Delayed maturation in plumage colour: Evidence for the female-mimicry hypothesis in the kestrel

Summary In many sexually dichromatic species, young males have female-like plumage during their first potential breeding year. The female-mimicry hypothesis (FMH) supposes that by possessing female-like plumage young males deceive older conspicuous males into believing that they are females, thus reducing competition from adult males. The status-signalling hypothesis (SSH) supposes that adult males can distinguish sex, but postulates that young males reduce competition from adult males by reliably signaling low status with their dull plumage. We tested these hypotheses in the European kestrel (Falco tinnunculus). Female-like young males settled to breed closer to adult males than did other adult males (Figs. 1a, b). By settling near adult males, young males seemed to increase their chance of mating with adult females. Adult female-young male pairs had better reproductive success than yearling-yearling pairs. These results suggest that there is an adaptive value in possessing a female-like plumage colour in the breeding season. To test the FMH, we measured sexual preference of adult males when adult females and young males were simultaneously shown in an aviary. Adult males were unable to recognize sex, because in half the cases they preferred young males (Fig. 3). However, when adult males and females were shown simultaneously, males preferred females (Fig. 2). Our results support the FMH rather than the SSH, because young males successfully deceived older males by their plumage.     

Male mating speed promote hybridization in the Rana lessonae–Rana esculenta waterfrog system

Hybridization is a widespread phenomenon in many vertebrate groups. Prezygotic isolating mechanisms, probably caused by selection against hybrids with reduced fitness, reduce the likelihood of such events. Although hybrid-reduced fitness relatively to parental species is common, hybridization can also be beneficial, and hybrids sometimes outperform the pure species type. In this study, we examined two potential processes, Hubbs’s principle and male–male competition, which could enhance hybridization in the waterfrog complex and thus explain the proportion of heterospecific pairs collected in a natural pond. Firstly, by collecting 791 frogs in the field to study pair and chorus composition, we showed that in a mixed Rana lessonae–Rana esculenta population, the scarcity of hybrid R. esculenta males did not account for the proportion of heterospecific pairs: indeed, when examining pairing composition in six different choruses, we found that hybrid males were always under-represented and that R. esculenta females were found paired with R. lessonae males. Secondly, we investigated experimentally whether or nor male–male competition mechanism could explain pair formation in waterfrogs. Our mating speed experiment highlights mechanisms that could explain heterospecific pairs in a context of promiscuous mating where scramble competition was intense. To measure the rapidity with which a male grasps a female, we placed males in a grid cage with a female, and the dynamics of pair formation was monitored. R. esculenta males showed a lower pairing success than R. lessonae males as a smaller proportion of them amplexed females, and more time was needed for them to get amplexed. Thus, a less adaptative mechanism than female mate choice may also explain the mating pattern observed in waterfrog species.     

Male approach and female avoidance as mechanisms of population discrimination in sagebrush lizards

Reproductive isolation and speciation can result from female choice for particular males. Isolation can also result, however, from male mating preferences or from aggressive encounters which then influence mating decisions. In this study, we use laboratory discrimination trials to study the behavioral mechanisms of population discrimination in sagebrush lizards (Sceloporus graciosus). We specifically ask three questions about population-level discrimination: (1) Does it vary in strength in relation to the geographic distance between the populations? (2) Is it more apparent in inter- or intra-sexual interactions? (3) Does it take the form of attraction or avoidance? We ran 890 trials that tested the ability of male and female sagebrush lizards from one population to discriminate their own population from four other populations. In addition, we utilized both sequential and simultaneous-choice designs, which enabled us to distinguish between attraction and avoidance. We found that most population-level discrimination was exhibited by male lizards preferring to associate with particular types of females, as well as female avoidance of particular types of males. The strength and direction of both discriminations depended on the populations compared and on whether the tests were conducted as sequential- or simultaneous-choice tests, producing a complex relationship between geographic distance and behavioral discrimination. Our results suggest that there are roles for male attraction and female avoidance in population discrimination, reproductive isolation, and speciation.     

