James N. Webb

An Evolutionary Approach to Offspring Desertion in Birds

Most parent birds face the decision of whether to spend time and energy caring for their offspring or to conserve their resources to survive and breed later. Should a parent care for the young or desert them? Analyzing this decision is complicated, as the rewards (in terms of reproductive success) depend upon the survival prospects of the offspring, the behavior of the mate, the parent’s chances of surviving and having future reproductive opportunities, and the behavior of other males and females in the population.

MATING PATTERNS, SEXUAL SELECTION AND PARENTAL CARE: AN INTEGRATIVE APPROACH

Mating patterns, sexual selection and parental care are central topics in behavioural ecology, but they are often analysed in isolation from each other. We propose a new conceptual framework to investigate these topics in relation to each other. We argue that it is beneficial to study both mating behaviour and parental care of all types of individual in a population, because the behaviours of different individuals are interrelated in many ways. In particular, we propose a framework in which the parental care adopted is the best response to the mating behaviour and the mating behaviour adopted is the best response to the parental behaviour. The backbone of the proposed framework is the feedback relationship between mating strategies (e.g. accepting or rejecting a mate), mating opportunities (related to the number and quality of animals searching for a mate) and parental care strategies (e.g. caring for the offspring or deserting them). For instance, mating opportunities should influence both the mating and parental strategies. The mating and parental strategies, in turn, have an effect on mating opportunities. We emphasise the conceptual significance of these feedback loops as well as referring to empirical studies which have demonstrated some of these feedbacks. The strength of these feedbacks probably vary between species and may be negligible in some systems. Unlike most previous approaches to mating behaviour and parental care, we do not assume that mating systems, parental investments by males and females, operational sex ratio, reproductive rates, or the intensity of sexual selection are fixed in a population. Rather, these characteristics emerge when one specifies the behavioural options of males and females, and their consequences. Mating and parental decisions can have consequences beyond the immediate breeding attempt and the proposed framework allows us to investigate such decisions from a life-history perspective. Mating and caring decisions involve various interactions among members of a population (e.g. conflicts between prospective mates, and between male and female parents), thus studying mating and caring behaviour benefits from the use of game theory. Since the state of animals (e.g. whether they are mated or not, their age, energy reserves or the number of their offspring) and the time in the breeding season commonly influence the payoffs from different behavioural options, we advocate the use of state-dependent dynamic game theory as a suitable approach for the analysis of such decisions. Finally, we call for a new generation of theoretical models and empirical studies to understand the diverse mating and parental behaviour of animals which have fascinated evolutionary biologists from Darwin onwards.

Dynamic Optimization in Fluctuating Environments

We consider the problem of finding an optimal strategy for an organism in a large population when the environment fluctuates from year to year and cannot be predicted beforehand. In fluctuating environments, geometric mean fitness is the appropriate measure and individual optimization fails. Consequently, optimal strategies cannot be found by stochastic dynamic programming alone. We consider a simplified model in which each year is divided into two non-overlapping time intervals. In the first interval, environmental conditions are the same each year; in the second, they fluctuate from year to year. During the first interval, which ends at time of year T, population members do not reproduce. The state and time dependent strategy employed during the interval determines the probability of survival till T and the probability distribution of possible states at T given survival. In the interval following T, population members reproduce. The state of an individual at T and the ensuing environmental conditions determine the number of surviving descendants left by the individual next year. In this paper, we give a general characterization of optimal dynamic strategies over the first time interval. We show that an optimal strategy is the equilibrium solution of a (non-fluctuating environment) dynamic game. As a consequence, the behaviour of an optimal individual over the first time interval maximizes the expected value of a reward R* obtained at the end of the interval. However,R* cannot be specified in advance and can only be found once an optimal strategy has been determined. We illustrate this procedure with an example based on the foraging decisions of a parasitoid.

Do parents make independent decisions about desertion

Maynard Smith (1977) provided the first game-theoreticalmodels of parental care. In his Model 2, Maynard Smithassumed that each parent had to choose whether to carefor the young or desert. Each parent made its choicewithout knowing the choice of its partner. This scenariois usually referred to as simultaneous choice, but it is thefact that choices are made independently, rather thantheir timing, that is central to the analysis of the game.The assumption that choices are independent is made inmost other models of parental care (e.g. Grafen & Sibly1978; Maynard Smith 1982; Houston & Davies 1985;Yamamura & Tsuji 1993; Balshine-Earn & Earn 1997;Webb et al. 1999). In this commentary, we examine theempirical evidence on whether members of a breedingpair do make their care decisions independently of oneanother.When all breeding pairs within a population adoptthe same pattern of care (for example, all males desertand all females care) it is trivially true that the decisionsof members of a pair are independent. In some popula-tions, however, there can be more than one pattern ofcare (Sze´kely et al. 1996). For example, in the popula-tion of the penduline tit,

Dynamic kin selection

When an animal performs an action that has consequences both for its own fitness and that of a relative, Hamilton’s rule tells us that the action will be favoured by natural selection if br > c, where c is the cost to the animal that performs the action (the ‘actor’), b is the benefit to its relative (the ‘recipient’), and r is the relatedness of the two individuals. We consider a period of time ending at T during which the actor makes a series of decisions. We show that the strategy predicted by Hamilton’s rule maximizes a particular form of the fitness function defined at T. Furthermore, each decision taken during the interval can be characterized by a form of Hamilton’s rule in which b depends not only on the state of the recipient but also on the state of the actor; similarly c depends on the states of both animals. We illustrate this with two schematic examples based on the actor controlling the delivery of food to itself and a relative.