vs 2024 / 2025
vs 1982–2025 normal
2024 · 2025 · 2026
East Pacific Olive Ridley
An Olive Ridley's sex is not in its genes. It is set by the temperature of the sand around the egg during the middle third of incubation, which runs about 45–55 days here. Warmer than the pivotal temperature and the clutch comes out mostly female; cooler and mostly male. The Olive Ridley nests on this coast from roughly July to December, and the owner's question this September was simple: the sea is warm this year; what does that do to the nests?
The honest starting point is that there are no sand or nest temperature measurements at La Saladita. What exists is the village weather station (air temperature and rain every five minutes) and the satellite sea-surface record. So this page builds a proxy, says exactly how, compares years with it rather than pretending it is sand, and takes the biology (pivotal temperature, the range that gives both sexes, the heat that kills embryos) only from published studies, cited below.
A week of air, minus the rain.
For each day: the mean of the last 7 daily-mean air temperatures, minus 0.02 °C for every millimetre of rain in those 7 days (capped at 2 °C). Seven days because a nest 30–50 cm down does not feel a single hot afternoon; it feels the week. The rain term because a soaked beach runs cooler. The 0.02 °C/mm is an assumption, not a fit; there is nothing local to fit it to. The comparison below is shown with no rain term and with double it, and the conclusion does not change.
| Week starting | 2024 proxy (°C) | 2025 | 2026 | Rain 2024 (mm) | 2025 | 2026 |
|---|---|---|---|---|---|---|
| Jul 1 | 26.6 | 27.1 | — | 41 | 27 | — |
| Jul 8 | 27.7 | 27.7 | — | 13 | 17 | — |
| Jul 15 | 28.5 | 28.9 | — | 103 | 2 | — |
| Jul 22 | 27.1 | 28.8 | 29.0 | 5 | 12 | 23 |
| Jul 29 | 26.6 | 27.7 | 27.7 | 104 | 20 | 55 |
| Aug 5 | 27.3 | 27.7 | 29.0 | 21 | 11 | 2 |
| Aug 12 | 27.5 | 27.7 | 29.8 | 31 | 9 | 0 |
| Aug 19 | — | 28.3 | 29.1 | — | 6 | 32 |
| Aug 26 | 28.3 | 28.1 | 28.3 | 7 | 15 | 18 |
| Sep 2 | 27.6 | 27.5 | 29.0 | 43 | 30 | 2 |
| Sep 9 | 25.5 | 26.6 | 29.4 | 110 | 72 | 6 |
| Sep 16 | 28.6 | 26.6 | — | 7 | 48 | — |
| Sep 23 | 25.7 | 26.2 | — | 234 | 25 | — |
From 25 July to 18 September, the 2026 air averaged 29.3 °C against 28.2 in 2024 and 28.0 in 2025: +1.1 and +1.2 °C. With less rain on top, the proxy gap widens to +1.7 and +1.3 °C.
Week by week, 2026 ran warmer than 2024 in 6 of 7 comparable weeks (mean +1.8 °C, largest +3.9 in the week of 9 September, when 2024 took 110 mm of rain and 2026 took 6) and warmer than 2025 in 7 of 8 (mean +1.1 °C). The weeks without a gap are 26 August against 2024 and 29 July against 2025, both level. Doubling the assumed rain term moves the gaps to +1.7 and +1.3 °C; dropping it entirely leaves the pure air gaps of +1.1 and +1.2. The sign never changes. The same station's hourly readings show the same thing on the heat and UV page: 2026 warmer at every hour and 6–10 humidity points drier.
Yes, the sea is warm. The sea is not what heats a nest.
July–September sea-surface temperature against the 1982–2025 average for each month at the 0.25° cell off Saladita.
| Year | Jul–Sep mean (°C) | Anomaly vs 1982–2025 | 25 Jul–18 Sep mean | Station air, same window | Rain, same window |
|---|---|---|---|---|---|
| 2024 | 30.58 | +0.55 | 31.03 | 28.2 | 325 mm |
| 2025 | 30.23 | +0.19 | 30.32 | 28.0 | 178 mm |
| 2026 | 31.27 | +1.21 | 31.29 | 29.3 | 134 mm |
2026 is the warmest of the three summers at sea (+1.21 °C: July +1.34, August +1.12, September 1–25 +1.16). But 2024's sea in the same window was nearly as warm (31.03 vs 31.29 °C) and its air was 1.1 °C cooler.
