Hanna Kokko

The evolution of cooperative breeding through group augmentation

Some individuals (helpers) in cooperatively breeding species provide alloparental care and often suppress their own reproduction. Kin selection is clearly an important explanation for such behaviour, but a possible alternative is group augmentation where individuals survive or reproduce better in large groups and where it therefore pays to recruit new members to the group. The evolutionary stability of group augmentation is currently disputed. We model evolutionarily stable helping strategies by following the dynamics of social groups with varying degrees of subordinate help. We also distinguish between passive augmentation, where a group member benefits from the mere presence of others, and active augmentation, where their presence as such is neutral or harmful, but where helping to recruit new group members may still be beneficial if they in turn actively provide help for the current reproductives (delayed reciprocity). The results show that group augmentation (either passive or active) can be evolutionarily stable and explain costly helping by non–reproductive subordinates, either alone or leading to elevated help levels when acting in concert with kin selection. Group augmentation can thus potentially explain the weak relationships between relatedness and helping behaviour that are observed in some cooperatively breeding species. In some cases, the superior mutualistic performance of cooperatively behaving groups can generate an incentive to stay and help which is strong enough to make ecological constraints unnecessary for explaining the stability of cooperatively breeding groups.

Social queuing in animal societies: a dynamic model of reproductive skew

Previously developed models of reproductive skew have overlooked one of the main reasons why subordinates might remain in a group despite restricted opportunities to breed: the possibility of social queuing, i.e. acquiring dominant status in the future. Here, we present a dynamic ESS model of skew in animal societies that incorporates both immediate and future fitness consequences of the decisions taken by group members, based on their probability of surviving from one season to the next (when post–breeding survival probabilities drop to zero, our analysis reduces to the model produced by Reeve and Ratnieks in 1993, which considered only a single breeding season). This allows us to compare the delayed benefits of philopatry and the immediate opportunities for independent breeding. We show that delayed benefits greatly reduce the need for dominants to offer reproductive concessions to retain subordinates peacefully in the group. Moreover, this effect is strong enough that differences in survival have a much greater impact on the group structure than differences in other parameters, such as relatedness. When the possibility of acceding to dominant status is taken into account, groups where the dominant completely monopolizes reproduction can be stable, even if they consist of unrelated individuals, and even if subordinates have a reasonably high probability of winning a fight for dominance. Finally, we show that stable groups are possible even if association leads to a decrease in current productivity. Subordinates may still stand to gain from group membership under these circumstances, as acquiring breeding positions by queuing may be more efficient than the attempt to establish a new territory. At the same time, the dominant may be unable to exclude unwelcome subordinates, may enjoy increased survival when they are present, or may gain indirect benefits from allowing relatives to stay and queue for dominance. We conclude that reproductive skew in animal groups, ranging from eusocial insect colonies to mating aggregations (leks), will be strongly influenced by the future prospects of group members.

Modelling sexual segregation in ungulates: effects of group size, activity budgets and synchrony

Abstract Sexual segregation is common in ungulates and some social mammals but its causes are still poorly understood. We developed an individual-based, spatially explicit simulation model to test whether sexual differences in activity could lead to sexual segregation. In our model, males and females differed only in their propensity to switch from an active to a passive state and vice versa, with males being more reluctant to get up and more ready to lie down than the more active females, or vice versa. The only factor in our model that affected sexual segregation was the sexual difference in the propensity to switch from an active to a passive state and vice versa. As differences in activity budgets increased, the degree of sexual segregation increased. Sexual segregation reached a peak when sexual differences in activity budgets were greatest. This high level of segregation remained, even when animals were programmed to adjust their activity to other animals in their vicinity. Our results verify the logic of the activity budget hypothesis that sexual differences in time spent active (grazing, walking) versus passive (lying, ruminating) can, in principle, result in sexual segregation. However, this does not exclude alternative mechanisms. Our model could also be applied to any social animal foraging in groups. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Sexual reproduction and population dynamics: the role of polygyny and demographic sex differences

Most models of population dynamics do not take sexual reproduction into account (i.e. they do not consider the role of males). However, assumptions behind this practice—that no demographic sex differences exist and males are always abundant enough to fertilize all the females—are usually not justified in natural populations. On the contrary, demographic sex differences are common, especially in polygynous species. Previous models that consider sexual reproduction report a stabilizing effect through mixing of different genotypes, thus suggesting a decrease in the propensity for complex dynamics in sexually reproducing populations. Here we show that considering the direct role of males in reproduction and density dependence leads to the conclusion that a two–sex model is not necessarily more stable compared with the corresponding one–sex model. Although solutions exist where sexual reproduction has a stabilizing effect even when no genotypic variability is included (primarily when associated with monogamy), factors like polygyny, sex differences in survival or density dependence, and possible alterations of the primary sex ratio (the Trivers–Willard mechanism), may enlarge the parametric region of complex dynamics. Sexual reproduction therefore does not necessarily increase the stability of population dynamics and can have destabilizing effects, at least in species with complicated mating systems and sexual dimorphism.

Sexually transmitted disease and the evolution of mating systems.

