Filial cannibalism in fishes: Why do parents eat their offspring?

Filial cannibalism (the eating of one's own offspring) occurs in a variety of taxa, but is especially prevalent in fishes with parental care. Recent research supports a central tenet of parental-investment theory; that is, parents consume their offspring when it maximizes their lifetime reproductive success. This review outlines the theoretical framework used to explain the adaptive significance of filial cannibalism, evaluates experimental studies to test some predictions of this theory and discusses how the occurrence of filial cannibalism affects other aspects of a species' reproductive ecology.     

Selection of aphid prey by a generalist predator: do prey chemical defences matter?

1. For predators, prey selection should maximise nutrition and minimise fitness costs. In the present study, it was investigated whether a generalist predator [Chrysoperla carnea (Stephens) lacewing larvae] rejected harmful, chemically‐defended prey [Brevicoryne brassicae (Linnaeus) aphids] when non‐defended prey [Myzus persicae (Sulzer) aphids] were available. 2. It was tested: (i) whether consuming different prey species affects predator mortality; (ii) whether naïve predators reject chemically‐defended prey while foraging when non‐defended prey are available; (iii) whether the relative abundance of each prey affects the predator's prey choice; and (iv) whether predators learn to avoid consuming chemically‐defended prey after exposure to both prey species. 3. Consumption of B. brassicae yielded greater C. carnea mortality than M. persicae consumption, but naïve C. carnea did not reject B. brassicae in favour of M. persicae during foraging. When presented at unequal abundances, naïve predators generally consumed each aphid species according to their initial relative abundance, although, predation of non‐defended prey was less than expected when defended prey were initially more abundant, indicating a high consumption of B. brassicae impeded M. persicae consumption. With experience, C. carnea maintained predation of both aphid species but consumed more M. persicae than B. brassicae, indicating a change in behaviour. 4. Although prey choice by C. carnea may change with experience of available prey, prey chemical defences do not appear to influence prey choice by naïve predators. This inability to avoid harmful prey could facilitate wider, indirect interactions. Myzus persicae may benefit where high consumption of B. brassicae hinders predators in the short term, and in the long term, increases predator mortality.     

Priority versus brute force: when should males begin guarding resources?

When should males begin guarding a resource when both resources and guarders vary in quality? This general problem applies, for example, to migrant birds occupying territories in the spring and to precopula in crustaceans where males grab females before they molt and become receptive. Previous work has produced conflicting predictions. Theory on migrant birds predicts that the strongest competitors should often arrive first, whereas some models of mate guarding have predicted that the strongest competitors wait and then simply usurp a female from a weaker competitor. We build a general model of resource guarding that allows varying the ease with which takeovers occur. The model is phrased in terms of mate-guarding crustaceans, but the same logic can be applied to other forms of resource acquisition where priority plays a role but takeovers might be possible too. The race to secure breeding positions can lead to strong competitors (large males) taking females earliest, even though this means accepting a lower-quality female. Paradoxically, this means that small males, which have fewer breeding opportunities, are more choosy than larger ones. Such solutions are found when takeovers are impossible. The easier the takeovers and the higher the rate of finding guarded resources, the more likely are solutions where guarding durations are short, where strong competitors begin guarding only just before breeding, and where they do this by usurping the resource. The relationship between an individual's competitive ability and its timing of resource acquisition can also be nonlinear if takeovers are moderately common; if this is the case, then males of intermediate size guard the longest.     

