Global Decline in the Size of Sea Turtles

Abstract

Changes in mean adult body size may be a universal response to global warming and sometimes lead to conservation concerns. We show that size reductions in sea turtles are now the norm and have another explanation. From 18,707 measurements of nester size (curve carapace length) for sea turtles spanning 30 years from Redang Island (Malaysia), where nearly all nesting individuals have been tagged, we show that the mean size was initially fairly stable and then decreased by 4.0 cm from 100.8 cm in 2005 to 96.8 cm in 2022, which likely translates to a change in mean mass from 120 to 105 kg. At the same time, nesting increased from around 300 to 2000 nests per year. Consistent with this finding of a size reduction in an expanding population, at 27 of 31 sites across the globe where changes in the mean size of nesting sea turtles have been assessed, mean size is decreasing, and the most marked decreases are at sites where population size is increasing most dramatically. Taken together, these focal and global findings suggest that an important driver of size reductions in sea turtles is an influx of small first-time nesters (neophytes) in expanding populations, and hence, size reductions are partially a consequence of successful sea turtle conservation measures and population recoveries. At the same time, the focal observations in Malaysia show that the mean size of neophytes has also been getting smaller over time: from 99.6 to 96.8 cm between 2005 and 2022, likely because of a change in foraging environments. While smaller turtles have lower reproductive output, this negative consequence of decreases in nester size will often be more than offset by increases in nesting numbers that are occurring widely.

Keywords: Chelonia mydas; Bergmann's rule; body size reduction; ectotherm; food availability; macroevolution; responses to climate change.

FIGURE 1

For nesting green turtles at Redang (Malaysia), the change in the annual number of nests (a) and of nesters (b) over 30 years between 1993 and 2022, (c) the change in mean size of all nesters (red line) and of neophytes (blue line). Annual mean CCL is shown as ± 1 SE. From 2005, there was a significant increase in nesting abundance of 13.6% per year, as well as reductions in the mean size of both all nesters and neophytes.

FIGURE 2

(a) Sites around the world where trends in the mean size of nesting turtles have been reported. Filled symbols indicate sites where both changes in mean size and nesting abundance were available. Open symbols indicate sites where only changes in mean size were available. Different symbols (e.g., circles and squares) indicate different species. (b) The change in mean nester size reported at sites around the world. Most data reports simply mean size of nesters, but in a few cases, the change in the mean size of remigrants and neophytes is both reported, and in these cases, there will be two points for one site. In one case (i.e., olive ridleys nesting in Orissa, India), mean sizes were reported for males and females separately. We removed males from our analysis. Including males in the analysis did not change the results, since males and females had a very similar decrease in size (decrease from 72.9 to 68.8 cm for males and a decrease from 72.6 to 68.6 cm for females over the same period of time), but since data from other sites are for females only, we did not include the males in our analysis. In only four cases reported around the world was there an increase in mean nester size, and in all these cases, the increase was very small. (c) The relationship between the rate of change of mean nester size (% per year) versus the rate of change in nesting abundance (% per year). All the decreases in mean size found in published papers were statistically significant. In general, the most marked decreases in mean turtle size are observed at sites with the most marked increases in population size. The line represents the fitted quadratic curve (F _2,14 = 6.081, p = 0.0126, r ² = 0.464). The dotted lines indicate the origin for each axis (i.e., x = 0 and y = 0). Point 1 indicates Redang Island (current study); point 2 represents Sal, Cape Verde, which has the highest reported rate of population growth; and point 3 represents Milman Island, Australia, where a decrease in population size was reported over 28 years. Data compiled from: This study, Bjorndal et al. (1985), Hatase et al. (2002), Limpus et al. (2003), Shanker et al. (2004), Ilgaz et al. (2007), Da Silva et al. (2007), Pérez‐Castañeda et al. (2007), Bellini et al. (2013), Tucek (2014), Weber et al. (2014), Balazs et al. (2015), Piacenza et al. (2016), Sönmez (2019), Bell et al. (2020), Le Gouvello et al. (2020), Phillips et al. (2021), Gulick et al. (2022), Hays et al. (2022, 2024), López‐Castro et al. (2022), Mortimer et al. (2022), and Evans et al. (2024).