What do foraging hummingbirds maximize?

Summary Hainsworth and Wolf (1976) reported that under certain conditions hummingbirds made food choices which did not maximize their net rate of energy intake while foraging. They concluded that the birds were not foraging optimally. We show here that their birds probably maximized a different utility function, the net energy per unit volume consumed (NEVC), which appears to be an optimal choice on a time scale longer than that of a foraging bout. Our own experiments with Archilochus colubris support the conclusion that hummingbirds make foraging decisions that maximize NEVC. A simulation model shows that, in nature, NEVC maximization would require fewer foraging trips and visits to fewer flowers per day to balance daily energy budgets. For territorial birds this can lead to smaller territory sizes and reduced costs of territorial defense. Plants that evolutionarily increase corolla length to enhance pollinator specificity need only increase nectar concentration slightly to maintain the same net energy per unit volume consumed (NEVC) by a given hummingbird pollinator.     

Fatty acid composition of membranes of L-forms ofStreptococcus faecalis andProteus mirabilis cultured at different osmolalities

The fatty acid composition of membranes of L-forms ofStreptococcus faecalis andProteus mirabilis cultured at different osmolalities and in different osmotic stabilizers was examined.S. faecalis L-forms cultured with sucrose in the medium showed a decrease in the unsaturated fatty acid C₁₈₁ and an increase in the C₁₈ fatty acid and C₁₉ cyclopropane fatty acid. Fatty acid composition ofS. faecalis L-forms cultured in medium containing 1.8% NaCl was similar to the fatty acid composition of L-forms cultured in brain-heart infusion broth (BHI) without osmotic stabilizer and was between the composition of fatty acids of L-forms in BHI with sucrose and that in BHI without 0.5 M sucrose. InProteus mirabilis L-forms, there were differences between L-forms cultured with and without sucrose, but these differences were not comparable to the changes observed inS. faecalis L-forms.P. mirabilis L-forms cultured with and without NaCl in the medium had similar fatty acid compositions.     

Concordant evolution of plumage colour, feather microstructure and a melanocortin receptor gene between mainland and island populations of a fairy-wren

Studies of the patterns of diversification of birds on islands have contributed a great deal to the development of evolutionary theory. In white-winged fairy-wrens, Malurus leucopterus, mainland males develop a striking blue nuptial plumage whereas those on nearby islands develop black nuptial plumage. We explore the proximate basis for this divergence by combining microstructural feather analysis with an investigation of genetic variation at the melanocortin-1 receptor locus (MC1R). Fourier analysis revealed that the medullary keratin matrix (spongy layer) of the feather barbs of blue males was ordered at the appropriate nanoscale to produce the observed blue colour by coherent light scattering. Surprisingly, the feather barbs of black males also contained a spongy layer that could produce a similar blue colour. However, black males had more melanin in their barbs than blue males, and this melanin may effectively mask any structural colour produced by the spongy layer. Moreover, the presence of this spongy layer suggests that black island males evolved from a blue-plumaged ancestor. We also document concordant patterns of variation at the MC1R locus, as five amino acid substitutions were perfectly associated with the divergent blue and black plumage phenotypes. Thus, with the possible involvement of a melanocortin receptor locus, increased melanin density may mask the blue-producing microstructure in black island males, resulting in the divergence of plumage coloration between mainland and island white-winged fairy-wrens. Such mechanisms may also be responsible for plumage colour diversity across broader geographical and evolutionary scales.     

Fertility Advertisement in Birds: a Means of Inciting Male-male Competition?

Certain loud calls made by female red junglefowl and Lapland longspurs are given most frequently immediately after egg laying, when a copulation should have the highest probability of fertilizing the next egg to be laid. In these species there is considerable male-male interaction for access to fertilizable females, and males are attracted to or follow females making these calls. Based on our interpretation of these calls, we develop a general hypothesis to predict the pattern of occurrence of fertility advertisement both within and among bird species. We hypothesize that certain loud calls given by female birds before and during the egg-laying period are designed to advertise fertility and thereby incite male-male competition. This hypothesis predicts that calls advertising female fertility should most often occur soon after an egg is laid (i.e. during the period of highest fertility) but may also occur at any time during the female's fertilizable period. Such calls are unlikely to be given by females in strictly monogamous species (especially those with long-term pair bonds) because in these species each female usually mates with only a single male and extra-pair copulations are avoided. Although reports of loud female calls associated with copulation are rare in the literature, the 18 examples we found (including junglefowl and longspurs) were predominantly (15/18) in species adopting mating systems other than strict monogamy, and these calls were most commonly and disproportionately reported in multi-male mating systems. This form of “estrus” in birds may be widespread because few species appear to be strictly monogamous, but it will be difficult to study because the period of high fertility for female birds is so short.     

