Anim. Behav., 1991, 41,963 970 Mate guarding as a key factor in the evolution of parental care in birds JOHAN G. VAN RHIJN* Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands and Department of Sciences, Open University, P.O. Box 2960, 6401 DL Heerlen, The Netherlands (Received 26 March 1990; initial acceptance 24 April 1990; final acceptance 18 October 1990; MS. number: 3553) Abstract. The evolution of male parental care in vertebrates with internal fertilization must have been preceded by a stage in which males profit by staying with the female after copulation. This paper discusses the results of a series of computer simulations to determine the pay-off to post-copulatory mate guarding under various conditions. Guarding is promoted by asynchrony in fertility of the females, high copulation frequencies of females, preference of females for males using the guarding strategy, and mate fidelity of guarded females. Moreover, it is demonstrated that, under several conditions, apparently those operating in a natural environment, the success of the guarding strategy is inversely related to its frequency in the population. This implies that both the guarding and the non-guarding strategy can be maintained in the same population. This phenomenon is put forward as a key factor determining the pathways in the evolution of parental care. The origin of parental care in birds was probably Smith 1977). Another obstacle to the sudden closely related to the acquisition of body tempera- appearance of biparental care is the slow rate of ture regulation, because this could also require change in the course of evolution in the need for regulation of egg temperature. Parental care must parental care by the offspring. It is therefore diffi- have been preceded by a stage without prolonged cult to imagine how the aid of a second parent care after egg laying and could be established as would further increase the number of surviving off- soon as individuals succeeded in rearing more sur- spring in species evolving from a stage without viving offspring by incubation or other care, than parental care. Their young may still be adapted to simply by leaving the eggs to their fate. Parental survive without care at all. Evidence for evolution care might have originated in three alternative of biparental care via a stage of care by one parent ways. has also been presented by Gittleman (1981) by First, both parents, female and male, could have pedigree analysis for fish. Thus, the idea of a direct evolved the ability to care for their offspring. There transition, from a stage without parental care to a are two reasons why such transition from no care at stage with biparental care, does not seem to be very all to biparental care cannot simply be rejected: realistic. (1) males and females possess almost the same set of Second, the female could have evolved the ability genes, and thus are almost equally affected by to care for her offspring. This possibility seems to be natural selection for parental care, and (2) continu- rather plausible because the female lays the eggs. ous regulation of egg temperature is not very easy The evolution of prolonged care of the eggs by the for one parent, especially as foraging for its own female requires, as a matter of course, that a new needs takes a lot of time. However, there is at least (mutant) type of female, who performs some care, one very important hindrance to the evolution of arises in the population. It also requires that this biparental care in species with internal fertilization investment results in more surviving offspring than (reptiles, birds and mammals): females and males the same investment in prolonged laying or the experience quite different circumstances at the time same investment in extra reserves for the eggs. If of egg laying. Mothers are necessarily present when these conditions are fulfilled, natural selection the eggs are laid, but fathers are not (Maynard should, after a number of generations, result in all *Address for correspondence: Slochterweg, 3, 9635 TA females having the ability to care for their off- Noordbroek, The Netherlands. spring. Thus, the idea of a transition from a stage 0003 3472/91/060963+ 08 $03.00/0 1991 The Association for the Study of Animal Behaviour 963 964 Animal Behaviour , 41, 6 without parental care to a stage with parental care The cost and benefits of guarding for an individ- by the female seems to be associated with simple ual are closely related to the frequency of guarding conditions. However, in birds the female allocates in the population (van Rhijn 1984). If the guard- considerable reproductive effort in the formation of ing strategy is rare, implying that the majority of eggs. It has been argued that this effort was even males try to copulate with any female, the benefits greater in the common ancestor of birds, probably of guarding are high, because many copulation before it performed any prolonged parental care attempts by other males can be prevented. The (Elzanowski 1985). It is reasonable to suggest that costs, however, are also high, because the guarding extra investment in care after laying necessarily male has to refrain from many copulation oppor- leads to lower investment in reserves before laying, tunities with other females. If, however, guarding is and possibly to fewer surviving offspring. This common, it may be difficult for a male to find an could have been an obstacle in the evolution of par- unguarded female. This implies that the costs of ental care by the female, which possibly facilitated guarding are comparatively low: the time needed to the evolution of male parental care. find a female is long in comparison with the time Thus, third, the male could have evolved the needed for guarding. It also implies that the ben- ability to care for his offspring. This possibility efits of guarding are low: the probability of an might have occurred if there was any other reason unguarded female copulating for a second time is for the male to stay after copulation with the much lower than in the situation with mainly non- female, at least until egg laying. There seems to be at guarding males. This could mean that both guard- least one situation in which it is advantageous to the ing and non-guarding strategies can be maintained male to stay with the female. If females tend to in the same population as a mixed evolutionarily Copulate with many different males for the same stable strategy (ESS; e.g. Maynard Smith 1982). clutch of eggs, and if males are able to keep other Earlier theoretical work, however, suggests that males away from females, the reproductive success precopulatory mate guarding, a kind of guarding of a male may increase when he guards females after promoting future access to receptive females, insemination until egg laying. Such a male would evolves only as a pure ESS (Grafen & Ridley 1983; ensure that his inseminations fertilize the eggs. This Ridley 1983). This should occur if females permit is, in fact, the beginning of a monogamous pair- males to copulate for a very short period of their bond. The establishment of such a bond may be the reproductive cycle, provided that males are able basis for the evolution of male parental care. to judge the female's reproductive state. In con- trast, models of post-copulatory mate guarding MATE GUARDING (Yamamura 1986; Yamamura & Tsuji 1989), pri- marily developed for insects, suggest that guard- The possibilityof the evolution of male parental care ing and non-guarding strategists may coexist in depends on the benefits and costs of post-copulatory the same population. The differences between the mate guarding. The benefits of the guarding strategy outcomes of these models might indicate that may be described as an increase in the efficiency of pre-copulatory and post-copulatory guarding are insemination (e.g. Birkhead et al. 1987). Guarding fundamentally distinct, or instead, that the assump- raises the probability that copulations with a certain tions do not correspond. Clearly, we do not yet female lead to fertilization. However, this strategy understand how mate guarding might have may also incur considerable costs for the male. evolved. In this paper, I use computer simulations During mate guarding a male is less able than non- to study the costs and benefits of post-copulatory guarding males to court other females and, thus, to mate guarding, especially under conditions that copulate with them. This may lead to a lower repro- apply to vertebrates with internal fertilization of ductive success than for non-guarding males. The eggs. costs of the guarding strategy may thus be described as a decrease in the number of females who can be PROCEDURES inseminated. In cases where the benefits outweigh the costs, natural selection should promote the My experimental subjects belonged to a hypotheti- evolution of guarding. Where costs exceed benefits, cal species without parental care. Two different natural selection should not promote mate types of male mating strategies could occur in this guarding. species: mate guarding and non-guarding. Males van Rhijn: Guarding and evolution of parental care 965 with the least complex non-guarding strategy population. All simulation programs were run on started to court females (searching) to test whether an IBM compatible personal computer, using they were willing to copulate, copulated as soon as Turbo Pascal as the programming language. they found a receptive female, and, subsequently, I use the following terminology in this paper. continued to search. Males with the more complex Males with the non-guarding strategy are 'non- guarding strategy also started to search and court guarders' (NG), and males with the guarding strat- to test whether a female was willing to copulate, egy 'guarders' (G).