Female control of paternity

Of the two components of sexual selection, female choice is much less obvious than male-male competition, and hence has always been considered to be of secondary importance. However, recent field observations and new theory have brought about a radical change of emphasis. It now appears that although a female's choice of who fathers her offspring often occurs in a subtle manner, it may be widespread and take place through a variety of behavioural and physiological mechanisms, including the manipulation of male behaviour and the selection of sperm within the female reproductive tract.     

Distribution of the bridled form of the Common guillemot Uria aalge in the North Atlantic

This study documents the fourth survey (198 1–1982) of bridled guillemots and provides the most complete information yet available on the distribution of the bridled form of the Common guillemot. The distribution has remained largely unchanged since the last survey (1959-60), although marked changes have occurred at a few colonies. Sea surface temperature "explains" only 35% of the variance in the proportion of bridled guillemots. Minimum air temperatures during the breeding season are much more closely correlated with bridling and "explain" 60% of the variance. No relationship exists between sea temperatures and the proportion of bridled guillemots in wintering areas. The distribution of bridled guillemots may represent the slow, southward spread of the bridled form. Alternatively, selection may favour the bridled form in cooler areas. There is insufficient evidence to distinguish between these two hypotheses, but if selection is important, the correlations with environmental factors suggests that it operates during the breeding season rather than at other times.     

New insights from old eggs – the shape and thickness of Great Auk Pinguinus impennis eggs

We compared the shape and eggshell thickness of Great Auk Pinguinus impennis eggs with those of its closest relatives, the Razorbill Alca torda, Common Guillemot Uria aalge and Brünnich's Guillemot Uria lomvia, in order to gain additional insights into the breeding biology of the extinct Great Auk. The egg of the Great Auk was most similar in shape to that of Brünnich's Guillemot. The absolute thickness of the Great Auk eggshell was greater than that of the Common Guillemot and Razorbill egg, which is as expected given its greater size, but the relative shell thickness at the equator and pointed end (compared with the blunt end) was more similar to that of the Common Guillemot. On the basis of these and other results we suggest that Great Auk incubated in an upright posture in open habitat with little or no nest, where its pyriform egg shape provided stability and allowed safe manoeuvrability during incubation. On the basis of a recent phylogeny of the Alcidae, we speculate that a single brood patch, a pyriform egg and upright incubation posture, as in the Great Auk and the two Uria guillemots, is the ancestral state, and that the Razorbill – the Great Auk's closest relative – secondarily evolved two brood patches and an elliptical egg as adaptations for horizontal incubation, which provides flexibility in incubation site selection, allowing breeding in enclosed spaces such as crevices, burrows or under boulders, as well as on open ledges.     

Sperm competition mechanisms in birds: models and data

This study presents three models to explain the mechanism oflast male sperm precedence in birds. Because passive loss ofsperm from the female reproductive tract occurs, all modelsincorporate this process. The three models are passive spermloss alone, stratification with passive sperm loss, and displacementwith passive sperm loss. With two inseminations containing thesame number of sperm, the models make the following predictions.For passive sperm loss alone, (1) differential paternity ispositively and linearly related to the time interval betweeninseminations, (2) with a slope that is equal to rate of lossof sperm from the female reproductive tract, (3) with an interceptthat is the same as the differential fertilizing capacity betweenthe semen of the two inseminations, and (4) the ratio of offspringfrom two inseminations remains constant over time. For stratification,(1) the relationship between differential paternity and theinterval between inseminations is nonlinear and exhibits a "brokenstick" pattern, with a substantial first-insemination precedencefor short intervals, and (2) the proportion of offspring fatheredby the first insemination increases over time. For displacement,the relationship between differential paternity and the intervalbetween inseminations is nonlinear and also exhibits a "brokenstick" pattern, but in contrast to the stratification model,sperm from the last insemination have precedence. Data fromthree experimental studies of the domestic fowl and one forthe turkey provide the opportunity to test these models, albeitto different extents. The data from all studies are consistentwith the passive sperm-loss model, except that one aspect ofone data set provided ambiguous support for stratification.None of the data provided any support for the displacement model.     

Changes in sperm quality and numbers in response to experimental manipulation of male social status and female attractiveness

In promiscuous species, male reproductive success is determined by the interaction between the ability to access and choose females of the highest reproductive quality and, after copulation, the ability to outcompete the ejaculates of rival males. Disentangling the factors regulating the interplay between traits conferring a reproductive advantage before and after copulation is therefore crucial to understanding how sexual strategies evolve. Here we show in the fowl Gallus gallus, where social status determines copulation success, that dominant males produce more sperm than subordinates but that the quality of dominant males' sperm decreases over successive copulations, whereas that of subordinates remains constant. Experimentally manipulating male social status confirmed that ejaculate quality (the number and quality of sperm produced) was a response to the social environment rather than the result of intrinsic differences between dominant and subordinate males. We further show that dominant males responded to variation in female sexual ornamentation, which signals reproductive quality, by adjusting the number and quality of sperm they transferred, whereas subordinate males did not: they transferred ejaculates of similar quality to females with different ornament sizes. These results indicate that trade-offs between traits influencing reproductive success before and after copulation, combined with variation in social dynamics and female quality, may favor the evolution of phenotypically plastic alternative reproductive strategies.