Discrimination of conspecifics from heterospecifics in a hybrid zone: Behavioral and chemical cues in ants

Species and nestmate recognition in social insects occurs mostly through cuticular hydrocarbons acting as chemical cues. These compounds generate a colony-specific odor profile depending on genetic and environmental factors. Species and nestmate recognition results in specific behavioral responses, regulating the level of aggression toward other individuals during an interaction. Although species discrimination and recognition cues have been poorly studied in the context of interspecific hybridization, such systems offer an opportunity to further investigate the influence of heritable and environmental factors on recognition. We explored the strength of discrimination in a hybrid zone between two ant species—Tetramorium immigrans and T. caespitum—by comparing cuticular hydrocarbon profiles and measuring intra- and interspecific worker aggression in both areas of sympatry and areas of allopatry among species. Species cuticular hydrocarbon profiles were well-differentiated and interspecific aggression was high, revealing highly discriminating species recognition cues. Hybrids’ cuticular hydrocarbon profiles consisted of a mixture of the parental bouquets, but also exhibited hybrid-specific patterns. Behavioral assays showed that T. immigrans is as aggressive toward hybrids as toward heterospecifics. Finally, aggression between heterospecific workers was lower when interacting individuals came from areas of sympatry among species than from areas of allopatry. Taken as a whole, these findings paint a particularly complex picture of the recognition system in T. immigrans, T. caespitum, and their hybrids, and highlight that hybrid zones afford a still underexplored opportunity for investigating recognition mechanisms and discrimination between species.

Artificial light at night decreases the pupillary light response of dark-adapted toads to bright light

Artificial light at night(ALAN)is expanding worldwide.Many physiological effects have been reported in animals,but we still know little about the consequences for the visual system.The pupil contributes to control incoming light onto the retina.Sudden increases in light intensity evokes the pupil light reflex(PLR).Intrinsically photosensitive retinal ganglion cells(ipRGC)affect PLR and melatonin expression,which largely regulate circadian rhythms and PLR itself.IpRCG receive inputs from various photoreptors with different peak sensitivities implying that PLR could be altered by a broad range of light sources.We predicted ALAN to enhance PLR.Contrary to our prediction,dark-adapted cane toads Rhinella marina,exposed to ALAN(5 lx)for 12 days,exhibited a lower PLR than controls and individuals exposed to 0.04 lx,even after 1 h in bright light.We cannot conclude whether ALAN induced a larger pupil size in dark-adapted toads or a slower initial contraction.Nevertheless,the response was triggered by a light source with an emission peak(590 nm)well above the sensitivity peak of melanopsin,the main photoreceptor involved in PLR.Therefore,ALAN alters the capacity of toads to regulate the incoming light in the eye at night,which may reduce the performance of visually guided behaviors,and increase mortality by predators or road kills at night.This first study emphasizes the need to focus on the effect of ALAN on the vision of nocturnal organisms to better understand how this sensory system is altered and anticipate the consequences for organisms.