Relocate nests/eggs for artificial incubation: Crocodilians

Supporting evidence from individual studies

1. A replicated study in 1967–1974 in three rearing stations along the Zambezi River in Zimbabwe (Blake & Loveridge 1975) found that three-quarters of artificially incubated Nile crocodile Crocodylus niloticus eggs hatched in captivity. Over seven years, artificially incubated Nile crocodile egg hatching success was 74% (16,697 of 22,697 eggs hatched). The authors reported that collecting eggs very soon after laying had a detrimental effect on hatching success. Nile crocodile eggs were collected from the wild, hatched and reared in three rearing stations (at Kariba Lake, Binga and Victoria Falls) as part of a crocodile farming initiative in 1967–1973 (128–2,475 eggs collected/station/year). Eggs were artificially incubated in captivity (no details are provided).

   Study and other actions tested

   Referenced paper

   Blake D.K. & Loveridge J.P. (1975) The role of commercial crocodile farming in crocodile conservation. Biological Conservation, 8, 261-272.

2. A replicated study in 2001–2002 in a laboratory in Santa Fe province, Argentina (Larriera et al. 2006) found that artificially incubated broad-snouted caiman Caiman latirostris eggs hatched in captivity. Hatching success of artificially incubated broad-snouted caiman eggs taken from the wild ranged from 43–100% (hatching success of seven caiman nests: 30 of 36 eggs hatched; 18 of 37 eggs hatched; 35 of 41 eggs hatched; 20 of 30 eggs hatched; 13 of 30 eggs hatched; 18 of 26 eggs hatched; 35 of 35 eggs hatched). Between 1990 and 2002, a head-starting programme collected caiman eggs from wild nests (December–January), artificially incubated the eggs and reared hatchlings for up to nine months before releasing caiman (with individual scale markings) back into the collection site (see original paper for details). In austral summer 2001–2002, clutches from seven head-started female broad-snouted caiman were collected from the wild (26–41 eggs/nest) and artificially incubated (at 31.5°C and 95% relative humidity) until hatching.

   Study and other actions tested

   Referenced paper

   Larriera A., Siroski P., Pina C.I. & Imhof A. (2006) Sexual maturity of farm-released Caiman latirosis (Crocodylia: Alligatoridae) in the wild. Herpetological Review, 37, 26-28.

3. A replicated, controlled, paired study in 1999–2004 in an area of marsh in Louisiana, USA (Elsey & Trosclair 2008) found that relocating American alligator Alligator mississippiensis eggs for artificial incubation soon after laying resulted in heavier, longer hatchlings compared to eggs left in the nests until just before hatching. Eggs relocated soon after laying produced heavier (after hatching: 39–56 g; 6–9 months old: 795–1,270 g) and longer (after hatching: 24–26 cm; 6–9 months old: 63–78 cm) hatchlings than eggs from naturally incubated nests (mass: after hatching: 37–53 g; 6–9 months old: 795–1,130 g; length: after hatching: 23–26 cm; 6–9 months old:  62–74 cm). Alligator nests were located by helicopter and ground surveys (three nests in 1999 and six nests in 2003). Half of each clutch was relocated for artificial incubation soon after laying, whereas the other half was left in the nest and collected just prior to hatching. Relocated eggs were artificially incubated at 31–32°C. Weight and length of all hatchlings was measured two days after hatching, and then three more times at 2–3 month intervals.

   Study and other actions tested

   Referenced paper

   Elsey R.M. & Trosclair Iii P.L. (2008) Effect of timing of egg collection on growth in hatchling and juvenile American alligators. The Herpetological Bulletin, 105, 13-18.

4. A study in 2009 in two river basins in Venezuela (Barros et al. 2010) found that most American crocodile Crocodylus acutus and Orinoco crocodile Crocodylus intermedius eggs hatched successfully after being collected from the wild and artificially incubated. Results were not statistically tested. In total 66% of American crocodile eggs (200 of 305 eggs) and 54% of Orinoco crocodile eggs (116 of 216 eggs) hatched successfully after artificial incubation. Egg collection was carried out in 2009 (521 eggs collected overall) in the Santa Ana (305 American crocodile eggs) and Manapire (216 Orinoco crocodile eggs) river basins. Eggs were transported to a brick-walled building in each location (25 m² and 5 m² in size) with a zinc roof closed to predator access. The larger incubation room temperature was maintained at 32ᵒC by five light bulbs. Eggs were stored in sand-filled insulated polystyrene boxes. The sand was kept damp by adding water at regular intervals.

   Study and other actions tested

   Referenced paper

   Barros T., Jiménez-Oraá M., Heredia H.J. & Seijas A.E. (2010) Artificial incubation of wild-collected eggs of American and Orinoco crocodiles (Crocodylus acutus and C. intermedius), Gua-rico and Zulia, Venezuela. Conservation Evidence, 7, 111-115.

5. A replicated study in 2007–2012 in hatching facilities across six counties in Texas, USA (Eversole et al. 2013) found that artificially incubating American alligator Alligator mississippiensis eggs resulted in more than half of eggs hatching successfully. Average hatching success was 61% (average of 23 of 37 eggs/nest) and hatching success of viable eggs was 71% (average of 23 of 32 viable eggs/nest). In 2007–2012, a total of 33,454 eggs were collected from 902 wild alligator nests, and the viability of eggs was determined by examining egg colour, odour and presence of an opaque band. Eggs and nesting materials were transported in wire baskets to hatching facilities, where they were incubated at 31–32.8°C and 100% humidity, buried inside the nesting material. Eggs were removed from 50% of nests that were discovered during surveys, and surveys were carried out on foot, by boat and by helicopter.

   Study and other actions tested

   Referenced paper

   Eversole C.B., Henke S.E., Powell R.L. & Janik L.W. (2013) Effect of drought on clutch size and hatchling production of American alligators (Alligator mississippiensis) in Texas. Herpetological Conservation and Biology, 8, 756-763.

6. A replicated, before-and-after study in 1989–2015 in hatching facilities within four river systems in Northern Territory, Australia (Corey et al. 2018) found that artificially incubated saltwater crocodile Crocodylus porosus eggs hatched in captivity, but hatching success rates differed between local and external management. Results were not statistically tested. Hatching success of saltwater crocodile eggs as part of a sustainable harvest programme was 49% when run by a local Indigenous community organisation (654 hatchlings from 1,396 live eggs/year) compared to 84% when it was run by an external management company (1,413 hatchlings from 1,659 live eggs/year). Saltwater crocodile eggs were collected and incubated as part of a regional government-led sustainable harvest initiative. In 1989–1997 an external management company ran the programme. In 1998–2015 it was run by a local Indigenous management company. In 1996–1997 eggs were harvested by the external company and incubated by the Indigenous management company. There was no harvest in 2007–2008. Annual quotas were 2,700–3,000 eggs/year (total limit of 70,000 eggs/year across the territory). Eggs were incubated at a constant temperature of 32°C and ≥99% humidity. Local workers were paid based on the number of eggs collected and hatchlings produced.

   Study and other actions tested

   Referenced paper

   Corey B., Webb G.J.W., Manolis S.C., Fordham A., Austin B.J., Fukuda Y., Nicholls D. & Saalfeld K. (2018) Commercial harvests of saltwater crocodile Crocodylus porosus eggs by Indigenous people in northern Australia: lessons for long-term viability and management. Oryx, 52, 697-708.