The beneficial sexually transmitted microbe hypothesis of avian copulation

Abstract

Several hypotheses have been proposed to explain why female birds either copulate repeatedly with a single mate or copulate with multiple partners even though only a single copulation may be sufficient to fertilize an entire clutch. We hypothesize that females may directly benefit from high frequencies of copulation and multiple copulation partners if they receive a cloacal inoculation of beneficial sexually transmitted microbes (STMs) that can either protect them against future encounters with pathogens and/or serve as therapy against present infections. Experiments in domestic animal production, wildlife rehabilitation, and clinical medicine indicate that inoculations of beneficial microbes derived from the indigenous microflora of hosts can lead to nutritional benefits, resistance to colonization by pathogens, the elimination of infection, and improved immune system functioning in recipients. Our hypothesis predicts greater copulatory rates when the probability of the transmission of beneficial microbes exceeds that of pathogens and when the positive effects of beneficial microbes on host fitness exceed the negative effects of pathogens. Patterns of copulatory behavior in birds suggest the potential utility of our hypothesis. We discuss our hypothesis in the context of observed patterns of copulation in birds and propose some ways to directly test our hypothesis. Information on the probabilities of transmission during copulation of beneficial and pathogenic microbes and their relative potencies in birds are needed to directly test the predictions of our hypothesis.

A plethora of hypotheses (e.g., Birkhead and Møller, 1992; Birkhead et al., 1987; Hunter et al., 1993; Keller and Reeve, 1995; Lumpkin, 1981; Petrie, 1992) have been proposed to explain the variations in copulatory behavior both among and within species of birds (Birkhead et al., 1987). Despite a great deal of theoretical musing over the potential costs and benefits of copulations to females, including extrapair copulations (EPCs), the realized costs and benefits are not very clear (e.g., Birkhead and Møller, 1992; Birkhead et al., 1990; Hamilton, 1990; Kempenaers et al., 1992; Lifjeld et al., 1993; Møller, 1994; Wagner, 1991, 1992; Westneat et al., 1990). Direct empirical evidence supporting or contradicting any hypothesized benefit to females is lacking for most species.

We propose that the cloacal inoculation of beneficial sexually transmitted microbes (STMs; viruses, bacteria, fungi) is a previously ignored direct benefit to females of participating in copulations. Our hypothesis explains why female birds might pursue multiple copulations with their mates or EPCs with males of higher quality than their social mates, and is at least as parsimonious as theories that predict only indirect female benefits (e.g.,“ good genes”; Hamilton and Zuk, 1982, see below). Although our hypothesis is directed at birds, it may also apply to other animals.

The avian cloaca serves the dual functions of excretion and gamete transfer. Microbes may be readily transmitted from males to females during copulation because intestinal microbes could become incorporated into an ejaculate (Sheldon, 1993). Thus, sexually transmitted diseases (STDs) may be important selective forces in the evolution of avian mate choice (Hamilton, 1990) and mating systems (Lombardo, 1998; Sheldon, 1993). But microbes can also have beneficial effects on host health (Herceg and Peterson, 1997; Hutchenson et al., 1991; Prescott et al., 1996; Savage, 1977; van der Waaij, 1989) and therefore on reproductive success. The benefits associated with the horizontal transmission of beneficial microbes may be a selective force helping to mold the evolution of mate choice and copulation behavior in birds.

A theory that posits that female birds seek multiple copulations with one or more partners in order to be inoculated with beneficial microbes must meet these requirements: (1) microbes can have beneficial effects, (2) birds transmit beneficial microbes during copulation, (3) the transmitted microbes produce beneficial effects in their recipients, and (4) the probability of transmission and potency of effect of beneficial microbes relative to that of potential pathogens influences copulatory behavior.

Requirement 1: the beneficial effects of microbes

Studies in commercial animal husbandry, wildlife rehabilitation, and clinical medicine demonstrate the beneficial effects of microbes on their hosts. The beneficial effects of gastrointestinal microbes are well documented in commercial animal husbandry (for reviews see Fuller, 1989; Hutchenson et al., 1991). Newborn and juvenile domestic animals inoculated with beneficial microbes are, on average, less likely to harbor potentially pathogenic species, grow more rapidly, and better resist challenges by pathogens than are uninoculated individuals (Hutchenson et al., 1991).

