Competition level determines compensatory growth abilities

In many animal taxa, size-selective predation favors fast growth early in life. However, same-aged juveniles can diverge in size due to differences in genotype, environmental conditions, and parental effects and thus may vary in competitive ability. Under food scarcity, competitively inferior juveniles may suffer suppressed growth, whereas under benign conditions, small juveniles may exhibit growth compensation and perform as well as large ones. However, studies testing this while controlling for parental effects are lacking. Here, we hand-raised cichlid, Simochromis pleurospilus, from a wide range of egg sizes and manipulated their size by differential feeding. Afterward, high- and low-ration siblings were kept in groups assigned to either a high- or low-competition environment. We investigated how the degree of competition affected aggressiveness and growth of juveniles with different feeding histories. As predicted, when competition was high, high-ration offspring grew fastest. Interestingly, when competition was weak, low-ration juveniles grew at a similar rate as high-ration ones and many were able to catch up in size. High-ration fish were more aggressive than low-ration ones, and this effect was strongest under high competition. Additionally, in the high-competition environment, received aggression was negatively related to growth, and inflicted aggression correlated positively with the growth of the aggressor. These relationships were absent under low competition. Our findings suggest that the abilities to compensate for early growth depression depend on the prevalent level of competition. Aggression is likely used to monopolize food by juvenile S. pleurospilus; however, when competition is strong, aggression cannot compensate for a size disadvantage

Juvenile exposure to predator cues induces a larger egg size in fish

When females anticipate a hazardous environment for their offspring, they can increase offspring survival by producing larger young. Early environmental experience determines egg size in different animal taxa. We predicted that a higher perceived predation risk by juveniles would cause an increase in the sizes of eggs that they produce as adults. To test this, we exposed juveniles of the mouthbrooding cichlid Eretmodus cyanostictus in a split-brood experiment either to cues of a natural predator or to a control situation. After maturation, females that had been confronted with predators produced heavier eggs, whereas clutch size itself was not affected by the treatment. This effect cannot be explained by a differential female body size because the predator treatment did not influence growth trajectories. The observed increase of egg mass is likely to be adaptive, as heavier eggs gave rise to larger young and in fish, juvenile predation risk drops sharply with increasing body size. This study provides the first evidence that predator cues perceived by females early in life positively affect egg mass, suggesting that these cues allow her to predict the predation risk for her offspring

Egg size-dependent expression of growth hormone receptor accompanies compensatory growth in fish

Large egg size usually boosts offspring survival, but mothers have to trade off egg size against egg number. Therefore, females often produce smaller eggs when environmental conditions for offspring are favourable, which is subsequently compensated for by accelerated juvenile growth. How this rapid growth is modulated on a molecular level is still unclear. As the somatotropic axis is a key regulator of early growth in vertebrates, we investigated the effect of egg size on three key genes belonging to this axis, at different ontogenetic stages in a mouthbrooding cichlid (Simochromis pleurospilus). The expression levels of one of them, the growth hormone receptor (GHR), were significantly higher in large than in small eggs, but remarkably, this pattern was reversed after hatching: young originating from small eggs had significantly higher GHR expression levels as yolk sac larvae and as juveniles. GHR expression in yolk sac larvae was positively correlated with juvenile growth rate and correspondingly fish originating from small eggs grew faster. This enabled them to catch up fully in size within eight weeks with conspecifics from larger eggs. This is the first evidence for a potential link between egg size, an important maternal effect, and offspring gene expression, which mediates an adaptive adjustment in a relevant hormonal axis

Prey selection by North Sea herring (Clupea harengus) with special reference to fish eggs

The herring stock in the North Sea in recent years has recovered to a relatively high biomass, and here we investigate prey selection of individual North Sea herring when population numbers are high. The diet composition, and specifically pelagic fish eggs, was investigated in February 2004. Samples of herring from the International Bottom Trawl Survey were used for stomach analysis, and ichthyoplankton samples from the southern North Sea were used to investigate selection. Crustaceans were the main diet component. The average diameter of the fish eggs recovered from the stomachs was significantly larger than that of the eggs collected in the field. In addition, the frequency at which the latest developmental stages occurred in the herring stomachs was significantly different from the frequency at which these stages were found in the field. This shows selective foraging. There was a relationship between the amount of food and the number of eggs in a herring stomach: the fullest stomachs tended to contain fewer fish eggs. This suggests that herring forage on eggs when other prey are not available. Hence, it is likely that the dynamics of multiple trophic levels influence the ecological impact of a large herring stock on the North Sea ecosystem

Warfare in stingless bees

Bees are well known for being industrious pollinators. Some species, however, have taken to invading the nests of other colonies to steal food, nest material or the nest site itself. Despite the potential mortality costs due to fighting with an aggressive opponent, the prospects of a large bounty can be worth the risk. In this review, we aim to bring together current knowledge on intercolony fighting with a view to better understand the evolution of warfare in bees and identify avenues for future research. A review of literature reveals that at least 60 species of stingless bees are involved in heterospecific conflicts, either as attacking or victim colonies. The threat of invasion has led to the evolution of architectural, behavioural and morphological adaptations, such as narrow entrance tunnels, mud balls to block the entrance, decoy nests that direct invaders away from the brood chamber, fighting swarms, and soldiers that are skilled at immobilising attackers. Little is known about how victim colonies are selected, but a phylogenetically controlled analysis suggests that the notorious robber bee Lestrimelitta preferentially attacks colonies of species with more concentrated honey. Warfare among bees poses many interesting questions, including why species differ so greatly in their response to attacks and how these alternative strategies of obtaining food or new nest sites have evolved

Egg size and food abundance interactively affect juvenile growth and behaviour

1. Comparative evidence from several animal taxa suggests that juveniles hatching from larger eggs have fitness benefits when growing up in a harsh environment, whereas under benign conditions egg size should be of less importance. However, the physiological and behavioural mechanisms responsible for these context-dependent fitness differences are as yet poorly understood. 2. We studied the interactions between the phenotype of developing offspring and their environment in the mouthbrooding cichlid Simochromis pleurospilus. We hand-raised young from large and small eggs, and measured their initial body size and burst swimming speed. Thereafter we raised half of each egg-size class on high and half on low food ration and followed their growth trajectories and behavioural development until the age of 12 weeks. 3. We found that larger eggs gave rise to larger young that had a higher burst swimming speed. Food ration greatly influenced long-term growth, while egg size predominantly affected fish size during the first 2 weeks of life. However, large egg size caused a size advantage of juveniles persisting throughout the experimental period. 4. Egg size and food ration interactively affected the hiding and foraging behaviour of young. In the low-food treatment, individuals from small eggs spent less time in shelter and showed a higher commitment to foraging than individuals from large eggs. In a natural setting, this should markedly increase predation risk of young originating from small eggs, particularly in poor environments. In contrast, when food was plentiful juveniles behaved similarly, irrespective of egg size. 5. Our results show that egg size affects juvenile growth trajectories and behaviour differently in different environments. While it is well-established that a large egg size raises offspring fitness particularly in harsh environments, our study suggests that this advantage arises through risk-averse behaviour being tightly linked to offspring size