Variation in testosterone levels and male reproductive effort: Insight from a polygynous human population

Recent evidence suggests that, in humans, variations in testosterone (T) levels between men reflect their differential allocation in mating versus parenting efforts. However, most studies have been conducted in urbanized, monogamous populations, making generalizations from them questionable. This study addresses the question of whether indicators of male reproductive effort are associated with variations in salivary T levels in a polygynous population of agriculturists in rural Senegal. We first show that pair-bonding and/or transition to fatherhood is associated with T profiles: married fathers (N = 53) have lower morning and afternoon T levels than unmarried non-fathers (N = 28). Second, among fathers, individual differences in parenting effort, as well as variations in mating effort, predict morning T levels. Indeed, men highly investing in parental care show lower morning T levels. Moreover, among men under 50, polygynous men show higher morning T levels than monogamous men. Taken together with previous results in monogamous settings, these findings suggest that the endocrine regulation of reproductive effort is probably a general feature of human populations.     

Testosterone and paternal care in East African foragers and pastoralists

The 'challenge hypothesis' posits that testosterone facilitates reproductive effort (investment in male-male competition and mate-seeking) at the expense of parenting effort (investment in offspring and mates). Multiple studies, primarily in North America, have shown that men in committed relationships, fathers, or both maintain lower levels of testosterone than unpaired men. Data from non-western populations, however, show inconsistent results. We hypothesized that much of this cross-cultural variation can be attributed to differential investment in mating versus parenting effort, even among married fathers. Here, we directly test this idea by comparing two neighbouring Tanzanian groups that exhibit divergent styles of paternal involvement: Hadza foragers and Datoga pastoralists. We predicted that high levels of paternal care by Hadza fathers would be associated with decreased testosterone in comparison with non-fathers, and that no such difference between fathers and non-fathers would be evident in Datoga men, who provide minimal direct paternal care. Twenty-seven Hadza men and 80 Datoga men between the ages of 17 and 60 provided morning and afternoon saliva samples from which testosterone was assayed. Measurements in both populations confirmed these predictions, adding further support to the hypothesis that paternal care is associated with decreased testosterone production in men.     

Male Participation in Nest Building in the Dung Beetle Scarabaeus catenatus (Coleoptera: Scarabaeidae): Mating Effort Versus Paternal Effort

The dung beetle, Scarabaeus catenatus, shows not only the rolling but also the tunneling tactic for nest building with bisexual cooperation. Sex roles, however, differed between the tactics. In rolling, the male took the initiative like that of ball-roller species: he rolled a dung ball away and buried it. In tunneling, in contrast, the male usually had a secondary role like that of tunnelers: he was less active in burrow excavation and provisioning. Regardless of the tactics, male participation did not increase female reproductive output measured by the number or size of brood balls in the field, but seemed to function as mate guarding against conspecific males. This suggests that, in both tactics, the male S. catenatus invests primarily in mating effort compared with paternal effort. The relative importance of mating effort in male participation seems to hold true in other dung beetles, irrespective of whether they are ball-roller or tunneler species. In addition, the male mating strategy of S. catenatus is compared with that of other ball-rollers.     

Multiple-year optimization of conservation effort and monitoring effort for a fluctuating population

We consider optimal conservation strategies for an endangered population. We assume that juvenile survival is affected by unpredictable environmental fluctuation and can be improved by costly conservation effort. The initial population size is not accurately known at the time that the conservation effort level is chosen, but the uncertainty of its estimate can be reduced by a costly monitoring effort. In a previous paper, we analysed the optimal management strategy that minimizes a weighted sum of extinction probability and economic costs when only a single year is considered. Here we examine the case in which the conservation period lasts for several years by dynamic programming with incompletely observed process states. We study the optimal levels of the conservation and the monitoring efforts, and their dependence on the length of the conservation period and other parameters. The main conclusions are: (1) The optimal conservation effort in the first year depends on the accuracy of the information on the population size in the first year, but is almost independent of the accuracy of the information in later years. (2) When the risk of population extinction is small, the optimal conservation effort increases with the uncertainty of the population size. In contrast when the population is endangered, the optimal conservation effort decreases with the uncertainty of the population size. (3) The optimal conservation and monitoring efforts both increase with the length of the conservation period, provided that the population is relatively safe. However, if the population is endangered, both types of effort become smaller when the conservation period increases.     

Optimal conservation strategy in fluctuating environments with species interactions: resource-enhancement of the native species versus extermination of the alien species

Alien species are often a major threat to native species. We consider optimal conservation strategies for a population whose viability is affected both by an alien species (such as a competitor, a predator, or a pathogen) and by random fluctuations of the environment (e.g. precipitation, temperature). We assume that the survivorship of the native population can be improved by providing resources such as food and shelter, and also by an extermination effort that decreases the abundance of the alien species. These efforts decrease the extinction probability of the native population, but they are accompanied by economic costs. We search for the optimal strategy that minimizes the weighted sum of the extinction probability and the economic costs over a single year. We derive conditions under which investment should be made in both resource-enhancement and extermination, and examine how the optimal effort levels change with parameters. When the optimal strategy includes both types of efforts, the optimal extermination effort level turns out to be independent of the density and economic value of the native species, or the variance of the environmental fluctuation. Furthermore, the optimal resource-enhancement effort is then independent of the density of the alien species. However, the parameter dependencies greatly change if one of the efforts becomes zero. We also examine the situation in which the impact of the alien species is uncertain. The optimal extermination effort increases with the uncertainty of this impact except when the cost of extermination is very high.     