The beneficial effects of microbes in wild birds is suggested by the use of adult saliva during the rehabilitation of chimney swifts (Chaetura pelagica; Kyle and Kyle, 1993). The adult saliva contained a variety of microbes. The saliva used in rehabilitation could come from any healthy adult swift. Nearly 100% of nestling swifts less than 6 days old died if given food lacking the adult saliva supplement, whereas nearly 100% of those fed food inoculated with saliva were rehabilitated and released (Kyle and Kyle, 1993).

Beneficial microbes have also been used as therapy against infection (e.g., Bruce and Reid, 1988; Gorbach et al., 1987). In humans, crude fecal suspensions obtained from healthy individuals and administered as enemas to sick individuals have been used effectively to treat enterocolitis caused by Clostridium difficile (Bowden et al., 1981; Schwann et al., 1984). In rhesus monkeys (Macca maculata), vaginal Escherichia coli infections have been cured by treatment with direct intravaginal application of vaginal microbes obtained from healthy monkeys (Herthelius et al., 1989).

Many studies have demonstrated that (1) “normal” indigenous microbes are important in providing resistance to intestinal pathogens and controlling the populations of opportunistic bacteria in the digestive and urogenital tracts in humans (e.g., Agnew and Hillier, 1995; Bruce and Reid, 1988; Hillier and Holmes, 1990; Lidbeck and Nord, 1993; Redondo-Lopez et al., 1990; Savage, 1977; Sobel, 1990; van der Waaij et al., 1971), other mammals (Savage, 1969), and birds (Fuller, 1973, 1989; Hutcheson et al., 1991; Nurmi and Rantala, 1973; Snoeyenbos et al., 1983; Weinack et al., 1981, 1982), and (2) there is a host genetic component to the development of the indigenous microflora (Stern et al., 1990; van der Waaij, 1989; van der Waaij et al., 1971).

Mechanisms by which females may receive therapeutic inoculations include (1) bacteriophagic viruses (e.g., Levin and Bull, 1996; Smith and Huggins, 1983), (2) less virulent strains of pathogens, which, in becoming established, limit the colonization abilities of more virulent strains (Sprunt and Leidy, 1988), and (3) beneficial microbes that produce bacteriocins lethal to already resident pathogenic strains (Daw and Falkiner, 1996; Jack et al., 1991; Waters and Crosa, 1991).

The microbial population of a healthy individual consists of a mixture of beneficial and potentially pathogenic microbes that can cause illness and death if the balance between them is disrupted (Fuller, 1989; Herceg and Peterson, 1997; Prescott et al., 1996; Savage, 1977; van der Waaij, 1989). Physical and psychological stress can disrupt the protective microflora (Tannock, 1983), leading to increases in pathogen populations and subsequent pathology (Fuller, 1989). One general effect of stress is for protective Lactobacilli spp. to decrease and pathogenic coliforms to increase (Fuller, 1989; Tannock, 1983). In addition, the hormonal state arising from changes in diet (Scrimshaw et al., 1968) and reproductive condition may affect the ability of the immune system to combat pathogens (Folstad and Karter, 1992). Thus, for many birds it is likely that the rigors of migration, territory establishment and defense, ecological and sexual competition, and reproduction itself influence the ecological balance of the microbes of the gastrointestinal and urogential tracts, making them more susceptible to pathogens (Tannock, 1983).

Reproduction may be especially stressful. For example, infections of the blood parasite Haemoproteus spp. and heterophile:lymphocyte ratios which indicate stress in birds were positively correlated with reproductive effort in great tits (Parus major, Ots and Horak, 1996). The fact that the ecological balance between microbes in the gastrointestinal and urogenital tracts can be disrupted by situations that are commonly encountered by wild birds establishes the conditions necessary for our hypothesis.

