Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable

In a genetic analysis of the mating system of cooperatively breeding Arabian babblers (Timalidae: Turdoides squamiceps), we identified which individuals in the population are breeding, and how reproductive success was distributed among group members with respect to their dominance rank, for both males and females. The population was characterized by an asymmetrical distribution of reproductive success; behaviorally dominant males produced 176 of 186 (95%) of the offspring in 44 social groups analyzed, and alpha females produced 185 of 186 (99.5%). We evaluated models of reproductive skew by examining genetic and demographic correlates of reproduction by␣subordinates. Subordinate (beta) males that sired young were more likely to be recent dispersers from their natal groups or members of newly formed groups than betas that did not reproduce. Breeding beta males had spent smaller proportions of their lives with the current alpha male and female as alphas than had beta males that did not sire young. One consequence of the linkage of dispersal with breeding in newly formed, nonnatal groups is that beta males that sired young had significantly lower genetic similarity to the alpha males in their groups (based on band-sharing coefficients using multilocus minisatellite DNA fingerprinting) than those that did not sire young. This pattern may occur generally in species in which group membership accrues both through nondispersal of young (forming groups of relatives) as well as through dispersal involving coalitions that sometimes include nonrelatives. Received: 22 July 1997 / Accepted after revision: 5 February 1998     

Brood sex ratio is dependent on female mating status in polygynous great reed warblers

Females capable of adjusting the sex ratio of their offspring should be more fit than females lacking such an ability. In polygynous birds where breeding success in males is more strongly influenced by body size and/or attractiveness than in females, females might produce more sons when predicting good conditions or when mating with attractive males. Polygynous great reed warbler, Acrocephalusarundinaceus, males direct most of their feeding effort to the primary (first-hatching) nest and in these nests increase their feeding effort in relation to the brood sex ratio (proportion of sons). Therefore, with the expectation of well-nourished sons, we would predict that females which start breeding first within harems might produce more sons than those which start breeding later, and in anticipation of sons with good genes, that females mated to polygynous males might produce more sons than females mated to monogamous males. I took blood samples from hatchlings and determined the sex using DNA markers. The sex ratio of primary (monogamous and polygynous primary) broods is more male-biased (mean 0.58 males, n = 50) than that of secondary (polygynous secondary and tertiary) broods (mean 0.46, n = 25). Moreover, in the secondary broods with the largest clutch (five eggs), in which offspring are most likely to suffer food shortage, the sex ratio was distinctively female biased (mean 0.33, n = 10). In the primary broods, sex ratio was correlated to harem size. The results suggest that great reed warbler females modify the brood sex ratio to produce both well-nourished sons and sons with good genes, but the former effect is probably stronger than the latter factor. Received: 11 March 1998 / Accepted after revision: 23 May 1998     

Male genital traits and mating interval affect male fertilization success in the water strider Gerris lacustris

Parker's seminal work brought attention to the possibility of postmating sexual selection by non-random fertilization success. Mechanisms for these processes are still only partly understood and there is clearly a need for more studies of intraspecific variation in sperm precedence. Here, we report results from an experimental study of the variation in fertilization success between males of the water strider Gerris lacustris. Genital morphology, male body size, and copulation duration were examined as possible correlates of paternity. The significance of guarding duration was also analysed. Only male genital morphology was correlated to fertilization success. This is one of the first studies showing a relationship between male genital traits and fertilization success, supporting the view that sexual selection may be responsible for the rapid and divergent evolution of genital structures in animals with internal fertilization. The fertilization success of last males varied considerably after double matings with a short mating interval (10 min). Last-male priority ranged from 0 to 100% and usually one of the males involved fertilized almost all the eggs. After double matings with a short mating interval, the proportion of eggs fertilized by the last male averaged 0.68 and was greater than 0.5. In contrast, the average fertilization success was biased towards the first male when the matings were more spread out over time (24 h). These results do not support earlier suggestions of a widespread last-male sperm priority in water striders. Received: 28 July 1998 / Received in revised form: 15 March 1999 / Accepted: 28 March 1999     

Relative importance of male and territory quality in pairing success of male rock ptarmigan (Lagopus mutus)

We studied pairing success in male rock ptarmigan (Lagopus mutus) in northern Alaska to learn whether males obtaining more females possessed phenotypic traits that influenced female choice directly, whether these traits permitted males to obtain territories favored by females, or whether both processes occurred. The number of females per male varied from zero to three. Several male and territory traits were significantly correlated with number of females per male. We used multiple regression to obtain a single measure of male quality and a single measure of territory quality. These measures of male and territory quality correlated with each other and with male pairing success. We used path analysis to separate direct effects of male quality on pairing success from indirect effects due to high-quality males obtaining high-quality territories. Both direct and indirect pathways had significant effects on pairing success, and direct and indirect effects of male traits on pairing success were about equal. This study illustrates an analytical approach for estimating the relative importance of direct and indirect causal relationships in natural systems. Received: 13 January 1998 / Accepted after revision: 7 November 1998     

