The frequency and costs of harassment in southern elephant seals

The female Euscepes postfasciatus refractory period is induced by the male but length is determined by the female

Females of many animals mate multiple times during their lives (i.e., polyandry). The period between matings (mating interval) is called the refractory period (RP). In the West Indian sweet potato weevil (Euscepes postfasciatus), males use the ejaculate to induce the RP in females to prevent re-mating. By measuring the RP, a large variation of 1-49 days was observed. This variation may be due to the males (ejaculate quantity and quality) and females (ejaculate sensitivity/degradation ability and body size) and their interactions, but the exact mechanisms are currently unclear. Here, we investigated a tendency towards a particular female RP duration and the associated traits of males and females to test the following three factors responsible for variation in the length of the RP: male manipulation of ejaculate volume, individual differences in male ejaculation substances, and ejaculate sensitivity/degradation ability in females. We prepared virgin males and females to create mating pairs. The following day, another mate was introduced to the females, and the first RP was measured. The same procedure was used for measuring the second RP. The males were also provided with another female (second female), mated, and then the RP of the second female was measured. In addition, the relationship between the length of the RP and female fitness was investigated. The results showed that there was a significant positive correlation between the first and second RP in the focal females, while no significant correlation was observed between the RP of the first and second females induced by the same male. It was also found that the length of the RP did not affect female fitness. This indicated that the males did not adaptively manipulate ejaculation volume depending on the quality of the females, and variance in the length of the RP may be explained by variation in the female physiological ability against ejaculate.

The conspecific avoidance strategies of adult female-calf humpback whales

During migration, humpback whale (Megaptera novaeangliae) adult females and their calves use acoustic calling to help maintain contact. The signals produced by these pairs, however, may unintentionally attract nearby breeding males, which can result in interactions that have negative physical and physiological effects on the calf. Therefore, maternal females must choose the vocal and/or behavioral strategy that most effectively balances intra-pair communication with male avoidance. Here, we analyzed differences in adult female-calf vocal activity and movement behavior according to the presence of, and distance to, singing whales and other groups likely to contain males. The results of this study found that these pairs make only minimal changes to their vocal behavior in response to nearby males, suggesting that they have instead evolved calls that are naturally difficult to detect (i.e., produced at significantly lower rates and acoustic levels than other whale groups, resulting in a restricted active space). In addition, they maintain spatial separation from nearby groups by moving to shallower, inshore waters, increasing their proportion of time spent near the surface, and favoring a direct migratory course. This combination of cryptic strategies balances avoidance of unwanted conspecific interaction with the necessity of continued contact between maternal female humpback whales and their calves.

Environment and Local Substrate Availability Effects on Harem Formation in a Polygynous Bark Beetle

Simple Summary

Harem polygyny is a mating system where a single male defends a group of females for the purpose of securing multiple mating. While this mating system is well-known in mammals it is uncommon in insect groups. The mating aggregations that occur in insect groups may be driven by environmental conditions or resources available for feeding and breeding. We aimed to determine how the local availability of breeding substrate affects the formation of harems in the five-spined bark beetle, Ips grandicollis. Aggregations are formed when a male bores under the bark of felled pine trees and makes a nuptial chamber. The male then releases an aggregation pheromone that attracts females for mating and other males to also exploit the resource. When the population density was higher the number of females associated with each male was greater. The population density was determined by environmental circumstances with higher density in a pine plantation that was being harvested than in a plantation that was still standing. The amount of substrate (logs per replicate pile) available to the bark beetles also influences the number of beetles attracted to a log and size of individual harems. The environment and local substrate availability did not affect how females distribute themselves around the male. Females did not actively avoid positioning themselves further from neighbouring females to avoid competition. Their arrangement within harems was equivalent to random positioning.

