To breed or not to breed: Physiological correlates of reproductive status in a facultatively biennial iguanid

It is unusual for seasonal breeders to frequently skip opportunities for reproduction. We investigated the relationship between physiological state and reproductive decision-making in Galápagos marine iguanas (Amblyrhynchus cristatus), a species in which females typically reproduce biennially, although the proportion of breeding individuals varies significantly across years. Nearly all adult-sized females initiated follicular development prior to the lekking period, but 38% of females resorbed all developing follicles 5-15 days before the start of copulations. Receptive and non-receptive females differed in reproductive hormones during the mate choice period. Testosterone peaked in receptive females immediately prior to copulation, indicating that testosterone or its derivative estradiol likely mediates female receptivity in Galápagos marine iguanas. Non-receptive females showed significant peaks in both testosterone and progesterone during follicular atresia, suggesting that these hormones may be involved in inhibiting vitellogenesis. Two to three weeks prior to the period of reproductive decision-making (and the onset of follicular atresia in non-receptive females) receptive females were in higher body condition, were developing larger follicles, and had lower levels of both baseline and stress-induced corticosterone. Reproduction is extremely costly in this long-lived species, and increases the likelihood of mortality in the year following breeding; females could therefore gain significant benefits from being attuned to indicators of reproductive success. We suggest that corticosterone may modulate reproductive decisions by altering individual sensitivity to both internal and external cues of the likelihood of successful reproduction.     

To breed or not to breed: a model of partial migration

Migration is used by a number of species as a strategy for dealing with a seasonally variable environment. In many migratory species, only some individuals migrate within a given season (migrants) while the rest remain in the same location (residents), a phenomenon called ‘partial migration’. Most examples of partial migration considered in the literature (both empirically and theoretically) fall into one of two categories: either species where residents and migrants share a breeding ground and winter apart, or species where residents and migrants share an overwintering ground and breed apart. However, a third form of partial migration can occur when non‐migrating individuals actually forgo reproduction, essentially a special form of low‐frequency reproduction. While this type of partial migration is well documented in many taxa, it is not often included in the partial migration literature, and has not been considered theoretically to date. In this paper we present a model for this partial migration scenario and determine under what conditions an individual should skip a breeding opportunity (resulting in partial migration), and under what conditions individuals should breed every chance they get (resulting in complete migration). In a constant environment, we find that partial migration is expected to occur when the mortality cost of migration is high, and when individuals can greatly increase their fecundity by skipping a year before breeding. In a stochastic environment, we find that an individual should skip migration more frequently with increased risk of a bad year (higher probability and severity), with higher mortality cost of migration, and with lower mortality cost of skipping. We discuss these results in the context of empirical data and existing life history theory.     

To breed or not to breed: a seabird's response to extreme climatic events

Intermittent breeding is an important life-history strategy that has rarely been quantified in the wild and for which drivers remain unclear. It may be the result of a trade-off between survival and reproduction, with individuals skipping breeding when breeding conditions are below a certain threshold. Heterogeneity in individual quality can also lead to heterogeneity in intermittent breeding. We modelled survival, recruitment and breeding probability of the red-footed booby (Sula sula), using a 19 year mark–recapture dataset involving more than 11 000 birds. We showed that skipping breeding was more likely in El-Niño years, correlated with an increase in the local sea surface temperature, supporting the hypothesis that it may be partly an adaptive strategy of birds to face the trade-off between survival and reproduction owing to environmental constraints. We also showed that the age-specific probability of first breeding attempt was synchronized among different age-classes and higher in El-Niño years. This result suggested that pre-breeders may benefit from lowered competition with experienced breeders in years of high skipping probabilities.     

