Polygynous Females Research Articles

Kin selection and polygyny: can relatedness lower the polygyny threshold?

Resource polygyny incurs costs of having to share breeding resources for female breeders. When breeding with a relative, however, such costs may be lessened by indirect fitness benefits through kin selection, while benefits from mutualistic behaviour, such as communal defence, may increase. If so, females should be less resistant to sharing a territory with a related female than with a non-related one. We investigated whether kin selection may lower the threshold of breeding polygynously, predicting a closer relatedness between polygynous females breeding on the same territory than between females breeding on different territories. Northern lapwings, Vanellus vanellus, are suitable for testing this hypothesis as they are commonly polygynous, both sexes take part in nest defence, and the efficiency of nest defence increases with the number of defenders. Using an index of relatedness derived from DNA fingerprinting, we found that female lapwings that shared polygynous dyads were on average twice as closely related as were random females. Furthermore, relatedness did not correlate with distance between breeders, indicating that our findings cannot be explained by natal philopatry alone. Our results suggest that the polygyny threshold in lapwings may be lowered by inclusive fitness advantages of kin selection.

The influence of phenology on double-brooding and polygyny incidence in the Sedge Warbler Acrocephalus schoenobaenus

There is growing evidence that the fitness of birds in temperate zones depends strongly on the match between the timing of breeding and local phenology. We have analysed data collected from a 15-year study on a Sedge Warbler (Acrocephalus schoenobaenus) population in its natural habitat. The highly skewed distribution of arrival dates of males indicated that they competed intensely to be the earliest arrivals. The distribution of mating dates, unlike arrival dates, was not skewed, rather it formed a bell-shaped distribution. Because females arrived much later, they could not base their mating choices on the arrival dates of males. Females, however, could use male song repertoire, a trait we found to be correlated with male arrival date. Double-brooded females mated early in the season, exactly within the peak of mating date distribution. Polygynous females, on the other hand, mated later in the season, choosing high-quality older males which had arrived significantly earlier. Male territory quality did not differ between double-brooded and polygynous females, nor did the quality of the territories occupied by an individual female change during her lifetime. Although longer lifespan and double-brooding influenced a female’s fitness over the course of her lifetime, polygyny had no significant effect. We conclude that Sedge Warbler females do not compete for territorial resources, but they do prefer to mate with high-quality early-arriving males. Polygyny is an effective strategy used by females to compensate for their late arrival and breeding onset, since they pair with high-quality males.

Mating strategy predicts the occurrence of the Bruce effect in the vlei rat Otomys irroratus

[The Bruce effect is a phenomenon in which pregnant females terminate their pregnancies when exposed to strange males, and is hypothesised to be a female counter-strategy to infanticide risk. Since uncertain paternity decreases the incidence of infanticide, we tested the prediction that the Bruce effect would occur in mating systems where paternity is more certain (polygyny) rather than in mating systems where paternity is less certain (promiscuity). Using populations of the vlei rat Otomys irroratus that were either polygynous or promiscuous in nature, we studied the Bruce effect in captivity. Pregnancy termination almost always occurred in polygynous females but never in promiscuous females ( N = 10 per population). We expected that the signal–receptor (male–female) system of pregnancy termination is present in polygynous but not promiscuous populations and predicted an absence of the Bruce effect in encounters between individuals of different mating systems. This occurred in promiscuous females. However, 50% of polygynous females terminated their pregnancies, indicating that males of both mating strategies might carry signals to disrupt pregnancy but only polygynous females respond to these signals. The Bruce effect may be an adaptive response to infanticide risk in polygynous populations, although studies of free-living populations would be important to confirm our findings., The Bruce effect is a phenomenon in which pregnant females terminate their pregnancies when exposed to strange males, and is hypothesised to be a female counter-strategy to infanticide risk. Since uncertain paternity decreases the incidence of infanticide, we tested the prediction that the Bruce effect would occur in mating systems where paternity is more certain (polygyny) rather than in mating systems where paternity is less certain (promiscuity). Using populations of the vlei rat Otomys irroratus that were either polygynous or promiscuous in nature, we studied the Bruce effect in captivity. Pregnancy termination almost always occurred in polygynous females but never in promiscuous females ( N = 10 per population). We expected that the signal–receptor (male–female) system of pregnancy termination is present in polygynous but not promiscuous populations and predicted an absence of the Bruce effect in encounters between individuals of different mating systems. This occurred in promiscuous females. However, 50% of polygynous females terminated their pregnancies, indicating that males of both mating strategies might carry signals to disrupt pregnancy but only polygynous females respond to these signals. The Bruce effect may be an adaptive response to infanticide risk in polygynous populations, although studies of free-living populations would be important to confirm our findings.]

