Growth trajectories in wild geladas (Theropithecus gelada)

Area of Science:

• Primate Ecology
• Vertebrate Growth Patterns
• Life History Theory

Background:

• Life history and socioecological factors influence vertebrate growth patterns, with greater maternal investment often linked to discrete growth and reproduction.
• Studying wild primate growth is challenging due to limited data collection methods.
• Geladas (Theropithecus gelada) exhibit a graminivorous diet and high infant mortality, suggesting low maternal investment in neonates.

Purpose of the Study:

• To investigate shoulder-rump (SR) growth patterns in wild geladas using a noninvasive photogrammetric method.
• To test hypotheses linking maternal investment to growth and reproductive timing in primates.
• To compare gelada growth patterns with those of related papionins.

Main Methods:

• Utilized parallel lasers, a noninvasive photogrammetric technique, to measure shoulder-rump (SR) length in wild geladas.
• Collected data from 154 female and 110 male geladas in Simien Mountains National Park, Ethiopia.
• Analyzed size-for-age patterns and growth velocity relative to key life history milestones like sexual swelling, reproduction, and dispersal.

Main Results:

• Female geladas completed 88.5% of SR growth by first sexual swelling and 97.2% by first reproduction, reaching adult size by 7.72 years.
• Gelada females finished more growth by first reproduction than related papionins, despite having smaller neonates.
• Male geladas grew for a longer duration than females, becoming larger around 6 years of age, and completed 85.4% of growth by dispersal.

Conclusions:

• Gelada females exhibit less discrete growth periods relative to reproduction compared to expectations based on maternal investment.
• The parallel laser method is a promising, noninvasive tool for studying life history in wild populations.
• Male gelada's larger size is due to prolonged growth, not faster growth rates, diverging from other polygynous primates.