Predator–prey interactions and the cannibalism of larvae of Armigeres subalbatus (Diptera: Culicidae)

Predation and cannibalism can affect the co-existence of mosquito species or the assemblage of mosquito species (i.e., community structure). In this study, predatory feeding patterns and cannibalism of larvae of Armigeres subalbatus mosquitoes were quantified under laboratory conditions using Aedes albopictus and Culex uniformis larvae as prey organisms. Rate of consumption of prey larvae and the rate of cannibalism of 1st to 4th instar larvae of Armigeres subalbatus were reported at 24 h intervals with and without alternative food supplement. Both the 3rd and 4th instar larvae of Ar. subalbatus showed a substantial predation on the larvae of Ae. albopictus (33.6 ± 4.4) and Cx uniformis (47.3 ± 7.6). The cannibalism of the predatory larvae was strongly correlated to the larval density (r = 0.9). The predator and the prey density and the given food type were significant factors that determined the rate of pupation, death and the emergence as adults. A significant relationship was shown for the co-occurrence of predatory larvae and the prey larvae at the 85 natural breeding habitats observed at the field (χ² = 74.4, p < 0.001). Nine breeding sites (10.6%) were positive for both prey and predatory larvae. Our study showed robust information about the predatory and cannibalistic behavior of Ar. subalbatus larvae emphasizing their role in managing the population structure of mosquito communities.     

Agonistic behavior in wild male Magellanic penguins: when and how do they interact?

Game-theory models predict that the frequency and type of agonistic interactions should vary with the value of the resource being contested. We describe bill duels and overt fighting in male Magellanic penguins (Spheniscus magellanicus) at a breeding colony and determine whether these behaviors change with the value of the nests over which they interact. Bill duels represent low levels of aggression while overt fighting high levels of aggression. Consistent with predictions, overt fighting was more common before egg laying when nests have the highest potential value while bill duels were more common at failed nests later in the season when nests are less valuable as they could not be used for reproduction until the next season. Contrary to expectations, overt fights were shorter and resulted in fewer cuts before egg laying than after egg laying. Large size asymmetries between opponents before egg laying may enable losers to quickly assess their opponents and leave before they are hurt. As predicted, the duration and damage occurring during overt fights were positively correlated with nest cover, which is correlated with higher reproductive success. We conclude that male Magellanic penguins have rules of engagement that in the most cases follow game-theory predictions on when and how to interact.     

How to analyse prey preference when prey density varies? A new method to discriminate between effects of gut fullness and prey type composition

Summary State-dependent changes in prey preference are among the phenomena to be expected in studies of predator behaviour. For example, the rate of attack on each prey type is well known to be affected by the state of satiation, the dynamics of which is often assumed to parallel that of gut fullness. An interesting question is whether satiation alone is the determinant of the attack rate or whether the particular mixture of prey types in the predator's direct environment has an additional influence by itself. To detect examples of the latter type the predictive method proposed by Cock (1978) may be useful. In the present paper the predictive tool is a model built on the assumption that gut fullness is the sole internal state variable determining the attack rate. It is provided with parameter estimates from observations in monocultures of each type and then used to predict predation in mixtures of prey types. When measured predation on these prey types differs from what is predicted, the model may be too simple in various respects, one of which is that predators change prey preference in response to their own sample estimates of the densities of each prey type and their (innate or sample) estimate of the profitability of each prey type in terms of reproductive success. Thus, the lack of fit of the model poses a challenging problem, for to explain it one must identify underlying causes, such as differences in prey quality with respect to scarce nutrients or noxious chemicals that need to be detoxified or rendered harmless in other ways. The predictive approach is illustrated by analysis of preference of predatory mites (Phytoseiulus persimilis Athias-Henriot and Typhlodromus occidentalis Nesbitt) with respect to various stages of development of their prey, the two-spotted spider mite (Tetranychus urticae Koch). The results show that the relation between attack rate and gut fullness might well explain prey stage preference of predatory mites when the prey stages are presented together rather than each alone. In another paper by Dicke et al. (1989) marked deviations between predicted and measured diet are reported when the predatory mite, Typhlodromus pyri Scheuten, was offered a choice between two prey species, i.e. apple rust mites and (larvae of) European red spider mites. The underlying causes are to be revealed by further research, the impetus of which is born out by use of the method proposed by Cock (1978) and extended in this paper.     

The timing of hibernation in Tasmanian echidnas: why do they do it when they do?