Staying with the young enhances the fathers’ attractiveness in burying beetles

Studying the relationship between parental and mating effort helps us to understand the evolution of parental care and, consequently, has been the subject of many theoretical and empirical investigations. Using burying beetles as a model, we found no correlation between the intensity of a sexual signal (sex pheromone quantity) and the amount of care provided by males. However, males that were given the opportunity to breed and care for young went on to produce a higher amount of their sexual signal and attracted three times more females in the field than control males that were not given the opportunity to breed. The likely explanation for our finding is that specific aspects of care in burying beetles, that is the defense and preservation of a nutrient rich breeding resource, a small vertebrate cadaver, is not only beneficial for the offspring but also for the adults themselves. Obtaining a good carrion meal possibly enables males to store resources that they can subsequently allocate toward sexual signaling. Collectively, our results highlight that conditions can exist where male participation in brood care has a positive effect on its sexual attractiveness. This in turn might have facilitated the evolution of male assistance in parental care.     

Alteration of reproductive effort by a monogenic,recessive mutation in jute (Corchorus capsularis L.)

Summary Reproductive effort (RE) of a day-length neutral mutant TCJ-5, its parent, and two other cultivars of jute (Corchorus capsularis) was estimated as reproductive biomass/aerial biomass. Plant height at flowering and aerial biomass were significantly higher in the mutant, while the reproductive biomass at 55 days after flowering was statistically equal. Therefore, the estimated RE was significantly lower in the mutant compared to the parent and other cultivars. The lower RE of the mutant was due to delayed initiation of flowering and additional vegetative growth in this period. The results show alteration of RE by a recessive mutation.     

Marriage, parenting, and testosterone variation among Kenyan Swahili men

Male variation in testosterone (T) levels may, in part, reflect a differential behavioral allocation to mating and parenting effort. This research tests whether demographic indicators of pair bonding and parenting were associated with salivary T levels among Kenyan Swahili men. Men in the sample were either unmarried (N = 17), monogamously married (N = 57), or polygynously married (N = 14), and between ages 29-52. In contrast with earlier findings among North American men, monogamously married men did not have lower T levels than unmarried men. However, among all married men, men with younger genetic children tended to have marginally lower T levels. Polygynously married men, all of whom had two wives, had higher T levels than all other Swahili men. Possible explanations of higher T levels among polygynously married men are explored.     

Lifetime reproductive effort in humans

Lifetime reproductive effort (LRE) measures the total amount of metabolized energy diverted to reproduction during the lifespan. LRE captures key components of the life history and is particularly useful for describing and comparing the life histories of different organisms. Given a simple energetic production constraint, LRE is predicted to be similar in value for very different life histories. However, humans have some unique ecological characteristics that may alter LRE, such as the long post-reproductive lifespan, lengthy juvenile period and the cooperative nature of human foraging and reproduction. We calculate LRE for natural fertility human populations, compare the findings to other mammals and discuss the implications for human life-history evolution. We find that human life-history traits combine to yield the theoretically predicted value (approx. 1.4). Thus, even with the subsidized energy budget and uniqueness of the adult lifespan, human reproductive strategies converge on the same optimal value of LRE. This suggests that the fundamental demographic variables contained in LRE trade-off against one another in a predictable and highly constrained manner.     

Physiological change in an insular lizard population confirms the reversed island syndrome

Unpredictable environmental conditions and highly fluctuating population densities are believed to have produced a ‘reversed island syndrome’ (RIS) in an insular population of the Wall lizard on Licosa Islet, Italy. Several of the physiological, behavioural, and life‐history changes that constitute the RIS could result from positive selection on increased activity of melanocortins. For example, increased levels of α‐melanocyte‐stimulating hormone (MSH) should lead to increased investment in reproduction and increased immunocompetence in the island population. We tested the crucial assumption of this idea that plasma levels of α‐MSH in Licosa Islet lizards are elevated compared to those of the mainland relatives. We also tested for differences in reproductive effort between populations, by measuring plasma levels of 5‐α‐dihydrotestosterone in males and clutch mass in females. In addition, we assessed ectoparasite load as an indicator for the lizards’ resistance to environmental stress. In agreement with the RIS, we found that insular lizards exhibit higher α‐MSH levels, allocate more energy to reproduction, and have a reduced ectoparasite load compared to the nearest mainland population. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Showoffs or Providers? The Parenting Effort of Hadza Men

Paternal care plays an important role in many scenarios of human evolution. Lately, however, this “Man the Provisioner” view has been challenged. The showoff hypothesis, for example, proposes that men hunt not to provision children but to gain extra mating opportunities, and some have suggested that male care among mammals is always a form of mating, rather than parenting, effort. This study, based on observation in a hunting and gathering society, the Hadza of Tanzania, tests whether men provide care as parenting effort. If male care were mating effort only, stepchildren should receive no less care than biological children. My data, however, reveal that stepchildren do receive less care. This suggests that care is provided, at least in part, as parenting effort. Although lower direct care implies stepfathers are less motivated to care for stepchildren, resource acquisition data raise the question of whether stepfathers are less motivated, or simply less skilled.     