Sand at nest depth is heated by the sun and cooled by rain and evaporation; the sea reaches it only indirectly, through the air and the humidity. Week to week over the three summers, sea and air move together only loosely (r = 0.58, 24 weeks). 2024 had a warm sea and a wet late summer (325 mm in the window, more after it); 2026 had a warm sea and a dry one. The owner's instinct is right that this is a warm year. The reason the nests are probably warmer is the warmer, drier air and the scarce rain, not the water directly. The sea record is still useful here: it is an independent instrument, and it agrees the year is warm at a time when the station itself had just been reinstalled (18 July), which is exactly when one should distrust a station.
What the literature says the sand has to do.
| Quantity | Value | Source (opened) |
|---|---|---|
| Pivotal temperature, East Pacific Olive Ridley (50% female at constant incubation) | 30.46 °C (95% 30.26–30.66) | Abreu-Grobois et al. 2020, Table 5, Bayesian logistic fit to incubations of eggs from La Escobilla, Playa La Destiladeras and El Verde Camacho (Mexico) and Nancite (Costa Rica) |
| Transitional range (5%→95% female) | width 2.24 °C, ≈29.3–31.6 °C | same |
| La Escobilla eggs at constant temperature | 27 °C all male; 32 °C all female | same, Table 1 data |
| Pivotal estimate for Mexican nesting populations | 29.95 °C | NMFS & USFWS 2014 Olive Ridley 5-Year Review (citing Sandoval-Espinoza 2011); also used by Sandoval et al. 2020 |
| When sex is set | middle third of incubation | NMFS & USFWS 2014 (citing Merchant-Larios et al. 1997); Sandoval et al. 2020 |
| Hatching success, Olive Ridley hatchery, Playa Grande, Costa Rica, 5 seasons | best at 29–32 °C mean in the 2nd third, 32–35 °C in the last third, 30–32.5 °C over the whole incubation; lower at both ends | Rutledge et al. 2024 |
| Nests reaching >35 °C at 25–30 cm, Playa Coyote, Costa Rica | emergence 78.9% vs 89.1%; 78.6% of nests female-biased 2013–2017; incubation 45–50 days | Binhammer, Beange & Arauz 2019 |
| Very hot sand | "can result in almost zero hatch success" | NMFS & USFWS 2014 (citing Sifuentes-Romero et al. 2013) |
The curve is steep. On the published fit, a clutch held 1 °C above the pivotal comes out about 93% female and one held 1 °C below about 7% (our arithmetic from the reported pivotal and range, assuming a symmetric logistic curve). A one-degree year is not a small thing if the sand is sitting inside that band.
More females. Mostly from the late nests.
Warmer, drier sand than 2024–25 tilts the season toward females. In the middle of the season the tilt is probably small, because those nests were likely female-heavy already. The place it can really move is the late season, October to December, when the sand cools through the transitional band and the males come from.
Two published observations support the timing. In a hatchery at Playa Ceuta, Sinaloa (six degrees further north, so cooler than here), sand temperatures logged at 30 cm in 2009 imply almost no males from the July, August and September nests (estimated 0%, 4% and 1%), then 50% in October, 98% in November and 100% in December (Sandoval et al. 2020; these are estimates from temperature, not sexed hatchlings). The US federal status review summarises Mexican data the same way: female-biased at the start of the season, male-biased at the end (NMFS & USFWS 2014). If this coast behaves like that, a warm July–September adds little to a batch that is mostly female already, and moves it closer to the heat that lowers hatching success. The sex-ratio question is decided in the autumn, and a sea running more than a degree warm can keep the autumn sand warm longer. This page will be re-run with October–December when that data exists.
