For India’s critically endangered gharial, a fish-eating crocodile with a slender snout that ends in a bulbous growth, heat is emerging as a major threat, reports contributor Sneha Mahale for Mongabay India.
Nest temperatures are rising, researchers wrote in a new study, which could be skewing the sex ratio of hatchling gharials (Gavialis gangeticus) more toward females.
The sex of gharial hatchlings, as with other crocodiles and sea turtles, is determined not by chromosomes but by nest temperatures during incubation. Temperatures at or close to 32° Celsius (89.6° Fahrenheit) produce males, while at temperatures at or below 31.5°C (88.7°F) and above 33.5°C (92.3°F), females dominate, Surya P. Sharma, study lead author and a scientist at the Wildlife Institute of India, told Mahale.
“Since crocodilians, including gharials, have temperature-dependent sex determination, we suspected that rising ambient and nest temperatures, likely driven by climate change and habitat alterations, might be altering hatchling sex ratios and shifting population dynamics,” Sharma said.
The researchers monitored 17 gharial nests over three breeding seasons between 2017 and 2019 in National Chambal Sanctuary in Madhya Pradesh state. The team inserted temperature loggers into the top, middle and bottom layers of each nest within 24 hours of a gharial laying eggs. The loggers continuously recorded temperature data over the eggs’ 60-day incubation cycle.
The researchers used the recorded data to model how temperatures shifted during the critical incubation window between days 20 and 40 when the embryo’s sex is determined. Then they estimated the likely sex ratios of hatchlings based on those findings.
The analysis showed that temperatures fluctuated from 30.5-34.0°C (86.9-93.2°F) during the critical sex-determining period, indicating the nests will likely produce more females than males, or three females for every one male, on average.
In fact, more than 35% of the nests showed such higher female-producing temperatures. In three nests, all hatchlings were expected to be female.
“We couldn’t physically confirm the hatchlings’ sex, as it’s still very difficult to determine the sex of such small individuals,” Sharma said. “Long-term monitoring is needed until they’re big enough for reliable sexing techniques.”
Even without actual sex ratio data for these nests, the field observations are “suggesting a female-biased sex ratio in gharials,” Sharma added.
In the long term, such a skewed sex ratio could weaken breeding patterns and threaten the species’ future.
“Gharials are polygynous, one male can mate with several females, but you still need enough reproductively active males to keep the population viable,” Sharma said.
“If the current trend continues, a persistent sex-ratio imbalance could lead to a smaller effective breeding population. That means fewer individuals able to reproduce, higher risk of inbreeding particularly in fragmented populations, loss of genetic diversity, and eventually, the collapse of breeding groups in isolated habitats.”
Read the full story by Sneha Mahale here.
Banner image of gharial hatchlings, courtesy of Surya P. Sharma.
