After nesting, adults disperse to feeding grounds. Outside of the
Mediterranean, migratory routes are not clearly defined although
hatchlings are thought to follow the warm currents such as the Gulf
Stream. Hatchlings and juveniles often congregate at sea fronts
(the meeting of oceanic currents) where food and floating animals
gather. It has been suggested that they get trapped in these currents
and only emerge back on their natal beaches once the circuit has
been completed - mature and ready to breed!
Loggerheads can migrate across large distances: one that had nested
and been tagged on a beach in Florida was recaptured less than 10
months later, 2,400 km away in the Dominican Republic!
The time elapsed (a year or more) that the hatchlings stay feeding
in these areas is called the Lost years. They are thought to drift
within the large mats of sargassum weed.
Mediterranean loggerheads rarely leave their enclosed sea and are
thought to be a genetically isolated population. There is some evidence
of recruitment via Gibralter and Suez, although this is probably
very small. Loggerheads are found all over the Mediterranean although
most activity occurs in the Eastern part of the basin and there
is firm evidence that there are important wintering areas located
off the south east coast of Turkey (Groombridge).
Outside
of the Mediterranean, there are several major nesting grounds. The
world’s major nesting sites are located in South-eastern USA
with annual nest counts of 28,000 and Oman with 30,000. Elsewhere,
there are 2,000 nests in Brazil, 1,000 in South Africa and 2,000
in Australia: smaller nesting sites also exist in numerous other
locations. Nests in the Indian Ocean are virtually unknown.
In the Mediterranean, the loggerhead is the most common nesting
turtle, with Greece hosting the highest nesting populations of more
than 2,000 individuals. Sekania beach on Zakynthos Island (Greece)
has the highest density of nests anywhere in the world. Recent surveys
have also shown significant nest numbers in Turkey and Libya.
Nesting periods
This usually occurs in spring or summer, depending upon location.
o Caribbean - April to early August
o NW Atlantic - April to September
o SW Atlantic - April to August
o Eastern Mediterranean - June to September
o Senegal - July to October
o South Africa - October to February
o China - April to August
o Australia - October to April
Nesting
behaviour
The female usually come ashore at night (records suggest that this
may well have once happened during the day in undisturbed sites).
Loggerheads are said to be highly philopatric: returning to the
same stretch of coastline to nest each year, probably to the same
area where they originally hatched. Some populations are more site
specific than others. Nesting sites are only found on sandy, shelving
beaches. The nest is usually made at the top of the beach and about
45 cm deep and flask shaped. After laying, the turtle will cover
the nest with sand and attempt to camouflage by scattering sand.
She will then return to the sea. Loggerheads have a characteristic
track as shown in the picture below.
Feeding
changes with age but loggerheads are mainly carnivorous. Loggerheads
will eat almost anything and show little real food preferences.
They have powerful jaws and can deal with many food types. Their
diet has been shown to include: jellyfish, crustaceans (lobsters,
crabs and shrimps), molluscs (clams, mussels, conchs etc) and encrusting
animals attached to reefs and rocks.
Some stomach content analysis has revealed weed and even plastic
bags mistaken for jelly-fish. Oil droplets have also been found
in the stomachs of hatchlings which have possibly been confused
as potential food particles (Wietrich, B. 1994).
Most females nest between 2 or 3 times in a season, with a gap of
about 14 days between each laying. They lay 40 to 190 eggs, depending
on the individual. A single female could lay 560 eggs per season.
The reproductive cycle using involves laying every two or three
years although some will lay every year. Clutch sizes vary greatly
as shown below.
o USA (N. Carolina) mean 123: 86-159 eggs: n=36
o USA (S. Carolina) mean 126: 64-198 eggs: n=71
o USA (Georgia) mean 120 eggs: n=2827
o USA (Florida) mean 107: 53-174: n=1928
o USA (Merrit Island) mean 12: 82-173: n=64
o Greece (Zakynthos) 52-114 eggs
o Turkey (Med. coast) mean 93: 55-160: n=50
• Egg size is usually proportional to the size of the turtle
e.g. smaller eggs from smaller turtles.
• Mean diameter ranges from 34.7-55.2 mm.
• Eggs in one clutch are very similar in size although a few
can be smaller. This contrasts to the Leatherback Sea Turtle, which
has a great variation in egg size within one clutch.
Incubation period
Incubation time varies with beach latitude.
o USA (Florida) mean 68 days (Lat-26)
o Mexico (Quintano Roo) mean 56 days (Lat-19)
o Turkey: mean 57 days (Lat-36)
o Greece (Zakynthos) mean 57 days (Lat-38)
o South Africa (Tongaland) mean 68 days (Lat-30)
o Japan (Hiwasa) mean 58 days (Lat-34)
There is some considerable degree of variation on any one beach,
due to local environmental conditions. Some sites also show considerable
variation from one year to the next.