For adult birds, potential pathways for the acquisition of beneficial microbes include copulation, mate feeding, coprophagy (cf. Troyer, 1982), and/or cloaca-pecking (cf. Davies, 1983). Coprophagy may be an inefficient way to obtain beneficial microbes that are obligate anaerobes (e.g., most Lactobacilli spp.) (Topley, 1983; Bokkenheuser, 1993), favoring the evolution of direct interindividual transmission by copulation or mouth-to-mouth transfer. The most direct pathway would be via copulation because it minimizes the exposure of gastrointestinal and urogential microbes to hostile aerobic environments.

Requirements 2 and 3: the sexual transmission of microbes and their effects

The existence of avian STDs (Lockhart et al., 1996; Sheldon, 1993; Stipkovits et al., 1986) is direct evidence that birds inoculate each other with microbes during copulation. Moreover, Perek et al. (1969) showed by experiment that male domestic cockerels with semen contaminated with bacteria infected the females with which they copulated.

For females to benefit from receiving cloacal inoculations of beneficial microbes, those microbes must become established in the gastrointestinal and/or urogenital tract. Corrier et al. (1991) showed that cloacal inoculation of turkey poults with beneficial microbes reduced the load of gastrointestinal Salmonella seftenberg after an oral challenge at 3 days of age. This finding shows that microbes introduced into the cloaca can become established in the gastrointestinal tract and may outcompete pathogens already resident in the host. In birds, beneficial STMs introduced into the cloaca have a direct route into the intestines and urogential system.

Requirement 4: copulatory behavior in relation to the benefits and risks of sexual transmission of microbes

Explanations for the adaptive significance of variation in copulatory behavior among different species (Birkhead et al., 1987), populations (e.g., synchronous versus asynchronous breeders) (Stutchbury and Morton, 1995), ecological communities (e.g., temperate versus tropical zones) (Stutchbury and Morton, 1995), and degrees of sociality (Møller and Birkhead, 1993) in birds have been proposed. However, none has directly considered the influence that STMs might have on copulatory behavior.

Female attempts to receive beneficial microbes may help explain why some female birds copulate outside of their fertile periods (e.g., Fitch and Shugart, 1984; Flood, 1985; Lombardo, 1986; Power and Doner, 1980; Quay, 1985, 1989; Wagner, 1991; Wolf, 1975). Moreover, it has been difficult to understand why many female birds copulate repeatedly with the same male or with multiple males (Birkhead and Møller, 1992; Hunter et al., 1993; Petrie, 1992) when only one or few ejaculates may provide enough sperm to fertilize all of a female's eggs (Adkins-Regan, 1995; Birkhead, 1988). Repeated inoculations of beneficial STMs may be a direct benefit of multiple copulations and may be necessary for female birds to receive inoculations large enough to produce benefits. In clinical situations and during domestic animal production, repeated inoculations of antibiotics and/or beneficial microbes are used to produce the desired prophylatic, therapeutic, or nutritional effects (Fuller, 1989; Hutcheson et al., 1991; Savage, 1969).

The beneficial STM hypothesis of copulation in birds

Our hypothesis is based upon the following assumptions:

1. In birds, large numbers of copulations are not necessary to fertilize all eggs (Adkins-Regan, 1995; Birkhead, 1988). Copulations in excess of the minimum number required for fertilization require explanation. Part of that explanation is probably that females seek the benefits of sperm competition (Birkhead and Møller, 1992; Keller and Reeve, 1995), but we also assume that females can be favored for seeking any beneficial component of male ejaculate, not just highly competitive sperm (Eberhard and Cordero, 1995).

2. Some birds in the local population carry both beneficial and pathogenic STMs, while others carry only one or the other, or neither (e.g., Brittingham et al., 1988; Calnek et al., 1991; Cooper et al., 1980; Flammer and Drewes, 1988; Fritz et al., 1992; Lombardo et al., 1996; Petrak, 1982). For example, of 30 tree swallow (Tachycineta bicolor) semen samples screened for microbes in 1998, 11 (37%) were negative, 11 (37%) contained both beneficial (e.g., Lactobacilli spp.) and potentially pathogenic (e.g., Salmonella spp., E. coli) microbes, 1 (3%) contained only beneficial microbes, and 7 (23%) contained only potentially pathogenic microbes (Lombardo and Thorpe, unpublished data)

3. Beneficial STMs increase the health and vigor of their recipients, enhance host resistance to pathogenic STMs, and positively affect host reproductive success; pathogenic STMs have the opposite effects.