Closely related species of birds differ more in body size when their ranges overlap—in warm,but not cool,climates

Differences in body size are widely thought to allow closely related species to coexist in sympatry, but body size also varies as an adaptive response to climate. Here, we use a sister lineage approach to test the prediction that body size differences between closely related species of birds worldwide are greater for species whose ranges are sympatric rather than allopatric. We further test if body size differences among sympatric versus allopatric species vary with geography, evolutionary distance, and environmental temperatures. We find greater differences in size among sympatric compared with allopatric lineages, but only in closely related species that live where mean annual temperatures are above 25°C. These size differences in warm environments declined with the evolutionary distance between sister lineages. In species living in cooler regions, closely related allopatric and sympatric species did not differ significantly in size, suggesting either that colder temperatures constrain the evolutionary divergence of size in sympatry, or that the biotic selective pressures favoring size differences in sympatry are weaker in colder environments. Our results are consistent with suggestions by Wallace, Darwin, and Dobzhansky that climatic selective pressures are more important in cooler environments (e.g., high elevations and latitudes) whereas biotic selective pressures dominate in warm environments (e.g., lowland tropics).     

Efficient copulation and the evolutionary loss of the avian intromittent organ

The absence of an intromittent organ (IO) in most species (>97%) of birds is an enigma: birds are the only terrestrial vertebrates in which insemination typically occurs without the use of an IO. In an earlier review, we evaluated six hypotheses to explain the loss of IOs among birds. We found some support for the influence of sperm competition and female choice, but other hypotheses (e.g. avoidance of sexually transmitted disease) could not be tested because of a lack of empirical data. Wesołowski (1999, J. Avian Biol. 30: 483–485) has criticised our female choice and sexually transmitted disease hypotheses as implausible and insufficient to explain the loss of the IO in birds. Instead, he proposes that IOs were lost to minimise the risk of predation (by shortening duration of copulation) and/or to increase the efficiency of sperm transfer. Here we respond to Wesołowski's (1999) criticisms and test his efficient copulation (EC) hypothesis with data on the duration of copulation in 243 species. Contrary to expectations from the EC hypothesis, copulation durations did not differ significantly between species with and without an IO. Thus we can reject the EC hypothesis as an explanation for the loss of IOs in birds.     

Motility, ATP levels and metabolic enzyme activity of sperm from bluegill (Lepomis macrochirus)

Male bluegill displays one of two life history tactics. Some males (termed "parentals") delay reproduction until ca. 7 years of age, at which time they build nests and actively courts females. Others mature precociously (sneakers) and obtain fertilizations by cuckolding parental males. In the current study, we studied the relations among sperm motility, ATP levels, and metabolic enzyme activity in parental and sneaker bluegill. In both reproductive tactics, sperm swimming speed and ATP levels declined in parallel over the first 60 s of motility. Although sneaker sperm initially had higher ATP levels than parental sperm, by approximately 30 s postactivation, no differences existed between tactics. No differences were noted between tactics in swimming speed, percent motility, or the activities of key metabolic enzymes, although sperm from parentals had a higher ratio of creatine phosphokinase (CPK) to citrate synthase (CS). In both tactics, with increasing CPK and CS activity, sperm ATP levels increased at 20 s postactivation, suggesting that capacities for phosphocreatine hydrolysis and aerobic metabolism may influence interindividual variation in rates of ATP depletion. Nonetheless, there was no relation between sperm ATP levels and either swimming speed or percent of sperm that were motile. This suggests that interindividual variation in ATP levels may not be the primary determinant of variation in sperm swimming performance in bluegill.     

Male reproductive suppression in the cooperatively breeding fish Neolamprologus pulcher

In most cooperative breeders, dominants suppress the reproductionof subordinates. However, two previous studies of Neolamprologuspulcher, a cooperatively breeding cichlid fish, have suggestedthat socially subordinate helper males sneak fertilizationsfrom dominant breeding males. If such sneaking does occur, boththeoretical work and empirical studies of other fish speciessuggest that sperm competition will select for increased reproductiveinvestment by sneaker males, relative to more dominant males.To address these issues, we quantified gonadal investment andsperm characteristics of 41 N. pulcher male breeders and 62male helpers from 55 groups in Lake Tanganyika. Gonadal investmentfollowed patterns consistent with reproductive suppression,with breeders having considerably larger testes masses thanhelpers. Breeders also had faster and longer swimming spermand a higher percentage of motile sperm compared to helpers.However, sperm characteristics of large helpers were similarto those of breeders, but these same helpers had lower testesmasses. Thus, large helpers had sperm that were physiologicallyequivalent to that of breeders, but their relatively small gonadsimply that they were reproductively suppressed.     

Osmotic Stability and Sodium and Potassium Content of L-Forms of Streptococcus faecalis

A stable L-form of Streptococcus faecalis (T(53)) was transferred in media containing decreasing concentrations of sucrose until it grew in medium without added osmotic stabilizer. This L-form (designated T(53I)) was compared with T(53) from which it was derived. The survival of these two L-forms suspended at different osmolalities showed that maximal survival for T(53I) was 350 to 400 milliosmolal and for T(53) was 900 to 1,000 milliosmolal. Both peaks were at the osmolality of their growth media. Measurement of intracellular potassium and sodium showed that the concentration of these ions was reduced in T(53I).