Physiological constraint on acrobatic courtship behavior underlies rapid sympatric speciation in bearded manakins

Abstract

Physiology's role in speciation is poorly understood. Motor systems, for example, are widely thought to shape this process because they can potentiate or constrain the evolution of key traits that help mediate speciation. Previously, we found that Neotropical manakin birds have evolved one of the fastest limb muscles on record to support innovations in acrobatic courtship display (Fuxjager et al., 2016a). Here, we show how this modification played an instrumental role in the sympatric speciation of a manakin genus, illustrating that muscle specializations fostered divergence in courtship display speed, which may generate assortative mating. However, innovations in contraction-relaxation cycling kinetics that underlie rapid muscle performance are also punctuated by a severe speed-endurance trade-off, blocking further exaggeration of display speed. Sexual selection therefore potentiated phenotypic displacement in a trait critical to mate choice, all during an extraordinarily fast species radiation-and in doing so, pushed muscle performance to a new boundary altogether.

Keywords: Skeletal muscle; calcium trafficking; courtship display; evolutionary biology; neuroscience; reproductive behavior; speciation; tropical bird.

Figure 1.. Evolutionary history of the bearded manakins.

(a) From a molecular phylogeny that represents our most up-to-date understanding of the group’s evolutionary history (see Materials and methods). (b) Ancestral state reconstruction of range polygons. 95% credible intervals are denoted by transparent outer edges (maximum extent) and the dotted lines (minimum extent). Golden-collared, white-collared, and orange-collared manakins arose from ancestors (2 and 3) that overlapped across 49% to 73% [95 CI] of their range. (c) Present-day species ranges are geographically isolated from one another.

Figure 1—figure supplement 1.. We reconstructed ancestral ranges by computing the coordinates for 12 index points on the range maps of each manakin species on our phylogeny.

The first four points (a) are simply those at which the minimum and maximum longitude/latitude occur. We then fit the minimum bounding polygon to each shapefile in ArcGIS and drew the diagonals and midlines of that rectangle (b), and the remaining eight points were drawn at the outermost intersection point between each line and the original polygon. Together, this collection of points provides an approximation of the original range (c) that can be reconstructed using phylogeographic modeling. To regenerate the range approximation from a set of points, connect them by proceeding radially (we connected counterclockwise, but the direction does not matter) from the cluster’s center.

Figure 2.. All bearded manakins use a unique wing-snapping display, called the roll-snap, for courtship.

This signal has phenotypically diverged in terms of (a) speed, or the rate (snaps sec⁻¹) at which an individual repeatedly hits its wings together above its back (p<0.05, with statistically significant differences between groups denoted by different letters atop the bars). (b) There is no apparent divergence in the display’s duration in terms of the total number of snap events within a single roll. Values plotted are estimated marginal means ± 1 SEM.

Figure 3.. The roll-snap display phenotype can be characterized as a bivariate distribution by plotting speed (# of snaps s⁻¹) as a function of length (total snaps in a roll) for (a) golden-collared, (b) white-collared, (c) white-bearded, and (d) orange-collared manakins.

Values are individual means computed from multiple roll-snap displays. (a) Golden-collared manakins and (b) white-collared manakins both have a significantly negative upper-bound (τ = 0.9, p<0.05) to the distribution, which is consistent with a performance constraint on speed. (a) For the golden-collared manakin, the negative bound extends continuously into the 20th quantile, thus impacting 80% of the population (at τ = 0.2: t = -2.95, p=0.004). (b) By contrast, in white-collared manakins, this constraint is only present at the uppermost end of the distribution (80th quantile and above; τ = 0.8, t = -2.33, p=0.028).

Figure 3—figure supplement 1.. Roll-snap display length (# snaps) as it predicts speed (#snaps/second) when evaluated at every quantile (τ) between 0.1 and 0.9 at intervals of 0.01.

Each point plotted represents the best-fit slope (±SEM) estimate at a given quantile, where values near 0 indicate no relationship between length and speed. For quantiles where the slope is significantly less than 0 (p<0.05), there is a trade-off between display speed and length.

Figure 4.. In bearded manakins, (a) the main humeral retractor, the scapulohumeralis caudalis (SH), actuates roll-snap display behavior.

Panel (b) is a representative in situ recording of twitch speed from the SH when stimulated with an 80 Hz pulse train. Representative recordings from both golden-collared and white-collared manakins are shown. Note that the first four individual stimulation pulses within the entire stimulation train are highlighted yellow, as these data are used for later analyses (See Figure 5). (c) We subjected the SH to pulse trains of varying frequencies and measured percent relaxation for each contraction in this series. We then averaged these values within each train to generate a plot of percent relaxation as it changes with stimulation frequency. From there, we fit a four-parameter logistic curve to the data that illustrates the twitch dynamics of each species’ SH muscle. In these models, the dark solid line reflects the best-fit model ±95% confidence bands (shaded area). (d) Our models also allow us to extract an inflection point, which corresponds to the tissue’s half-relaxation frequency. This is an index of twitch speed that we can use to compare between species. Bars represent mean ±1 SEM, with the asterisk (*) denoting a significant different between species (F_1,49=79.8, p<0.001).

Figure 5.. Change in percent relaxation of the SH muscle across a single stimulation train at the different stimulation frequencies.

(a) SH performance in the golden-collared manakin, whereby SH percent relaxation declines across stimulation trains that are greater than 80 Hz. This is just above the species maximum observed roll-snap speed of 68 Hz. Solid lines represent represent significant regression slopes (p<0.05, β <0), with corresponding shaded areas denoting 95% confidence bands. Non-significant slopes (p>0.05) are indicated by dotted lines. (b) SH performance in the white-collared manakin. Note that all slopes are non-significant (p>0.05), as denoted by the dotted lines; however, the y-intercept appears to progressively decrease as the stimulation frequency increases, suggesting that the muscle fuses at the onset of stimulation and stays that way across the entire stimulation train. (c) Regression lines of the slope of the lines in (a) and (b) plotted as a function of stimulation train frequency. Note that SH performance—as measured by the SH’s ability to resist ‘rapid fatigue’ during high frequency stimulations—declines in golden-collared manakins (F_1,6=27.7, p=0.004, R² = 0.82), but not in white-collared manakins (F_1,7=0.15, p=0.711). Solid lines associated with each species represent the mean change in percent relaxation at a given frequency,±95% confidence bands (shaded areas). The red line indicates the stimulation frequency at which the 95% CI of the SH performance line in golden-collared manakins intersects with the slope = 0 point. In theory, this represents that maximum twitch speed that the muscle can attain without incurring an endurance cost, and it notably corresponds to the species’ average roll-snap speed (denoted by horizontal box and whisker plot, where the vertical line is at the mean, shaded box indicates ±1 SEM, and whiskers extend to the species range).

Figure 5—figure supplement 1.. In the golden-collared manakin, roll-snap displays performed >60 Hz are significantly shorter (t = 6.5, df = 152, p<0.0001) than those ≤60 Hz.

Value plotted is mean ±SEM computed from individual roll-snap displays.

Figure 6.. One muscle twitch consists of two phases: contraction, or shortening (a–d) and relaxation, or lengthening (e–h).

To better pinpoint the physiological mechanisms generating endurance costs in the golden-collared manakin SH (see Figure 5), we measured the duration of each phase and tested whether contractile timing also changes over repeated stimulations administered near the species’ roll-snap display speed (a, b, e, f) or at a high speed of 80 Hz (c, d, g, h). We found no change throughout the stimulation series for any measure, which means the observed decline in percent relaxation shown in Figure 5 occurs independently of shifts in contractile timing.