Alternative mating tactics in the Italian treefrog, <Emphasis Type="Italic">Hyla intermedia</Emphasis>

As in many lekking anurans, Italian treefrog males use two mating tactics: they can attract females by calling vigorously or be satellites, that is, they can remain silent in proximity of a calling male and try to intercept females attracted by their neighbour. We investigated the factors that affected the expression of this mating tactic. Consistent with the conditional mating tactic hypothesis, satellites were smaller than average and smaller than their parasitised calling males. They spent a larger-than-average number of nights at the breeding site, where most of them were also observed calling. Moreover, satellites showed lower call rates and lower mating success than those of males they parasitise but not lower than those of males they did not parasitise. Overall, these results, together with those derived from the analyses of the seasonal and spatial distribution of males, provide evidence for a non-random association between satellites and calling males and are consistent with the hypothesis that satellites have spectral and temporal acoustic preferences that parallel those of females. By adopting the less-successful satellite mating tactic, competitively inferior males can nevertheless maximise their potential reproductive fitness by sexually parasitising the most attractive chorusing males.     

Territorial counter-singing in male sac-winged bats (<Emphasis Type="Italic">Saccopteryx bilineata</Emphasis>): low-frequency songs trigger a stronger response

In many animal species, individuals compete for resources but avoid escalated conflicts by threat displays, i.e. a mutual signalling behaviour that enables the opponents to predict the outcome of the conflict without the necessity of actual fighting. For example, territory holders may use acoustic signals to communicate not only their own identity and the borders of their territory but also their competitive quality, fighting ability and motivation. Here, we show that male sac-winged bats, Saccopteryx bilineata, adjust their vocal territorial displays according to the fundamental frequency of territorial songs of their opponents. In playback experiments with territorial males, low-frequency stimuli elicited a higher territorial song rate and length than high-frequency stimuli. Male S. bilineata that sing more often and with lower fundamental frequencies have been shown to sire more offspring than their competitors. Fundamental frequency of territorial songs, hence, may reveal male quality and, consequently, the resulting threat posed to competing males. We argue that this is reflected in the increased response of competitors to low-frequency territorial songs shown here. Such competitive signalling behaviour has been shown in a few mammal species like red deer and baboons but, thus far, not in bats. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The meat-scrap hypothesis: small quantities of meat may promote cooperative hunting in wild chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>)

A common explanation for hunting in groups is that doing so yields a greater per capita caloric benefit than hunting solitarily. This is logical for social carnivores, which rely exclusively on meat for energy, but arguably not for omnivores, which obtain calories from either plant or animal matter. The common chimpanzee, Pan troglodytes, is one of the few true omnivores that regularly hunts in groups. Studies to date have yielded conflicting data regarding the payoffs of group hunting in chimpanzees. Here, we interpret chimpanzee hunting patterns using a new approach. In contrast to the classical assumption that hunting with others maximizes per capita caloric intake, we propose that group hunting is favored because it maximizes an individual’s likelihood of obtaining important micronutrients that may be found in small quantities of meat. We describe a mathematical model demonstrating that group hunting may evolve when individuals can obtain micronutrients more frequently by hunting in groups than by hunting solitarily, provided that group size is below a certain threshold. Twenty five years of data from Gombe National Park, Tanzania are consistent with this prediction. We propose that our ‘meat-scrap’ hypothesis is a unifying approach that may explain group hunting by chimpanzees and other social omnivores.     

Behavioral evolution in the major worker subcaste of twig-nesting <Emphasis Type="Italic">Pheidole</Emphasis> (Hymenoptera: Formicidae): does morphological specialization influence task plasticity?