Abstract

Many forms of polygyny are observed across different animal groups. In some species, groups of females may remain with a single male for breeding, often referred to as “harem polygyny”. The environment and the amount of habitat available for feeding, mating and oviposition may have an effect on the formation of harems. We aimed to determine how the surrounding environment (a harvested or unharvested pine plantation) and availability of local substrate affect the harems of the bark beetle, Ips grandicollis (Coleoptera: Curculionidae: Scolytinae). In a harvested pine plantation with large amounts of available habitat, the population density of these beetles is much higher than in unharvested plantations. We found the number of females per male to be significantly greater in the harvested plantation than the unharvested one. Additionally, the amount of substrate available in the immediate local vicinity (the number of logs in replicate piles) also influences the number of beetles attracted to a log and size of individual harems. We also examined how females were distributing themselves in their galleries around the males’ nuptial chamber, as previous work has demonstrated the potential for competition between neighbouring females and their offspring. Females do not perform clumping, suggesting some avoidance when females make their galleries, but they also do not distribute themselves evenly. Female distribution around the male’s nuptial chamber appears to be random, and not influenced by other females or external conditions.

Trade-offs between age-related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better

Life history trade-off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding seasons, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade-offs are present in the highly polygynous male southern elephant seal (Mirounga leonina) using 34-years of individual-level data. We compare age-specific survival, recruitment and future breeding success probabilities of pre-breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre-breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre-breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age-specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population-level trade-off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade-off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.

Social bonds between unrelated females increase reproductive success in feral horses

In many mammals, females form close social bonds with members of their group, usually between kin. Studies of social bonds and their fitness benefits have not been investigated outside primates, and are confounded by the relatedness between individuals in primate groups. Bonds may arise from kin selection and inclusive fitness rather than through direct benefits of association. However, female equids live in long-term social groups with unrelated members. We present 4 years of behavioral data, which demonstrate that social integration between unrelated females increases both foal birth rates and survival, independent of maternal habitat quality, social group type, dominance status, and age. Also, we show that such social integration reduces harassment by males. Consequently, social integration has strong direct fitness consequences between nonrelatives, suggesting that social bonds can evolve based on these direct benefits alone. Our results support recent studies highlighting the importance of direct benefits in maintaining cooperative behavior, while controlling for the confounding influence of kinship.

Sexual harassment and female gregariousness in the South American sea lion, Otaria flavescens

Most colonial pinnipeds form extreme clusters of breeding females that cannot be entirely explained by the distribution of sites for reproduction. Avoidance of male harassment has been postulated as an important determinant of reproductive aggregation in this group of mammals. Female gregariousness can reduce harassment by resident males by two mechanisms; directly by the 'dilution effect' or indirectly because resident males that defend large female groups are less harassing. In order to investigate the relationship between male harassment and female gregariousness in relation to the size of breeding groups, we analysed the behaviour of dominant males and their females in a breeding colony of Otaria flavescens. Females in large breeding groups received less harassment by resident males due to dilution effects and because males that defended a large group interacted less frequently with females than males with small groups.

Physiology and behavior influence lactation efficiency in northern elephant seals (Mirounga angustirostris)

The efficiency with which mothers convert acquired energy into milk is a key determinant of the magnitude of parental investment in mammals; however, the mechanisms underlying lactation efficiency are poorly understood. Investigations on northern elephant seals have shown lactation efficiency, measured as the proportion of total energy expenditure that goes to the pup as milk, increases with age. In a cross-sectional study the physiological and behavioral determinants of lactation efficiency were investigated in eight young and seven prime (older) elephant seals by conducting behavioral observations and collecting milk, blood, and tissue on days 3 and 22 of lactation. Milk composition, circulating fatty acid and triglyceride concentrations, and mammary and blubber lipoprotein lipase activity were determined. Prime females had significantly greater percent milk fat and circulating fatty acids on day 3 than did young females, but these differences disappeared by day 22. The ability for prime females to produce higher-energy milk early in lactation may allow them to increase lactation efficiency by increasing the rate of energy transfer. In addition, prime females spent significantly more time resting. A combination of reduced activity and more rapid energy delivery likely explains the increase in lactation efficiency with age found in a previous study.