Enhanced UV-B radiation increases the reproductive effort in the Mediterranean shrub Cistus creticus under field conditions

Seedlings of the Mediterranean shrub Cistus creticus L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras, 38.3°W, 29.1°E) for 20 months. During this period, measurements of photosynthetic capacity, photochemical efficiency of PS II, chlorophylls and carotenoids were performed once per season. Supplemental UV-B radiation had no significant effect on these parameters nor on the total, above ground biomass accumulation, plant height and leaf specific mass measured at plant harvest. It was observed, however, that UV-B supplementation increased the number of seeds per fruit as well as mean individual seed mass. As a result, seed number and total seed mass per plant were considerably increased. Germination rates of produced seeds were not affected. We may conclude that C. creticus is a UV-B resistant plant whose competitive ability may be improved by enhanced UV-B radiation through an increase in its reproductive effort and a higher contribution to the seed bank.     

Integrating cues of social interest and voice pitch in men's preferences for women's voices

Most previous studies of vocal attractiveness have focused on preferences for physical characteristics of voices such as pitch. Here we examine the content of vocalizations in interaction with such physical traits, finding that vocal cues of social interest modulate the strength of men's preferences for raised pitch in women's voices. Men showed stronger preferences for raised pitch when judging the voices of women who appeared interested in the listener than when judging the voices of women who appeared relatively disinterested in the listener. These findings show that voice preferences are not determined solely by physical properties of voices and that men integrate information about voice pitch and the degree of social interest expressed by women when forming voice preferences. Women's preferences for raised pitch in women's voices were not modulated by cues of social interest, suggesting that the integration of cues of social interest and voice pitch when men judge the attractiveness of women's voices may reflect adaptations that promote efficient allocation of men's mating effort.     

Variation in nuptial gift quality in bush crickets (Orthoptera: Tettigoniidae)

Males of many insect species provide the female, during courtshipand copulation, with a nuptial gift consisting of a prey itemor synthesized material (e.g., spermatophores). The studiesperformed so far have mainly focused on effects of nuptial giftsize on male and female reproductive success. However, the qualityof the nuptial gift can differ substantially between taxa andmay potentially have a large impact on male and female reproductiveperformance. In this study the effects of. variation in dieton nuptial gift quality is investigated in several bush cricketspecies with different diets. The effect of diet on nuptialgift quality (e.g., protein in the spermatophylax) and femalereproductive output and, in turn, die effect of variation inspermatophylax quality on female reproductive output are investigated.Female reproductive output and male spermatophore size weremainly found to be influenced by differences in diet betweenspecies. Spermatophylax quality (i.e., protein concentration)was also correlated with differences in diet. There was a largevariation in protein content of the spermatophylax widiin aswell as between species. Moreover, larger spermatophylaxes hada lower protein concentration, indicating a possible trade-offbetween spermatophylax size and quality. Consequendy, productionof larger spermatophylaxes, required for protection of the male'ssperm carrying ampulla during insemination, can lead to a reducedprotein concentration, because the total amount of protein maybe limited. This pattern is also consistent with die idea diatthe spermatophylax functions primarily to ensure sperm transfer.Finally, there was no correlation between the amount of proteinin the spermatophylax and female reproductive output eitheracross diets or within each diet category, further supportingthe finding that female reproductive output is mainly affectedby differences in diet.     

Reproductive traits and change in body shape of neonates in the Oak Forest Skink,Plestiodon lynxe

Reproductive traits are critically important for understanding how organisms adapt to their respective environments. In this study, we provide information on relative litter mass (RLM) and other litter and neonate related characters of nine female Plestiodon lynxe captured in the field. We also recorded seven body dimensions in 16 neonates and 15 two-month juveniles, and on the basis of these dimensions we compared the body shape of these two age classes to detect changes in the proportions of body parts. The average litter size (4.55) is larger than that found in other viviparous species of Plestiodon, but smaller than those of congeneric oviparous species of similar size. However, the average body size of newborns (25.49 mm) is similar to that of other oviparous and viviparous species of Plestiodon. The average RLM was relatively high (0.36). The relative size of the head and limbs becomes proportionally smaller, whereas the axilla-groin length becomes proportionally larger in the first 2 months of life. We suggest that these changes are related to changes in locomotion and microhabitat use.