Allocation in reproduction is not tailored to the probable number of matings in common toad (<Emphasis Type="Italic">Bufo bufo</Emphasis>) males

The theory of life history evolution assumes trade-offs between competing fitness traits such as reproduction, somatic growth, and maintenance. One prediction of this theory is that if large individuals have a higher reproductive success, small/young individuals should invest less in reproduction and allocate more resources in growth than large/old individuals. We tested this prediction using the common toad (Bufo bufo), a species where mating success of males is positively related to their body size. We measured testes mass, soma mass, and sperm stock size in males of varying sizes that were either (1) re-hibernated at the start of the breeding season, (2) kept without females throughout the breeding season, or (3) repeatedly provided with gravid females. In the latter group, we also estimated fertilization success and readiness to re-mate. Contrary to our predictions, the relationship between testes mass and soma mass was isometric, sperm stock size relative to testes mass was unrelated to male size, fertilization success was not higher in matings with larger males, and smaller males were not less likely to engage in repeated matings than larger males. These results consistently suggest that smaller males did not invest less in reproduction to be able to allocate more in growth than larger males. Causes for this unexpected result may include relatively low year-to-year survival, unpredictable between-year variation in the strength of sexual selection and low return rates of lowered reproductive investment.     

The costs of male parental care and its evolution in a neotropical frog

Summary Parental care is practiced exclusively by males of the Puerto Rican frog, Eleutherodactylus coqui. Males brood clutches of direct-developing eggs in non-aquatic nest sites and defend eggs against cannibalistic nest intruders. Here, I report on energetic and mating costs incurred by males that provide parental care, and suggest how these proximate costs affect male fitness and the evolution of male parental care in this species. Energetic costs are small for brooding males in comparison to non-brooding, calling males. Brooding males had a higher frequency of empty stomachs and lost small, but significant, fractions of their initial body mass during parental care. Abdominal fat bodies of brooding males during the middle third of parental care were significantly smaller than those of calling males; those of males brooding eggs in earlier or later stages were not different. The mating cost of parental care is greater. Most brooding males cease calling during parental care. However, gravid females are available (i.e., known to mate) on most nights during the principal breeding season; hence non-calling males miss potential opportunities to mate. A mating cost was estimated by calculating nightly mating probabilities for calling males in a plot where nightly calling male densities and daily oviposition schedules were known. On average, a male exhibiting normal calling behavior would be expected to obtain a new mate once every 35.7 days. Hence a brooding male that ceased calling for a 20-day parental care period would miss, on average, 0.56 additional mates. Males that were more successful than average in attracting mates could miss up to 1.63 matings. A marginal value model (Fig. 1) is used to analyze the net effect on male fitness of parental care benefits and costs in E. coqui (Fig. 3). The model indicates that males garner the highest reproductive success by providing care from oviposition through hatching. There is no stage during the pre-hatching period at which a desertion strategy would yield higher reproductive success. In fact, the model suggests that males should provide full parental care even in the face of much higher mating costs than currently obtain in the system.     

Environmental and social factors influence chorusing behaviour in a tropical frog: examining various temporal and spatial scales

Many animals use conspicuous display to attract mates, and there should be selection for displays to occur at times and places that maximise the probability of mating, while minimising energetic costs and predator attraction. To select the best times for display, individuals may use environmental cues, the presence of other individuals, or both, but few studies have examined these sources of variation in display activity. In this study, we examined physical environmental and social factors triggering displays in a tropical, terrestrially breeding frog, Cophixalus ornatus. To measure the influence of physical environmental conditions on calling activity, we recorded temperature, rainfall, moon illumination/visibility, humidity, barometric pressure and intensity of calling activity throughout a breeding season at six locations along a 560-m transect. The intensity of calling varied daily, seasonally, and at a small spatial scale. Variation in calling activity from day to day was large. There was also a strong seasonal trend in calling activity: few males called at the start of the season, activity peaked shortly after the beginning of the season, and then declined linearly from the peak to the end of the season. There was also consistent variation among sites along the transect, which may have been due to variations in frog density at each site, or to consistent microscale variations in physical conditions, or both. After statistically removing consistent local variation among sites, a principal components analysis suggested that a maximum of 35.8% of the variation in calling activity among days was due to factors common to all sites, such as weather, moon illumination, or large-scale social facilitation (e.g. of choruses by other choruses). The remainder of the variation among sites (64.2%) was due to site-specific factors, such as small-scale social facilitation or unmeasured, apparently stochastic effects, such as microenvironmental physical factors that do not vary consistently over sites. Regressions of environmental variables on residual calling activity (after removing consistent effects of site and season), alone or in combination, accounted for very little of the variation in the number of calling males (maximum 10%). Thus, our data, showing strong seasonal effects and consistent variation among sites combined with large amounts of variation in the number of calling males at small spatial scales, suggest that environmental conditions, such as temperature, rainfall, moon illumination and barometric pressure, which act over large spatial scales, may determine the overall environmental envelope within which calling can occur but do not account for most of the variation in the number of calling males on a day-to-day or site-to-site basis. Similarly, variations in the number of calling males at small spatial scales suggest that social facilitation is a relatively unimportant trigger for displays on a large scale in these frogs. On the other hand, our data suggest that social facilitation may have important effects on variation in the number of calling males on a day-to-day and site-to-site basis. We used playback experiments to assess whether the sound of calling could initiate displays. We played either a taped chorus or white noise in areas where few (zero to two) males were calling. The number of calling males increased both during and after the chorus stimulus, whereas there was no increase in calling in response to white noise. These data suggest that examining variation in calling activity at small spatial scales can reveal the sources of variation for the number of calling males, and indicate that, in these frogs, males tend to use the calling of other individuals as a cue to determine when to display. Received: 19 October 1999 / Revised: 30 June 2000 / Accepted: 26 August 2000     

