Tropical bird species have less variable body sizes

Quentin D. Read*, Benjamin Baiser, John M. Grady, Phoebe L. Zarnetske, Sydne Record, Jonathan Belmaker

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Ecologists have often predicted that species' niche breadths should decline towards the Equator. Dan Janzen arrived at this prediction based on climatic constraints, while Robert MacArthur argued that a latitudinal gradient in resource specialization drives the pattern. This idea has some support when it comes to thermal niches, but has rarely been explored for other niche dimensions. Body size is linked to niche dimensions related to diet, competition and environmental tolerance in vertebrates. We identified 68 pairs of tropical and nontropical sister bird species using a comprehensive phylogeny and used the VertNet specimen database to ask whether tropical birds have lower intraspecific body-size variation than their nontropical sister species. Our results show that tropical species have less intraspecific variability in body mass (; p = 0.009). Variation in body-size variability was poorly explained by both abiotic and biotic drivers; thus the mechanisms underlying the pattern are still unclear. The lower variation in body size of tropical bird species may have evolved in response to more stable climates and resource environments.

Original languageEnglish
Article number20170453
JournalBiology Letters
Issue number1
StatePublished - 1 Jan 2018


  • birds
  • body size
  • macroecology
  • niche


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