Why does a grasshopper have fewer, larger offspring at its range limits?

Area of Science:

• Ecology
• Evolutionary Biology
• Animal Behavior

Background:

• Geographic variation in animal traits can be driven by environmental pressures and genetic adaptation.
• Offspring size and clutch size represent a critical trade-off influencing maternal fitness.
• The field grasshopper (Chorthippus brunneus) is a suitable model for studying adaptation across environmental gradients.

Purpose of the Study:

• To investigate geographic variation in offspring size in Chorthippus brunneus populations across the British Isles.
• To determine the genetic basis and environmental influences on offspring size variation.
• To explore the relationship between offspring size, clutch size, and maternal fitness under varying conditions.

Main Methods:

• Analysis of offspring size across three laboratory generations from 27 field populations.
• Assessment of geographic variation in relation to climatic factors (temperature, precipitation).
• Evaluation of trade-offs between clutch size and offspring size, and their impact on geometric mean fitness.

Main Results:

• Offspring size increased in cooler, wetter conditions at the western and northern range limits.
• Offspring size variation exhibited a significant genetic component.
• A trade-off between clutch size and offspring size was observed, with optimal maternal fitness strategies varying with environmental conditions.
• Significant environment-genotype interactions affecting egg size in relation to temperature were identified.

Conclusions:

• Geographic variation in offspring size in Chorthippus brunneus is genetically based and adaptive to local environmental conditions.
• Larger offspring at range edges serve as a conservative risk-spreading strategy, enhancing long-term maternal fitness.
• Environmental factors, particularly temperature, interact with genotype to influence egg size, highlighting the complexity of adaptation.