Compensating for the costs of polygyny in hen harriers Circus cyaneus

In polygynous species, the adults are faced with a dilemma during chick rearing. Males must decide how to distribute food between their females and food allocation patterns are often highly unequal. In turn, the females that receive less food from males have to decide how much time to invest in additional hunting. If they spend more time hunting, then they leave their young exposed to weather and predators. However, if they stay at the nest, they increase the risk of their chicks starving. One way that birds may compensate for reduced provisioning is by increasing the size of prey caught. We tested this hypothesis by comparing prey deliveries to nests of hen harriers, Circus cyaneus, with females of different breeding status. As expected, male harriers delivered less food items to the nests of polygynous females, and especially their secondary, or β females. However, both sexes were able to compensate by delivering larger items and there was no difference in the overall mass of food delivered to nests. Moreover, females spent a similar amount of time at the nest, irrespective of status, and there were no overall differences in breeding success. Our results show that polygynous female harriers can compensate for the costs of polygyny, but we suggest that their ability to do so will vary according to the abundance of both large prey and predators.

Competition for male reproductive investment elevates testosterone levels in female dunnocks, Prunella modularis.

In many songbirds, females occasionally sing in contexts of high female-female competition. Testosterone may be involved in the activation of song, because testosterone implants elicit female song in many species with rare female song. A possible mechanism for the hormonal control of female song is provided by the challenge hypothesis, which predicts a rise in testosterone in response to aggressive interactions during socially unstable situations. We tested this by comparing faecal testosterone levels in polygynandrous and monogamous female dunnocks. In groups with two to three females (polygynandry and polygyny) males provide less help at each nest than in groups with a single female (monogamy and polyandry). Polygynandrous and polygynous females are aggressive towards one another and attempt to expel rivals. Polygynandrous females had significantly higher testosterone levels than monogamous females. Competition between females that was induced by removal of males caused testosterone levels to rise. Further, female testosterone levels were correlated with the rate of 'tseep' calls, which are produced during aggressive encounters between females. Finally, polygynandrous and polygynous females sang significantly more than monogamous females. To the best of our knowledge, these results provide the first experimental support for the challenge hypothesis in female birds, and suggest that testosterone can regulate facultative female song in songbirds.

The Role of Female Behavior in Maintaining Monogamy of a Coral‐Reef Filefish

The corallivorous filefish Oxymonacanthus longirostris is a predominantly monogamous species without parental care. Polygyny reduces female fitness because monogamous females benefit from male assistance with feeding–territory defense that secures both a feeding area and time to feed. I studied the strategy of females to prevent polygyny, with the help of field observations and manipulative field experiments. Monogamous females always behaved aggressively towards intruding floater females. When a caged male or female was placed within the territory of a monogamous pair, the resident female was more likely to pay attention to the caged female than to the caged male. Experimentally transplanted floater females succeeded in settling as secondary females in the monogamous territory more frequently when the resident female was caged. These results suggest that monogamous females behave aggressively towards floater females to defend their mating status; such aggressive behavior is successful in preventing polygyny. Furthermore, the removal experiment showed that polygynous (especially secondary) females moved to territories of widowed males located near them more frequently than monogamous females did. This outcome indicates that polygynous females can change their mating status and escape polygyny by moving to another territory if the opportunity occurs. Thus, female behavior can play an important role in the maintenance of monogamy and in lowering the level of polygyny within populations of this species.