Polymorphism frequently correlates with specialized labor in social insects, but extreme morphologies may compromise behavioral flexibility and thus limit caste evolution. The ant genus Pheidole has dimorphic worker subcastes in which major workers appear limited due to their morphology to performing defensive or trophic functions, thus providing an ideal model to investigate specialization and plasticity. We examined worker morphology, brood-care flexibility, and subcaste ratio in 17 species of tropical twig-nesting Pheidole by quantifying nursing by major workers in natural colonies and in subcolonies lacking minors, in which we also measured brood survival and growth. Across species, majors performed significantly less brood care than minors in intact colonies, but increased rates of brood care 20-fold in subcolonies lacking minors. Brood nursed by majors had lower survival than brood tended by minors, although rates of brood growth did not vary between subcastes. Significant interspecific variation in rates of brood care by major workers did not lead to significant differences in brood growth or survival. Additionally, we did not find a significant association between the degree of major worker morphometric specialization and rates of nursing, growth, or survival of brood among species. Therefore, major workers showed reduced efficacy of brood care, but the degree of morphological specialization among species did not directly compromise task plasticity. The compact nests and all-or-nothing consequences of predation or disturbance on colony fitness may have influenced the evolution of major worker brood-care competency in twig-nesting Pheidole. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Dedicated to Professor Edward O. Wilson on the occasion of his 80th birthday.     

Does the risk of encountering lions influence African herbivore behaviour at waterholes?

A central question in the study of predator–prey relationships is to what extent prey behaviour is determined by avoidance of predators. Here, we test whether the long-term risk of encountering lions and the presence of lions in the vicinity influence the behaviour of large African herbivores at waterholes through avoidance of high-risk areas, increases in group size, changes in temporal niche or changes in the time spent in waterhole areas. In Hwange National Park, Zimbabwe, we monitored waterholes to study the behaviour of nine herbivore species under different risks of encountering lions. We radio-collared 26 lions in the study area which provided the opportunity to monitor whether lions were present during observation sessions and to map longer-term seasonal landscapes of risk of encountering lions. Our results show that the preferred prey species for lions (buffalo, kudu and giraffe) avoided risky waterholes. Group size increased as encounter risk increased for only two species (wildebeest and zebra), but this effect was not strong. Interestingly, buffalo avoided the hours of the day which are dangerous when the long-term and short-term risks of encountering lions were high, and all species showed avoidance of waterhole use at night times when lions were in the vicinity. This illustrates well how prey can make temporal adjustments to avoid dangerous periods coinciding with predator hunting. Additionally, many herbivores spent more time accessing water to drink when the long- and short-term risks of encountering lions were high, and they showed longer potential drinking time when the long-term risk of encountering lions was high, suggesting higher levels of vigilance. This study illustrates the diversity of behavioural adjustments to the risk of encountering a predator and how prey respond differently to temporal variations in this risk.     

The downfall of mating: the effect of mate-carrying and flight muscle ratio on the escape ability of a pierid butterfly

In many species, the ability to evade predators is known to be periodically impaired by increased weight loads due to feeding and reproduction. Not only may extra weight reduce escape speed, but feeding and mating can also make the prey more noticeable to visually hunting predators. A number of butterfly species mate for hours, and if a mating couple is disturbed, one of the butterflies is responsible for flying, whereas its partner remains still. This study investigated the ability of male Pieris napi butterflies to fly while mating, with the prediction that mate carrying impairs flight ability compared to single flying males and that males with relatively high flight muscle ratios (FMR; male thorax mass/male + female body mass) will have better flight performance in copula. Our results clearly show that whereas single males always take off at steep angles and fly upwards, couples invariably have a negative take-off angle and rarely gain height. Moreover, landing height of the couples is positively associated with higher FMR. Hence, male flight ability when in copula is positively associated with a high relative thorax mass. Butterfly pairs may thus be at greater risk of predation as a consequence of their impaired flight ability, especially couples with critically low FMRs (<16%).     

Radio tagging reveals the roles of corpulence,experience and social information in ant decision making

Ant colonies are factories within fortresses (Oster and Wilson 1978). They run on resources foraged from an outside world fraught with danger. On what basis do individual ants decide to leave the safety of the nest? We investigated the relative roles of social information (returning nestmates), individual experience and physiology (lipid stores/corpulence) in predicting which ants leave the nest and when. We monitored Temnothorax albipennis workers individually using passive radio-frequency identification technology, a novel procedure as applied to ants. This method allowed the matching of individual corpulence measurements to activity patterns of large numbers of individuals over several days. Social information and physiology are both good predictors of when an ant leaves the nest. Positive feedback from social information causes bouts of activity at the colony level. When certain social information is removed from the system by preventing ants returning, physiology best predicts which ants leave the nest and when. Individual experience is strongly related to physiology. A small number of lean individuals are responsible for most external trips. An individual’s nutrient status could be a useful cue in division of labour, especially when public information from other ants is unavailable.     

