Parturition Synchrony Index: A Method for Assessing Individual Parturition Synchrony Within a Group or Population

Area of Science:

• Ecology
• Evolutionary Biology
• Behavioral Ecology

Background:

• Parturition synchrony, or mothers timing births together, is a key population-level phenomenon studied for decades.
• Existing methods lack a standardized approach to quantify individual mothers' tendency to synchronize parturition timing within a group.

Purpose of the Study:

• To develop and validate a novel Parturition Synchrony Index (PSI) for quantifying individual-level synchrony in birth timing.
• To assess the influence of parturition season duration, group size, and birth date distribution on the PSI and its variability.

Main Methods:

• Developed the Parturition Synchrony Index (PSI) to measure individual synchrony, with 1 indicating complete synchrony and values near 0 indicating asynchrony.
• Conducted extensive simulations varying parturition-date distribution, season duration, and group size across 140,490 simulated mothers in 168 groups.
• Analyzed mean PSI and coefficient of variation (CV) of PSI values for each simulated group.

Main Results:

• Mean PSI values were higher in shorter parturition seasons across all distributions (except complete synchrony).
• Mean PSI was notably higher in larger groups for lognormal and normal birth distributions.
• Coefficient of variation (CV) of PSI increased with longer seasons and smaller group sizes across all distributions.

Conclusions:

• The PSI is a robust and flexible tool applicable to diverse socio-ecological contexts for analyzing individual parturition synchrony in vertebrates.
• The index's reliability is reduced under constrained conditions like very short seasons or extremely small groups.
• The PSI offers a functional approach to understanding the dynamics of synchronized reproduction in animal populations.