CONSEQUENCES OF MATE SHARING FOR FIRST-MATED FEMALES IN A POLYGYNOUS SONGBIRD, THE HOUSE WREN

We investigated whether male parental assistance, reproductive output, and overwinter survival differed for female House Wrens (Troglodytes aedon) that did and did not have to share their mate's territories with a later-settling female during the nestling stage of breeding. During first breeding attempts of the season, when mate sharing occurred, primary females in polygynous trios and females from monogamous pairs fledged equal numbers of offspring and fledged offspring of similar mass. Rates at which males fed broods of primary females were not significantly different than rates at which males fed broods of monogamous females. Primary females did not make significantly more trips to nests to feed offspring than did monogamous females. In the first year of study, primary females were more likely than monogamous females to attempt a second brood. In the second year of the study, however, primary females tended to be less likely to attempt a second brood, took longer to start such broods, and tended to be less successful in fledging offspring from second broods. This suggests that in some years there may be a delayed cost of mate sharing. Females with primary and monogamous status for first breeding attempts in a particular year were equally likely to return to the study area the next year. Overall, our results indicate that mate sharing does not appear to affect the reproductive output of first mated females in our House Wren population in the first breeding attempts of the season, but may affect output in second attempts (and hence possibly annual and lifetime success), at least in some years.

Parental behaviour in the Lapwing Vanellus vanellus

Parental behaviour of monogamous and polygynous Lapwings was studied during incubation and brood care. Both parents attended the nest in 86% of monogamous pairs (n= 29 pairs). In 14% of pairs, only the male parent continued incubation until the eggs hatched, whereas the female deserted the clutch before or at the end of incubation. There was a clear division of parental roles during incubation. Females spent more time incubating (64% of time) than their mates (27%), whereas males spent more time defending the nest (3%) than females (>1%). Time spent incubating did not differ between monogamous and polygynous males. However, polygynous females spent more time incubating (primary females: 95%; secondary females: 97%) than monogamous females. Biparental care was the most common pattern of post‐hatching care, although in some broods either the male or the female parent deserted before the chicks fledged. Division of sex roles was less pronounced in brood care than during incubation. Females spent more time brooding (21%) than males (3%), and females attended their chicks more closely than males. Nevertheless, males and females spent similar amounts of time defending the brood from predators and conspecifics. We suggest that the apparent division of parental roles may be explained by sexual selection, i.e. the remating opportunities for male Lapwings might be reduced if they increase their share in incubation. However, the different efficiency of care provision, for example in ability to defend offspring, may also influence the roles of the sexes in parental care.

Parental investment, reproductive success and polygyny in the lapwing,Vanellus vanellus

We studied the consequences of monogamy and polygyny for male and female lapwings at a site in northern England between 1993 and 1995. Males and females differed in breeding behaviour, and thus the pattern of reproductive investment: males contributed less time than females to the care of their offspring and more time to mating behaviour. We argue that this has resulted from sexual selection. Reproductive behaviour was similar in monogamous and polygynous individuals of both sexes. Male mating success was related to territory size, with males on the largest territories gaining more females. Polygynous male lapwings reared on average between 58 and 100% more chicks each year than monogamous males because of fewer complete breeding failures; between-year return rates of males to the area were similar. This would result in a strong advantage in terms of lifetime reproductive success for polygynous male lapwings. The seasonal breeding success of polygynous females was marginally, but not significantly, lower than that of monogamous females. Between-year return rates of monogamous and polygynous females were similar.

Female-Female Competition in a Coral Reef Fish and a Test of the Temporal Threshold Model of Polygynous Mating

The temporal threshold model of polygynous mating (TMPM) predicts that polygynous females will compete for breeding order to avoid polygyny costs It asserts that environmental cycles create optimal times to mate, but polygynous females can avoid polygyny costs by mating less synchronously. A polygynous angelfish, Centropyge potteri, was used to test the model. It is known to show daily rhythmicity in mating behavior. Females can spawn once per day near dusk on several days of the month Captive fish were observed in a tidal pond for a month. Six females were each given a male with whom to mate, and another six females were given a single male to share. Daily patterns of mating support the TMPM. Haremic females interfered with the mating of other females. The variance in mean time of mating was significantly greater for haremic females than for paired females (for both grand means and individual means). The time of mating analyzed was adjusted for "handling time"; thus the statistics reflect time a female spent choosing to mate, not time waiting because the male was mating with another female. Results of competition also manifest themselves another way: subordinate haremic females spawned significantly less often than paired females.