Male harassment of female New Zealand sea lions, Phocarctos hookeri: mortality, injury, and harassment avoidance

Sexual aggression by male pinnipeds during breeding can lead to female injury and death, affecting reproductive success, fecundity, and increasing the cost of mating for females. Thus, females that employ strategies to minimize the probability of being injured will be at an advantage. Here we investigate the extent of injuries and the number of deaths attributed to male harassment, and test the hypothesis of whether the arrival and departure behaviour of female New Zealand sea lions (Phocarctos hookeri (Gray, 1844); NZSL) at Sandy Bay, Enderby Island, the Auckland Islands, is adapted to reduce the chance of injury or death from encounters with male NZSLs. During the breeding season, harassment by non-territorial male NZSLs causes mortality in adult female NZSLs, approximately 5 in every 1000 females breeding each year. Permanent scars from male bites are observed on 84% of adult females. This mortality and visible injury rate only represents the direct impacts on female NZSL from male harassment. Indirect impacts, such as the time and energy cost of avoidance behaviour, pup separation, and pup injury and death, can have as significant long-term effects on individuals and the population. We find that male harassment can influence the behaviour of individuals in NZSL breeding harems.

Costs and benefits of joining South American sea lion breeding groups: testing the assumptions of a model of female breeding dispersion

A recent cost-benefit model has been proposed (M.H. Cassini. 1999. Behav. Ecol. 10: 612–616; M.H. Cassini. 2000. Behav. Processes, 51: 93–99) to predict the dispersion of female mammals when breeding resources are distributed in fixed and predictable patches. The benefit of the model is a reduction in male harassment when females join breeding groups, and the cost is an increase in female–female competition for breeding resources. We tested the main assumptions of this model in a breeding colony of South American sea lions (Otaria flavescens), a sexually dimorphic, polygynous pinniped. The rate of female–female agonistic interactions increased with the number of females, which suggests that higher levels of female–female competition in denser breeding groups could reduce pup survival, owing to mother–pup separation effects. The rate of male–female interactions per female decreased with the number of females defended by a male, the trend being nonlinear, and males did not modify the frequency of interaction with females according to variations in the size of breeding groups. This evidence supports the advantage of female gregariousness in reducing the reproductive costs of interacting with males. We concluded that avoidance of male disturbance through dilution effects may have played an important role in the evolution of this species' mating system.

Human disturbance, nursing behaviour, and lactational pup growth in a declining southern elephant seal (Mirounga leonina) population

We studied lactation behaviour in relation to pup growth in southern elephant seals (Mirounga leonina) at Macquarie Island, and compared harems in areas of high and low human presence to determine if there is an effect attributable to human activities, including scientific research. Pup weaning mass, a known correlate of first–year survival, was positively influenced by suckle bout durations during early and middle lactation and by maternal aggression during late lactation; no other behavioural variables were associated with weaning mass. In the area of high human presence, we observed from a distance the behaviour of mother–pup pairs directly before, during, and after visits to harems by other researchers. Alertness was raised threefold in the presence of people but quickly returned to predisturbance levels after their departure; there were no significant short-term effects on other behavioural variables. In the areas of high and low human presence, we observed the undisturbed behaviour of the seals in the absence of other people. No significant differences in any behavioural variables examined were found, indicating no long-term changes in behaviour resulting from human presence. Human disturbance therefore appears not to have significantly contributed to the population decline observed at Macquarie Island, but the conclusion requires caution given the fairly low power of our analyses.

Measures of breeding inequality: a case study in southern elephant seals

Inequality in distribution of resources is a key aspect of evolutionary biology particularly in relation to distribution of mates and copulations. Notwithstanding its important role, inequality is not easily defined, and its measurement is complicated by theoretical and methodological issues. Although the formal treatment of inequality has been mostly limited to the evolution of lek mating system, a methodologically correct approach to measurement of inequality is generally valid for the study of any kind of mating system. In this paper, we analyze inequality in a large set of southern elephant seal (Mirounga leonina) harems. The observed distribution of fertilizations was significantly different from both the expected distribution with equal shares of resources and the expected distribution with equal propensities to acquire resources. We calculate and compare various measures of inequality, observing a wide variation particularly among unbounded and bounded indices. We check the effect of choosing a specific measure of inequality by considering the effect of two aspects of harem socionomy, the number of females in the harem (i.e., the total amount of resources to be shared) and the number of males associated with the harem (i.e., the number of competitors). The choice of a specific measure of inequality had a strong impact on the results obtained and should be considered a critical step in every study of functional and evolutionary correlates of inequality. Unbounded indices showed a strong relationship with both harem size and number of males, while no effect was evident in the analysis of bounded indices. This demonstrates that, in this species, the despotism of the mating system remains high even in large harems and with many competitors, i.e., the worst conditions for monopolization.