Timing, for anyone matching dates: with 45–55 days of incubation, the middle third falls roughly 15 to 37 days after laying. A warm week in the chart matters most for nests laid two to five weeks before it.
A little, in the short run. Not if it becomes normal.
A female-leaning year can raise future egg output, because one male can fertilise several females.
Clutches with more than one father are documented in Olive Ridleys, more often on the mass-nesting beaches (NMFS & USFWS 2014, citing Jensen et al. 2006). A female lays about two clutches of 100–110 eggs in a season (same review). So a cohort with extra females is, up to a point, a cohort with extra eggs in its future, provided enough males still exist to find them.
The warning case is the northern Great Barrier Reef: green turtles from its warm nesting beaches are 99.1% female as juveniles and 99.8% as subadults, and the authors say complete feminization of that population is possible in the near future (Jensen et al. 2018).
Two things go wrong when warm years stack up: the male supply thins out, and the hot end of the range starts killing embryos and weakening hatchlings (hatching success falls above the optima in the table; a Mexican Pacific study of hatchlings from a mass-nesting beach found incubation temperature affected hatchling body condition and swimming thrust, Labastida-Estrada et al. 2024).
One year is one cohort among many. Olive Ridleys mature at a median of about 13 years (range 10–18), so this summer's hatchlings will not nest until around 2039.
Age from the north-central Pacific skeletochronology study, as summarised in the federal review (Zug et al. 2006 in NMFS & USFWS 2014). One warm year shifts one cohort. The number that would matter is how many of the next ten look like 2026.
Most protected nests here are not on the open beach.
The camps near Saladita collect eggs and rebury them in fenced hatcheries (local camps: Campamento Tortuguero Playa Petatillo, about 10 km away; ECOT in Troncones; Ayotlcalli at Playa Blanca near Zihuatanejo). A hatchery's sand can be managed, so its nests may run cooler or warmer than this proxy.
Shade cloth over the hatchery is used on the Guerrero coast.
At the Los Quelonios camp at Playa Ventura, Copala (Costa Chica, Guerrero), relocated Olive Ridley clutches hatched at 83.88% over the 2021–22 season, and the study concludes that the shade cloth (malla sombra) buffers the sun and helps keep incubation temperatures suitable (Galván Moscaira 2024). Some camps also water hatchery sand in hot spells; I found no Mexican Pacific study measuring how much that cools a nest, so no number is given here. Shading cools every nest under it and so pushes toward males as well as away from heat deaths, and a camp manager trades those off. The camps are the people to ask what their hatchery sand actually does.
Air is not sand. Dry sand in the sun heats well above the air, and wet sand stays cooler. Nest thermometers in Costa Rica read 28.6–34.6 °C at 25–30 cm (Binhammer et al. 2019), a range well above the daily air means here. Nothing at Saladita says how far above, so the proxy's level against the band means nothing; only the difference between years is used.
Depth and lag. Nests sit roughly 30–50 cm down, where the daily swing is damped and changes arrive days late. A 7-day mean is a crude stand-in for that.
Hatcheries. Relocated nests under shade or on managed sand do not follow the open-beach proxy.
Metabolic heating. In the last third of incubation, a clutch of about a hundred embryos warms its own nest. That matters more for survival than for sex, which is set earlier, and the proxy cannot see it.
The station. It sits among trees in the village, not on the beach. Its sensor array was dead from 15 May to 18 July 2026 and stuck on 18–21 August 2024; both gaps are removed, not filled, and the comparison uses only 25 July–18 September, the window all three years share (50, 56 and 56 valid days). The 2026 warm-dry signal began the day the array was reinstalled; the heat page labels it probably real, and the independent sea record points the same way.
The rain coefficient is assumed. 0.02 °C per mm per week, capped at 2 °C. The ranking of the years holds at zero and at double.
Thresholds travel imperfectly. The pivotal estimate pools Mexican and Costa Rican eggs incubated at constant temperature; field nests fluctuate, and the pivotal temperature can vary between beaches.