Male Feeding of Nestlings in the Facultatively Polygynous European Starling: Allocation Patterns and Effect On Female Reproductive Success

Abstract Parental investment by males is less common among birds with polygynous mating systems than in monogamous species. Here, we examined the contribution of males in feeding nestlings in the facultatively polygynous European starling, Sturnus vulgaris. 1. In nestbox colonies around Antwerp, Belgium, males and females within monogamous pairs divided the feeding duties about equally, with a slight bias towards the female, and responded similarly to changes in brood size and age of nestlings. 2. The proportion of primary females receiving male assistance in feeding nestlings was significantly smaller than the proportion of monogamous females during each of the three nestling age stages (early, mid and late) we considered. In most primary broods, the strong decrease in male assistance was not due to the male directing part of his feeding effort toward the brood of the secondary female, but was due to the fact that the male's investment in feeding primary nestlings was negatively affected by his polygynous behaviour. As prospecting females were present after hatching of the primary broods (as contrasted to other studied starling populations), most males spent time trying to attract and courting additional females instead of giving parental care to the primary brood. This suggests that males trade off the attraction of additional females against giving parental care to an existing brood. The proportion of secondary females receiving male assistance in feeding was significantly smaller than the proportion of monogamous females during the early- and mid nestling stages. Overall, secondary females received less male assistance than primary females. The amount of male help to primary and secondary broods was not related to the hatching interval between the primary and secondary brood. 3. Primary females did not suffer reduced breeding success compared to monogamous females. In secondary broods, nestling mortality (partial brood loss) was significantly higher than in both primary and monogamous broods, while average nestling weights were significantly lower. These results suggest that secondary females, as contrasted to primary females, are not able to compensate fully for the reduction in male assistance. 4. During the mid-nestling stage, when nestlings grow most rapidly, but not during the early- and late-nestling stages, polygynously mated females feeding young without male assistance significantly increased their per-caput feeding rate compared with aided polygynous females and monogamous females, and made as many feeding visits as did polygynous pairs in which the male assisted, and as monogamous pairs. The higher nestling mortality rates in polygynous broods without male help during the early and mid-nestling stages suggest that unaided females cannot compensate fully (in terms of quantity or quality of food delivered), that male starlings can improve female fledging success by assisting in feeding nestlings, and that the reduced reproductive success of secondary females is directly linked to the strongly reduced male assistance in feeding.

Shift in red fox, Vulpes vulpes, mating system associated with El Niño in the Bering Sea

Fifteen reproductive groups of red foxes were observed on Round Island, Alaska during five field seasons. Polygyny occurred among these foxes, correlated with abundant food resources. The predictions of the polygyny threshold model were supported, i.e. polygynous females had equal or better reproductive success than monogamous females. However, beginning in 1982, widespread nesting failure of seabirds occurred (the primary prey item of the foxes), corresponding with the occurrence of El Niño in the Bering Sea. This change in food resources apparently caused red foxes to switch their diet from the most abundant to rarer prey species, and caused a shift from facultative polygyny to monogamy within this population.

Polygynous Blue Tits and Monogamous Great Tits: Does the Polygyny-Threshold Model Hold?

Using the polygyny-threshold model as a starting point, and knowing that great tits are monogamous and blue tits occasionally polygynous, I test the hypothesis that quality differences between habitats should be larger for blue tits than for great tits. Comparisons are made among study plots at Ghent (10-30 ha) differing in habitat and hence in density and among 10 subplots of about 1 ha each in a single study plot of optimal habitat for tits near Antwerp. Both analyses show a larger difference in reproductive rate for blue tits than for great tits. In the Antwerp study, polygyny was found only on the subplots where monogamous females produced more surviving offspring. Production of offspring surviving for at least 3 mo was greatest for monogamous females in the better subplots, next greatest for polygynous females, and least for monogamous females in the poorer subplots. My data support the polygyny-threshold model.

