Abundance of Saltwater Crocodile Hatchlings is Related to Rainfall in the Preceding Wet Season in Northern Australia

Lost to the Sea: Predicted Climate Change Threats to Saltwater Crocodile Nesting Habitat

Climate change is predicted to have devastating impacts on apex predators such as eliminating their required habitats. Crocodilians are no exception as most species require freshwater for nesting, and such freshwater habitats are particularly vulnerable to saltwater inundation (SWI) caused by the sea level rise (SLR) from global warming. Here, we examined the impacts of climate change on saltwater crocodiles Crocodylus porosus in terms of the potential loss of nesting habitat in the Northern Territory, Australia; an area that contains the world’s most extensive nesting habitat for the species. Our spatial model, derived from 730 nest locations and selected environmental features, estimated a total of 32,306.91 km2 of current suitable habitat across the study region. The most important variable was distance to perennial lakes (71.0% contribution, 87.5% permutation importance), which is negatively correlated with nesting habitat suitability. We found that projected changes in temperature and rainfall by 2100 could impact the area of suitable nesting habitat negatively or positively (0.33% decrease under low future emission climate scenario, and 32.30% increase under high emission scenario). Nevertheless, this can be canceled by the strong negative impact of SLR and concomitant SWI on nesting areas. A portion (16.40%) of the modeled suitable habitat for a subsection of our study area, the Kakadu Region, were already subject to > 0.25 m SWI in 2013. The suitable area for nesting in this region is predicted to be further reduced to 1775.70 km2 with 1.1 m SLR predicted for 2100, representing 49.81% loss between 2013 and 2100. Although the estimates of habitat loss do not account for the potential creation of new habitat, nor for the uncertainty in the degree of future SLR, our results suggest that SLR driven by continuing global warming can be the major threat to mound-nest-building crocodilians including C. porosus, rather than direct impacts from changes in temperature and rainfall. The degree of impact on saltwater crocodiles will be determined by the interplay between the loss of nesting habitat, which would appear inevitable under current global warming, and the ability to expand into new areas created by the expansion of the tropics.

Environmental resistance and habitat quality influence dispersal of the saltwater crocodile

Landscape genetics commonly focuses on the effects of environmental resistance on animal dispersal patterns, but there is an emerging focus on testing environmental effects on emigration and settlement choices. In this study, we used landscape genetics approaches to quantify dispersal patterns in the world's largest crocodilian, the saltwater crocodile (Crocodylus porosus), and demonstrated environmental influences on three processes that comprise dispersal: emigration, movement and settlement. We found that both environmental resistance and properties of the source and destination catchments (proportion of breeding habitat) were important factors influencing observed dispersal events. Our habitat quality variables related to hypotheses about resource competition and represented the ratio of breeding habitat (which limits carrying capacity), suggesting that competition for habitat influences emigration and settlement choices, together with the strong effect of environmental resistance to movement (where high-quality habitat was associated with greatest environmental permeability). Approximately 42% of crocodiles were migrants from populations other than their sampling locations and some outstandingly productive populations had a much higher proportion of emigration rather than immigration. The distance most commonly travelled between source and destination was 150-200 km although a few travelled much longer distances, up to 600-700 km. Given the extensive dispersal range, individual catchments or hydrographic regions that combine two or three adjacent catchments are an appropriate scale for population management.

Characterisation of sperm production and morphology in the male Philippine crocodile Crocodylus mindorensis via voluntary behavioural training

Philippine crocodiles Crocodylus mindorensis are critically endangered due to agricultural and fishing threats that have severely fragmented their habitat and population in the Philippines. Captive management plans are important to safeguard against their extinction, but the current population in US zoos is small, and breeding is hampered by the slow growth of this species and the danger of introducing differently sized animals for breeding. There is little information regarding the sperm characteristics of crocodilians, and none for Philippine crocodiles. In this study, we sought to characterise sperm production in the male Philippine crocodile (n=1) by performing voluntary (without sedation or restraint) collections (n=181) over a 3.5-year period. Peak sperm production in this individual occurs from January to July, when the mean (±s.e.m.) total number of spermatozoa recovered was 10.2×106±3.8×106 (n=104), compared with 0.3×106±0.2×106 (n=71) for all other months of the year. Analysis of sperm morphology indicated that 15.9% of spermatozoa exhibited normal morphology. A bent tail was the most common abnormality (48.2%) observed. Understanding the basic reproductive biology of the male Philippine crocodile will facilitate the development of artificial reproductive technologies to improve captive propagation and genetic management of this species.

