Morphological and functional maturity of the oral jaws covary with offspring size in Trinidadian guppies

T R Dial,L P Hernandez,E L Brainerd

doi:10.1038/s41598-017-06414-6

Abstract

Large size of individual offspring is routinely selected for in highly competitive environments, such as in low-predation populations of the Trinidadian guppy (Poecilia reticulata). Large guppy offspring outcompete their smaller conspecifics, but the functional mechanisms underlying this advantage are unknown. We measured jaw kinematics during benthic feeding and cranial musculoskeletal morphologies in neonates and juveniles from five populations of Trinidadian guppy and found that both kinematics and morphologies vary substantially with neonatal size. Rotation at the intramandibular joint (IMJ), but not the quadratomandibular joint (QMJ), increases with size among guppy offspring, from 11.7° in the smallest neonates to 22.9° in the largest neonates. Ossification of the cranial skeleton varies from 20% in the smallest neonates to 90% in the largest. Relative to standard length (SL; jaw tip to caudal fin base distance), the surface area of jaw-closing musculature scales with positive allometry (SL2.72) indicating that muscle growth outpaces body growth. Maximum gape also scales with positive allometry (SL1.20), indicating that larger neonates are capable of greater jaw excursions. These findings indicate that size is not the sole adaptive benefit to producing larger offspring; maturation provides a potential functional mechanism underlying the competitive advantage of large offspring size among Trinidadian guppies.

Highlights

Competition tends to select for large size of individual offspring, balanced by a relatively smaller total number of offspring[1,2,3,4,5,6,7,8]
We predicted that rotation at both the intramandibular and quadratomandibular joints would increase with neonatal size, but the data show that guppy offspring only exhibit a positive relationship between joint mobility and standard length at the intramandibular joint (IMJ; Fig. 5)
Intramandibular joint rotation is positively allometric among neonates and postnatal juveniles, such that larger offspring employ greater movement at the IMJ while feeding (Figs 3, 4 and 5)

Summary

Introduction

Competition tends to select for large size of individual offspring, balanced by a relatively smaller total number of offspring[1,2,3,4,5,6,7,8]. Primary production within LP environments has been shown to constrain guppy population size, indicating that individuals indirectly compete over a limited supply of available food[18] In such environments, large guppy offspring have higher fitness than their smaller high-predation (HP) counterparts[19]. To distinguish between size and population effects, we will collect data at two time points for each population: first at birth, where size is predicted to vary among populations; and second at later stages, where juvenile size ranges overlap This functional and morphological investigation of neonates will offer insights into the advantage of large offspring size and will provide a better understanding of how size and maturity influence organismal growth and development, and how evolution shapes these parameters at the population level

Methods

Results

Conclusion