Satellite tracking reveals sex-specific migration distance in green turtles (Chelonia mydas)

1. Introduction

Sea turtles spend most of their lives at sea, making observation of their movements and behaviour a challenge. In recent decades, satellite tracking has become a key tool for unravelling enigmatic aspects of their ecology [1]. Dozens of satellite-tracking studies on sea turtles are published each year on a wide variety of topics, such as habitat selection, navigation and interactions with human activities [2–4]. As female sea turtles are readily captured while nesting on beaches, they are particularly amenable to the deployment of tracking devices. However, focusing tracking efforts on adult females have left the movement ecology of immature and male turtles poorly understood [3]. To date, satellite tracking of adult males has been conducted on all seven sea turtle species, however, often with sample sizes of as few as one or two individuals [5]. Further tracking of adult males can therefore help address long-standing questions, such as whether sea turtles display sex-specific migratory behaviour or use different foraging habitats.

Results from tracking studies on loggerhead (Caretta caretta), hawksbill (Eretmochelys imbricata) and Kemp's ridley turtles (Lepidochelys kempii) suggest that male sea turtles tend to remain closer to breeding grounds than females [6–8]. Additionally, reconstructive paternity analyses and satellite tracking indicate that, in some populations, male sea turtles also return to breed at shorter intervals than do females [9,10]. It has been hypothesized that if males breed more frequently than females, then foraging closer to the breeding grounds may confer fitness benefits by both maximizing mating opportunities and optimizing travel costs [11]. In green turtles (Chelonia mydas), evidence from both population censusing and tracking studies suggest that the sexes use similar foraging habitats [12,13]. However, as few male green turtles have successfully been tracked from breeding sites to foraging grounds, it remains uncertain whether males of this species choose foraging grounds closer to breeding sites [12,14,15].

Here we report on the satellite tracking of male green turtles in West Africa. Our aim was to describe movement behaviours of this under-studied population segment, from the breeding season through migration and into the post-breeding foraging period. To do so, we first quantify the ranging distances and space use areas of males during breeding, when they are presumably attempting to mate with receptive females. Next, we test whether males and females breeding in the same year migrate different distances to the foraging grounds. Finally, we assess whether sex differences in migratory distance can be explained by differential habitat use, by comparing the locations of foraging grounds used by each sex.

2. Methods

3. Results

The duration of data transmission varied across individuals, spanning a median of 109 days (min = 17 days, max = 158 days, n = 25, table 1). The devices of four individuals (three males and one female) stopped transmitting prior to migration and one failed during migration (one female), leaving data for nine males and 11 females from breeding through migration and into foraging. During breeding, males ranged a median maximum distance of 13.9 km (min = 5.7 km, max = 99.5 km, n = 12) from Poilão Island. Two males shifted away from Poilão before migration, and two others performed short-duration looping trips (figure 1). Additionally, individual 213039 left Poilão shortly after device attachment, making long looping movements in the Bijagós, before migrating (figure 1). For the 11 males showing periods of stationarity during breeding (i.e. excluding ID 213039), 50% core-use areas and 95% home ranges averaged 9 km² (min = 6 km², max = 28 km²) and 92 km² (min = 32 km², max = 210 km²) in size, respectively. Tracked males migrated away from the breeding grounds after a mean of 40 days (min = 8.2 days, max = 62 days, n = 9).

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Of the nine males tracked throughout migration, two remained in the Bijagós Archipelago, five travelled to coastal waters of The Gambia and Senegal, and two went to the Banc d'Arguin in northern Mauritania (figure 2). For females, 10 of 11 individuals migrated to northern Mauritania, while the remaining female travelled to a hitherto unknown foraging area for this population near Accra, Ghana (figure 2). The median distance to the foraging grounds used by males was significantly shorter than females, both in terms of the great circle distance (W = 82, p = 0.01, n = 20) and total distance travelled (W = 76, p = <0.001, n = 20, table 1). There was no significant effect of advancing season on the probability of a female turtle migrating to Mauritania, rather than moving a shorter distance to The Gambia/Senegal or remaining within the larger Bijagós Archipelago (z = 1.02, estimate = 0.18, s.e. = 0.01, p = 0.3, electronic supplementary material, figure S2).

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During foraging, tracking devices on males transmitted a median of 32 days (min = 3 days, max = 73 days, n = 9), revealing that seven of nine males overlapped with foraging areas used by females in 2018–2020 (electronic supplementary material, figure S3). The remaining two males settled within 2.8 km and 6.1 km of known female foraging areas.

4. Discussion

By tracking a robust sample of male green turtles from a breeding site to the foraging grounds, we reveal novel behaviours of this under-studied population segment. During the breeding period, we found that males can both remain near a single rookery or perform exploratory movements, presumably to visit other breeding sites [10]. We show for the first time in green turtles that males tend to migrate shorter distances to foraging areas compared to females. Importantly, the foraging sites chosen by males were also known foraging areas for females, indicating that differences in migratory behaviour are not likely caused by differential foraging habitat selection. We also identified a novel foraging ground for females of this population in Ghana, located further away than all previously known sites.

