Crocodile mothers’ response to hatching calls

In animal species with parental care, caregivers' investment is often driven by offspring solicitation signals. Previous studies, particularly in birds and mammals, show that parents modulate their response to these signals according to characteristics such as their intensity and number per unit of time. In crocodilians, mature embryos still in the egg emit hatching calls perceived by other embryos and by the adult guarding the nest. These calls help to synchronize hatching within the nest and encourage the adult to dig the nest and help the young to hatch. However, the embryos do not start calling all at once, and it can take several hours or even days after embryos have started to vocalize before the adult digs the nest. When and why does the crocodile parent decide to respond to the hatching calls of its offspring? Here we show that Nile crocodile, Crocodylus niloticus, mothers only start digging their nests if the hatching calls are emitted in a sustained manner by the offspring. While females react with head and body orientation movements to isolated calls, only continuous bursts of calls lasting several tens of seconds induce digging behaviour. This high response threshold on the part of the mothers probably limits the untimely digging up of eggs that may not have reached full maturity, thus avoiding damaging them, and is likely to favour the synchronization of offspring hatching. Overall, this study suggests that the parental response of Nile crocodiles to offspring solicitations is not necessarily linearly correlated with the number of begging signals received but may require a sufficient number of begging signals to exceed a threshold.