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

## TL;DR

A new index called PSI measures how synchronized individual mothers' births are within a group, and simulations show it works well in most scenarios.

## Contribution

The novel Parturition Synchrony Index (PSI) quantifies individual-level parturition synchrony within groups or populations.

## Key findings

- Mean PSI values were higher in shorter parturition seasons across all distributions except complete synchrony.
- The coefficient of variation (CV) of PSI was higher in longer seasons and smaller groups.
- PSI is robust in diverse socio-ecological contexts but less reliable in very short seasons or small groups.

## Abstract

The phenomenon of parturition synchrony at the population level has been studied for decades using various methods. To assess the tendency of individual mothers to synchronize their parturition timing with others, we developed a novel parturition synchrony index (PSI). The PSI quantifies an individual mother's degree of parturition synchrony within a group or population by detecting variation in the synchrony of parturitions, regardless of their actual timing within the season. A value of 1 indicates complete synchrony (i.e., a parturition occurring on the same day as others), whereas values approaching 0 reflect increasing asynchrony. To evaluate the robustness of the PSI, we conducted simulations examining how parturition‐date distribution (completely synchronous, lognormal, normal, bimodal, and uniform), parturition‐season duration (1, 10, 50, 100, 200, and 365 days), and group size (5, 10, 25, 50, 100, 500, 1000, and 5000 mothers) influence the PSI and its inter‐individual variability. A PSI value was calculated for each of the 140,490 individual mothers in the 168 simulated groups. For each group, we subsequently calculated both the mean PSI and the coefficient of variation (CV) of PSI values. All examined parameters influenced both metrics. Across all distributions (excluding complete synchrony), mean PSI values were higher in shorter than in longer parturition seasons. However, only in lognormal and normal distributions were mean PSI values markedly higher in larger than in smaller groups. Conversely, the CV was higher in longer seasons and in smaller groups across all distributions. Our results highlight the PSI's broad applicability to diverse socio‐ecological contexts. However, its reliability decreases under constrained conditions, such as very short seasons or extremely small groups, where wider confidence intervals were observed. The PSI thus provides a robust and flexible tool for quantifying and functionally analyzing individual parturition synchrony in mammals, and potentially in other viviparous vertebrates.

We developed a novel parturition synchrony index (PSI) to quantify the degree of parturition synchrony for an individual mother within a group (or population). We evaluated the robustness of the PSI by simulating parturition synchrony at the individual level under a range of scenarios. The PSI provides a robust and flexible tool for quantifying and functionally analyzing individual parturition synchrony in mammals and potentially in other viviparous vertebrates.