Polygyny and the breeding success of the great reed warbler Acrocephalus arundinaceus

SummarySettlement date, mating status, and breeding success of individually marked great reed warblers, Acrocephalus arundinaceus, were studied during the 1980–84 breeding seasons in Kahokugata, Ishikawa Prefecture, Japan. Twenty‐five per cent of the territorial males were polygynous, of which the majority were bigamous. The settling periods of both sexes were long, extending for 65 days in males and 49 days in females. About 80% of males and females settled in the first half of the settling period, and the settlement date of 28–54% males overlapped with that of females. Many of the late settlers were bachelors and the males which mated earlier tended to be polygynous. The timing of a male's settlement is important in acquiring mates. Fifty‐five per cent of eggs laid were lost before fledging, mainly due to predation. The mean number of fledglings was 3.19 per primary female, 2.41 per secondary female, and 2.80 per monogamous female. Comparison of the number of fledglings of females which mated during the same period showed that the presence of another female in the same territory did not adversely affect the breeding success of either of the polygynous females. Polygynous males have the advantage of decreasing the risk of breeding failure under high predation pressure.

Did Tree Swallows Cross a Polygyny Threshold?

Recently, Quinney (1983) demonstrated that female Swallows (Tachycineta bicolor) involved in bigamous relationships had poorer reproductive success than monogamous females. Despite this, he entitled his paper Tree Swallows cross a threshold. Because Quinney did not assess the reproductive success of polygynous females (those females choosing already-mated males), a comparison between concurrently settling females was not made, as is usual in assessments of threshold models (Garson et al. 1981). Quinney's conclusions therefore are premature. Quinney couched his arguments in terms of the Verner-Willson-Orians (VWO) model for the evolution of (Verner 1964, Verner and Willson 1966, Orians 1969). This model assumes that is adaptive and will evolve if a female mating with an already-mated male has a reproductive success equal to or greater than that of a female mating with a bachelor male. However, tests based on this premise often are invalid due to the potential depressing effects of beta females on alpha females (Altmann et al. 1977) or the effects of discontinuous habitat or male quality on female success (Simmons MS). Using the VWO model as his criterion, Quinney showed that monogamous females were significantly more successful in fledging young than were alpha and beta females (4.7 vs. 2.1, P < 0.01) and that monogamous females raised slightly heavier young. Assuming that alpha plus beta (= polygynous, Quinney 1983) productivity is equivalent to the (more correct) beta female productivity, then according to the VWO model appears maladaptive for polygynous females. However, Quinney states that polygyny is advantageous to secondary [beta] females at the Sewage Lagoon if these females could not breed otherwise. First, it is possible that the beta females could have bred at Backus Field (or other nearby sites), since not all of the nest boxes were occupied there. Second, if a lack of males was preventing nesting at alternate sites, Quinney is invoking the desperation hypothesis (Simmons 1983), which assumes that arises from a forced choice among females who cannot breed without choosing a mated male. This should not be confused

Offspring Quality and the Polygyny Threshold: A New Model for the "Sexy Son" Hypothesis

Recent models for the evolution of polygyny in territorial birds predict that a female should choose to mate polygynously only if her expected number of off-spring is equal to or greater than her expected number if she mated monogamously. A recent model by Weatherhead and Robertson (1979) suggests that polygynous females can, in fact, tolerate lower reproductive success relative to available monogamous breeding situations if their sons inherit from their fathers an enhanced tendency to mate polygynously. Weatherhead and Robertson's model contains several inconsistencies that lead to misleading numerical predictions. These problems are indicated and a new model is presented. The model supports Weatherhead and Robertson's qualitative expectations. It predicts that an increase in the adult sex ratio, the average harem size, the frequency of polygynous males, or the degree of father-son resemblance in pairing success will all increase the size of the initial reproductive loss that can eventually be recovered. Consideration of the effect of reproduction over subsequent generations indicates that there is a limit to the number of generations over which a polygynously mated female can recoup an initial reproductive loss. A method is suggested by which the model may be applied to field studies of natural populations. Finally, limitations on the value of nongenetical approaches in modeling the evolution of female choice are discussed.