The sea is warm this year, and so is the air over the beach: about a degree warmer than the last two summers, with less rain. That tilts the nests toward females, and the tilt that matters will come from the October–December nests. A few extra females can mean more eggs in thirteen years. A decade of summers like this one would mean too few males and too many cooked eggs.Synthesis · 27 September 2026
Sources and method
Data: on-site Ambient Weather station, 5-minute temperature, cleaned exactly as on the heat page (68 spikes >5 °F from a centred 1-h or 3-h median or outside 50–100 °F removed; stuck array 2024-08-18 12:00 → 08-21 06:00 blanked; the dead-array zeros of 2026-05-15 → 07-18 fall outside the bounds). Daily mean = mean of hourly means, ≥20 hours required. Rain from the corrected daily file. Local day UTC−6. Sea: NOAA OISST v2.1 daily, cell 17.875 °N 101.875 °W; monthly normals 1982–2025 from the site's SST climatology.
- Proxy: 7-day trailing mean of daily-mean air (≥5 valid days) minus min(0.02 °C × 7-day rain mm, 2 °C); not extended past the last measured day. Weekly value = mean of the daily proxy in weeks starting 1 July (≥4 days).
- Common window: 25 July–18 September, the first and last dates 2026 can supply.
- Sex-ratio arithmetic: logistic with the reported pivotal (30.46 °C) and 5–95% range (2.24 °C), slope 2·ln19/2.24.
Script: scripts/analyze_nest_temperature_proxy.py → functions/api/_findings_nest_temperature.js (export FINDINGS_NEST_TEMPERATURE, generated 2026-09-27), served at /api/findings-nest-temperature.
Literature (each opened for this page):
- Abreu-Grobois FA, Morales-Mérida BA, Hart CE, Guillon J-M, Godfrey MH, Navarro E, Girondot M (2020). Recent advances on the estimation of the thermal reaction norm for sex ratios. PeerJ 8:e8451. doi:10.7717/peerj.8451
- NMFS & USFWS (2014). Olive Ridley Sea Turtle (Lepidochelys olivacea) 5-Year Review: Summary and Evaluation. NOAA repository 17036
- Sandoval S, Gómez-Muñoz VM, Porta-Gándara MÁ (2020). Nuevo método para simplificar la estimación de la proporción sexual en crías de tortuga marina utilizando datos de temperatura en corrales de incubación. Investigación y Ciencia 28(80):14–21. redalyc
- Rutledge ML, Paladino FV, Spotila JR, Santidrián Tomillo P (2024). Shifting thermal tolerance windows throughout incubation affects sea turtle hatching success. Marine Biology 171. doi:10.1007/s00227-024-04417-2
- Binhammer MR, Beange M, Arauz R (2019). Sand temperature, sex ratios, and nest success in Olive Ridley sea turtles. Marine Turtle Newsletter 159:5–9. seaturtle.org
- Galván Moscaira BA (2024). Análisis del éxito de la eclosión y emergencia en tortugas marinas… en Playa Ventura, Copala, Guerrero. MSc thesis, Universidad Autónoma del Estado de Morelos. riaa.uaem.mx
- Jensen MP, Allen CD, Eguchi T, Bell IP, LaCasella EL, Hilton WA, Hof CAM, Dutton PH (2018). Environmental warming and feminization of one of the largest sea turtle populations in the world. Current Biology 28:154–159. doi:10.1016/j.cub.2017.11.057
- Labastida-Estrada E et al. (2024). Influence of incubation temperature, maternal effects, and paternity on quality of olive ridley hatchlings from a mass-nesting beach in the Mexican Pacific. J Exp Zool A 341:563–577. doi:10.1002/jez.2804
Wibbels (2003) in The Biology of Sea Turtles vol. II is the standard review behind these numbers and is cited by the federal review; I did not open it, so no figure here is taken from it.
Cite as: Conner, A. (2026). "Nest Temperature This Season: A Proxy." La Saladita Field Guide, lasaladita.com/findings/turtle-nest-temperature/, 27 September 2026. Data: on-site station 2024–2026, NOAA OISST v2.1; artifact /api/findings-nest-temperature.