4. The probability that a female becomes colonized by STMs increases with the number of copulations she participates in (and/or partners she copulates with).

5. Birds will generally make risk-averting (sensu Kahneman et al., 1982) decisions (e.g., avoid partners infected with pathogenic STMs) while pursuing copulations because this will promote their survival and hence reproductive success. However, when their probability of survival has already been compromised by acquisition of pathogenic STMs, birds may be risk seeking (sensu Kahneman et al., 1982) to improve their odds of acquiring beneficial STMs as an antidote. Thus, ill females may increase their number of copulations/partner or number of partners even though this will inevitably also increase their chances of acquiring additional pathogenic STMs and thus further decrease their odds for survival and/or nesting success.

Given these assumptions, we hypothesize that females should pursue copulations to obtain STMs when these conditions obtain: (1) the probability of obtaining beneficial STMs exceeds that of obtaining pathogenic STMs, (2) the positive effects of beneficial STMs are greater than the negative effects of pathogenic STMs, and (3) the opportunity costs of obtaining copulations (i.e, the time, energy, and risk costs) are not too great for the female's budget. Additionally, if increased microbial diversity is beneficial, then mated females should pursue EPCs as well as copulations with their mates. However, the opportunity costs of EPCs will average higher because of mate guarding by the female's own mate, possible attack by the target male's female, and greater average distances between nonmated than mated individuals.

The identification of potential partners

Positive correlations between the presence of beneficial STMs and host nutrition and resistance to pathogens (i.e., health) (Fuller, 1989; Herceg and Peterson, 1997; Hutcheson et al., 1991; Prescott et al., 1996; Savage, 1977; van der Waaij, 1989) strongly suggest a similar positive relationship between beneficial microbes and competitive ability. If carrying beneficial microbes is associated with good health (Fuller, 1989; Hutcheson et al., 1991; Savage, 1977; van der Waaij, 1989), then the ability of individuals to identify potential donors of beneficial STMs should be favored. Furthermore, if individuals that avoid copulating with carriers of pathogenic STMs have a selective advantage over those that do not, then the ability to detect infected partners by one sex would favor the ability to advertise freedom from disease by the other (Hamilton, 1990). Because there is a host genetic component to the establishment of symbiotic microbes (Stern et al., 1990; van der Waaij, 1989), carriers of strains of the most beneficial microbes will also simultaneously display their genetic quality.

An increasing body of empirical evidence suggests that welldeveloped secondary sexual characters may be reliable signals of health because they may be positively correlated with superior immunocompetence (sensu Folstad and Karter, 1992; Møller and Saino, 1994; Ros et al., 1997; Saino et al., 1995, 1997). Likewise, well-developed secondary sexual characters might also be reliable signals that an individual carries large numbers of and/or highly potent beneficial microbes. Therefore, female choice for showy males (Andersson, 1982; Møller, 1988; von Schantz et al., 1989) may have begun with females choosing the healthiest looking males as mates and EPC partners because they not only avoided STDs and other infectious diseases, but also received inoculations of superior beneficial STMs.

Our argument is parallel to the Hamilton and Zuk (1982) “good genes” model of female choice, except that in our hypothesis females receive both direct and indirect benefits from choice. Our hypthesis and good genes models of choice are complementary, not mutually exclusive. The direct benefit of avoiding STDs and other infectious diseases by mating with showy males is implicit in Hamilton and Zuk (1982). Thus some current models of parasite-mediated sexual selection (Clayton, 1991; Hamilton, 1990; Hamilton and Zuk, 1982; Møller, 1994) may be useful in understanding the dynamics of choice for partners that are likely to transmit beneficial microbes. Good genes models (Hamilton and Zuk, 1982) may be applicable when females pursue copulations during their fertile periods, although health considerations should always be present. In contrast, if females pursue copulations outside of their fertile periods (e.g., during migration; cf. Quay, 1985) or when their indigenous microflora has been disturbed, then no assumptions about the genetic quality of potential partners is necessary (cf. Clayton, 1991), and our hypothesis is more applicable.

