a predisposition to refrain from drinking salt solutions. Providing
that reptiles maintain the relative impermeability of their integument, dispersal of
terrestrial species through the seas is physiologically conceivable.
Reptiles have a water content equivalent to about 70% of their body weight,
an amount similar to that of birds and mammals, though less than that of the Amphibia. The concentration of the body fluids conforms to the usual tetrapod pattern, being about 250 to 300 m-osmole/l. Variations in the concentration and electrolyte content of the body fluids do, however, occur; reptiles in a marine environment have a slightly higher plasma concentration than those living on land (Table
5.1) though they remain hypoosmotic to sea-water. Reptiles can tolerate quite
large changes in the concentrations of their body fluids; the sodium concentration
in the plasma of the lizard, Trachysaurus rugosus, may rise from 150 to nearly 200
m-equiv/l during the dry Australian summer (Table 5.1) and comparable increases
have also been observed in the lizard, Amphibolurus ornatus, and the desert tortoise
(BRADSHAW and SHOEMAKER, 1967; DANTZLER and SCHMIDT-NIELSEN, 1966). Decreased plasma electrolyte levels can also be readily tolerated; the softshell turtle,
Trionyx spinifer, normally has a plasma sodium concentration of about 150 mequiv/l, but in healthy hibernating individuals this may decrease to 80 m-equiv/l
(Table 5.1). Birds and mammals do not seem to be able to withstand such variations
in the concentrations of their body fluids, but in some reptiles such tolerance may
be an important factor in their ability to survive adverse conditions when the possibility of osmotic regulation is limited.
Reptiles are poikilotherms and so, unlike birds and mammals, do not utilize
evaporative water loss for thermal cooling. Nevertheless, they do attempt to
136
the oceans is often diffiGult to envisage, but this has undoubtedly occurred among
the reptiles (DARLINGTON, 1957), a feat that must largely reflect their relative
osmotic independence from their environment. Reptiles are thought to have
originated in the old world and migrated to the new world, with a subsequent
pilgrimage from South America to Australia. DARLINGTON states that these
movements can be explained 'without resorting to special land bridges or
continental drift' presumably by making transoceanic voyages. Fossil representatives of an extinct group of land tortoises (Meiolaniidae) from South America have
been found on Lord Howe Island and Walpole Island in the eastern Pacific Ocean.
It seems likely that these reptiles floated across the many hundreds of kilometers
of ocean to occupy these remote islands . Turtles have a remarkable propensity to
survive such conditions. When at Duke University I placed three turtles, Pseudemys scripta, in a tank of sea-water, two of these animals died within 14 days but
the third was still alive after 31 days. I replaced it in fresh water after this time,
and it proceeded to carryon with its normal existence. Among the snakes, the genus
Natrix has a remarkably wide geographic distribution and is found in Europe, Asia,
Africa, Australia, America and islands such as Cuba and Madagascar. These snakes
are primarily aquatic, but have probably attained their cosmopolitan status by
crossing the seas. PETTUS (1958) has .cornpared the fluid metabolism of two races
of Natrix sipedon living in the southern United States. One of these groups normally lives in fresh water and the other in brackish water; but the former cannot
usually survive transfer to a saline medium. The success of such adaptation seems
to reside solely in a predisposition to refrain from drinking salt solutions. Providing
that reptiles maintain the relative impermeability of their integument, dispersal of
terrestrial species through the seas is physiologically conceivable.
Reptiles have a water content equivalent to about 70% of their body weight,
an amount similar to that of birds and mammals, though less than that of the Amphibia. The concentration of the body fluids conforms to the usual tetrapod pattern, being about 250 to 300 m-osmole/l. Variations in the concentration and electrolyte content of the body fluids do, however, occur; reptiles in a marine environment have a slightly higher plasma concentration than those living on land (Table
5.1) though they remain hypoosmotic to sea-water. Reptiles can tolerate quite
large changes in the concentrations of their body fluids; the sodium concentration
in the plasma of the lizard, Trachysaurus rugosus, may rise from 150 to nearly 200
m-equiv/l during the dry Australian summer (Table 5.1) and comparable increases
have also been observed in the lizard, Amphibolurus ornatus, and the desert tortoise
(BRADSHAW and SHOEMAKER, 1967; DANTZLER and SCHMIDT-NIELSEN, 1966). Decreased plasma electrolyte levels can also be readily tolerated; the softshell turtle,
Trionyx spinifer, normally has a plasma sodium concentration of about 150 mequiv/l, but in healthy hibernating individuals this may decrease to 80 m-equiv/l
(Table 5.1). Birds and mammals do not seem to be able to withstand such variations
in the concentrations of their body fluids, but in some reptiles such tolerance may
be an important factor in their ability to survive adverse conditions when the possibility of osmotic regulation is limited.
Reptiles are poikilotherms and so, unlike birds and mammals, do not utilize
evaporative water loss for thermal cooling. Nevertheless, they do attempt to
136