Jack W. Bradbury

Social organization and foraging in emballonurid bats

Summary1.A general model of mating system evolution in mammals is developed, which takes into account the different male strategies of resource defense, female group defense, and male mating aggregations. The critical environmental variables determining differential defensibility of females and resources are identified by generalizing the resource defense model of Orians (1969). The model is then applied to available data on African antelopes (Jarman, 1974) to establish a set of hypothetical relations between certain patterns of habitat use and mating structures. The resulting relations are only likely to apply to species in which food determines female dispersion and in which any resource defense exhibited by males is directed towards food supplies.2.The relations developed for antelopes are then compared to recently published data on mating systems in five neotropical emballonurid bats (Bradbury and Vehrencamp, 1976a).3.Antelopes and the bats are found to share the following features. Species living in wet and stable forests tend to be fine-grained socially and to have groups consisting of monogamous pairs or nested male-female territories. Species in more seasonal habitats show an inverse relation between the size stability of groups and the duration of use of a given foraging site. As the model predicts, in both groups resource defense occurs where groups are least stable and female defense where groups are most stable. Also as the model predicts, the numbers of females accessible to each male and the number of reproductive males per group can be anticipated in each of the two taxa wherever sufficient data for the critical variables are available.4.Antelopes and bats differ in the following ways. Whereas body size is a good predictor of antelope habitat use and social dispersions, it is a poor predictor for emballonurid patterns. Similarly, although the numbers of females per male generally increase with group size in antelopes, this correlation does not hold for the bats in this study. These differences lead to the conclusion that application of the general model cannot be simplified by measurement of a few variables such as body size or group size, but instead will generally require actual measurements of the critical resource dispersion parameters in the field.

The energetic cost of display in male sage grouse

The energetic expenditure of displaying male sage grouse, Centrocercus urophasianus, was measured for 18 individuals in the field using the doubly labelled water technique. Daily energy expenditure increased significantly with increased display rate, increased time spent on the lek, and decreased ambient temperature. Daily energy expenditure for the most vigorously displaying males was two times higher than for a non-displaying male and four times higher than basal metabolic rate. Estimates of the instantaneous rate of energy expenditure during display ranged from 13·9 to 17·4 times basal metabolic rate. The effort devoted to display differed markedly among males and was correlated with certain other male characteristics. Males that attended leks were in better condition (higher body weight relative to size) than non-attenders, but among lek attenders condition was negatively correlated with increased display effort. Active displayers lost less weight per day and foraged further from the lek than less active males, suggesting that differences in foraging and food intake affect daily energy output. Neither blood parasites nor the potential effects of other diseases as determined by haematocrit levels were associated with display effort.

Sexual Selection in Lekking Sage Grouse: Phenotypic Correlates of Male Mating Success

SummaryMate choice cues in sage grouse were reinvestigated by analyzing relationships between male mating success and a range of suggested cues. Display cues were implicated by significant relationships between mating status (whether or not a male mated) and lek attendance, display rate (corrected for effects of female proximity and time of day) and an acoustic component related to temporal and frequency measure of a whistle emltted during the strut display. Although display rate and the acoustic component were intercorrelated, both exerted significant partial effects on mating success in mutivariate analyses. These display measures also differed significantly between males. In contrast, mating success was not significantly related to measures of territory characteristics, including size and proximity to the lek center, or to body size. These results resolve discrepancies between previous studies and provide a basis for experimental analysis of the role of female choice in this lek system.

Paternity and Genetic Heterogeneity in the Polygynous Bat, Phyllostomus hastatus

Wild colonies of greater spearnose bats were marked, censused regularly, and genotyped at three polymorphic allozyme loci. While adult composition of social units is very stable and strong polygyny results in marked changes in gene frequencies between generations, dispersal of offspring is sufficient to prevent significant genetic heterogeneities between social units. Kin selection cannot explain social cohesiveness in these highly social mammals.

Hotspots and the dispersion of leks

Abstract The settlement of promiscuous males on sites where they are most likely to encounter females was examined by computer simulation. The study extended an earlier model of Parker (1978) , which dealt with male settlement on environmentally fixed mating sites, to include populations of mobile females who can mate at any point in their home ranges. Males in the simulations were expected to settle at sites with high levels of female traffic (hotspots) and to correct for the sharing of females between adjacent sites. As a results, males became clustered into fewer and more compact aggregations as female home range size was increased. Increasing female density or allowing males to settle despotically instead of in a free manner had an opposite effect. The same results were found for both bounded and unbounded surfaces, although there was increased aggregation of males at the centre of bounded surfaces as female size was increased. The model may be relevant to the diversity of male dispersions seen in lek and swarm mating animals.

Economic models of animal communication.

Many models of animal signal evolution fail to incorporate an explicit strategy for receivers prior to the evolution of signals. When reasonable assumptions are made for such strategies, we have shown that there is a minimal accuracy of signal coding that is required before receivers should attend to signals (Bradbury & Vehrencamp 1998, Principles of Animal Communication). Depending upon the relative payoffs of correct and incorrect decisions by receivers, this minimal accuracy can be quite high. Here we use this result to explain why so many signals appear to be traits that provided useful information to receivers before becoming ritualized into signals. Our model also supports one prediction of sensory drive models: that latent preferences may selectively favour some signal precursors over others. However, it imposes a serious constraint on sensory drive by requiring that there be sufficient benefits to a receiver to compensate for the costs of disrupting the optimal receiver strategy used before exploitation. Finally, we discuss the overlap between signal honesty and accuracy and show how senders that completely disagree with receivers about appropriate receiver decisions may still benefit by providing moderately honest and accurate signals. Copyright 2000 The Association for the Study of Animal Behaviour.