To breed or not to breed: endocrine response to mercury contamination by an Arctic seabird

Mercury, a ubiquitous toxic element, is known to alter expression of sex steroids and to impair reproduction across vertebrates but the mechanisms underlying these effects are not clearly identified. We examined whether contamination by mercury predicts the probability to skip reproduction in black-legged kittiwakes (Rissa tridactyla) from Svalbard. We also manipulated the endocrine system to investigate the mechanism underlying this relationship. During the pre-laying period, we injected exogenous GnRH (gonadotropin-releasing hormone) to test the ability of the pituitary to release luteinizing hormone (LH, a key hormone for the release of sex steroids and hence breeding) in relation to mercury burden. Birds that skipped reproduction had significantly higher mercury concentration in blood than breeders. Endocrine profiles of these birds also varied based on breeding status (breeders versus non-breeders), mercury contamination and sex. Specifically, in skippers (birds that did not breed), baseline LH decreased with increasing mercury concentration in males, whereas it increased in females. GnRH-induced LH levels increased with increasing mercury concentration in both sexes. These results suggest that mercury contamination may disrupt GnRH input to the pituitary. Thus, high mercury concentration could affect the ability of long-lived birds to modulate their reproductive effort (skipping or breeding) according to ongoing environmental changes in the Arctic, thereby impacting population dynamics.     

To breed or not to breed: causes and implications of non-breeding habit in the willow tit Parus montanus

Causes and consequences of non-breeding in willow tits were studied in northern Finland during 1986–1992. The breeding status was sex and age biased; males and yearling birds were in excess among the non-reproducers. Due to sex bias in the population it appeared detrimental for males to lose a mate, especially shortly before breeding. Lack of a mate was a important factor for males not reproducing (37% of non-breeding males) than for females (14%). Most of the non-breeding birds maintained a pair bond which only rarely broke up for the next breeding season (divorce rate 5.5%). This implies that parental incompatibility is not a possible explanation for pairs not reproducing. Males that did not breed tended to survive better than reproducing ones, whereas such a relationship was not found for females. It is possible that this sex-related difference in survival cost is attributable to quality differences among non-breeding individuals. It was especially low-quality yearling females, with low survival prospects, that were responsible for the discrepancy. The proportion of non-breeding females in the population correlated highly with clutch size and subsequent juvenile survival. It is therefore suggested that for most of these females non-breeding is a phenotypic response to low offspring value in the prevailing circumstances (inter-generational tradeoff). However, it is uncertain whether willow tits in a northern population can use breeding density as an indicator of changing survival prospects of their descendants, as suggested by Ekman and Askenmo (1986) for southern Sweden.     

To stay or not to stay: the contribution of tactile and thermal cues to coming to rest in newborn rabbits

Newborn rabbits, like other altricial mammals, demonstrate thermotaxis and when placed on a thermal gradient locate and come to rest at physiologically appropriate temperatures. Little is known, however, about the sensory-motor components contributing to the in energetic terms important decision of the young to cease locomotion and come to rest. We investigated the behavior of newborn rabbits on two thermal gradients; linear in which pups could use tactile cues from the arena wall, and concentric in which pups were unable to use such cues. On both gradients pups located the warm, thermal-neutral area within the 200-s test time, thereby demonstrating their ability to orient appropriately using thermal cues alone. Unexpectedly, however, pups on the concentric gradient failed, or took significantly longer, to come to rest than pups on the linear gradient. Since the speed of locomotion of pups on the linear gradient was significantly slowed when they were in contact with the arena wall, and in most cases they came to rest in contact with it, we suggest that not only thermal but also tactile cues may be important in bringing young mammals to rest in a thermally appropriate environment.     

To breed or not to breed: an analysis of the social and density-dependent constraints on the fecundity of female badgers (Meles meles).

