IntroductionThe differential allocation hypothesis (DAH) predicts that individuals should adjust their parental investment to their current mate’s quality. Although in principle the DAH holds for both sexes, male adjustment of parental investment has only been tested in a few experimental studies, revealing contradictory results. We conducted a field experiment to test whether male blue tits (Cyanistes caeruleus) allocate their parental effort in relation to female ornamentation (ultraviolet colouration of the crown), as predicted by the DAH.ResultsWe reduced the UV reflectance in a sample of females and compared parental care by their mates with that of males paired to sham-manipulated control females. As predicted by the DAH our results demonstrate that males paired with UV-reduced females invested less in feeding effort but did not defend the chicks less than males paired with control females.ConclusionsTo our knowledge, this is one of the first studies providing support for male differential allocation in response to female ornamentation.

The widely accepted functions of complex bird song - to defend a territory or attract a mate, or both - have generally been tested in northern hemisphere species in which males produce the song and females choose the singer. In our study species, the Superb Fairy-wren (Malurus cyaneus), both males and females sing a solo song throughout the year. We compare the chatter song in males and females of two genetically distinct subspecies, and test if resident birds respond to the sex and subspecies of the intruder song. Compared with island birds (M. c. ashbyi), mainland Superb Fairy-wrens (M. c. leggei) produced songs with lower frequency and fewer elements. Compared with females, males produced longer songs with more elements. Resident birds showed acoustical discrimination for the sex and subspecies of the intruder bird. The response of resident pairs was positively correlated, but each sex showed a solo response. Resident males were the first to respond to male intruders, and resident females were the first to respond to female intruders. Fairy-wrens had the strongest response towards (1) intruders of the same subspecies and (2) male intruders. The finding of signal divergence and acoustical discrimination in males and females makes this a model system to test the mechanism of reproductive isolation when both sexes sing

In Coregonus sp. the period between hatching and metamorphosis is characterized by an enhanced involvement of glycolysis in energy metabolism (as reflected by oxygen consumption and enzyme activities) and by the differentiation of the red and pink muscle fibers on which the increasing versatility of swimming performance of the larvae appears to depend. The larval weight of Coregonus sp. increased during the first 150 d of development at 10 °C from 6 to 2000 mg, but the average rate of oxygen consumption decreased only from 639 to 419 μg∙g−1∙h−1. Four types of muscle fibers were distinguished, each with a distinct developmental pattern: red and pink fibers first become observable 25–34 d after hatching, the latter growing more slowly than the former; before this, only white muscles and a characteristic layer of small diameter red fibers are present in Coregonus sp. The activities of the two oxidative enzymes, citrate synthase and cytochrome oxidase, and the activity of hexokinase, increased directly after hatching, reaching a peak within 20–47 d. On the other hand, the three glycolytic enzymes, phosphofructokinase, pyruvate kinase, and lactate dehydrogenase, remained at a constantly low level from hatching to day 40, whereafter their rates of activity began to increase rapidly.

Female song is an ancestral trait in songbirds, yet extant females generally sing less than males. Here, we examine sex differences in the predation cost of singing behaviour. The superb fairy-wren (Malurus cyaneus) is a Southern Hemisphere songbird; males and females provision the brood and produce solo song year-round. Both sexes had higher song rate during the fertile period and lower song rate during incubation and chick feeding. Females were more likely than males to sing close to or inside the nest. For this reason, female but not male song rate predicted egg and nestling predation. This study identifies a high fitness cost of song when a parent bird attends offspring inside a nest and explains gender differences in singing when there are gender differences in parental care.

Do parents defend their offspring whenever necessary, and do self-sacrificing parents really exist? Studies recognized that parent defence is dynamic, mainly depending on the threat predators pose. In this context, parental risk management should consider the threat to themselves and to their offspring. Consequently, the observed defence should be a composite of both risk components. Surprisingly, no study so far has determined the influence of these two threat components on parental decision rules. In a field experiment, we investigated parental risk taking in relation to the threat posed to themselves and their offspring. To disentangle the two threat components, we examined defence behaviours of parent blue tits Cyanistes caeruleus towards three different predators and during different nestling developmental stages. Nest defence strategies in terms of alarm call intensity and nearest predator approach differed between the three predators. Defence intensity was only partly explained by threat level. Most importantly, parental risk management varied in relation to their own, but not offspring risk. Parent defence investment was independent of nestling risk when parents followed a high-risk strategy. However, parents considered nestling as well as parental risk when following a low-risk strategy. Our findings could have general implications for the economy of risk management and decision-making strategies in living beings, including humans.