Phenotypic Plasticity in Response to the Social Environment: Effects of Density and Sex Ratio on Mating Behaviour Following Ecotype Divergence

The ability to express phenotypically plastic responses to environmental cues might be adaptive in changing environments. We studied phenotypic plasticity in mating behaviour as a response to population density and adult sex ratio in a freshwater isopod (Asellus aquaticus). A. aquaticus has recently diverged into two distinct ecotypes, inhabiting different lake habitats (reed Phragmites australis and stonewort Chara tomentosa, respectively). In field surveys, we found that these habitats differ markedly in isopod population densities and adult sex ratios. These spatially and temporally demographic differences are likely to affect mating behaviour. We performed behavioural experiments using animals from both the ancestral ecotype (“reed” isopods) and from the novel ecotype (“stonewort” isopods) population. We found that neither ecotype adjusted their behaviour in response to population density. However, the reed ecotype had a higher intrinsic mating propensity across densities. In contrast to the effects of density, we found ecotype differences in plasticity in response to sex ratio. The stonewort ecotype show pronounced phenotypic plasticity in mating propensity to adult sex ratio, whereas the reed ecotype showed a more canalised behaviour with respect to this demographic factor. We suggest that the lower overall mating propensity and the phenotypic plasticity in response to sex ratio have evolved in the novel stonewort ecotype following invasion of the novel habitat. Plasticity in mating behaviour may in turn have effects on the direction and intensity of sexual selection in the stonewort habitat, which may fuel further ecotype divergence.     

Why Parental Sex Ratio Manipulation is Rare in Higher Vertebrates (Invited Article)

The debate over adaptive parental sex ratio adjustment in higher vertebrates appears neither to be resolvable by the current approach, nor does it necessarily make sense. It rests on the a priori supposition of parental manipulation, which is questioned here from first principle. It is considered an unlikely biological hypothesis if we extend our perspective to gametic and offspring optimal strategies, and to the potential mechanisms existing in the avian and mammalian reproductive systems. Evenness of primary sex ratios is expected to be optimal from the gametic point of view and is supposed here to be the more likely evolutionary outcome. Also, manipulations by sex-selective offspring mortality is argued to be unlikely as usually the benefits will be surpassed by the costs incurred. Furthermore, parents can adjust behavioural and energetic investment patterns to their offspring sex (ratio), thereby minimizing any costs of sex ratio control inability. Slight biases in offspring sex ratios are then viewed as resulting from physiological limitations ultimately relating to sex differences in embryonic development. Contrary to recent attempts to understand higher vertebrate sex ratio variation by further refinement of functional models (of parental optima) and data analysis, Bayesian logic precludes those approaches to gain useful new insights. To prove the basic assumption of parental manipulation, apart from defining gametic and offspring optima, the emphasis should lie on identifying control mechanisms by experimental verification.     

Brood Relief Behaviour of the Cichlid Fish Etroplus maculatus

Quantitative recordings were made of the behaviour of Etroplus maculatus parents caring for eggs and wrigglers. Spatial and temporal aspects of brood relief behaviour are examined in connection with parent size, time of day, interval since spawning and the frequencies of occurrence of other behaviours. Function aspect of Cichild brood relief behaviour are discussed and compared with those of avian nest relief.     

窝雏数处理对两种雀形目幼鸟生长的影响

基于 1 997～ 1 999年野外实验 ,对高寒草甸小云雀和黄嘴朱顶雀两种雀形目鸟的窝雏数进行增减处理。结果表明 ,对照组的幼鸟生长率和离巢体重都大于增加组 ,说明窝雏数增加后 ,幼鸟质量下降。随着窝雏数增加 ,这两种幼鸟生长率显著下降 (小云雀 :r =-0 965 ,P =0 0 3 5 <0 0 5 ;朱顶雀 :r =-0 82 8,P =0 0 2 2 <0 0 5 )。窝雏数改变对小云雀幼鸟出飞重影响不显著 (r =-0 41 8,P =0 5 2 8>0 0 5 ) ,而对黄嘴朱顶雀有显著的影响 (r=-0 90 1 ,P =0 0 1 4<0 0 5 )。     

