Do crocodiles know their way home?
American crocodile translocation study: Tracking survival, movement, and habitat use in a threatened South Florida species.
The American Crocodile (Crocodylus acutus) is one of two crocodilian species native to Florida and is listed as threatened by the state and federal government. In the U.S., the range of this species is limited to the southernmost parts of Florida where it primarily inhabits coastal areas. Despite its limited range and threatened status, crocodile populations and human-crocodile conflicts have increased substantially in the past decade.
Map of the current range of American crocodiles in Florida (left image) vs. hotspot map (right image) generated using crocodile complaint data from 2005-2022. Warm colors show the areas of greatest human-crocodile conflict. Drag the arrows in the center of the map left and right to compare the two images!
In 2005, FWC partnered with the U.S. Fish and Wildlife Service, National Park Service, and the University of Florida to develop the "American Crocodile-Human Interaction Response Plan".
This plan allows for the translocation of crocodiles under certain circumstances as a means to address human-crocodile conflict. However, some translocated crocodiles return to their original capture sites, requiring additional translocations, while many were not observed again, leaving their fate unknown.
To gain a better understanding of the effects translocation have on the survival, movement, and habitat use of American crocodiles, FWC partnered with The University of Florida to capture two groups of American crocodiles and outfit them with GPS transmitters.
The transmitters were designed to acquire GPS positions every 4 hours while the animals were being tracked. This data was uploaded to a server by satellite transmission every 2 days.
One group consisted of 10 free-ranging crocodiles that were captured, fitted with a GPS tag, and released at their capture site. These crocodiles acted as a control or reference group.
Capture sites of reference crocodiles are represented by red pins.
The other group consisted of 7 crocodiles that were captured, fitted with a GPS tag, and translocated to Collier-Seminole State Park or the C-111 Canal according to existing FWC protocols.
Capture sites of translocated crocodiles are represented by colored pins. Arrows point to the release sites of each translocated crocodile.
The tracking data from reference crocodile RF1 provides us with a good example of the typical movements of a wild American crocodile in South Florida.
After being fitted with a GPS tag in October of 2018 and released at her capture site, she remains within a very consistent area, moving primarily within the canal where she was captured.
This movement pattern continues until mid-March 2019 when she begins to make forays across Barnes Sound into Crocodile Lake National Wildlife Refuge, presumably to find a suitable nest site and lay her eggs.
Refuge biologists were able to photograph her in May with a trail camera placed at a crocodile nest that had been predated by raccoons earlier in the spring. Coupled with her tracked movements, this suggests she laid her eggs at this site and continued to periodically check her nest.
This GPS data is then analyzed to give us a clear picture of the animals' home range, a useful tool to compare our reference and translocated crocodiles, and to compare Florida crocodiles to crocodiles elsewhere.
Home range calculation methods used were Bias Random Bridge (A), Kernel Density Estimate (B), and Minimum Convex Polygon. (C). Red polygons enclose 50% home ranges and light yellow polygons enclose 95% home ranges. Capture and release site is indicated by a yellow circle. Black dots indicate recorded GPS locations.
Reference crocodiles were characterized by slower, meandering movements within stable home ranges, suggesting a familiarity with their surroundings.
The home range of this reference crocodile was in the southwestern portion of Everglades National Park in an area known as Cape Sable. It made regular trips from the the coast to a group of small inland lakes over the course of several months.
In contrast, translocated crocodiles were characterized by fast, active directional movements, suggesting an unfamiliarity with their surroundings and a search for more suitable or familiar habitats.
This animal (TR5) covered a lot of ground in a few months after a translocation of 152 km, moving from its release site at Collier-Seminole State Park (at the top left of the map) in the direction of its original capture site on Tavernier in the upper Keys (located outside of the map extent, at the bottom right).
After 17 months, TR5 crossed Florida Bay to reach Long Key (at the bottom right of the map), 39 km from her original capture site. Shortly after making this crossing, location data downloads from the GPS tag ceased.
However, we were able to recapture this individual 0.42 km from her original capture site over 2.5 years after release! Since we were not able to collect location data for this entire time, we do not know exactly when she returned home.
The movements of this translocated crocodile (TR1) highlight differences in behavior between animals translocated shorter distances (45 km or less) and those translocated longer distances (110 km or greater).
TR1 was captured on Key Largo (right side of the map) and released 14 km away at the C-111 canal (bottom left corner of the map). Notice how this animal showed fast, active directional movement until slowing considerably upon reaching its original capture site a few days later- in this case a golf course in Key Largo. Once there, its movements began to look more like those of reference crocodiles.
All of the crocodiles translocated 45 km or less were able to return to within 3 km of their capture site in 13 days or less. In this video, crocodile TR2 is shown traveling from its release site at the C-111 canal (bottom left of the map) back to its original capture location 45 km away in the village of Pinecrest, just south of Miami (top right of the map). The journey home took only 9 days.
Our findings suggest that translocation distances over 110 km pose significant challenges to a crocodile's ability to navigate back to its original capture site.
Movements of all 17 American Crocodiles followed in this study.
Based on our tracking of crocodile movements, it appears the distance a crocodile is translocated affects its ability to return to the capture site. Crocodiles that are moved less than 45 km are likely to return within a few days to weeks. There is no guarantee that crocodiles translocated at least 110 km will not return, but it is unlikely they will return quickly.
Because of concerns regarding return rates and heightened stress as a result of capture and translocation, we suggest crocodile translocations have limited conservation value and may only be worth considering once all other reasonable options are exhausted.
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This project was funded with the help of the Fish & Wildlife Foundation of Florida via proceeds from the "Conserve Wildlife" license plate.