Size
and weight of hatchlings
Size and weight of the hatchlings correlates directly to the egg
size. The mean Straight Carapace Length (SCL) ranges from 33.5-55
mm. Weights vary between 18.8-21.1 g.
Hatchlings
swimming out to sea - the Lost Years
(continued from Eggs & Hatchlings box…)
Hatching
Hatchlings emerge at night, with a peak time between 21:00 and 02:00
hours. On cool cloudy days this time-period will be longer. Together,
the hatchlings dig their way out of the nest. Usually emerging at
night, the group makes its way down the beach and enters the sea.
This race to the sea is important for the hatchlings' biological
cycle.
Sex determination
Incubation occurs between 26-32°C. Evidence that sex determination
is temperature dependent.
Male-biased in the cool. Key or pivotal temperature in which sex
ratio is 1:1 is 30°C for the Loggerheads. There is some evidence
that this key temperature varies from one location to another.
The age of maturity is not clear but research in captive specimens
puts it between 6 and 20 years. Figures taken from the wild
involving back calculations (recapture data from tagged females)
suggests 12 to 30 years to maturity. The main difference probably
depends on latitude and food availability .
Courtship and Mating
Unlike most other sea turtle species, courtship and mating
is not performed near the nesting beaches but during migration
from the feeding to breeding grounds. Copulation is most accomplished
whilst floating but does occur underwater. Several matings
make take place and there is evidence that sperm from several
males may be stored in the oviducts. All eggs for one season
can be fertilised from this stored sperm. Mating normally
takes place a few weeks before laying.
n.b. Observations in Dalyan ( June Haimoff, Conservationist),
Turkey and in Kefalonia, Greece suggest that males do move
close in shore to mate and even in fresh water lagoons linked
to the sea. Reports from Kefalonia suggest that males seem
to set up 'mating' areas which might further suggest some
selection process (Kefalonia Marine Turtle Project - Tom Stringell)
Mortality
Egg mortality and predation
In some locations, predation on eggs can be extremely high as shown
by work carried out on Dalyan Beach in Turkey (Erk'akan, 1991).
Out of an estimated 17,254 eggs laid, 12,078 and 1,725 were predated
by foxes and ghost crabs respectively. This predation rate is estimated
as high as 70%. On the Northern shores of Cyprus, predation rates
are much lower - 38% predated, 8% hatched and predated and 31% unknown
(Broderick, 1995) .
Loggerhead turtles are more prone to predation than Green turtles
because their eggs are located closer to the surface of the beach.
Hatchling mortality and predation
Lights near the beach disorientate hatchlings, causing them to wander
in the wrong direction. If this happens, they may die of dehydration
or are eaten by predators. The highest mortality occurs at this
stage. This route to the sea is made more difficult by vehicle tracks
and sand pits made during the day by tourists. Hatchlings are too
small to climb out of the ruts and soon die of dehydration in the
hot sun of the morning. Many eggs are destroyed by natural activities
such as erosion or sea over-wash, while others are destroyed in
the nest by bacteria and fungi, or predators such as raccoons (In
Florida, 40-50% of the clutch is destroyed by raccoons, skunks etc).
In Zakynthos (Greece), there has been some damage caused by plant
roots entering nests and from other nesting females damaging nests
on high density laying beaches, such as Sekania. Mortality varies
greatly from site to site.
Little is known about predation on juveniles and adults but they
are generally too large and well armoured for most predators. However,
large carnivores like sharks do take turtles. When attacked, adults
present their flanks to the predator to prevent biting.
There have been few quantitative stomach content studies on most
sharks although qualitative studies have shown that turtle parts
are common in big sharks such as the tiger shark. One recent study
showed a link between the increase in viral infections of sea turtles
and the decline in shark numbers.
Monk seals in Greece are also predators of loggerhead turtles, although
the numbers they kill are small (Margaritoulis.D, 1995).
Commensals and disease
Loggerheads carry more epibiontic animals than the other species
of Sea Turtle. There is one barnacle which is specific to the loggerhead,
pictured below:
Recent work has suggested that barnacles
can cause harm to the turtle and there is a possible correlation
between the health of the turtle and the number of barnacles it
carries on its carapace. (e-mail via the CTURTLE network, Beasley
J, sept. 1996). Leeches can cause skin damage and secondary infection
(this may cause the tissue to degenerate, which is then known as
a papillomae).
The most important recent disease of sea turtles, especially Greens,
is a disorder known as fibropapillomatosis. The disease is characterised
by one or more fibrous tumours, which are located on areas of soft
skin. The tumours are debilitating and can prove fatal. The cause
is unknown but a viral infection is suspected. Research is underway
in a number of Universities. The disease is becoming more common.