Differing migration distances between the sexes are a common feature in birds [26], and evidence is mounting to suggest the same in sea turtles [15]. Such sex-specific behaviours may reflect reproductive role specialization, which in turn may be influenced by cost–benefit trade-offs related to the frequency of breeding, migration distance and foraging habitat quality [8]. For example, if male sea turtles breed more frequently than females, they may choose to forage closer to breeding sites to reduce the time and energy costs of migration [6]. Migrating shorter distances may also allow males to maximize mating opportunities, either by intercepting females en route to breeding sites or by scheduling their arrival according to the timing of female receptiveness [11,27]. Evidence from at-sea surveys of breeding loggerheads suggests that males attempting to mate outside the receptive period are normally unsuccessful, implicating a role for female receptiveness in driving male behaviour during migration and breeding [28]. However, spatio-temporal patterns of female sexual receptiveness in the wild remain poorly understood [29].

Sex-specific migration distances could have implications for population structuring at foraging sites. In Nicaragua, catch records from sea turtle fisheries identified male-biased sex ratios at foraging grounds located closer to the main source rookery at Tortuguero, Costa Rica [30,31]. Understanding whether sex ratios are biased at foraging grounds is relevant to understanding wider population dynamics, as mortality rates can differ across sites due to varying exposure to threats [32]. Females previously tracked from Poilão indicate that the foraging destinations used can vary among annual breeding cohorts, with four of four turtles migrating to Mauritania in 2001, seven of 15 in 2018, none of eight in 2019 and six of 12 in 2020 [19,33]. However, without contemporaneous data on male movements, it remains unclear whether sex-specific migration distances remain consistent despite annual variation in foraging destinations. A study tracking loggerheads of both sexes across several years found that males consistently foraged in areas located closer to breeding sites [6], suggesting such differences can persist across years in sea turtles. As the distances green turtles migrate vary widely across populations, it would be relevant to compare movements between the sexes at other sites, for example to understand potential relationships between sex, migratory distance, and foraging habitat (e.g. oceanic versus coastal, [14,34]). Continuing to expand efforts to study breeding males as well as juveniles is crucial, both to gaining a more complete picture of sea turtle ecology and to informing their effective conservation.

Ethics

The animal study was reviewed and approved by Orgão Responsável pelo Bem-estar Animal do ISPA (ORBEA – ISPA). Research permits were granted by the Instituto da Biodiversidade e das Áreas Protegidas (IBAP), of Guinea-Bissau. Fieldwork involving sea turtles followed recommended guidelines (NMFS-SFC, 2008), to reduce possible disturbance to the turtles, and were conducted by trained personnel.

Data accessibility

Data derivatives and analysis code are available in a Zenodo repository (Beal et al. [35]); tracking data are stored in a different Zenodo repository (Beal et al. [36]), which will become available after a one-year embargo period, ending 31 August 2023. The tracking datasets analysed herein can alternatively be requested from the authors on Movebank (www.movebank.org) under the study names ‘Chelonia mydas_bijagos_females_2021’ (study ID: 2173907778), and ’Chelonia mydas_bijagos_males_2021’ (study ID: 2173900562).

Supplementary material is available online [37].

Authors' contributions

M.B.: conceptualization, data curation, formal analysis, methodology, software, visualization and writing—original draft; P.C.: conceptualization, funding acquisition, methodology, project administration, resources, supervision and writing—review and editing; A.R.: project administration, resources and writing—review and editing; A.J.P.: resources and writing—review and editing; C.B.: resources and writing—review and editing; J.M.: formal analysis, methodology, software and writing—review and editing; C.S.: project administration, resources and writing—review and editing; E.S.: project administration, resources and writing—review and editing; R.P.: conceptualization, data curation, funding acquisition, methodology, project administration, supervision and writing—review and editing.

All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

Conflict of interest declaration

We declare we have no competing interests.

Funding

This study was funded by the MAVA Foundation through the projects ‘Consolidation of sea turtle conservation at the Bijagós Archipelago, Guinea-Bissau’ and ‘Tortue d'Arguin’; the Regional Partnership for Coastal and Marine Conservation (PRCM), through the project ‘Survies des Tortues Marines’; the ‘La Caixa’ Foundation (ID 100010434) through a fellowship awarded to A.R.P. (LCF/BQ/PR20/11770003); the Fundação para a Ciência e a Tecnologia, Portugal, through a grant (UIDB/04292/2020 and UIDP/04292/2020) awarded to MARE and the project LA/P/0069/2020 granted to the Associate Laboratory ARNET.