Male care and mating effort among Hadza foragers

Paternal care figures prominently in many scenarios of human evolution. Recently, however, such scenarios have been challenged on two scores. First, the level of male contribution may be insignificant. Second, male care may be provided as a form of mating effort, rather than parenting effort. In theory, since men can enhance their Darwinian fitness both by providing care to their own offspring if this raises offspring fitness and by pursuing additional mates if this leads to additional offspring, men should respond to payoffs from both mating and parenting effort. If men respond to payoffs from parenting effort, paternity ought to make a difference. And if men respond to payoffs from mating effort, mating opportunities ought to make a difference. I analyzed the impact of these two factors on variation in male care among the Hadza, a foraging society in Tanzania. Two predictions were tested: (1) biological children will receive more care than stepchildren, and (2) men will provide less care to their biological children as their mating opportunities increase. Both predictions were supported. These results suggest men provide care, in part, as parenting effort, and that they trade off parenting effort for mating effort when they have greater mating opportunities. Received: 21 January 1998 / Received in revised form: 24 January 1999 / Accepted: 1 February 1999     

Male reproductive success in free-ranging feral horses

In the social organization of feral horses, adult males compete to monopolize groups or bands of females, sometimes called harems. Alternative male strategies are to remain alone or with other bachelors or, less commonly, to accept subordinate status within a harem. The hypothesis that dominant harem stallion status confers a reproductive advantage was tested in free-ranging feral horses. The presence of foals in harems headed by vasectomized (VSX) versus intact stallions was used to assess the ability of these stallions to control reproduction in their harems. Of harems headed by VSX stallions, 17 and 33% contained foals during years 2 and 3 post-treatment, respectively. In contrast, 86 and 80% of harems headed by non-VSX stallions contained foals in those years. Acquisition of pregnant mares appeared more likely than sneak copulations by bachelor stallions to account for foals in harems with a single stallion. However, most foals were born into harems that included a subordinate stallion, an occurrence that was undoubtedly exacerbated by the extended breeding season resulting from the sterility of the harem stallion. Thus, in comparing alternative reproductive tactics, bachelors appeared less successful than subordinate stallions within a harem. However, the highest reproductive success was achieved by the harem stallion, further demonstrating that alternative tactics are not equally profitable. Received: 13 July 1999 / Received in revised form: 24 July 1999 / Accepted: 24 July 1999     

Trade-off between fitness components in males of the polygynous butterfly Callophrys xami (Lycaenidae): the effect of multiple mating on longevity

The differential costs of mating paid by males and females influence the nature and strength of sexual selection. In butterflies, males invest a relatively large amount of time and resources in each mating, but male survival costs of mating have not been demonstrated. I present the results of experiments designed to measure the effect of different aspects of mating on male longevity in the polygynous butterfly Callophrys xami. In experiment 1, I compared the longevity of pairs of males that produced similar amounts of spermatophore, but that mated at different rates, a different numbers of times, and that produced spermatophores at different rates, and found that the longevity of ”low-mating-rate” males was not different from that of ”high-mating-rate” males. In experiment 2, the longevity of virgin males was not significantly different from that of multiply mated males. In experiment 3, I used resource-limited males resulting from experimental food limitation of last-instar larvae; resource-limited virgin males lived significantly more days than resource-limited multiply mated males. Since ecological costs of mating (e.g., disease transmission, predation risk) were excluded in the experiment, diminished male longevity was a product of physiological costs of sexual interactions. These results suggest that the cost of ejaculate production is an important cause of longevity reduction when there are resource limitations; however, the role of other possible physiological costs of mating in longevity reduction is still unknown. Received: 21 March 2000 / Accepted: 26 August 2000     

Female feeding regime and polyandry in the nuptially feeding nursery web spider, Pisaura mirabilis

We examined the influence of female feeding regime on polyandry in the nuptially feeding nursery web spider (Pisaura mirabilis). In this species, the nuptial gift, a dead prey item wrapped in the male’s silk, is physically separate from the ejaculate. We manipulated female feeding regime (starved or fed) and the presence or absence of a gift with three successive males to test direct-benefits hypotheses (nuptial gift or sperm supply) for the expression of polyandry. The presence of a gift was necessary for copulation, as no male without a gift successfully copulated. Female mating behavior most strongly supports polyandry due to the accumulation of gifted food items (“nuptial gift” direct-benefits hypothesis). Starved females that were presented with a gift accepted significantly more gifts and inseminations than fed females. Most starved females (74%) copulated two or more times, as opposed to only 3% of the fed females. Nearly all of the females that accepted a gift subsequently copulated. The nuptial gift item seems to function as male mating effort and females appear to receive multiple matings as part of a feeding strategy.