Behav Ecol Sociobiol Research Articles

Weighting waiting in collective decision-making

Animals searching for food, mates, or a home often need to decide when to stop looking and choose the best option found so far. By re-analyzing experimental data from experiments by Mallon et al. (Behav Ecol Sociobiol 50:352–359, 2001), we demonstrate that house-hunting ant colonies are gradually more committed to new nests during the emigration. Early in house-hunting, individual ants were flexibly committed to new nest sites. However, when carrying to a new nest had started, ants hardly ever switched preference. Using a theoretical model based on experimental data, we test at which stage flexible commitment influences speed and accuracy most. We demonstrate that ant colonies have found a good compromise between impatience and procrastination. Early flexibility combined with later rigidity is identically effective as other strategies that include flexible commitment, but it is particularly good when emigration conditions are harsh.

Do honey bees tune error in their dances in nectar-foraging and house-hunting?

The "tuned-error" hypothesis states that natural selection has tuned the divergence angle in the dances of the honey bee to produce an optimal scatter of recruits across a resource. Weidenmuller and Seeley (Behav Ecol Sociobiol 46:190–199, 1999) supported this hypothesis by finding smaller divergence angles in dances indicating potential home sites, which are always point sources, than in dances indicating food sources, which often occur in patches. This study tested for the same effect, but controlled for variables, e.g., substrate and context, that may have confounded those results. When performed on the same substrate, divergence angle does not differ between dances for the two resources. Furthermore, dances performed for food within an observation hive exhibit significantly greater divergence angle when performed on comb (as Weidenmuller and Seeley measured food dances) than on hardware cloth (as they measured home-site dances on a swarm). These findings suggest that the angular variance in direction indication in dances is more likely an artifact of physical constraints, rather than an adaptive modification of a behavior that a bee could perform more precisely.

Current versus future reproduction: an experimental test of parental investment decisions using nest desertion by mallards ( Anas platyrhynchos )

Past investment in offspring may be important in determining a parent's ability to reproduce in the future and, hence, should affect the relative value of current offspring. However, there have been surprisingly few clear tests of whether animals actually adjust parental care in response to diminished opportunities for future reproduction. We modified the experimental protocol of Sargent and Gross [Behav Ecol Sociobiol (1985) 17:43–45] to examine offspring desertion by mallards (Anas platyrhynchos), and decoupled the influence of past investment from expected current benefits by controlling for the effect of offspring age on clutch value. Using 9 years of nest mortality data, we accounted for the increasing prospects of egg survival with clutch age by calculating clutch sizes throughout incubation with equivalent expected benefits. Applying this approach, we experimentally reduced 203 clutches at two different incubation stages such that they had equivalent expected benefits but differed in the amount of past investment. Nest desertion rates did not differ between early- and late-incubated clutches that had equivalent expected benefits. Rather, the probability of desertion increased with the severity of the clutch reduction treatment. These results suggest that female mallards adjust parental care according to the expected benefits of current offspring, rather than to diminished prospects for future reproduction due to past investment. We further examined whether females assessed expected benefits on the basis of clutch size alone or clutch size adjusted for the age of the clutch. Using Akaike's Information Criterion, the most parsimonious model to explain the probability of deserting an experimentally reduced clutch included both the proportion of the clutch remaining and clutch age. Thus, female mallards appear to fine-tune their level of parental care not only according to the relative number of offspring in the clutch, but also to the increased prospects for offspring survival as they age.

The value of titration experiments: a reply to Moody et al. (1996)