Data from post-mortem examinations, population density estimates and long term capture-mark-recapture studies have been combined to look at the pattern of reproductive behaviour and the social factors leading to reproductive failure in badgers in Britain. The results are used to evaluate whether the hypothesis that the defence of oestrous females (as opposed to defence of food resources) best explains territorial behaviour and the social organization of badgers. Badgers in Britain have two peaks of reproductive activity, one immediately post partum and one in the summer/autumn. These coincide with two peaks of ovulation, and in the late winter/spring there is a steep rise in the number of sows carrying blastocysts, to reach an asymptote in June for yearling sows and April in older sows. Measured by their contribution to overall productivity, winter/spring matings were much more important than summer/autumn matings, contributing 65% of total autumn blastocysts in yearling sows and 71% of autumn blastocysts in older sows. The relative importance of the two mating periods is reflected in the seasonal pattern of bite wounding in adult male badgers; minor bite wounding in January-March was 2.3 times as frequent as in August-October, and moderate-extensive bite wounding was 3.1 times more frequent. In the populations studied, pre- and post-natal losses were high, with reproductive failure occurring at all stages of the breeding cycle, so that less than 30% of potential productivity was achieved. Indeed 22% of sows failed to develop blastocysts; these had a lower body mass, less body fat, larger adrenal glands, poorer health and higher bite wound scores than sows with blastocysts. Only 44% of adult sows implanted their blastocysts and proceeded to the end of pregnancy. However, it was less easy to identify features characteristic of sows that did or did not go on to implant their blastocysts. Finally, 35% of sows that produced cubs ceased lactation early, and this loss of entire litters was thought to be due to infanticide by dominant sows. The presence of annexe setts correlates with increased productivity in younger sows, and this is thought to be because annexe setts enable younger sows and their cubs to avoid the aggression of older, more dominant sows. Living in large social groups has no net reproductive gain for adult males or females, and there was a decline in productivity (per adult) with increasing group size.(ABSTRACT TRUNCATED AT 400 WORDS)     

Resource allocation in a social wasp: effects of breeding system and life cycle on reproductive decisions

Organisms must make important decisions on how to allocate resources to reproduction. We investigated allocation decisions in the social wasp Vespula maculifrons to understand how social insects make reproductive choices. We first determined how annual colonies apportioned resources to growth and reproduction by analysing developing brood. In contrast to expectations, colonies invested in both growth (workers) and reproduction (males) simultaneously. In addition, colonies showed evidence of producing males in pulses and reversing their reproductive choices by decreasing investment in males late in the season. This reversal is consistent with theory suggesting that colonies decrease production in males if fitness of late emerging males is low. To further investigate reproductive decisions within colonies, we determined if the male mates of multiply-mated queens varied in their reproductive success over time. Sperm use by queens did vary over time suggesting that male success may depend on sperm clumping within the female reproductive tract. Finally, we tested if colony sex ratio conformed to expectations under kin selection theory that nestmate relatedness would positively correlate with investment in new queens if workers controlled sex allocation. Surprisingly, the proportion of queens produced by colonies was negatively correlated with nestmate relatedness, suggesting that allocation may be shaped by advantages arising from increased genetic diversity resulting from multiple mating by queens. Overall, our study suggests that the reproductive decisions of colonies are flexible and may depend both on environmental cues arising from energetic needs of the colony and genetic cues arising from mating behaviours of queens.     

To feed or to breed: morphological constraints of mouthbrooding in coral reef cardinalfishes

Functionally coupled biomechanical systems are widespread in nature and are viewed as major constraints on evolutionary diversification, yet there have been few attempts to explore the implications of performing multiple functions within a single anatomical structure. Paternally mouthbrooding cardinalfishes present an ideal system to investigate the constraints of functional coupling as the oral jaws of male fishes are directly responsible for both feeding and reproductive functions. To test the effects of (i) mouthbrooding on feeding and (ii) feeding on reproductive potential we compared the feeding apparatus between sexes of nine species of cardinalfish and compared brood characteristics among species from different trophic groups, respectively. Mouthbrooding was strongly associated with the morphology of the feeding apparatus in males. Male cardinalfishes possessed longer heads, snouts and jaws than female conspecifics irrespective of body size, trophic group or evolutionary history. Conversely, reproductive potential also appeared to be related to trophic morphology. Piscivorous cardinalfishes produced larger, but fewer eggs, and had smaller brood volumes than species from the two invertebrate feeding groups. These interrelationships suggest that feeding and reproduction in the mouth of cardinalfishes may be tightly coupled. If so this may, in part, have contributed to the limited morphological diversification exhibited by cardinalfishes.     

Social and environmental cues drive the intra-population variation in courtship behavior of a neotropical lekking bird

acta ethologica - Sexual selection predicts evolution of secondary sexual traits favoring mating. Here, we address the within population variation in courtship display behavior among male...     