Egg coloration in ring-billed gulls (<Emphasis Type="Italic">Larus delawarensis</Emphasis>): a test of the sexual signaling hypothesis

Although many avian eggs appear to be cryptically colored, many species also lay vibrant blue green eggs. This seemingly conspicuous coloration has puzzled biologists since Wallace, as natural selection should favor reduced egg visibility to minimize predation pressure. The sexual signaling hypothesis posits that blue green egg coloration serves as a signal of female quality and that males exert post-mating sexual selection on this trait by investing more in the nests of females laying more intensely blue green eggs. This hypothesis has received mixed support to date, and most previous studies have been conducted in cavity-nesting species where male evaluation of his partner’s egg coloration, relative to that of other females, may be somewhat limited. In this study, we test the sexual signaling hypothesis in colonially nesting ring-billed gulls (Larus delawarensis) where males have ample opportunity to assess their mate’s egg coloration relative to that of other females. We used correlational data and an experimental manipulation to test four assumptions and predictions of the sexual signaling hypothesis: (1) blue green pigmentation should be limiting to females; (2) extent of blue green egg coloration should relate to female quality; (3) extent of blue green egg coloration should relate to offspring quality; and (4) males should provide more care to clutches with higher blue green chroma. Our data provide little support for these predictions of the sexual signaling hypothesis in ring-billed gulls. In light of this and other empirical data, we encourage future studies to consider additional hypotheses for the evolution of blue green egg coloration.     

The rarity of twins: a result of an evolutionary battle between mothers and daughters—or do they agree?

The aim of this study was to test the hypothesis that parent–offspring conflict over parental investment might have led to the rarity of dizygotic twins in humans. We explored the theoretical prediction that twins maximize their inclusive fitness by the death of a co-twin, while mothers of twins maximize fitness by raising both twins to independence. We used life history data (1700–1900) from two parishes in Central Norway to compare differences in inclusive fitness (measured as number of children reared to the age of 16 years, using Hamilton's rule) between twins and mothers of twins. Our results show that twins maximize their inclusive fitness by the death of a co-twin, while mothers of twins raise more children by rearing both twins to adulthood. However, because twins growing up as singletons may produce higher or at least equal number of offspring than the sum of the two twins growing up together, mothers might gain more grandchildren by allowing twins to grow up as singletons. To conclude, both selfish twins and their mothers might benefit by the death of a co-twin, indicating that there is no parent–offspring conflict responsible for the rareness of twins in these human populations. Finally, we discuss the results in the light of “The Insurance Egg Hypothesis” and “The Natural Selection Hypothesis”.     

Invasive species impacts on ecosystem structure and function: A comparison of Oneida Lake, New York, USA, before and after zebra mussel invasion

Exotic species invasion is widely considered to affect ecosystem structure and function. Yet, few contemporary approaches can assess the effects of exotic species invasion at such an inclusive level. Our research presents one of the first attempts to examine the effects of an exotic species at the ecosystem level in a quantifiable manner. We used ecological network analysis (ENA) and a social network analysis (SNA) method called cohesion analysis to examine the effect of zebra mussel (Dreissena polymorpha) invasion on the Oneida Lake, New York, USA, food web. We used ENA to quantify ecosystem function through an analysis of food web carbon transfer that explicitly incorporated flow over all food web paths (direct and indirect). The cohesion analysis assessed ecosystem structure through an organization of food web members into subgroups of strongly interacting predators and prey. Our analysis detected effects of zebra mussel invasion throughout the entire Oneida Lake food web, including changes in trophic flow efficiency (i.e., carbon flow among trophic levels) and alterations of food web organization (i.e., paths of carbon flow) and ecosystem activity (i.e., total carbon flow). ENA indicated that zebra mussels altered food web function by shunting carbon from pelagic to benthic pathways, increasing dissipative flow loss, and decreasing ecosystem activity. SNA revealed the strength of zebra mussel perturbation as evidenced by a reorganization of food web subgroup structure, with a decrease in importance of pelagic pathways, a concomitant rise of benthic pathways, and a reorganization of interactions between top predator fish. Together, these analyses allowed for a holistic understanding of the effects of zebra mussel invasion on the Oneida Lake food web.