A high aggression strategy for smaller males

Male-male conflict is common among animals, but questions remain as to when, how and by whom aggression should be initiated. Factors that affect agonistic strategies include residency, the value of the contested resource and the fighting ability of the two contestants. We quantified initiation of aggression in a fish, the desert goby, Chlamydogobius eremius, by exposing nest-holding males to a male intruder. The perceived value of the resource (the nest) was manipulated by exposing half of the residents to sexually receptive females for two days before the trial. Resident male aggression, however, was unaffected by perceived mating opportunities. It was also unaffected by the absolute and relative size of the intruder. Instead resident aggression was negatively related to resident male size. In particular, smaller residents attacked sooner and with greater intensity compared to larger residents. These results suggest that resident desert goby males used set, rather than conditional, strategies for initiating aggression. If intruders are more likely to flee than retaliate, small males may benefit from attacking intruders before these have had an opportunity to assess the resident and/or the resource.     

Sexually selected nest-building--Pomatoschistus minutus males build smaller nest-openings in the presence of sneaker males

Both natural selection and sexual selection may act on nest-building. We tested experimentally how different regimes of egg-predation and male-male competition influence nest-building before mating, using the marine fish sand goby, Pomatoschistus minutus. Males with sneaker males present built the smallest nest-openings, smaller than males held alone or with Pomatoschistus microps males (which may predate eggs and compete over nest-sites but not compete over fertilizations). Males with visual access to other nest-building males tended also to build smaller openings than males held alone or with P. microps. Males with egg-predators present built nests with openings not differing significantly from any other treatment. Our results indicate that the small nest-openings found in the sneaker male treatment are sexually selected through protection against sneaking or by female choice. Across treatments, time span before a male started to build his nest also explained variation in nest-opening width; males starting late built larger nest-openings.     

Mate selection in Litoria chloris and Litoria xanthomera: Females prefer smaller males

It is generally accepted that high quality males are those that succeed in male–male competition: in either aggression or rivalry to attract and be selected by females. Previous studies of amphibians have suggested that the main characters influencing male mating success include variation in call characteristics (e.g. call rate, call intensity), calling behaviour, body condition, age and chorus tenure. In the present paper, several of the characters influencing female mate choice (male body size, body condition, call rate, call frequency and chorus tenure) are investigated in two closely related, explosive breeding frog species Litoria chloris and Litoria xanthomera. Smaller males of both species are shown to be more successful than larger males and this success is attributed to the increased chorus tenure of smaller males in L. xanthomera. This increased chorus tenure was attributed to the lower total energy used per call by a small male calling at a higher frequency. Whether increased chorus tenure explains female mate choice in L. chloris is uncertain but is highly probable given the strong similarity between the two species in both ecology and call characteristics.     

Reversed sexual size dimorphism in microtines: Are females larger than males or are males smaller than females?

Summary We analysed sexual size dimorphism for 21 populations of microtine rodents. Female to male size ratio varied considerably among populations from females significantly larger than males (ratio=1.18) to males larger than females (ratio=0.78). In a multiple regression analysis female to male home range size ratio explained 94% of the total variation in body size dimorphism and was the only one of eight independent variables that was selected in a stepwise regression procedure. When females are the larger sex, males have home range sizes much larger than females. We suggest that the relationship between home range size ratio and body weight size dimorphism reflects different selection pressures on males and females in competition for resources and mates.     

Predation, sensitivity, and sex: why female black rhinoceroses outlive males

Among sexually dimorphic, polygynous mammals, adult femalestend to outlive males and respond more strongly to predatorsthan males. We asked whether a monomorphic, polygynous speciesvirtually immune to predation due to large size (black rhinoceros,Diceros bicornis) conforms to this pattern. Data on 193 interactionswith lions (Panthera leo) and spotted hyenas (Crocuta crocuta)in two nonhabituated populations in Namibia studied from 1991to 1993 revealed that: (1) females were more vigilant or aggressivethan males to either of the potential predators; and (2) whethersolitary or with calves, females attacked more often than males.Although solitary females tended to be more aggressive to lionsthan to hyenas, neither females with calves or males seemedto discriminate between the two carnivores. We also simulatedthe behavior of human predators (poachers) during 69 encounterswith rhinoceroses. While both sexes abandoned local sites becauseof our presence, females ran farther than males, covering upto 40 km in a day. These findings implicate a behavioral mechanismto explain why secondary sex ratios favor females - males aremore prone to human predation, a prediction consistent withdata from 12 populations throughout Africa. Black rhinocerosesappear to be an unanticipated exception to the well-establishedpattern of male-biased mortality in polygynous mammals; in theabsence of intense human predation (a recent event), male mortalityfails to exceed that of females, suggesting that intrasexualcompetition in a polygynous mammal: may not be the primary causeof unbalanced secondary sex ratios. Our results on the causesof sex differences in mortality and in responsiveness to differentpredators reinforce the relevance of behavioral ecology to conservation;such information is necessary for planning how best to minimizenegative human influences on the few remaining wild Africanrhinos.     