Evidence for Strategic Sex Allocation in the Guppy (Poecilia reticulata): Brood Sex Ratio and Size Predict Subsequent Adult Male Mating Success

Sex allocation theory predicts that females should produce more sons when the reproductive success of sons is expected to be high, whereas they should produce more daughters, not daughters when the reproductive success of sons is expected to be low. The guppy (Poecilia reticulata) is a live‐bearing fish, and female guppies are known to produce broods with biased sex ratios. In this study, we examined the relationship between brood sex ratio and reproductive success of sons and daughters, to determine whether female guppies benefit from producing broods with biased sex ratios. We found that sons in male‐biased broods had greater mating success at maturity than sons in female‐biased broods when brood sizes were larger. On the other hand, the reproductive output of daughters was not significantly affected by brood sizes and sex ratios. Our results suggest that female guppies benefit from producing large, male‐biased brood when the reproductive success of sons is expected to be high.     

Variation in Brood Sex Ratios of Texas Rio Grande Wild Turkeys

Abstract: We evaluated brood sex ratio (BSR) variation in Rio Grande wild turkeys (RGWT; Meleagris gallopavo intermedia) in the Edwards Plateau and South Texas Plains of Texas, USA, during 2005-2006. Offspring sex was determined from DNA extracted from tissue biopsies of embryos from unhatched eggs or vascular tissue from eggshells of hatched and depredated eggs. Sex ratio across all eggs was 56.3% male (135/240; X²₁ = 3.75, P = 0.053). We found that mean population growth rate based on a population simulation with BSR at unity averaged 1.02 (range = 0.924-1.058), whereas it declined to 0.978 (range = 0.816-1.037) using BSR estimates from our study. Although our statistical analyses did not detect BSRs different from unity in BSR, our simulation modeling demonstrated that BSR variation caused biologically significant differences in mean population growth rates. Even though the biological mechanism controlling primary sex ratio remains unknown, our estimates of BSR should allow managers to more reliably predict population dynamics insuring viable RGWT populations across Texas.     

The First Sexual Lineage and the Relevance of Facultative Sex

Models for the origin of the sex incorporate either obligate or facultative sexual cycles. The relevance of each assumption to the ancestral sexual population can be examined by surveying the sexual cycles of eukaryotes, and by determining the first lineage to diverge after sexuality evolved. Two protistan groups, the parabasalids and the oxymonads, have been suggested to be early-branching sexual lineages. A maximum-likelihood analysis of elongation factor-1α sequences shows that the parabasalids diverged prior to the oxymonads and thus represent the earliest sexual lineage of eukaryotes. Since both of these protist lineages and most other eukaryotes are facultatively sexual, it is likely that the common ancestor of all known eukaryotes was facultatively sexual as well. This finding has important implications for the ``Best-Man hypothesis' and other models for the origin of sex. Received: 21 August 1998 / Accepted: 26 December 1998     

Parental Sex Differences in Food Allocation to Junior Brood Members as Mediated by Prey Size

Individual offspring within a brood may receive different amounts of provisioning from the male and female parents. Some hypotheses suggest that this bias is the result of an active and adaptive choice by parents. An alternative hypothesis is that feeding biases arise as a result of a constraint of fitting large prey items into small gapes. In an experiment with pied flycatchers, Ficedula hypoleuca , we tested for sex-biased allocation to junior nestlings in asynchronous broods and whether this could be explained by active parental choice or by passive allocation according to prey size and gape size. In both control broods and broods with experimentally increased degree of asynchrony, prey types did not differ between parents but females brought smaller prey than males at younger but not older nestling stages. At younger but not older nestling stages, the majority of feeds to junior nestlings were from females, and the smaller nestlings consumed smaller prey than older siblings. However, there was no evidence of active preference of small nestlings by females as parents did not differ in the tendency to bypass a begging senior nestling in order to feed a junior nestling. Provisioning rates by females were lower than those by males when nestlings were young and we suggest that foraging time constraints caused by the need to brood offspring result in females bringing smaller prey than males. In turn, the larger prey brought by males was more often transferred to larger offspring after the smaller ones failed to swallow it. In such cases, 'preferential' feeding of small nestlings by females may simply be a passive side effect of foraging constraints and gape-size limitations.     