We investigated the patterns of hibernation and arousals in seven free-ranging echidnas Tachyglossus aculeatus setosus (two male, five female) in Tasmania using implanted temperature data loggers. All echidnas showed a ‘classical’ pattern of mammalian hibernation, with bouts of deep torpor interrupted by periodic arousals to euthermia (mean duration 1.04±0.05 (n=146). Torpor bout length increased as body temperature fell during the hibernation season, and became more variable as temperature rose again. Hibernation started in late summer (February 28±5 days, n=6) and males aroused just before the winter solstice (June 15±3 days, n=3), females that subsequently produced young aroused 40 days later (July 25±3, n=4) while females that did not produce young hibernated for a further two months (arousal Sept 27±5, n=7). We suggest that hibernation in Tasmanian echidnas can be divided into two phases, the first phase, marked by declining minimum body temperatures as ambient temperature falls, appears to be obligatory for all animals, while the second phase is ‘optional’ and is utilised to varying amounts by females. We suggest that early arousal and breeding is the favoured option for females in good condition, and that the ability to completely omit breeding in some years, and hibernate through to spring is an adaptation to an uncertain climate.     

Exosomes and retroviruses: the chicken or the egg?

Retroviruses appropriate pre‐existing cellular machineries to propagate. In the last decade, impressive similarities have been observed in the generation and dissemination in the host cells of retroviruses and small cellular vesicles known as exosomes. These cellular vesicles are thought to facilitate intercellular communication processes and mediate immune functions. However, their link to the retroviral life cycle has given rise to distinct hypotheses and puzzling dilemmas. Are exosomes the antecessors of retroviruses or do retroviruses merely exploit the same cellular machinery designated for exosome biosynthesis? Here, we address these fascinating evolutionary questions by reviewing recent discoveries and analysing the controversies surrounding them.     

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.     

Apoptosis in the heart: when and why?

Since mammalian cardiac myocytes essentially rely on aerobic energy metabolism, it has been assumed that cardiocytes die in a catastrophic breakdown of cellular homeostasis (i.e. necrosis), if oxygen supply remains below a critical limit. Recent observations, however, indicate that a process of gene-directed cellular suicide (i.e. apoptosis) is activated in terminally differentiated cardiocytes of the adult mammalian heart by ischemia and reperfusion, and by cardiac overload as well. Apoptosis or programmed cell death is an actively regulated process of cellular self destruction, which requires energy and de novo gene expression, and which is directed by an inborn genetic program. The final result of this program is the fragmentation of nuclear DNA into typical nucleosomal ladders, while the functional integrity of the cell membrane and of other cellular organelles is still maintained. The critical step in this regulated apoptotic DNA fragmentation is the proteolytic inactivation of poly-[ADPribose]-polymerase (PARP) by a group of cysteine proteases with some structural homologies to interleukin-1-converting enzyme (ICE-related proteases [IRPs] such as apopain, yama and others). PARP catalyzes the ADP-ribosylation of nuclear proteins at the sites of spontaneous DNA strand breaks and thereby facilitates the repair of this DNA damage. IRP-mediated destruction of PARP, the supervisor of the genome, can be induced by activation of membrane receptors (e.g. FAS or APOI) and other signals, and is inhibited by activation of anti-death genes (e.g. bcl-2). Overload-triggered myocyte apoptosis appears to contribute to the transition to cardiac failure, which can be prevented by therapeutic hemodynamic unloading. In myocardial ischemia, the activation of the apoptotic program in cardiocytes does not exclude their final destiny to catastrophic necrosis with release of cytosolic enzymes, but might be considered as an adaptive process in hypoperfused ventricular zones, sacrificing some jeopardized myocytes to regulated apoptosis, which may by less arrhythmogenic than necrosis with the primary disturbance of membrane function.     

Divide and conquer: when and how should competitors share?