American crocodiles (Crocodylus acutus) as restoration bioindicators in the Florida Everglades

The federally threatened American crocodile (Crocodylus acutus) is a flagship species and ecological indicator of hydrologic restoration in the Florida Everglades. We conducted a long-term capture-recapture study on the South Florida population of American crocodiles from 1978 to 2015 to evaluate the effects of restoration efforts to more historic hydrologic conditions. The study produced 10,040 crocodile capture events of 9,865 individuals and more than 90% of captures were of hatchlings. Body condition and growth rates of crocodiles were highly age-structured with younger crocodiles presenting with the poorest body condition and highest growth rates. Mean crocodile body condition in this study was 2.14±0.35 SD across the South Florida population. Crocodiles exposed to hypersaline conditions (> 40 psu) during the dry season maintained lower body condition scores and reduced growth rate by 13% after one year, by 24% after five years, and by 29% after ten years. Estimated hatchling survival for the South Florida population was 25% increasing with ontogeny and reaching near 90% survival at year six. Hatchling survival was 34% in NE Florida Bay relative to a 69% hatchling survival at Crocodile Lake National Wildlife Refuge and 53% in Flamingo area of Everglades National Park. Hypersaline conditions negatively affected survival, growth and body condition and was most pronounced in NE Florida Bay, where the hydrologic conditions have been most disturbed. The American crocodile, a long-lived animal, with relatively slow growth rate provides an excellent model system to measure the effects of altered hydropatterns in the Everglades landscape. These results illustrate the need for continued long-term monitoring to assess system-wide restoration outcomes and inform resource managers.

Assisted breeding technology in the saltwater crocodile Crocodylus porosus: a review and look to the future

This review reports the current status of artificial breeding technology in the Crocodylia and the future requirements for the establishment of AI in the saltwater crocodile. Although there are challenges regarding safe restraint and immobilisation, semen collection of the saltwater crocodile by manual stimulation has proven effective in yielding sufficient volume and sperm concentrations for empirical and molecular analyses of sperm preservation and physiology. Nevertheless, there is still much to learn with respect to fundamental anatomy, physiology and behaviour in both sexes, but particularly in the female. Although lessons can be learned from successful AI in the alligator, the details of this research are not readily accessible. Future research needs to focus on the proximate factors of seasonality and the underlying control of the female's annual reproductive cycle; this will require novel and innovative ways to collect blood samples without causing stress or injury, and ideally a dedicated crocodile research breeding colony. Because the saltwater crocodile is a farmed species, there is likely to be sufficient impetus for the application of assisted breeding technology to drive future productivity in the industry. These developments will also have benefits for the genetic and reproductive management of endangered captive populations.

Ontogenetic shifts in the nesting behaviour of female crocodiles

Body size and age are crucial factors influencing reproductive capacity and success. As females grow, their reproductive investment and success often increase due to improved overall physiological condition and experience gained through successive reproductive events. While much of this work has been conducted on birds and mammals, surprisingly little is known on how body size affects nesting decisions in other long-lived vertebrates. We monitored the movements and nesting behaviour of 57 wild female estuarine crocodiles Crocodylus porosus over a 10-year period (and across consecutive nesting seasons) using externally mounted satellite tags, implanted acoustic transmitters and a network of submerged acoustic receivers. Applying Hidden Markov models to the telemetry-derived location data revealed that female nesting behaviours could be split into three distinct states: (i) ranging movements within home ranges and at nesting sites; (ii) migrations to and from nesting sites; (iii) and nesting/nest guarding. We found that during migration events, larger females migrated further and remained away from dry season territories for longer periods than smaller individuals. Furthermore, not only were migratory movements stimulated by increases in rainfall, larger females migrated to nest sites at lower rainfall thresholds than smaller females. We provide some of the first evidence of body size influencing nesting decisions in an ectothermic vertebrate, with shifts likely resulting from an increased willingness to invest in nest protection among larger and more experienced females.