Shifts in morphological covariation and evolutionary rates across multiple acquisitions of the trap-jaw mechanism in Strumigenys

Abstract

A long-standing question in comparative biology is how the evolution of biomechanical systems influences morphological evolution. The need for functional fidelity implies that the evolution of such systems should be associated with tighter morphological covariation, which may promote or dampen rates of morphological evolution. I examine this question across multiple evolutionary origins of the trap-jaw mechanism in the genus Strumigenys. Trap-jaw ants have latch-mediated, spring-actuated systems that amplify the power output of their mandibles. I use Bayesian estimates of covariation and evolutionary rates to test the hypotheses that the evolution of this high-performance system is associated with tighter morphological covariation in the head and mandibles relative to nontrap-jaw forms and that this leads to shifts in rates of morphological evolution. Contrary to these hypotheses, there is no evidence of a large-scale shift to higher covariation in trap-jaw forms, while different traits show both increased and decreased evolutionary rates between forms. These patterns may be indicative of many-to-one mapping and/or mechanical sensitivity in the trap-jaw LaMSA system. Overall, it appears that the evolution of trap-jaw forms in Strumigenys did not require a correlated increase in morphological covariation, partly explaining the proclivity with which the system has evolved.

Keywords: biomechanics; covariation; evolutionary rates; trap-jaw ants.

Figure 1

A small sample of the diversity of Strumigenys species used for this study. The red boxes are long‐mandible trap‐jaw species (L‐Trap): (A) S. bibliona, ID # CASENT0005494 (photo by April Nobile). Scale bar = 0.2 mm. (B) S. ignota ID #CASENT0102610 (photo by April Nobile). Scale bar = 0.2 mm. Black boxes are nontrap‐jaw species (GRP): (C) S. ambatrix ID #CASENT0005990 (photo by April Nobile). Scale bar = 0.2 mm. (D) S. roomi ID #CASENT0914713 (photo by Zach Lieberman). Scale bar = 0.1 mm. Blue boxes are short‐mandible trap‐jaw species (S‐Trap): (E) S. azteca ID #CASENT0281949 (photo by Shannon Hartman). Scale bar = 0.1 mm. (F) S. truncatidens ID #CASENT0235691 (photo by Will Ericson). Scale bar = 0.2 mm. All photos are from antweb.org.

Figure 2

Sketches illustrating the five morphological traits used in this study. The outline is based on the S. ignota image from figure 1. For all traits, the length shown as a solid line is regressed against the length shown with a dotted line. Full measurement definitions are in the main text. Anterior‐posterior orientation based on Bolton and Boudinot et al. .

Figure 3

Comparison of the posterior distributions of estimated covariation between MusLen and the other four traits across the three types of Strumigenys species. There was no overall trend toward higher covariations in the trap‐jaw groups. However, some individual shifts can be seen: L‐Trap species show an increase in covariation between MusLen and MusWide compared to other forms. On the other hand, L‐Trap forms show a reduction in MandLen‐MandAR covariation.

Figure 4

Comparison of the posterior distributions of estimated covariation for the remaining six potential trait pairs across the three types of Strumigenys species. There was a general lack of increased covariation in the trap‐jaw groups. However, comparing MusWide and MandAR, S‐Trap forms show an increase in covariation relative to the other two forms.

Figure 5

Comparison of the posterior distributions of estimated evolutionary rates between three forms of Strumigenys across all five morphological traits. Although L‐Trap shows significant shifts in evolutionary rate compared with both GRP and S‐Trap taxa for 3 out of 5 traits, the direction of the shift is not consistent. L‐Traps have higher rates for MusLen but lower rates for MandLen and MandAR.