leatherback life history |
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status |
Status
Despite being listed as critically endangered both internationally and in the US, leatherback nesting populations in the Atlantic are generally considered to be stable. Contrary to this, Pacific populations have recently shown precipitous declines, and there is concern that the population in the Atlantic will follow a similar trend unless measures are taken to identify and manage threats to leatherback survival.
Leatherbacks are the only extant member of the family Dermochelyidae, with all other species of sea turtle being in the family Cheloniidae. One of the most fundamental anatomical differences between the leatherback turtle and the other species of sea turtles is the skeletal composition. In all other species of sea turtle, as they mature, the shell becomes increasingly ossified with dermal bones growing outwards between the ribs to form a hard carapace covered in keratinous scutes. In the leatherback the bone structure of the shell comprises thousands of small interconnected bones embedded in a layer of oil-saturated connective tissue covered with a waxy black skin. These and other structural adaptations (e.g. counter-current heat exchangers) permit the unusual lifestyle of the leatherback, frequenting waters approaching freezing and diving to depths of over 1000m.
Leatherbacks are one of the few apex predators which forage almost exclusively on gelatinous zooplankton. Around the UK leatherback sightings have recently been linked to areas of high jellyfish abundance. Foraging on such a temporally and spatially sporadic prey is the primary driving force behind the annual migration of leatherbacks.
The leatherback turtle is often considered to be a tropical species which annually migrates away from more equatorial climes to cooler waters in search of fields of gelatinous prey. It may, in fact, be more appropriate to consider leatherbacks to be a cold-water species that annually migrates to tropical oceans due to thermal constraints imposed on them during the winter months in more northern climes and, every two or three years, to breed. During the non-breeding season, the oceans around the tropics are generally less productive than in cooler waters. The consistency of productivity in the tropics, however, means that although foraging success will be lower in the topics than during summers in more temperate zones, prey will be available at all times of year. Time spent by leatherbacks in tropical waters are therefore comparatively lean times.
Despite being capable of frequenting cold waters, leatherback nesting is restricted to tropical and sub-tropical regions. The process of nesting for leatherbacks is very similar to that of all sea turtles and can be roughly divided into six stages; ascending, digging a body pit, digging an egg chamber, laying eggs, camouflaging and descending.
Leatherbacks will deposit up to ten clutches of eggs in a season, with a period between nesting events of 9-11 days. The period between nesting seasons is generally two years.
Leatherback nests are unique amongst sea turtles in that, as well as containing large yolked eggs, they also contain small yolkless eggs. The yolkless eggs are concentrated in the upper layer of the nests, and although the purpose of these eggs is not completely understand they may serve to cap the nests, aiding gas exchange.
The sex ratio of sea turtle hatchlings, as for many reptiles, is determined by temperature. If the mean temperature during the middle third of the incubation period is below a certain temperature (~29ºC), the sex ratio will be male biased and vice versa.
Mating in leatherbacks is likely to be opportunistic and probably occurs at the start of the breeding season. The life history of the male leatherback is even more poorly understood than that of the female as they never leave the ocean and so their behaviour cannot easily be observed, either directly or by means of data logging devices. However it has recently been show that the migratory route of males is similar to that of females. If males were to mate with females at the start of the season then the females would not require further copulation in the season due to the ability to store sperm for several months. A recent study using a turtle mounted camera showed that although males persist in their attempts to mate later on in the season, their advances are rebuffed once the female has been mated.
The early life history of leatherbacks is still somewhat of an enigma with very few records of sightings/strandings of small (<1m) turtles. Opportunistic sightings of these smaller leatherbacks indicate that leatherbacks smaller than 1m are restricted to waters warmer than 26°C.