THE COMPARISON OF HARMONICALLY RICH SOUNDS USING SPECTROGRAPHIC CROSS-CORRELATION AND PRINCIPAL COORDINATES ANALYSIS

ABSTRACT We explore the effectiveness of spectrographs cross-correlation (SPCC) combined with principal coordinates (PCO) analysis as a method for sound comparison. We do this using synthetic sounds modeled after the individually-distinctive, harmonically-rich contact calls of wild orange-fronted conures Aratinga canicularis. Calls with acoustic properties similar to Aratinga contact calls are common in other taxa including non-oscine birds, primates and cetaceans. We generated signals with known variations in time-frequency pattern, duration, noise level, harmonic content and harmonic weighting, and applied SPCC-PCO analysis to obtain an ordering of sounds in n-dimensional space. We find that shared time-frequency patterns dominate the positioning of sounds in PCO space. This was true despite high variability in signal-to-noise ratio (from −60 to +40 dB) and duration (150–275 ms). Furthermore, inclusion of naturally-weighted harmonics (versus fundamentals only) enhances, rather than obscures, the separation of call types. We conclude that SPCC-PCO is an effective method for sorting sounds based on overall time-frequency pattern. In addition, the resulting PCO measures can be used in statistical tests of association with extrinsic variables. The method is thus an effective starting point for examining most bioacoustic hypotheses.

Dispersion of displaying male sage grouse

SummaryThe distribution of lek sizes was examined in each of three populations of sage grouse in eastern California. Peak seasonal lek sizes collected over a 35 year period were found to covary among the three sites indicating that some global environmental or demographic features modulated male attendance in any given year. Despite these annual variations, the ranks of the three populations with regard to mean lek size remained stable. In all three populations, there was a persistent excess of small and large leks, compared to random settlement on the same number of sites, and a consistency in the ranking by size of particular sites in successive years. The sequential phenology of lek site occupation in each population was correlated with recolonization of habitats surrounding central wintering refuges each spring. Some lek sites utilized for display in early spring were regularly abandoned prior to the onset of mating as more peripheral leks became active. On top of population, site, and seasonal variations in lek size, pronounced daily fluctuations in attendance were common. Multivariate regressions indicated that an average 36% of the daily variation in male numbers was correlated with weather variables, female attendance levels, and prior raptor harassment. Several outcomes of the analyses support the notion that dispersion of males is partly determined by male settlement on current female traffic patterns (hotspot settlement). The analyses also suggest that display is sufficiently costly that variations in male attendance are in part a result of conflicts between strutting and thermoregulatory expenditures.

GEOGRAPHICAL VARIATION IN THE CONTACT CALLS OF ORANGE-FRONTED PARAKEETS

Abstract We examined patterns of geographic variation in contact calls of Orange-fronted Parakeets (Aratinga canicularis) during the nonbreeding season. Calls were recorded throughout the range of that species in Costa Rica. Recordings of wild-caught birds held for one to two weeks indicated that each individual favors one dominant call type and different birds use different favored calls. We used that fact to cull replicate calls from the same individual in field recordings of flocks observed at various locations throughout the sample area. Remaining recordings from a given year were submitted to spectrographic cross-correlation and principal coordinates analysis as described by Cortopassi and Bradbury (2000). Principal coordinates were then correlated with site location using MANOVA. Call durations were also examined for geographical patterns. Like sympatric Yellow-naped Parrots (Amazona auropalliata) studied over the same region by Wright (1996), Orange-fronted Parakeets exhibited significant geographic variation but, unlike the larger species, showed no discrete dialect patterns. Call duration varied clinally but with different patterns for the Nicoya Peninsula and the Guanacaste mainland. Two principal coordinates also showed clinal variation even after removing any correlated duration effects. Scale over which local calls were statistically homogeneous was 7–10 km. We compared that figure to home ranges of 18 birds radio-tracked concurrently with call sampling. Both range areas and range lengths were asymptotic after a week of tracking. Asymptotic range lengths were 2–9 km. Scale of movements of birds, at least during that period, was thus similar to distance within which calls tended to be statistically homogeneous. This study and that by Wright show that several well-known patterns of geographical variation seen with passerine song are replicated quite closely in contact calls of parrots, despite the fact that the functions of vocal signals are quite different in the two taxa.

Observations on the Foraging Behavior and Avian Prey of the Neotropical Carnivorous Bat, Vampyrum spectrum

The foraging behavior of Vampyrum spectrum was studied in the field by radio-tracking, evening roost observations, and systematic collection of the prey parts found in the bottom of a roost containing five bats. Field observations, roost monitoring, and netting records all suggest that the bats hunt solitarily. One radiotracked bat hunted over an area of 3.2 hectares, and spent most of its time in deciduous woodland, secondary growth, and forest edge, rather than in the riparian forest where the roost was located. The avian prey species were identified from the feathers collected in the roost over a one-year period. Of the 18 species (about 86 individuals) identified, most were common residents of the tropical dry deciduous forest. Non-passerines were significantly preferred over passerines. Vampyrum appear to select birds which weigh between 20 and 150 grams (g), sleep in foliage rather than in holes or burrows, and either roost communally or have a strong body odor. It appears that the bats locate their avian prey by scent rather than by vision or echolocation.