Experimental evidence of a link between breeding conditions and the decision to breed or to help in a colonial cooperative bird

In many species mature individuals delay independent reproduction and may help others to reproduce. This behaviour is often explained through ecological constraints, although recently attention has also been paid to the variation in habitat quality. If the quality of vacant habitat influences the fitness trade-off between delaying reproduction and breeding independently, individuals should delay reproduction when conditions for breeding are poor. Yet, no study has experimentally manipulated habitat quality or the conditions experienced during the breeding period to test this assertion conclusively. We report results from an experiment conducted on a colonial cooperative bird with no territory constraints on reproduction. We artificially improved breeding conditions in several colonies of sociable weavers, Philetairus socius, through the provision of an easily obtainable and unlimited supply of food. We provide experimental evidence showing that under enhanced conditions some individuals reduce their age at first reproduction, a greater proportion of colony members engage in independent breeding and proportionally fewer birds act as helpers. Hence, these results also provide evidence for a direct influence of reproductive costs on life-history decisions such as age at first reproduction and breeding and helping behaviours.     

To close or not to close: Plasmodesmata in defense

Cell death is a biological process that occurs during differentiation and maturation of certain cell types, during senescence, or as part of a defense mechanism against microbial pathogens. Intercellular coordination is thought to be necessary to restrict the spread of death signals, although little is known about how cell death is controlled at the tissue level. The recent characterization of a plasmodesmal protein, PDLP5, has revealed an important role for plasmodesmal control during salicylic acid-mediated cell death responses. Here, we discuss molecular factors that are potentially involved in PDLP5 expression, and explore possible signaling networks that PDLP5 interacts with during basal defense responses.     

To be or not to be solitary: Phytophthora infestans' dilemma for optimizing its reproductive fitness in multiple infections

The success of parasitic life lies in an optimal exploitation of the host to satisfy key functions directly involved in reproductive fitness. Resource availability generally decreases over time with host mortality, but also during multiple infections, where different strains of parasite share host resources. During multiple infections, the number of parasite strains and their genetic relatedness are known to influence their reproductive rates. Using infections of the potato plant Solanum tuberosum with the parasite Phytophthora infestans, we set up an experimental design to separate dose effects (double- vs. single-site infections) from genetic relatedness (different vs. identical genotypes) on the reproductive fitness of competing parasite genotypes. We showed the existence of two basic response patterns--increase or decrease in reproductive fitness in multiple infections- depending on the parasite genotype. In all cases, the intensity of the response of any genotype depended on the genotype of the competing strain. This diversity of responses to multiple infections is probably maintained by the fluctuating frequencies of multiple infections in nature, arising from variations in disease pressure over the course of an epidemic and between successive epidemics. It allows a rapid response of parasitic populations to changing environments, which are particularly intense in agricultural systems.     

Fertility in South Australian commercial Merino flocks: relationships between reproductive traits and environmental cues

High levels of reproductive loss have been reported in commercial Merino flocks (n=68) from the cereal/livestock and high rainfall zones in South Australia (Kleemann DO, Walker SK. Fertility in South Australian commercial Merino flocks: sources of reproductive wastage. Theriogenology 2005;63:2075-88). Relationships between reproductive traits (estrus, ovulation, fertility, fecundity, lamb survival) and environmental end points (liveweight, condition score, temperature at mating, chill index at lambing) are reported in this paper. They were analysed within season of mating (October to December; January to March) and age of ewe (maiden as 1.5-year-old, mature as older). Incidence of estrus was positively related (P<0.05) to condition score in the October to November interval. Return rate to service was positively (P<0.05) influenced and fertility was negatively (P<0.05) influenced by the number of days ambient temperatures were > or =32.0 degrees C during mating, indicating that high ambient temperatures may reduce embryo survival. Liveweight and, to a lesser extent, condition score, accounted for significant proportions of variation associated with ovulation rate (39.3 and 12.7%, respectively). Ovulations per 100 ewes increased by 1.8 per kg increase in liveweight over all flocks. Ovulatory response to liveweight increased (P<0.01) from the October to December to the January to March period of mating (1.5 versus 3.4 ovulations per kg, respectively). Overall, a flock's fertility and fecundity increased by 0.27 and 1.42% per kg increase in liveweight, respectively. Reproductive wastage from partial failure of multiple ovulations (PFMO) was positively related to liveweight (P<0.01) and ovulation rate (P<0.001). Survival of single lambs was positively and curvilinearly related to maternal liveweight and condition score measured in late pregnancy (P<0.05). Linear relationship of these variables for twin lamb survival was significant for condition score only. Single and twin lamb survival were also positively related to liveweight and condition score at mating (P<0.05). We concluded that nutritional cues have a major impact on reproductive traits in commercial Merino flocks in South Australia; it sets the potential number of lambs (ovulation rate) and influences survival of lambs as early as at mating. Indications are that high ambient temperatures may influence embryo survival. It is recommended that future research efforts focus on: (a) prenatal nutritional influences on the physiology of mother-offspring behaviours at birth; and (b) possible peri-conceptional dietary factors controlling embryo loss resulting from partial failure of twin ovulations, to improve reproductive efficiency in the Merino.     