Sex Determination in Freshwater Eels and Management Options for Manipulation of Sex

Catadromous eels enter fresh water as sexually undifferentiated glass eels and develop into males and females before migrating back to sea as silver eels. Females develop ovaries directly from the ambiguous primordial gonad whereas males pass through a transitional intersexual stage before developing testes. Eels have sex-specific life-history strategies. Males may grow faster than females initially, but this difference is soon reversed and females attain a greater age- and size-at-metamorphosis than males. Male fitness is maximized by maturing at the smallest size that allows a successful spawning migration (a time-minimizing strategy) whereas females adopt a more flexible size-maximizing strategy that trades off pre-reproductive mortality against fecundity. Although heteromorphic sex chromosomes have been identified in some species, the sex of developing gonads is labile and gender is determined principally by environmental factors. Individuals experiencing rapid growth prior to gonad differentiation tend to develop as males, whereas eels that grow slowly initially are more likely to develop as females. Paradoxically, males tend to predominate under conditions of high density, which may be because a male “grow quickly, mature early” strategy increases an individual’s chances of survival during periods of intraspecific competition. High temperatures and saline conditions have also been proposed to favor development as males but experimental studies have failed to demonstrate a clear effect of either on sex determination. High proportions of female silver eels migrating from some upstream areas, lakes and large rivers may be due to low population density or poor conditions for growth in these habitats. Manipulating sex ratios in favor of females has the potential to increase eel production in aquaculture and to buffer natural populations against fishing pressure. Sex steroids (oestrogens and phytoestrogens) have a strong feminizing effect on undifferentiated individuals and are most effective when targeted at younger eels and administered at high doses for prolonged periods. Modifying local environmental conditions, in particular reducing eel density and limiting interference and social stress, may also promote the development of females. Further research into the timing and mechanisms of sex determination in eels is required to effectively and efficiently manipulate sex for conservation and/or economic benefit.     

Change of Genetic Architecture in Response to Sex

A traditional view is that sexual reproduction increases the potential for phenotypic evolution by expanding the range of genetic variation upon which natural selection can act. However, when nonadditive genetic effects and genetic disequilibria underlie a genetic system, genetic slippage (a change in the mean genotypic value contrary to that promoted by selection) in response to sex may occur. Additionally, depending on whether natural selection is predominantly stabilizing or disruptive, recombination may either enhance or reduce the level of expressed genetic variance. Thus, the role of sexual reproduction in the dynamics of phenotypic evolution depends heavily upon the nature of natural selection and the genetic system of the study population. In the present study, on a permanent lake Daphnia pulicaria population, sexual reproduction resulted in significant genetic slippage and a significant increase in expressed genetic variance for several traits. These observations provide evidence for substantial genetic disequilibria and nonadditive genetic effects underlying the genetic system of the study population. From these results, the fitness function of the previous clonal selection phase is inferred to be directional and/or stabilizing. The data are also used to infer the effects of natural selection on the mean and the genetic variance of the population.     

