Figure 1. Six freshly laid clutches of the water snake Natrix maura were split into 12 half clutches: 6 half clutches were incubated in close contact with slightly older eggs (mixed) and the remaining 6 half clutches were used as controls.

Heart rates (Y axis) were recorded for each egg throughout the incubation period (X axis). Heart rates naturally increase during the incubation, therefore eggs mixed with older eggs were consistently exposed, from incubation day 17 onwards, and until older eggs hatched (black arrow) to higher heart rates (on average 89.81 ± 3.92 bpm) than they would have experienced if not mixed (i.e. control eggs averaged 82.52 ± 3.39 bpm). Means ± SE are plotted.