Both partners may be inoculated with STMs during copulation. However, because most birds lack an intromittant organ and ejaculates move from male to female, the transmission of microbes during copulation is likely to be asymmetrical with the probability of transmission from male to female being greater than that from female to male (Perek et al., 1969). Thus, while both sexes are favored for detection and signaling ability, because of the asymmetry of risk, females would be favored for greater detection ability and males for greater signaling ability. However, because males should avoid copulating with females infected with pathogenic STMs, females may be favored for signaling their freedom from infection (cf. Hamilton, 1990)

Copulatory behavior in birds and tests of the hypothesis

Directly comparing the predictions of our hypothesis to data on avian copulatory behavior (see Birkhead et al., 1987; Birkhead and Møller, 1992, for reviews) is difficult because observations of copulation are biased in that they are most often of within-pair copulations during the breeding season. If females copulate to achieve fertilization, promote sperm competition, assess potential future partners, and acquire beneficial STMs, we have no a priori reason to predict patterns of copulatory behavior different from those already observed. However, our hypothesis also predicts that females pursue EPCs and copulations outside of their fertile periods to acquire beneficial STMs. Copulations performed solely to acquire beneficial STMs might be difficult to observe. Many more careful observations of EPCs and of birds outside of their breeding seasons are required before our hypothesis can be properly evaluated.

The observations that extrapair mating systems are more common among synchronously breeding songbirds than among asynchronously breeding songbirds and more common in the temperate zone than in the tropics (Stutchbury and Morton, 1995) are consistent with our hypothesis. Synchronous breeding and the short breeding seasons of the temperate zone limit the dispersal opportunities of microbes and thus favor the evolution of less virulent strains of pathogens (Ewald, 1994). We predict that copulations, including EPCs, will be more common when the probability of transmission of pathogenic microbes is low. In contrast, asynchronous breeding and the long breeding seasons of the tropics provide more dispersal opportunities for microbes and thus favor the evolution of more virulent strains of pathogens (Ewald, 1994). We predict that copulations, including EPCs, will be relatively uncommon when the probability of transmission of pathogenic microbes is high.

We also predict the evolution of female traits that facilitate colonization by beneficial microbes and impede colonization by pathogens obtained via copulation. First, females in a variety of bird species have the ability to selectively retain or expel ejaculates based on the identity of their copulatory partner (Adkins-Regan, 1995; Birkhead and Møller, 1992). Thus, we predict that females will be found to be able to retain or expel semen based on whether it contains beneficial or pathogenic microbes, respectively. Second, we predict the existence of mechanical and physiological impediments to colonization by pathogenic STMs in female reproductive tracts. For example, human females have a variety of defenses that help them avoid being colonized by pathogenic STMs (reviewed in Holmes et al., 1990; Profet, 1993). It is highly probable that females in all species with internal fertilization have evolved defense mechanisms to prevent colonization by pathogenic STMs.

Here we provide a short list of some ways to test our hypothesis:

1. Potential beneficial or pathogenic STMs can be identified by the association between their presence and/or abundance on host health, growth, social status, development of secondary sexual characters, and reproductive success.

2. The effects of experimental cloacal inoculation on individual health, growth, status, and expression of secondary sexual characters and on reproductive success could provide a direct way to identify both beneficial and pathogenic STMs.

3. Observations of the copulatory behavior of females of different ages could be used to determine whether younger females copulate more frequently with each partner and/or have more partners than older females on the grounds that younger females need to be “vaccinated” against future infections because of a lack of prior exposure to pathogens.

4. An experiment that induces copulations outside of female fertile periods by females that have been experimentally infected with pathogenic STMs could show that females pursue copulations as a way of acquiring beneficial STMs.

We thank C. J. Bajema, two anonymous reviewers, and especially L. L. Wolf for comments on previous versions of the manuscript. M.P.L. benefited from conversations with P. W. Turke. M.P.L. was supported by a sabbatical leave from Grand Valley State University during the writing of the manuscript.