Nest Transplantation As A Deterrent To Mammalian Predation At Tortuguero, Costa Rica

Nest Transplantation As A Deterrent To Mammalian Predation At Tortuguero, Costa Rica

Stephen E. Stancyk
Department of Biology and Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, SC 29208 USA

Simple nest transplantation has been touted as a way to protect newly laid turtle nests from predation by mammalian predators, particularly in localities where predation is high, labor scarce, and logistics difficult (Stancyk, Talbert and Dean, 1980; Stancyk, 1981). Our arguments have been based on work done on two barrier islands in South Carolina, where predation by raccoons ranged from 55-93% (about half first-night predation), and where removal of nests from cues left by nesting females to nearby open, uncaged areas reduced predation to between 6 and 19% (Stancyk, Talbert and Dean, 1980).

Further tests of the method have not been published, and a visit to Tortuguero, Costa Rica, presented the opportunity to examine the usefulness of transplanting on a more tropical beach with different turtle species c h e l o d my&& and different predat01.s. Predat~~on was studied at Tortuguero by Fowler (1979). who found that 38% of marked nests were taken. In 1977, dogs took about 80% of the nests and coatis & took only 8%. With the establishment of Tortuguero as a national park, however, free-roaming dogs were eliminated to a great extent, and the impact of coatis as predators has increased.

To test whether removal of freshly-laid turtle eggs from cues left by the nesting female reduces predation in a location where the primary mammalian predator is the coati, 39 nests were transplanted between July 18 – August 11, 1989. Nest were moved within two hours of the time; they were laid to sites 10-30 m from their original location, and were marked, but left unprotected. An additional 61 unexcavated, suspected nests were marked as controls during the same period. At 2-3 day intervals until August 10, 1989, surveys were conducted to document the rate of first-night predation and to examine the fat’s of marked nests. After August 10, marked nests were surveyed at 10-16 day intervals until October 17,1989.

Table 1 shows the fates of all marked nests. Nearly half (38% of transplants, 49% of controls) of the marked nests were obliterated by subsequent turtle nesting activity, reducing the sample size considerably. In addition, 12% of control nests did not hatch after 72-91 days, and probably were not nests. With obliterated nests included, transplants and controls both suffered 28% depredation; if only hatchedldepredated nests are counted, transplants experienced 46% depredation, controls, 65%. This difference is not significant, however, and the basic result is that nest transplantation has little effect on the rate of mammalian predation on nests.

There are other characteristics of the data which relate to differences in predation behavior, and probably the cues used, by raccoons, dogs and coatis. Table 2 shows that although there were no differences in predation rate on transplants and controls in the vegetated or vegetation border zon~es (33 and 31%, respectively), predation is higher in these zones than on the open parts of the beach. This pattern was also observed by Fowler (1979) and the implication for nest transplantation is that one should transplant to unvegetated sites, if such sites can be found where erosion won’t take the nests.

First-night predation of nests was relatively infrequent. Of an estimated 174 new nests counted during 10 surveys, only 5 (2.9%) were depredated. Figure 1 shows how many days after laying or transplantation marked – nests were depredated. More transplanted nests were destroyed within 5 days of laying (18 vs. 6%), but fewer during the remaining observation period (10 vs. 23%). The pattern in Figure 1 is very different from that which would be expected if raccoons were the major predators. Raccoons take 50-90% of the nests on the first night (Stancyk, Talbert and Dean, 1980). Dogs also appear to be strong first-night predators; most of the first-night predation in Figure 1 was by dogs, and 5 of the 7 transplant predations under 5 days were due to dogs. Coatis, however, are diurnal predators which do not take markedly more newly-laid nests. I observed many examples of coatis exploring body pits for several consecutive days before finally excavating eggs. In Figure 1, about 67-73% of the known coati predation occurred after 5 days. This pattern is similar to that found by Fowler (1979), except that she found relatively constant predation throughout the first 6 weeks of incubation, while these data indicate that predation drops off between the third and the eighth week. In addition, Fowler found significantly more predation of 6-9 week-old nests than younger nests; although the sample size was small, this pattern does not appear in Figure 1.

In conclusion, the transplant method was not particularly successful on a tropical beach where predation was about 30% and the major predator was the coati. Patterns of predation appear to differ between predators such as dogs, raccoons and coatis, which may imply that they use different cues, or use the same cues differently. Transplantation may be more effective against first- and second-right predators such as dogs and raccoons, or on heavily-nested, heavily-depredated beaches with associated logistic difficulties.

Thanks are due to J.P. Ross, K. Horikoshi, B. Witherington, A. Chaves Quiros and the members of the short course on Sea Turtle Biology and Conservation for help in the field and stimulating discussion. This research was supported by the Caribbean Conservation Corporation.

Fowler, L.E.. 1979. Hatching success and nest predation in the grcen sea turtle, Chelonia m, at Toxtuguem, Costa Rica. Ecology. 60(5): 946-955.

Stancyk, SE. 1981. Non-human predators of sea turtles and their control. In: Bjomdahl, K.A. (ed.) Biology and conservation of sea turtles, p. 139-152. Smithsonian Institutio~n Press, Washington, D.C.

Stancyk, S.E., O.R. Talbert and J.M. Dean. 1980. Nesting activity of he loggerhead turtle QEX& caret& in South Carolina, 11. Protection of nests from raccoon predation by transplantation. Biol. Conserv. 18: 289-298.

Paper presented at 10th International Symposium, 1990