Consistency in Egg Rejection Behaviour: Responses to Repeated Brood Parasitism in the Blackcap (Sylvia atricapilla)

To evaluate host responses towards repeated brood parasitism we experimentally parasitized and continuously videotaped blackcap nests in two consecutive trials. The ejection of a foreign egg was the most common response (94.5%) in both trials, but desertion (4.1%) and acceptance (1.4%) also occurred. The general method of ejection was puncturing. In 9.8% of identified ejections, already punctured eggs stuck to the abdominal feathers of the incubating bird and were carried out of the nest. Females were responsible for the majority of ejections in both trials and their response time was significantly shorter than that of males. Blackcaps exhibited consistency in the sex responsible for egg ejection over the two trials; but in five (20.8%) experiments individuals changed their behaviour. Repeatability for host responses within the nest was very high. In ejections accomplished by the same bird, the response was significantly quicker in the second trial, indicating the presence of certain learning abilities. Our results suggest that cuckoo hosts are quite consistent in their responses towards parasitic eggs when parasitized repeatedly within one breeding attempt.     

Indirect Genetic Effects and Housing Conditions in Relation to Aggressive Behaviour in Pigs

Indirect Genetic Effects (IGEs), also known as associative effects, are the heritable effects that an individual has on the phenotype of its social partners. Selection for IGEs has been proposed as a method to reduce harmful behaviours, in particular aggression, in livestock and aquaculture. The mechanisms behind IGEs, however, have rarely been studied. The objective was therefore to assess aggression in pigs which were divergently selected for IGEs on growth (IGEg). In a one generation selection experiment, we studied 480 offspring of pigs (Sus scrofa) that were selected for relatively high or low IGEg and housed in homogeneous IGEg groups in either barren or enriched environments. Skin lesion scores, a proxy measure of aggression, and aggressive behaviours were recorded. The two distinct IGEg groups did not differ in number of skin lesions, or in amount of reciprocal fighting, both under stable social conditions and in confrontation with unfamiliar pigs in a 24 h regrouping test. Pigs selected for a positive effect on the growth of their group members, however, performed less non-reciprocal biting and showed considerably less aggression at reunion with familiar group members after they had been separated during a 24 h regrouping test. The enriched environment was associated with more skin lesions but less non-reciprocal biting under stable social conditions. Changes in aggression between pigs selected for IGEg were not influenced by G×E interactions with regard to the level of environmental enrichment. It is likely that selection on IGEg targets a behavioural strategy, rather than a single behavioural trait such as aggressiveness.     

Facultative Sex Allocation and Sex‐Specific Offspring Survival in Barrow's Goldeneyes

Sex allocation theory predicts that females should bias their reproductive investment towards the sex generating the greatest fitness returns. The fitness of male offspring is often more dependent upon maternal investment, and therefore, high‐quality mothers should invest in sons. However, the local resource competition hypothesis postulates that when offspring quality is determined by maternal quality or when nest site and maternal quality are related, high‐quality females should invest in the philopatric sex. Waterfowl – showing male‐biased size dimorphism but female‐biased philopatry – are ideal for differentiating between these alternatives. We utilized molecular sexing methods and high‐resolution maternity tests to study the occurrence and fitness consequences of facultative sex allocation in Barrow's goldeneyes (Bucephala islandica). We determined how female structural size, body condition, nest‐site safety and timing of reproduction affected sex allocation and offspring survival. We found that the overall sex ratio was unbiased, but in line with the local resource competition hypothesis, larger females produced female‐biased broods and their broods survived better than those of smaller females. This bias occurred despite male offspring being larger and tending to have lower post‐hatching survival. The species shows strong female breeding territoriality, so the benefit of inheriting maternal quality by philopatric daughters may exceed the potential mating benefit for sons of high‐quality females.     

Response to a Dangerous Enemy: Should a Brood Parasite be Mobbed?

