# Early‐growth trajectories affect juvenile survival, age at first reproduction and lifetime fitness in a long‐lived seabird, the little penguin

## TL;DR

Fast-growing little penguin chicks survive better, reproduce earlier, and have higher lifetime fitness compared to slow or light-growing chicks.

## Contribution

This study reveals how natural variation in chick growth trajectories affects long-term survival and reproductive success in a wild seabird population.

## Key findings

- Fast-growing penguin chicks had 19% survival to yearling age, compared to 3% for light-growing chicks.
- Fast individuals reproduced 12% earlier and produced 1.2–3.8 times more eggs than slow and light individuals.
- Fast growth did not reduce longevity, suggesting a 'silver spoon' effect where early advantages persist.

## Abstract

Early environmental conditions experienced during juvenile growth are known to have marked effects on adult phenotypes in animal populations. Yet, the life‐history outcomes of variable growth strategies have rarely been investigated in wild populations.The aim of this study was to examine the natural variation of growth patterns displayed within a seabird population and assess their impact on juvenile survival, age at first reproduction, lifetime reproductive outputs (LRO) and longevity.Using a 26‐year study on the ecology of little penguins, we compiled over 2200 chick growth curves and defined 11 growth parameters classified by magnitude, form and rate. Although the growth curves formed a continuum according to these 11 growth parameters, non‐supervised statistical clustering showed that growth trajectories clustered into three main groups: fast, slow and light. Fast chicks (n = 48%) attained the highest maximum mass in the shortest amount of time, whereas slow chicks (n = 33%) stood out by a prolonged (+7 days, i.e. +13% in comparison to fast chicks) and irregular period of juvenile growth. Finally, light chicks (n = 19%) reached low maximum and fledging masses (~−350 g; −37% and −36% of fast and slow chicks).We tested for the effects of chick growth parameters on subsequent annual vital rates estimated through capture–mark–recapture methods as well as longer term effects on life‐history outcomes using Markov chain models. Fast and slow individuals had the highest survival rates from hatching to yearling age (19% and 17%, respectively), while light chicks were at a disadvantage during this initial period (3% survival). Fast individuals reproduced 12% earlier (2.6 years old) than slow individuals, had 12.5%–88% greater longevity (up to 21 years old), and produced 1.2–3.8 times more eggs over their lifespan than slow and light individuals, respectively.Fast chicks reached maturity faster and produced more offspring during their lifetime without discernible negative effects to their longevity, highlighting possible silver spoon effects.

Early environmental conditions experienced during juvenile growth are known to have marked effects on adult phenotypes in animal populations. Yet, the life‐history outcomes of variable growth strategies have rarely been investigated in wild populations.

The aim of this study was to examine the natural variation of growth patterns displayed within a seabird population and assess their impact on juvenile survival, age at first reproduction, lifetime reproductive outputs (LRO) and longevity.

Using a 26‐year study on the ecology of little penguins, we compiled over 2200 chick growth curves and defined 11 growth parameters classified by magnitude, form and rate. Although the growth curves formed a continuum according to these 11 growth parameters, non‐supervised statistical clustering showed that growth trajectories clustered into three main groups: fast, slow and light. Fast chicks (n = 48%) attained the highest maximum mass in the shortest amount of time, whereas slow chicks (n = 33%) stood out by a prolonged (+7 days, i.e. +13% in comparison to fast chicks) and irregular period of juvenile growth. Finally, light chicks (n = 19%) reached low maximum and fledging masses (~−350 g; −37% and −36% of fast and slow chicks).

We tested for the effects of chick growth parameters on subsequent annual vital rates estimated through capture–mark–recapture methods as well as longer term effects on life‐history outcomes using Markov chain models. Fast and slow individuals had the highest survival rates from hatching to yearling age (19% and 17%, respectively), while light chicks were at a disadvantage during this initial period (3% survival). Fast individuals reproduced 12% earlier (2.6 years old) than slow individuals, had 12.5%–88% greater longevity (up to 21 years old), and produced 1.2–3.8 times more eggs over their lifespan than slow and light individuals, respectively.

Fast chicks reached maturity faster and produced more offspring during their lifetime without discernible negative effects to their longevity, highlighting possible silver spoon effects.

This study highlights the short‐ and long‐term effects of early development on life‐history outcomes in little penguins. By describing chick growth using 11 different parameters (including growth irregularity), weshowed that different growth trajectories lead to differences in long‐term fitness. In particular, chicks that grew the fastest and with the highest maximum mass exhibited the highest  reproductive success and longevity. Conversely, weak or irregular growth imposed costs in terms of individual fitness.