Multi-Modal Homing in Sea Turtles: Modeling Dual Use of Geomagnetic and Chemical Cues in Island-Finding

Abstract

Sea turtles are capable of navigating across large expanses of ocean to arrive at remote islands for nesting, but how they do so has remained enigmatic. An interesting example involves green turtles (Chelonia mydas) that nest on Ascension Island, a tiny land mass located approximately 2000 km from the turtles' foraging grounds along the coast of Brazil. Sensory cues that turtles are known to detect, and which might hypothetically be used to help locate Ascension Island, include the geomagnetic field, airborne odorants, and waterborne odorants. One possibility is that turtles use magnetic cues to arrive in the vicinity of the island, then use chemical cues to pinpoint its location. As a first step toward investigating this hypothesis, we used oceanic, atmospheric, and geomagnetic models to assess whether magnetic and chemical cues might plausibly be used by turtles to locate Ascension Island. Results suggest that waterborne and airborne odorants alone are insufficient to guide turtles from Brazil to Ascension, but might permit localization of the island once turtles arrive in its vicinity. By contrast, magnetic cues might lead turtles into the vicinity of the island, but would not typically permit its localization because the field shifts gradually over time. Simulations reveal, however, that the sequential use of magnetic and chemical cues can potentially provide a robust navigational strategy for locating Ascension Island. Specifically, one strategy that appears viable is following a magnetic isoline into the vicinity of Ascension Island until an odor plume emanating from the island is encountered, after which turtles might either: (1) initiate a search strategy; or (2) follow the plume to its island source. These findings are consistent with the hypothesis that sea turtles, and perhaps other marine animals, use a multi-modal navigational strategy for locating remote islands.

Keywords: homing; magnetism; navigation; olfaction; sea turtles.

Figure 1

(A) A map depicting a magnetic isoline that runs from Brazil to Ascension Island. Brazil is on the left, marked by hash marks, while Ascension Island is the small black dot. The black line represents the magnetic intensity isoline that intersected both Ascension Island and Brazil in 1985. (B) Map as depicted in (A) but 25 years later, in 2010. The same isoline shown in (A), which intersected both Ascension and Brazil in 1985, has now shifted so that it no longer intersects Ascension Island but instead runs south of the island. However, a waterborne odor plume, depicted in gray, emanates from the island and intersects the intensity isoline. Thus, if turtles swim along the isoline in the later stages of their migration (or as part of a search strategy), then they might plausibly encounter a plume of waterborne odorants that could lead them to the island.

Figure 2

Maps of waterborne odor plumes in combination with the isolines of magnetic intensity and inclination angle at Ascension Island. (A) The small white circle represents Ascension Island. The intensity (solid black) and inclination angle (dashed black) isolines that existed at Ascension Island in the year 1900 have been plotted in the year 1925, when turtles that left the island in 1900 (as hatchlings) would be expected to return for the first time to mate and nest (as adults). Colored swirls emanating from the island represent the dispersal of waterborne odorant particles within the top 50 m of the ocean surface at the beginning of the nesting season. Red, purple, and blue swirls represent odors that persist in the environment for 15, 30, and 45-day respectively. (B) Same as in (A), but with simulated odorants released towards the end of the nesting season. (C,D) Same as in (A,B), but with magnetic isolines associated with Ascension Island in 1940 plotted 25 years later, in 1965. (E,F) Same as in (A,B), but with magnetic isolines associated with Ascension Island in 1980 plotted 25 years later, in 2005. Hypothetically, a turtle returning to Ascension Island after a 25 year absence might follow the intensity isoline on which it imprinted as a hatchling (Lohmann et al., ; Brothers and Lohmann, 2015) to arrive in the vicinity of the island, where it would then detect olfactory cues which might guide it the rest of the way to the island.

Figure 3

Maps of airborne odor plumes in combination with the isolines of magnetic intensity and inclination angle at Ascension Island. (A) The small white circle represents Ascension Island. The intensity (solid black) and inclination angle (dashed black) isolines that existed at Ascension Island in the year 1900 have been plotted in the year 1925, when turtles that left the island in 1900 (as hatchlings) would be expected to return for the first time to mate and nest (as adults). Colored plumes emanating from the island represent the dispersal of airborne odorant particles for a duration of 48 h. Purple-colored and green-colored plumes represent simulations toward the beginning and end of the nesting season, respectively (lighter colors indicate simulations from the same year). (B) Same as in (A), but with magnetic isolines associated with Ascension Island in 1940 plotted 25 years later, in 1965. (C) Same as in (A), but with magnetic isolines associated with Ascension Island in 1980 plotted 25 years later, in 2005. Similar to waterborne cues (Figure 2), airborne cues might enlarge the homing target for a turtle initially using the geomagnetic field to return to the vicinity of its natal site.

Figure 4

Proportion of time that particles intersect the 25-year intensity isoline for 15, 30 and 45-day odorant durations (averages from years 2004–2007). Error bars represent 95% confidence intervals.

Figure 5

Proportion of time that particles intersect the 25-year intensity isoline for 15, 30 and 45-day odorant durations (yearly average). Error bars represent 95% confidence intervals.

Figure 6

Proportion of time that airborne odor particles intersect the 25-year intensity isoline (yearly averages). Error bars represent 95% confidence intervals.

Figure 7

Proportion of time that airborne odor particles intersect the 5-year intensity isoline (yearly averages). Error bars represent 95% confidence intervals.

Figure 8

Proportion of time that particles intersect 25-year intensity isoline (4-year average) over the past century.

Figure 9

Distances that an intensity isoline intersecting Ascension Island at a point in time moved during the next 5 and 25 years. For example, the intensity isoline intersecting Ascension Island in the year 1900 moved approximately 5 km by 1905 but nearly 350 km by 1925.