Behavioural ecology has been remarkably successful as a discipline, in part because much of the theory that has been developed is readily testable through manipulative experiments in the laboratory and the ®eld. Although the preferred currency for most theories is ®tness, it has usually been possible to incorporate modi®cations or simplifying assumptions that allow the use of more readily observed, surrogate measures of ®tness. Admittedly this does require some compromise, but in the grand tradition of behavioural ecology this trade-o€ is easy to solve: the bene®ts outweigh the costs. Moody et al. (1996) (henceforth referred to as MHM), explored the role that risk dilution will have on the results of experiments that investigate predation risk ± foraging trade-o€s. Assuming that the probability of being killed by a predator is inversely related to the number of animals feeding at a site, they developed a model that incorporates risk dilution into ideal free distributions (IFDs) under predation risk. Due to risk dilution, their model suggests that patch choice decisions will be a€ected not only by the presence of predators and relative food availability, but also by the total number of animals. Their suggestions are very useful in advancing our theoretical understanding of this problem. However, we disagree with their critical assessment of the use of the IFDs as an experimental tool to quantify decisions involving con icting demands. In 1989, Abrahams and Dill (henceforth referred to as AD) published a paper that described an experimental technique that could be used to quantify the energetic equivalence of the risk of predation and, more importantly, presented the results of experiments that tested its validity. The approach is based upon a continuous-input IFD. With this type of IFD, when food is the only parameter that describes patch quality, the spatial distribution of the foragers will match the spatial distribution of their food. A large number of authors have demonstrated that under experimental conditions, animals closely conform to an IFD (for a review see Milinski and Parker 1991). Incorporation of an additional patch parameter (i.e. the risk of predation) should result in a change in the spatial distribution of animals that re ects the relative change in patch quality. Di€erences in individual intake rates will then provide a measure of the assessed value of this parameter from the animals' perspective, and can be determined empirically. This approach describes our ®rst experiment. Our second experiment then tested a prediction (derived from the results of the ®rst experiment) concerning how much additional food must be added to the dangerous patch in order for the foragers to consider it equivalent to the safe patch. To make this prediction, we assumed a linear relationship between energy and ®tness. This was not intended to describe the actual relationship between these two parameters, but to make it possible to interpret deviations in the spatial distribution with respect to the relationship between energy and ®tness. For example, if more animals than we predicted used the dangerous location after the addition of extra food, then we concluded that the ®tness bene®ts of the patch increased by more than the manipulated energy bene®ts. Conversely, if the addition of extra food resulted in fewer animals than predicted using the dangerous location, then we concluded that the ®tness bene®ts of the patch increased by less than the manipulated energy bene®ts. These experiments demonstrated that even with the linearity assumption, we could fairly accurately predict the amount of additional food that must be added to a dangerous patch to o€set the risk of predation. MHM were critical of this approach for three reasons: (1) our calculations did not incorporate risk dilution; (2) the relationship between ®tness and energy should have no in uence on the calculations necessary to Behav Ecol Sociobiol (1998) 44: 147±148 O Springer-Verlag 1998

Commentary: Solitary behavior in social bees

Professor George Eickwort was killed in an automobile accident on 11 July 1994 (see Wcislo et al. 1994). The preceding manuscript (Eickwort et al. 1996) was essentially complete, except for revisions, up-dating references, and related editorial matters. I attended to these matters at the request of the second and fourth authors, both of whom participated in the field work and were listed as authors on the original manuscript. I have the notes, data, and original manuscript pertaining to this study. Revisions to a posthumous manuscript pose a problem, because the senior author might not have agreed to proposed changes. The aim of this commentary is to clarify what substantive revisions I made to the original manuscript, so readers can distinguish my additions from George Eickwort’s original thinking. Revisions were made using three formal reviews from the journal, comments from an associate editor, two informal reviews previously solicited by Eickwort (see Acknowledgements, Eickwort et al. 1996), and three reviews that I solicited. The original manuscript exclusively focused on the hypothesis that the Gothic population of Halictus rubicundus represented an evolutionary reversion to solitary behavior from an eusocial population. The authors equated loss of sociality with “brood loss”, and asked whether solitary behavior is based on an evolutionary deletion of the first brood of their social ancestors (i.e., foundresses immediately produce a brood of reproductives), or on deletion of the second brood. In an extensive discussion of offspring sex ratio, they reasoned that these two evolutionary pathways to solitary behavior generate different predictions about the observed sex ratio. Since sex ratio differs between broods of eusocial populations, an evolutionary retention of the first brood (deleting the second) predicts a femalebiased sex ratio, while retention of the second brood predicts an equal or male-biased sex ratio. The observed sex ratio in solitary H. rubicundus is more similar to the second brood of social populations, so they concluded that the solitary population deleted the first brood. Their argument assumed, however, that sex ratio evolution was slow relative to the rate at which social behavior was lost, and therefore observed patterns of sex ratio investment are not obscured by the length of time a population is solitary. This may be unlikely (e.g. Fisher 1958). Secondly, they did not consider the hypothesis that observed differences in sex ratio might be due to a facultative mechanism which alters the sex ratio in response to environmental conditions (e.g. Mueller 1991), and differences do not necessarily imply there has been an evolutionary reversion. I revised the manuscript to address these alternative interpretations. A different, and more conventional, way to view loss of sociality is as the loss or suppression of worker behavior. The original manuscript contained no mention of this possibility, nor did it discuss mechanisms which might be involved in the loss or suppression of worker behavior. This is a particular problem because the original focus on brood deletion implicitly assumed that all (or most) females of the first brood become workers (i.e., one way to lose worker behavior is to delete the first brood). In fact, however, first-brood halictine females can become reproductives or workers, depending on environmental and social conditions (e.g. Michener 1990). These considerations were incorporated into the revisions. The occurrence of solitary behavior (or modified brood structure) in a high-altitude population is consistent with a second hypothesis that was not mentioned in the original manuscript, but is discussed Solitary behavior in a high-altitude population of the social sweat bee Halictus rubicundus (Hymonoptera: Halictidae). Behav Ecol Sociobiol 38:227–233