Social network theory in the behavioural sciences: potential applications

Social network theory has made major contributions to our understanding of human social organisation but has found relatively little application in the field of animal behaviour. In this review, we identify several broad research areas where the networks approach could greatly enhance our understanding of social patterns and processes in animals. The network theory provides a quantitative framework that can be used to characterise social structure both at the level of the individual and the population. These novel quantitative variables may provide a new tool in addressing key questions in behavioural ecology particularly in relation to the evolution of social organisation and the impact of social structure on evolutionary processes. For example, network measures could be used to compare social networks of different species or populations making full use of the comparative approach. However, the networks approach can in principle go beyond identifying structural patterns and also can help with the understanding of processes within animal populations such as disease transmission and information transfer. Finally, understanding the pattern of interactions in the network (i.e. who is connected to whom) can also shed some light on the evolution of behavioural strategies.     

Courtship display by great frigatebirds, Fregata minor: an energetically costly handicap signal?

We used detailed time-budget observations, mark-resight data, and doubly labeled water estimates of energy expenditure to test whether energy spent on courtship display by male frigatebirds functions as a sexually selected handicap signal. During a 2-day period of time budget observations, males displayed on average 29.5% of the time (range 0–100% for 15 different males), and this value was correlated with an index of season-long display effort. Season-long display effort was strongly predictive of pairing success. Average field metabolic rate (FMR) during the 2-day time budget period was 676.5 kJ/day (range 464.8–1035.0), substantially lower than the mass-specific FMR predicted from studies of other seabirds during incubation or chick-rearing. Despite a low overall FMR, display effort could function as an energetic handicap, either if FMR correlates positively with the amount of courtship performed or if high-quality males display at a high rate because they pay a lower energy cost per unit of display than do low-quality males. We found no relationship between FMR and display rate, despite good power for doing so. We also did not find a significant difference in energy spent divided by courtship time for males that were or were not chosen by females (inferred to be high- and low-quality males, respectively), though the medians differed in the predicted direction (preferred males having lower relative costs than non-preferred males) and the confidence interval on the difference between groups was very wide. Thus, we found evidence that male courtship effort is predictive of pairing success, we rejected one mechanism by which energetic cost of display could function as a handicap, and our test of the alternate energetic handicap mechanism was equivocal.     

Mate compatibility,parental allocation and fitness consequences of mate choice in the sand goby <Emphasis Type="Italic">Pomatoschistus minutus</Emphasis>

Recent theory and empirical work suggests that there may be variation among females in mate preferences that is adaptive. One of the possible mechanisms maintaining variability in preferences and preferred traits is that the benefits of mate choice may depend on compatibility with potential mating partners. We examined fitness consequences of mate choice in a species of fish, the sand goby, Pomatoschistus minutus with a special focus on mate compatibility. Females were given the opportunity to establish their mate preferences in a dichotomous mate choice experiment. This information was then applied by mating the focal or control female with either the preferred or the non-preferred male. The parental performance of the males of these four mating combinations was then measured. In a separate experiment, we assessed the female differential allocation by determining the residual gonad weight of spawned females as a measure of the proportion of eggs spawned. We also estimated the amount of filial cannibalism separately for both sexes. Our results show that preferred males provided benefits in the form of an increased number of hatching eggs. This benefit was the same when the male was mated with a focal or a control female. Hence, we found no support for benefits that depend on mate compatibility. Neither did we find support for the hypothesis that females would lay a different number of eggs depending on the male status. The results also indicate that male filial cannibalism has a strong role in determining hatching success in this species.     

Extra-pair paternity in alpine marmots, <Emphasis Type="Italic">Marmota marmota</Emphasis>: genetic quality and genetic diversity effects

Assuming that a male’s genetic characteristics affect those of his offspring, extra-pair copulation has been hypothesized to increase heterozygosity of the progeny—the “genetic compatibility” hypothesis—and the genetic diversity within litters—the “genetic diversity” hypothesis. We tested these two hypotheses in the alpine marmot (Marmota marmota), a socially monogamous mammal showing a high rate of extra-pair paternity (EPP). In a first step, we tested the assumption that a male’s genetic characteristics (heterozygosity and genetic similarity to the female) affect those of his offspring. Genetic similarity between parents influenced offspring heterozygosity, offspring genetic similarity to their mother, and litter genetic diversity. The father’s heterozygosity also influenced litter genetic diversity but did not affect offspring heterozygosity. Hence, heterozygosity seems not to be heritable in the alpine marmot. In a second step, we compared genetic characteristics of extra-pair young (EPY) and within-pair young (WPY). EPY were less genetically similar to their mother but not more heterozygous than WPY. EPY siblings were also less genetically similar than their WPY half siblings. Finally, the presence of EPY promoted genetic diversity within the litter. Thus, our data support both the “genetic compatibility” and the “genetic diversity” hypotheses. We discuss further investigations needed to determine the primary causes of EPP in this species.     

