Animal eyes are some of the most widely recognisable structures in nature. Due to their salience to predators and prey, most research has focused on how animals hide or camouflage their eyes [1]. However, across all vertebrate Classes, many species actually express brightly coloured or conspicuous eyes, suggesting they may have also evolved a signalling function. Nevertheless, perhaps due to the difficulty with experimentally manipulating eye appearance, very few species beyond humans [2] have been experimentally shown to use eyes as signals [3]. Using staged behavioural trials we show that Trinidadian guppies (Poecilia reticulata), which can rapidly change their iris colour, predominantly express conspicuous eye colouration when performing aggressive behaviours towards smaller conspecifics. Furthermore, using a novel, visually realistic robotic system to create a mismatch between signal and relative competitive ability, we show that eye colour is used to honestly signal aggressive motivation. Specifically, robotic 'cheats' (that is, smaller, less-competitive robotic fish that display aggressive eye colouration when defending a food patch) attracted greater food competition from larger real fish. Our study suggests that eye colour may be an under-appreciated aspect of signalling in animals, shows the utility of our biomimetic robotic system for investigating animal behaviour, and provides experimental evidence that socially mediated costs towards low-quality individuals may maintain the honesty of dynamic colour signals.

As telomere length (TL) often predicts survival and lifespan, there is considerable interest in the origins of inter-individual variation in TL. Cross-generational effects of parental age on offspring TL are thought to be a key source of variation, but the rarity of longitudinal studies that examine the telomeres of successive offspring born throughout the lives of parents leaves such effects poorly understood. Here, we exploit TL measures of successive offspring produced throughout the long breeding tenures of parents in wild white-browed sparrow weaver ( Plocepasser mahali ) societies, to isolate the effects of within-parent changes in age on offspring TLs. Our analyses reveal the first evidence to date of a positive within-parent effect of advancing age on offspring TL: as individual parents age, they produce offspring with longer telomeres (a modest effect that persists into offspring adulthood). We consider the potential for pre- and post-natal mechanisms to explain our findings. As telomere attrition predicts offspring survival to adulthood in this species, this positive parental age effect could impact parent and offspring fitness if it arose via differential telomere attrition during offspring development. Our findings support the view that cross-generational effects of parental age can be a source of inter-individual variation in TL.

Highlights d Guppies, particularly large females, blacken their eyes when encountering predators d Predatory fish attack black-eyed guppies toward their head d Guppies more easily evade these diverted attacks using a pivoting evasive maneuver d This offsets the reduced agility of larger prey, possibly allowing the evolution of larger body sizes

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Investigatio-ns on the changes in the blood and blood pressure in relation to burns (Brown, 1944) have shown that even with minor injuries the blood pressure may be raised and slight haemoconcentration may occur. These findings may be related, and both may be due to excitement. Adrenaline plays a prominent part in the production of the excitement syndrome, and it was thought that, by observing the effect of the drug on haemoglobin levels, some idea would be gained as to the possible magnitude of the resulting changes in man caused by excitement. METHODSIn the course of investigations on the glycogenolytic action of adrenaline in cases of hepatic disease, advantage was taken of the opportunity to observe the effect of the drug on haemoglobin levels. The nature of the primary investigation explains the relative frequency of liver disease among the patients selected. Several relatively normal individuals have also been included.The patients were confined to bed on the day of the test which began at about 11.15 a.m. and ended before the midday meal. Samples of blood were obtained 5 min. before, and 10, 30 and 60 min. after, the subcutaneous injection of 1 ml. of 0-1 % adrenaline hydrochloride. Two patients were given 1 ml. of 0-85 % sodium chloride instead of adrenaline.Venous stasis of short duration (less than 60 sec.) was used to facilitate venepuncture. Such stasis does not influence haemoglobin levels (Brown, 1945). Blood was collected in a clean dry tube containing a suitable quantity of heparin as an anticoagulant.Haemoglobin was estimated in a Spekker photoelectric colorimeter with oxyhaemoglobin as the pigment, 0-100 ml. of blood being added to 10 ml. of 0.4 % ammonia. The pipettes used were Grade 'A' (N.P.L.). All estimations were performed in duplicate, and no two such estimations differed by more than 0*15 g. %. RESULTSThe changes in haemoglobin levels in sixteen patients given 1 mg. of adrenaline hydrochloride are shown in Table 1. With three exceptions, a significant rise in haemoglobin followed the injection. It reached a maximum in 10 min. when an average rise of 0 73 g., equivalent to 5-53 % of the mean original level, was recorded. A fall occurred in 30 min., and return to pre-adrenaline levels was noted at the end of 1 hr. There is no evidence that the results were affected by the nature of the illness present.