Many taxa show substantial differences in lifespan between the sexes. However, these differences are not always in the same direction. In mammals, females tend to live longer than males, while in birds, males tend to live longer than females. One possible explanation for these differences in lifespan is the unguarded X hypothesis, which suggests that the reduced or absent chromosome in the heterogametic sex (e.g. the Y chromosome in mammals and the W chromosome in birds) exposes recessive deleterious mutations on the other sex chromosome. While the unguarded X hypothesis is intuitively appealing, it had never been subject to a broad test. We compiled male and female longevity data for 229 species spanning 99 families, 38 orders and eight classes across the tree of life. Consistent with the unguarded X hypothesis, a meta-analysis showed that the homogametic sex, on average, lives 17.6% longer than the heterogametic sex. Surprisingly, we found substantial differences in lifespan dimorphism between female heterogametic species (in which the homogametic sex lives 7.1% longer) and male heterogametic species (in which the homogametic sex lives 20.9% longer). Our findings demonstrate the importance of considering chromosome morphology in addition to sexual selection and environment as potential drivers of sexual dimorphism, and advance our fundamental understanding of the mechanisms that shape an organism's lifespan.

Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.

1. Plants lose a remarkable amount of energy to herbivorous animals, and this damage has substantial impacts on plant fitness and species' distributions. There are many ways ecologists can measure leaf damage, with some methods being more time-consuming than others. Due to a high variance in herbivory, accurate quantification of damage at the population level requires sampling of many leaves. A simple yet effective solution to this problem is to estimate leaf damage visually.2. Visually estimating leaf damage may be less accurate than scanning methods, but visual estimates of leaf damage are much faster than digital measurements.Using simulations, we show that gathering larger quantities of data at a slightly higher level of inaccuracy gives a more accurate estimate of a population's overall leaf damage than fewer, exact measurements.3. We then introduce the ZAX Herbivory Trainer, a free online application that teaches researchers to accurately visually estimate leaf damage. On average, users took ~9 min and 48 images to complete our trainer which significantly decreased their estimate inaccuracy from 13.2% to 6%. This low level of inaccuracy can be retained up to 3 months post-training so researchers can use the ZAX Herbivory Trainer once prior to short fieldwork or every 3 months for extensive fieldwork. We also recommend a cut-off point, whereby if a person has not completed the app in 17.5 min or 85 images (90th percentile), they may not be suitable to estimate herbivory for research purposes.4. The ZAX Herbivory Trainer will allow researchers of any experience level to assess herbivory quickly and accurately in a globally standardised way.International collaborators, students and citizen scientists can all find use in this app, no matter the scale of their projects. From this we can gather better data to address big picture questions in ecology such as patterns in herbivory relating to latitude or climate change.