American Society of Naturalists

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“Within-individual canalization contributes to age-related increases in trait repeatability: a longitudinal experiment in red knots”

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Eva M. A. Kok, Joseph B. Burant, Anne Dekinga, Petra Manche, Darren Saintonge, Theunis Piersma, and Kimberley J. Mathot (Sep 2019)

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Longitudinal lab experiment shows how canalization contributes to age-related changes in trait repeatability

Birds change their stomachs but not their behavior when they get older

Exploring juvenile Red Knots in the Dutch Wadden Sea.<br />(Credit: Jan van de Kam)
Exploring juvenile Red Knots in the Dutch Wadden Sea.
(Credit: Jan van de Kam)

Individual variation is at the core of Darwin’s theory of evolution. Yet in ecology, variation between individuals was often considered as ‘noise’ or a nuisance. However, under changing environmental conditions, variation between individuals increases resilience for a population as a whole. Therefore, understanding what processes generate variation between individuals is important.

This study focuses on individual variation and, in particular, what factors promote the development of variation between individuals. Researchers from the Royal Netherlands Institute for Sea Research (NL) and the University of Alberta (CA) looked at changes in bird ‘character’ with age, and if these changes are the unavoidable consequence of ageing or whether different experiences during life cause individuals to diverge.

Red Knots weighing up their differences.<br/>(Credit: Ysbrand Galama)
Red Knots weighing up their differences.
(Credit: Ysbrand Galama)

The scientists study the behavior of red knots—migratory shorebirds—exploring their surroundings in search of food. These birds differ in how they search (exploration behavior) and in the size of their stomachs (physiology). To investigate how these differences come about, 90 birds were brought into captivity either as juvenile or as adult, and then given identical experiences over the next two years. While the birds age, exploration behavior and stomach size are measured repeatedly to tease apart the effects of age, experience, and time in captivity on the amount of variation in both traits. If the birds’ individual experiences help maintain the differences between individuals, this variation should disappear during the course of the study because in these experiments, all birds have the same experience.

Although it may seem easier to change one’s behavior than one’s physiology, these birds maintain their exploratory character while their stomach size changes. The next step is to follow the birds after their release back into the wild, to examine how the changes measured in the lab translate to real life.

Working with Red Knots.<br/>(Credit: Jan J. Wijmenga)
Working with Red Knots.
(Credit: Jan J. Wijmenga)


Age-related increases in the repeatable expression of labile phenotypic traits are often assumed to arise from an increase in among-individual variance due to differences in developmental plasticity or by means of state-behavior feedbacks. However, age-related increases in repeatability could also arise from a decrease in within-individual variance as a result of stabilizing trait expression, i.e. canalization. Here we describe age-related changes in within- and among-individual variance components in two correlated traits, gizzard mass and exploration behavior, in a medium-sized shorebird, the red knot (Calidris canutus). Increased repeatability of gizzard mass came about due to an increase in among-individual variance, unrelated to differences in developmental plasticity, together with decreases in within-individual variance, consistent with canalization. We also found canalization of exploration, but no age-related increase in overall repeatability, which suggests that showing predictable expression of exploration behavior may be advantageous from a very young age onward. Contrasts between juveniles and adults in the first year after their capture provide support for the idea that environmental conditions play a key role in generating among-individual variation in both gizzard mass and exploration behavior. Our study shows that stabilization of traits occurs under constant conditions: with increased exposure to predictable cues, individuals may become more certain in their assessment of the environment allowing traits to become canalized.

De foarspelbere ûntwikkeling fan lichems- en gedrachseigenskippen: in eksperiment oer welhelberhyd fan eigenskippen by mientsen

Alhoewol guon lichemseigenskippen fan jonge yndividuën hyltiten wer feroarje kinne, komme by it âlder wurden sokke eigenskippen ornaris dochs hyltiten mear fêst te lizzen. Soks kin komme troch it feroarjen fan yndividuele plastisiteit en troch weromkeppelingen tusken it gedrach en de steat fan sa’n bist. Yndividuele ferskillen yn it fêstlizzen fan eigenskippen fergrutsje de fariaasje tusken yndividuën. In technysk begrip om yndividuele fariaasje fan eigenskippen te kwantifisearjen is ‘repeatability’ (‘werhelberhyd’), mar it euvel is dat in taname fan dizze statistyske maat sawol komme kin troch in taname yn ‘e fariaasje tusken yndividuën en troch in ôfname fan de fariaasje binnen yndividuën; dit lêste neame wy ‘kanalisaasje’. Yn dit artikel beskriuwe wy hoe’t dizze twa boarnen fan fariaasje by it âlder wurden feroarje kinne by mientsen (Calidris canutus), en dat dogge wy oan ‘e hân fan twa besibbe eigenskippen: (1) it gewicht fan de spiermage, en (2) de wize werop mientsen yn in eksperimentele romte lytse stikjes waad ferkenne (der binne fûgels dy’t bot eksplorearje, en guon dy’t ôfwachtsje). It die bliken dat de werhelberhyd fan it magegewicht feroare troch tanimmende ferskillen tusken yndividuën en in ôfname fan de fariaasje binnen yndividuën, in kombinaasje fan plastisiteit en kanalisaasje dus. By eksploraasje-gedrach fûnen wy by it âlder wurden oanwizings foar kanalisaasje. Dat soe betsjutte kinne dat der foardielen binne om al op jonge leeftyd in bepaalde yndividuele wize fan eksploraasje oan te hâlden. Út in fergeliking tusken jonge en âlde fûgels (dy’t in ferskil yn ûntwikkeling yn it frije fjild wjerspegelje), blykte it bestean fan weromkeppelingen tusken de steat fan it lichem (magegewicht) en it gedrach (eksploraasje). Ús stúdzje lit lykwols foaral sjen dat eigenskippen, sels yn sitewaasjes dy’t net feroarje, fêst komme te lizzen. Miskien makket it fenomeen dat fûgels har omjouwing hyltiten better foarspelle kinne sokke kanalisaasje mooglik.