Vigilance in wild Thomas's langurs (Presbytis thomasi ): the importance of infanticide risk

This study examines vigilance as a behavioural indicator of the importance of infanticide risk by comparing the infanticide avoidance hypothesis with the predation avoidance and mate defence hypotheses for wild Thomas's langurs (Presbytis thomasi) in Sumatra. We found that all individuals were more vigilant in situations of high predation risk, i.e. lower in the trees and in the absence of neighbours. Females were also more vigilant on the periphery of the group. However, there were variations in vigilance levels that could not be accounted for by the predation avoidance hypothesis. Males without infants showed higher levels of vigilance in areas of home range overlap than in non-overlap areas during the early phase of their tenure, strongly suggesting mate defence. In these areas of home range overlap where Thomas's langur groups can interact, males may attack females and infants, and so the infanticide risk for males and females with infants is likely to be high in these areas. Only females with infants, but not males with infants or females without infants, showed higher vigilance levels in overlap areas than in non-overlap areas; in addition, in overlap areas, females with an infant were more vigilant than females without an infant, while this was not the case in non-overlap areas. Both females and males with infants were more vigilant high in the trees than at medium heights in overlap areas but not elsewhere. These findings can only be explained by the infanticide avoidance hypothesis. In contrast to predator attacks, infanticidal male attacks come from high in the canopy, and only occur in overlap areas. There was a significant sex difference in vigilance, but males were only more vigilant than females without an infant, and not more vigilant than females with an infant. We conclude that vigilance varied mainly in relation to the risk of predation and infanticide. Mate competition only played a role for males during the early phase of their tenure. Predation risk seems to offer the best explanation for vigilance for all individuals in the absence of infants. Both predation risk and infanticide risk played a role for females and males with infants. Received: 4 April 1998 / Accepted after revision: 6 September 1998     

Mating system evolution in response to search costs in the speckled wood butterfly, Pararge aegeria

A general and intuitive prediction from models of mate preference is that when the cost of searching for mates increases, individuals should become less choosy. Here, we test this prediction by comparing the mating propensity of females in two populations of the butterfly Pararge aegeria. The populations originated from southern Sweden and Madeira and due to different adult emergence patterns throughout the year, the average density of males per female is likely to be lower on Madeira. Therefore, we expected that the cost of searching should be greater on Madeira and, consequently, that the Madeiran females should be less choosy. In line with predictions, the Madeiran females mated significantly sooner after the first interaction with males than did females from southern Sweden. This difference may reflect a weaker preference for territorial males over non-territorial patrollers in the Madeiran population, because of the greater costs of searching. The Madeiran females also showed a shorter time lag between mating and the start of oviposition. We discuss this unexpected result and propose that the same mechanism could also explain this population difference, i.e. different costs of searching for suitable host plants. Both search processes are fundamental for female reproductive success and we find it plausible that they can be generalised into the same theory of optimal search behaviour. Received: 14 May 1998 / Accepted after revision: 13 December 1998     

Imperfect female choice and male mating skew on leks of different sizes

We present a model of error-prone female choice on leks, and investigate the effects of different degrees of error on the distribution of mating success among males present at leks of different sizes. At higher levels of error, the best male is predicted to gain a smaller share of matings, while low-ranking males gain a larger share. Males who are of high rank but not the most desirable on the lek do best at intermediate levels of error, since the top-ranked male does not then claim all the matings, but assessment is still sufficiently accurate for females to discriminate between high-ranking and low-ranking competitors. The effects of error are shown to be more pronounced on larger leks, due to smaller expected differences in mating value between males of adjacent ranks. This interaction between lek size and error suggests that observed negative relationships between lek size and mating skew need not be attributed solely to intrasexual competition, as previously suggested, but could also be a result of imperfect choice. Received: 20 February 1998 / Accepted after revision: 25 October 1998