Abstract Sexually transmitted diseases (STDs) have been shown to increase the costs of multiple mating and therefore favor relatively monogamous mating strategies. We examine another way in which STDs can influence mating systems in species in which female choice is important. Because more popular males are more likely to become infected, STDs can counteract any selective pressure that generates strong mating skews. We build two models to investigate female mate choice when the sexual behavior of females determines the prevalence of infection in the population. The first model has no explicit social structure. The second model considers the spatial distribution of matings under social monogamy, when females mated to unattractive males seek extrapair fertilizations from attractive males. In both cases, the STD has the potential to drastically reduce the mating skew. However, this reduction does not always happen. If the per contact transmission probability is low, the disease dies out and is of no consequence. In contrast, if the transmission probability is very high, males are likely to be infected regardless of their attractiveness, and mating with the most attractive males imposes again no extra cost for the female. We also show that optimal female responses to the risk of STDs can buffer the prevalence of infection to remain constant, or even decrease, with increasing per contact transmission probabilities. In all cases considered, the feedback between mate choice strategies and STD prevalence creates frequency-dependent fitness benefits for the two alternative female phenotypes considered (choosy vs. randomly mating females or faithful vs. unfaithful females). This maintains mixed evolutionarily stable strategies or polymorphisms in female behavior. In this way, a sexually transmitted disease can stabilize the populationwide proportion of females that mate with the most attractive males or that seek extrapair copulations.

The role of kin recognition in the evolution of conspecific brood parasitism

Abstract Conspecific brood parasitism (CBP) is a common strategy in several species of birds. Currently, some studies suggest that relatedness between host and parasite enhances CBP, since indirect fitness benefits could select for acceptance of related eggs by hosts. Conversely, parasites should avoid laying eggs in nests of relatives if this is costly for the host. Based on the latter argument, kinship should not promote brood parasitism. A recent model clarified this relationship, and showed that kinship can promote brood parasitism, assuming kin recognition. However, in that model kin recognition was assumed perfect. Here we present a model that addresses the role of relatedness and kin selection in CBP, when kin recognition is not perfect and hosts do not always detect parasitism. We consider both the indirect fitness of the parasite and the possible responses of the host. Our results indicate that the existence and accuracy of a kin recognition system is crucial to the final outcome. When CBP represents a cost to the host, a parasitic female that has the choice should avoid parasitizing relatives, unless (1) the costs are not too high and (2) hosts can accurately enough recognize eggs laid by relatives, rejecting them less often than eggs laid by nonkin. But if ‘parasitism’ enhances the direct fitness of the host (which is possible in species with precocial young) parasites should choose relatives whenever possible, even if hosts do not recognize kin eggs. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .

Role models for behavioural ecologists

View Large Image | Download PowerPoint SlideModel Systems in Behavioral Ecology. Integrating Conceptual, Theoretical and Empirical Approachesedited by L.A. Dugatkin. Princeton University Press, 2001. £24.95 pbk (584 pages) ISBN 0 691 00653 9Model systems in behavioural ecology are species or populations that we study in particular detail, and Dugatkins volume draws together 25 examples, ranging from dungflies to gorillas and cheetahs. The tone of the book is optimistic, and so is the title, promising integration of conceptual, theoretical and empirical approaches.Synthesis would indeed be welcome. Behavioural ecology is not particularly explicit about the ways in which one should interact – advocating ones own approach is the norm. In Chapter 4, Reeve strongly argues against ‘pluralism’, finding isolated explanations for particular phenomena. We must always strive towards a common, theoretical framework, he says. However, two chapters earlier, Seeley describes how much behaviour would have been neglected without a deep knowledge of one particular species. Both authors have a point; but it smacks of contradiction.In practice, people do what suits them best: those struck by the grace of cheetahs study cheetahs, those with an eye for mathematical beauty develop theory, and some add phylogenies and meta-analyses to it all. And we all hope to interact enough that it results in something positive. Perhaps this is sufficient, but I would have liked to have seen one, two, or maybe three chapters devoted to synthesizing different approaches, and discussing the benefits and limitations of the study of model systems (as opposed to, say, large-scale comparative analyses). But we are simply left with the often disparate opinions of different researchers. So I cannot judge whether the book succeeds in the integrative task it sets itself – it does not even properly attempt it.Waste of money, then? Absolutely not. Its merits lie elsewhere. This is not a book on how behavioural ecology should be done. Instead, it is a superb introduction to how it is done. Reading it is like going to ones first behavioural ecology conference: it does not teach explicitly, but you cannot help that the examples trigger new connections in your brain. The scientific content is lucidly presented and avoids too much detail – it is like hearing 25 very well prepared plenary talks. Implicit facts one learns also include the all too familiar taxonomic bias (based on this book, one would think that 72% of animals are vertebrates, and 12% primates), and the fact that scientists talk mainly to their buddies (only six of the 30 contributors are from outside the USA).And here comes the best bit: reading the book is like getting to know the plenary speakers at a conference. The authors were asked to reveal how they ended up where they are, and mostly, they have dared to do so. Where else would one read that Geoff Parker spent the first year of his PhD achieving nothing, Anders Mollers first postdoc applications were turned down, or that Paul Sherman felt a total loser when he initially could not make any sense of wood duck behaviour? I am not reciting these gems of information out of schadenfreude: entering any field of science is scary, and it can help enormously to know that the great gurus have had difficulties too.Equally important are the expressions of enthusiasm, excitement, even love, that are so essential to our devotion to the subject, but which are always censored from dry journal pages. If we are honest (and many of the authors admit to this), no one studies big fierce mammals because they are brilliant model systems, but because they fascinate us. For a would-be behavioural ecologist feeling this kind of passion, the authors willingly share their experiences: watch your animals, endlessly; believe in serendipity; keep your eyes and ears open – pick someones brain at a conference, on the way to work, or even on a holiday; fail in one task, come up with something far better. That is what mentors are for, and lucky is the student who has a good one. Reading this book is like having gained not one, but 25 mentors, and I dearly wish I had had it on my bed-side table when I was starting my career. I highly recommend it to anyone who now has that chance.