Human language is based on grammatical rules 1-4 . Cultural evolution allows these rules to change over time 5 . Rules compete with each other: as new rules rise to prominence, old ones die away. To quantify the dynamics of language evolution, we studied the regularization of English verbs over the last 1200 years. Although an elaborate system of productive conjugations existed in English's protoGermanic ancestor, modern English uses the dental suffix, -ed, to signify past tense 6 . Here, we describe the emergence of this linguistic rule amidst the evolutionary decay of its exceptions, known to us as irregular verbs. We have generated a dataset of verbs whose conjugations have been evolving for over a millennium, tracking inflectional changes to 177 Old English irregulars. Of these irregulars, 145 remained irregular in Middle English and 98 are still irregular today. We study how the rate of regularization depends on the frequency of word usage. The half-life of an irregular verb scales as the square root of its usage frequency: a verb that is 100 times less frequent regularizes 10 times as fast. Our study provides a quantitative analysis of the regularization process by which ancestral forms gradually yield to an emerging linguistic rule.Natural languages comprise elaborate systems of rules which enable one speaker to communicate with another 7 . These rules serve to simplify the production of language and enable an infinite array of comprehensible formulations 8-10 . Yet each rule has exceptions, and even the rules themselves wax and wane over centuries and millennia 11,12 .Verbs which obey standard rules of conjugation in their native language are called regular verbs 13 . In the modern English language, regular verbs are conjugated into the simple past and past participial forms by appending the dental suffix -ed to the root (for instance, talk/talked/ talked). Irregular verbs obey antiquated rules (sing/sang/sung) or in some cases, no rule at all (go/went) 14,15 .New verbs entering English universally obey the regular conjugation (google/googled/ googled), and many irregular verbs eventually regularize. Regular verbs become irregular much more rarely: for every sneak that snuck in 16 , there are many more flews that flied out.Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to M. A. N. (martin_nowak@harvard.edu). * These authors contributed equally to this work.Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Although less than 3% of modern verbs are irregular, the ten most common verbs are all irregular (be, have, do, go, say, can, will, see, take, get). The irregular verbs are heavily biased towards high frequencies of occurrence 17,18 . Linguists have suggested an evolutionary hypothesis underlying the frequency distribution of irregular verbs: uncommon irregular verbs tend to disappear more r...

A revolutionary advance in ecological immunology is that postgenomic technologies now allow molecular mediators defined in laboratory models to be measured at the mRNA level in field studies of many naturally occurring species. Here, we demonstrate the application of such an approach to generate meaningful immunological profiles for wild mammals. We sampled a natural field vole population across the year (n = 307) and developed a battery of cellular assays in which functionally different pro- and anti-inflammatory signalling responses (transcription factors and cytokines) were activated and quantified by Q-PCR. Temporal trends were the strongest feature in the expression data, although some life history stages (mating vs. nonmating males and pregnant females) were also associated with significant variation. There was a striking set of significant negative associations between inflammatory mediators and condition indices reflecting packed erythrocyte volume and relative liver size, spleen size and splenocyte count. Grouped (principal component) measures of inflammatory and anti-inflammatory expression were high in winter, with minima in the breeding season that occurred earlier for grouped anti-inflammatory responses than for grouped inflammatory responses. Some individual immunological mediators also showed patterns unrelated to the breeding season or annual periodic cues. For example, interferon regulatory factor 5 (IRF5) expression declined throughout the study period, indicating a systematic trend in antimicrobial defences. Pinpointing the causes and consequences of such variation may help identify underlying environmental drivers of individual fitness and demographic fluctuation.

SummaryAlthough the molecules and cells involved in triggering immune responses against parasitic worms (helminths) remain enigmatic, research has continued to implicate expansions of T-helper type 2 (Th2) cells and regulatory T-helper (T reg ) cells as a characteristic response to these organisms. An intimate association has also emerged between Th2 responses and wound-healing functions. As helminth infections in humans are associated with a strong Th2/T reg immunoregulatory footprint (often termed a 'modified Th2' response), plausible links have been made to increased susceptibility to microbial pathogens in helminth-infected populations in the tropics and to the breakdowns in immunological control (allergy and autoimmunity) that are increasing in frequency in helminth-free developed countries. Removal of helminths and their anti-inflammatory influence may also have hazards for populations exposed to infectious agents, such as malaria and influenza, whose worst effects are mediated by excessive inflammatory reactions. The patterns seen in the control of helminth immunity are discussed from an evolutionary perspective. Whilst an inability to correctly regulate the immune system in the absence of helminth infection might seem highly counter-adaptive, the very ancient and pervasive relationship between vertebrates and helminths supports a view that immunological control networks have been selected to function within the context of a modified Th2 environment. The absence of immunoregulatory stimuli from helminths may therefore uncover maladaptations that were not previously exposed to selection.