Introduction
Within an individual, the expression of labile traits (e.g.,
behavioural, physiological or morphological traits such as body mass)
can exhibit variation with individuals adjusting their phenotypes over
time or in response to environmental changes. Moreover,
within-individual plastic changes in multiple traits can also be
associated across traits (Dingemanse et al. 2012; Dingemanse and
Dochtermann 2013). If the expression of multiple traits is based on a
shared underlying mechanism or these traits affect each other via a
feedback loop (Sih et al. 2015), we would expect to find an association
between within-individual changes in multiple traits. For instance, if
exercise elevates the plasma level of corticosterone in mice (Girard et
al. 2002), the daily pattern of exercise is expected to align with that
of corticosterone levels. In addition, as a high metabolic rate is
needed to express energetically demanding behaviour (such as risk-taking behaviour), within-individual changes in the resting metabolic rate are expected to parallel those in risk-taking
behaviour (e.g, Cornwell et al. 2020). The correlation between
changes in multiple traits within an individual is termed a
”within-individual trait correlation” (Dingemanse and Dochtermann 2013).
Multiple traits may change (or fluctuate) in the same direction within
an individual, causing a positive within-individual trait correlation
(scenarios 1 and 3 in Figure 1). Conversely, traits may change (or
fluctuate) in opposite directions within an individual, causing a
negative within-individual trait correlation (scenarios 2 and 4 in
Figure 1).
Within-individual trait correlations are strongly expected when
trait expression is dependent on age (scenarios 1 and 2 in Figure 1).
Notably, since individual differences in behaviour can stem from
variations in state variables (Biro and Stamps 2008; Dingemanse
and Wolf 2010; Wolf and Weissing 2010; Sih et al. 2015), age-related
decreases in labile state variables (e.g., body mass, metabolic rate and
hormone levels) are expected to correspond with decreases in
behaviours, leading to positive within-individual correlations between
state variables and behaviours (scenario 1 in Figure 1). In this
scenario, physiological deterioration with ageing may diminish state
trait levels, subsequently reducing the expression of state-dependent
behavioural traits and resulting in a positive within-individual
correlation between state and behaviour. For example, in male
field crickets Gryllus campestris , both body mass and exploration
decrease with age, leading to a positive within-individual correlation
between body mass and exploration (Santostefano et al. 2016). In
addition to ageing, there may be a positive feedback loop between body
mass and exploration in the field cricket (e.g., reduced body mass
decreases exploration, and vice versa), further contributing to a
positive within-individual correlation between body mass and
exploration.
In addition, within-individual trait correlation can be an
outcome of age-related changes in future fitness expectations (i.e.,
residual reproductive value, Williams 1966). According to the asset
protection principle (Clark 1994), individuals tend to take more risks
when their future fitness expectation is low, and when the reproductive
costs from injury or death are less important (Clark 1994; Luttbeg
2017). Consequently, older individuals with lower expected future
fitness are more likely to exhibit risk-taking behaviour (e.g., Dammhahn
2012; Fisher et al. 2015; Ory et al. 2015; Moschilla et al. 2018),
potentially resulting in strongly positive within-individual
correlations among risk-taking behaviours. Moreover, if age-related
decreases in state variables such as body mass reflect changes in future
fitness expectations (e.g., body mass determines dominance), a decrease
in body mass with age would be integrated with the age-related increase
in multiple risk-taking behaviours, resulting in a negative
within-individual correlation between body mass and risk-taking
behaviour (scenario 2 in Figure 1).
Overall, the direction of within-individual trait correlations between
state and behaviour can vary depending on the trait-specific mechanisms
leading to age-related increase/decrease or fluctuation (Figure 1).
Recent meta-analyses have indicated a weak relationship between state
and behaviour at both the within-individual and among-individual levels
(Niemelä and Dingemanse 2018); however, this might be attributed to a
paucity of studies conducting frequent repeated measurements of labile
traits with longer intervals between repeated measures. Therefore, it is
imperative to assess within-individual trait correlations to identify
the underlying trait-specific or common mechanisms shaping age-related
plasticity. However, within-individual correlations have garnered
considerably less attention from behavioural ecologists (Dochtermann
2023).
In this study, we tracked individual male cicadas Tettigetta
isshikii (Figure 2) in their natural habitat and repeatedly assessed
their plant use, calling activity, and body mass. We investigated
age-related changes in these traits, partitioned trait variance into
among- and within-individual levels, and estimated among- and
within-individual correlations among these traits. In this study, we
predicted that two behaviours, plant use and calling activity, would
reflect the boldness of each male. Cicadas actively use plants as
shelters, adjusting their location within the plant to hide themselves
(Steward et al. 1988). Different plant species vary in structural
characteristics, such as leaf size and density, suggesting the shelter
they provide for cicadas may differ by species. Cicadas might prefer
plants with larger, denser leaves to protect them from aerial predators
(e.g., robber fly). T. isshikii also feeds on herbaceous plant
sap and lays its eggs in plant stems, potentially using the plants as
shelters by hiding underneath the leaves (Jiman Heo, personal
observation). In our study area, plants such as Erigeron annuus (Daisy fleabane), Thalictrum aquilegiifolium (Meadow rue), orMiscanthus sinensis (Chinese silver grass) have narrow or small
leaves, whereas only 2 plants, Convallaria majalis (Lily of the
valley) and Polygonatum odoratum (Solomon’s seal), have large
leaves and occurs in discrete clusters (Figure 3). As a result, onlyC. majalis and P. odoratum may effectively function as a
shelter for T. isshikii. Thus, we predicted that the increased
use of narrow-leafed plants might indicate greater boldness in the face
of potential predators. In addition, male T. isshikii produces
calls to attract females, and females respond with wing-flicking sounds
(Jiman Heo, personal observation). However, calling songs of cicadas
increases predation risk by predators such as the robber fly (Hou et al.
2017), suggesting that calling can also be considered a risk-taking
behaviour of male T. isshikii .
We predicted that trait-specific mechanisms shaping age-related
plasticity would determine within-individual trait correlations. If
physiological deterioration leads to both reductions in calling activity
and boldness with age as well as weight loss in male cicadas after
eclosion and maturation, positive within-individual correlations between
body mass and risk-taking behaviours are expected (scenario 1 in Figure
1). In contrast, according to the asset protection principle, male
cicadas might increase the expression of risk-taking behaviours with
age, even as body mass decreases (scenario 2 in Figure 1). This scenario
could lead to negative within-individual correlations between body mass
and risk-taking behaviours. Consequently, we predicted that the
integration of age-related plasticity in multiple traits of male cicadas
would hinge on the trait-specific mechanisms governing age-related
changes.