Models of a long-tailed cuckoo and a song thrush (as a control) were presented 1.5 to 3 m from the nests of whiteheads, a species that the cuckoo is known to parasitize. Tests were conducted early in incubation (days 3 to 8) and during the nestling period. Most breeding females responded to the cuckoo by remaining inconspicuous or hiding. Other members of the cooperatively breeding group mobbed the cuckoo if they saw it. This difference in behaviour resulted in differences in overall response between incubation and nestling periods; the cuckoo was only occasionally mobbed during incubation because birds other than the female rarely approached the nest, whereas the cuckoo was almost always mobbed after eggs hatched. Cuckoos benefit from group breeding by whiteheads because chicks receive more food if reared by larger groups. I suggest that inconspicuous behaviour by the female in the presence of a cuckoo results in at least two effects: first, females can determine what the intentions of the cuckoo are, particularly if it intends to lay in the nest; second, no cues about size of breeding group are given to the cuckoo. Comparisons are drawn with the recent study of PAYNE et al. (1985) on splendid wrens and Chrysococcyx cuckoos. Despite similar social behaviour, splendid wrens and whiteheads behaved differently in the presence of a cuckoo. Mobbing may not always be the most appropriate tactic when faced with a brood parasite.     

Lignocellulose Mineralization by Arctic Lake Sediments in Response to Nutrient Manipulation

Mineralization of specifically labeled ¹⁴C-cellulose- and ¹⁴C-lignin-labeled lignocelluloses by Toolik Lake, Alaska, sediments was examined in response to manipulation of various environmental factors. Mineralization was measured by quantifying the amount of labeled CO₂ released from the specifically labeled substrates. Nitrogen (NH₄NO₃) and, to a greater degree, phosphorus (PO₄⁻³) additions enhanced the mineralization of white pine (Pinus strobus) cellulose during the summer of 1978. Nitrogen and phosphorus together had no cumulative effect. During the summer of 1979, nitrogen or phosphorus alone had only a slight stimulatory effect on the mineralization of a sedge (Carex aquatilis) cellulose; however, together, they had a dramatic effect. This variable response of mineralization to nutrient addition between 1978 and 1979 was probably attributable to year-to-year variation in nutrient availability within the lake. Cellobiose addition and oxygen depletion inhibited the amount of pine cellulose mineralized. Whereas addition of nitrogen to oxygen-depleted treatments had limited effect, addition of phosphorus resulted in mineralizations equal to or greater than that of the controls. Nitrogen had no effect on mineralization of pine or Carex lignins. Phosphorus, however, inhibited mineralization of both lignins. With Carex lignin, the phosphorus inhibition occurred at a concentration as low as 0.1 μM. The antagonistic role of phosphorus in cellulose and lignin mineralizations may be of significance in understanding the increased proportion of lignin relative to cellulose in decomposing litter.     

Experimental Manipulation of Food Accessibility Affects Conflict Management Behaviour in Ravens

Conflict management strategies such as reconciliation and bystander affiliation have been described for a variety of species. A common determinant seems to be a ‘complex’ social life, with individuals relying on affiliate relationships or social bonds. Little is known, however, about the strategic and flexible use of conflict management skills in experimental settings in species other than primates. We here investigated conflict and post‐conflict behaviour of ravens by manipulating the accessibility of food and, thus, the likelihood of aggressive interactions while foraging. Specifically, we presented birds with a certain amount of highly preferred food that varied in the number of pieces (one piece, two pieces or, as a control, small pieces matching the number of participating birds) and observed their agonistic behaviour during feeding and their affiliative behaviour afterwards. The results showed that high levels of conflicts during feeding in the 1‐piece and 2‐piece conditions led to high levels of affiliation after feeding. Depending on the experimental condition, this effect is best explained (a) by the affiliative behaviour of former aggressors (1‐piece condition) and (b) by the affiliation directed to the receivers of aggression after feeding (2‐piece condition). Those dyads that engaged in allo‐preening after feeding also engaged in allo‐preening outside the experimental setting, suggesting that socially bonded individuals provided third‐party affiliation to victims of aggression. Moreover, socially bonded ravens fed close to each other in the experiment when food was clumped, indicating that they actively coordinated their behaviour when there was a high conflict potential. Taken together, these findings support the assumption that ravens use their social bonds to avoid conflicts by choosing with whom to feed, and to buffer effects of conflicts by engaging in third‐party affiliation as post‐conflict behaviour.