Size-dependent selection on arrival times in sticklebacks: why small males arrive first

Abstract Studies on arrival time to breeding areas show that high-quality males usually arrive first and gain the highest reproductive success. This is generally assumed to be due to phenotype-dependent costs and benefits of early arrival. We show that the opposite arrival order can occur, probably due to selection on poor-quality males to increase their chances of reproduction. In a fish species, the threespine stickleback, Gasterosteus aculeatus , small males arrived before larger males at the breeding grounds. Early arrival was costly because predation risk was at its highest at the start of the season and early territory establishment was selected against, as demonstrated by selection coefficients for territory maintenance and hatching success. Large males probably postponed arrival until females were available to decrease predation risk costs and increase offspring production. An experimental study showed that a delay in arrival of large males does not decrease their probability of reproduction, because large males are able to take over nest sites from small males. Small males, on the other hand, are less likely to establish territories in competition with large males but can pay the costs of early arrival in exchange for the benefit of access to territories. Thus, whereas natural selection favors later arrival, sexual selection through competition for breeding territories favors early arrival in small, competitively inferior males. This results in the benefits of early arrival depending on the competitive ability of the male, which favors size-dependent optimal arrival times.     

Unexpected female preference for smaller males in the marbled goby Pomatoschistus marmoratus

The study tested the role of body size and of nest size in female mate choice in the marbled goby, Pomatoschistus marmoratus. The results show a female preference for smaller males, supporting the idea that smaller males may be preferred to larger ones in the absence of male–male competition. No effect of nest size was detected, suggesting that other nest characteristics, beyond nest size, may be implicated.     

Large males have a mating advantage in a species of darter with smaller,allopaternal males Etheostoma olmstedi

<正> Theory suggests that males that are larger than their competitors may have increased mating success, due to bothgreater competitive ability and increased attractiveness to females. We examined how male mating success varies with male sizein the tessellated darter Etheostoma olmstedi. Previous work has shown that large males tend to move around and breed in vacantbreeding sites, and consequently provide less care for their eggs, while smaller individuals can be allopaternal, caring for the eggsof other males as well as for their own. We studied female egg deposition in a natural breeding population using artificial breedingsites and in the laboratory, where female choice of spawning site was restricted to two breeding sites tended by two males ofdifferent sizes. In both the field and the laboratory, nests tended by larger males were more likely to receive new eggs. Additionally,the mean size of males associated with a nest was positively correlated with both the maximum coverage of eggs at the nestand the number of times new eggs were deposited. We discuss how the increased mating success of larger males, despite their decreasedparental care, may help explain allopaternal care in this species     

Large males have a mating advantage in a species of darter with smaller, allopaternal males Etheostoma olmstedi

Theory suggests that males that are larger than their competitors may have increased mating success, due to both greater competitive ability and increased attractiveness to females. We examined how male mating success varies with male size in the tessellated darter Etheostoma olmstedi. Previous work has shown that large males tend to move around and breed in vacant breeding sites, and consequently provide less care for their eggs, while smaller individuals can be allopaternal, caring for the eggs of other males as well as for their own. We studied female egg deposition in a natural breeding population using artificial breeding sites and in the laboratory, where female choice of spawning site was restricted to two breeding sites tended by two males of different sizes. In both the field and the laboratory, nests tended by larger males were more likely to receive new eggs. Additionally, the mean size of males associated with a nest was positively correlated with both the maximum coverage of eggs at the nest and the number of times new eggs were deposited. We discuss how the increased mating success of larger males, despite their decreased parental care, may help explain allopaternal care in this species [Current Zoology 56 (1): 1-5, 2010].     

Female call preferences in tree-hole frogs: why are there so many unattractive males?

In tree-hole frogs, Metaphrynella sundana, the fundamental call frequency varies widely between males. In field playback experiments, females strongly preferred calls from the lower range of frequencies found in the population. There was no correlation, however, between male size and call frequency, as is normally the case for anurans, so large males were not necessarily more attractive to females. Presence or absence of upper harmonics in the call had no effect on female choice. Tree holes with shallow air columns were more often used by calling frogs, and were presumably more common, than deep holes. Since male M. sundana actively exploit the resonant properties of tree holes for mate attraction, and high frequencies match comparatively shallow holes, the benefits of attaining acoustic matching probably select for high-frequency calls. In addition, males with high-frequency calls may be heard from a greater distance in the vicinity of torrent streams. Since the level of such noise in the forest varies in time and space, different frequencies may prove optimal in different contexts, thereby preserving the observed variation within the population. Having an ‘unattractive’ high-frequency call should be potentially beneficial only when calling males do not congregate, a condition that our data suggest is fulfilled in this system.     

In memoriam: Napoleon A. Chagnon

Anthropologist Napoleon A. Chagnon was a central figure in the development and foundation of evolutionary approaches to human behavior. We highlight his ethnographic fieldwork, contributions to studies of kinship and marriage, and his foundational role in the development of evolutionary approaches to human behavior. As a holistic anthropologist Chagnon led anthropology toward the integration of cultural and evolutionary theory. Finally, his leadership was central in the foundation of the Human Behavior and Evolution Society.EpigraphRicardo, a Brazilian Protestant missionary who was assisting the scientific team and who has worked with the Yanomami for over 25 years … told me to run and get my camera. “There's going to be a fight,” he said. Then he turned to leave. “Aren't you going to stay?” I asked. “Nah, happens all the time. You stay, you're an anthropologist, should be interesting. Call me if anyone gets hurt.”