To be or not to be a hamlet pair in sympatry

Recent adaptive radiations, such as the fringillid finches on the Galapagos archipelago or the cichlid fishes in the East African Great Lakes, are invaluable model systems for evolutionary and ecological research. Puebla et al. , in this issue of Molecular Ecology , have established a group of colourful coral reef fishes from the Caribbean sea for studying the early phases of species formation in a marine adaptive radiation. It appears that local evolutionary processes are important in this system, which might have even triggered in situ speciation.     

To pair or not to pair: chromosome pairing and evolution

Chromosome pairing in wild-type wheat closely resembles the process in both yeast and Drosophila. The recent characterisation of a mutant Ph1 wheat and the observation that chromosome pairing in the absence of Ph1 more closely resembles that of mammals and maize has shed light on the evolution of chromosome pairing in the cereals.     

To die or not to die: DNA repair in neurons

One of the critical emerging problems in modern pathobiology is how cells govern the decision to live or die, and the cost of making such a decision. Nowhere are these questions more poignant than in deciphering the tissue-specific responses to DNA damage. Mutations in DNA repair enzymes, malfunctions in cell cycle regulation, and genetic instability are associated with most somatic cancers. However, in many hereditary diseases arising from mutations in DNA repair proteins, the same dominant mutations that cause cancer in dividing cells are often associated with cell death in terminally differentiated neurons. Context dependent differences in the response to DNA damage are used to make fundamental choices as to cell fate, and are likely to shed light on the mechanisms underlying human disease.     

抑制还是转导:信号分子调节机体健康与疾病

细胞内的信号转导网络是由多条功能特异且彼此关联的信号通路所构成,它们赋予了细胞功能的多样性和可塑性,同时也必须受到精细严谨的调控。一些功能广泛的信号调节因子,如β-抑制蛋白(β-arrestin),在细胞信号转导网络完整性的维持中扮演着重要的角色。β-arrestin分子的经典功能是终止G-蛋白偶联受体(G-protein-coupled receptors)下游信号转导,即受体脱敏,但最近许多研究证据表明,这种脱敏功能(负调控)还可以针对其他的信号转导途径。例如,β-arrestin能够通过不同的机制负调控三条重要的NF-κB激活通路,该功能异常则导致NF-κB持续激活以及下游炎性因子的过度分泌。此外,近年来发现β-arrestin还能作为支架蛋白介导功能性信号复合物的形成。例如,在特定外界信号刺激下,β-arrestin1能够转移至细胞核内并与组蛋白乙酰化酶p300相互作用而调控基因表达。该机制的生理意义之一反映在多发性硬化症的小鼠模型中,β-arrestin1在发病小鼠中较正常小鼠表达上调并能够显著加重病情。与之相反,在细胞质中富集的β-arrestin2参与了胰岛素激活时InsR/Akt/β-arrestin2/Src信号复合体的形成,它的缺失能够导致胰岛素耐受和2型糖尿病的发生。因此,在特定的条件下,β-arrestin对于胞内信号的传递究竟是抑制还是激活,已成为细胞信号转导中的关键问题,并在机体健康和疾病状态的相互转化中的起着重要作用。