When two individuals are unwilling to fight over a valuable resource, then they may obtain it with equal probability, or they may choose to divide the resource in some way. Although both strategies have been observed in nature, modelers have so far implicitly assumed that their long-term payoffs are the same. First we show that increasing returns to size in the value of a resource favor random allocation over sharing, whereas diminishing returns favor the reverse. Next we extend our approach to understand the conditions under which sharing will evolve when contestants vary in their resource-holding potential. We show that although closely matched individuals are more likely to share, it is by no means a prerequisite when contestants have limited information about one another’s abilities. Collectively, our models support recent observations of physical sharing as a solution to conflict resolution, and elucidate the conditions under which sharing will arise.     

Blue tail and striped body: why do lizards change their infant costume when growing up?

Ontogenetic changes in color and pattern that are not directlyrelated to reproduction are very common yet remain a poorlyunderstood phenomenon. One example is conspicuous colors inthe tails of fish, amphibians, and reptiles that fade out laterin life. We suggest a novel hypothesis: conspicuous tail colorsthat appear only in juveniles compensate for an increased activitylevel, deflecting imminent attacks to the tail. We observedblue-tailed, newly hatched lizards (Acanthodactylus beershebensis)in the field and compared 5 behavioral parameters with thoseof older individuals that had already lost their neonate coloration.In addition, we explored whether tail displays, often assumedto direct a predator's attention to the tail, disappear withthe color change. Striped blue-tailed hatchlings foraged moreactively than 3-week-old juveniles, spent a longer time in openmicrohabitats, and performed deflective tail displays. In comparison,2 other lacertids that do not undergo ontogenetic change didnot switch to safer foraging when growing up. The results suggestthat activity alteration may be a major factor affecting theontogenetic color and pattern change. Active lizards that foragein open habitats increase their probability of attack by ambushpredators. Conspicuous colors and deflection displays may shiftattacks to the expendable tail, increasing the prey's overallprobability of surviving attacks. The persistence of both stripedbody pattern and blue tail fits the active foraging period ofneonates and hence may be appropriate for other species thatdisplay a conspicuous tail accompanied by a striped pattern.     

Predator–prey interactions and changing environments: who benefits?

While aquatic environments have long been thought to be more moderate environments than their terrestrial cousins, environmental data demonstrate that for some systems this is not so. Numerous important environmental parameters can fluctuate dramatically, notably dissolved oxygen, turbidity and temperature. The roles of dissolved oxygen and turbidity on predator-prey interactions have been discussed in detail elsewhere within this issue and will be considered only briefly here. Here, we will focus primarily on the role of temperature and its potential impact upon predator-prey interactions. Two key properties are of particular note. For temperate aquatic ecosystems, all piscine and invertebrate piscivores and their prey are ectothermic. They will therefore be subject to energetic demands that are significantly affected by environmental temperature. Furthermore, the physical properties of water, particularly its high thermal conductivity, mean that thermal microenvironments will not exist so that fine-scale habitat movements will not be an option for dealing with changing water temperature in lentic environments. Unfortunately, there has been little experimental analysis of the role of temperature on such predator-prey interactions, so we will instead focus on theoretical work, indicating that potential implications associated with thermal change are unlikely to be straightforward and may present a greater threat to predators than to their prey. Specifically, we demonstrate that changes in the thermal environment can result in a net benefit to cold-adapted species through the mechanism of predator-prey interactions.     

Mechanosensitive channels: what can they do and how do they do it?

While mechanobiological processes employ diverse mechanisms, at their heart are force-induced perturbations in the structure and dynamics of molecules capable of triggering subsequent events. Among the best characterized force-sensing systems are bacterial mechanosensitive channels. These channels reflect an intimate coupling of protein conformation with the mechanics of the surrounding membrane; the membrane serves as an adaptable sensor that responds to an input of applied force and converts it into an output signal, interpreted for the cell by mechanosensitive channels. The cell can exploit this information in a number of ways: ensuring cellular viability in the presence of osmotic stress and perhaps also serving as a signal transducer for membrane tension or other functions. This review focuses on the bacterial mechanosensitive channels of large (MscL) and small (MscS) conductance and their eukaryotic homologs, with an emphasis on the outstanding issues surrounding the function and mechanism of this fascinating class of molecules.