Mortality in the pheasant Phasianus colchicus during the breeding season

Summary Mortality of wild radio-tagged pheasants was analysed over six breeding seasons (1984–1987 and 1989–1990) in the Revinge area in southernmost Sweden. The aim of the study was to compare morphology and behaviour of birds found dead with those that survived the breeding season (from the first week of April to 30 June). The pheasants were kept over winter in an outdoor enclosure, equipped with radio transmitters, and released during the first week of April. In all, 49 females and 43 males were found dead and the average breeding-season mortality was 25% in males and 26% in females (not including birds that disappeared). The main predators were goshawk Accipiter gentilis and red fox Vulpes vulpes. Mortality among males peaked in late April whereas female mortality was highest in late May. Measurements of morphology included: length of tail, tarsus and wing, body mass and for males, spur length. The only morphological trait that differed between survivors and non-survivors was spur length in males; survivors had longer spurs. This difference was only found when allometric effects of age and body size on spur length were controlled for. Attractiveness and spacing behaviour of non-surviving males were recorded by telemetry from release until the last day of life, and compared with the average performance of survivors in that year. Non-surviving males moved shorter distances between days but did not differ from survivors in calling activity and attractiveness. Calling activity was higher in males that died early in the season compared to those dying late in the season. No differences were found between surviving and non-surviving females.     

Parental investment, potential reproductive rates, and mating system in the strawberry dart-poison frog, Dendrobates pumilio

We studied the effect of relative parental investment on potential reproductive rates (PRRs) to explain sex differences in selectivity and competition in the dart-poison frog Dendrobates pumilio. We recorded the reproductive behavior of this species in a Costa Rican lowland rainforest for almost 6 months. Females spent more time on parental care than males, and `time out' estimates suggest that PRRs of males are much higher than than those of females, rendering females the limiting sex in the mating process. Males defended territories that provide suitable calling sites, space for courtship and oviposition, and prevent interference by competitors. Male mating success was highly variable, from 0 to 12 matings, and was significantly correlated with calling activity and average perch height, but was independent of body size and weight. Estimates of opportunity for sexual selection and variation in male mating success are given. The mating system is polygamous: males and females mated several times with different mates. Females were more selective than males and may sample males between matings. The discrepancy in PRRs between the sexes due to differences in parental investment and the prolonged breeding season is sufficient to explain the observed mating pattern i.e., selective females, high variance in male mating success, and the considerable opportunity for sexual selection. Received: 9 June 1998 / Received in revised form: 27 March 1999 / Accepted: 3 April 1999     

Parental care and the cost of reproduction in a Mediterranean fish

Summary Symphodus tinca is a common near-shore Mediterranean labrid fish in which females may sometimes spawn their eggs over hundreds of square meters, or alternatively spawn into well-defined algal nests. Eggs spawned in either manner are fertilized, but widely scattered eggs receive no parental care, whereas eggs spawned into nests are usually guarded by the male until they hatch. Here, I report weight changes of individual marked fish that engaged in a variety of different reproductive behaviors during three breeding seasons. Males gained weight at 0.15% per day outside the spawning season, and added 29–78% to their overall body weight between reproductive seasons, even following substantive weight losses in a spawning season (up to 20% among nesting males). Nesting and nest-guarding males lost an average of 0.32% and 0.41% of their body weight per day in 1986 and 1987. This cost is four times greater than reproduction for nonnesting males, which registered a 0.03% daily weight gain. Actively spawning females lost 0.06% of their body weight daily during the spawning season. While long-term growth rates did not appear to be substantially affected by reproduction in either sex or by parental care in males, present work does not exclude the possibility that such long-term effects may exist.     

Age-specific filial cannibalism in a paternal mouthbrooding fish

Entire-brood cannibalism by mouthbrooding males of the cardinal fish Apogon doederleini was investigated in temperate waters of southern Japan during two breeding seasons. The rate of cannibalism was 17–18% in each season and did not differ among age-groups. However, the seasonal pattern of cannibalism differed markedly among age-groups: young (1- and 2-year-old) males frequently cannibalized early broods, especially the first brood, of the season, whereas cannibalism by middle-aged (3- and 4-year-old) and old (5- and 6-year-old) males mainly occurred late in the breeding season. We explain this difference in terms of trade-offs between current and future reproduction. Young males, whose future reproductive success is enhanced by the growth increment, may allocate more time and energy to growth by cannibalizing early broods. In contrast, for older males who have had more breeding cycles and grow little, cannibalism could be a way to reverse the deterioration in their somatic condition that occurs as the breeding season progresses. It is also likely that the current reproductive loss entailed by the cannibalism is effectively compensated by quick re-mating with another female. Received: 24 February 1997 / Accepted after revision: 20 July 1997     

The reproductive behavior and energetics of male gray seals (Halichoerus grypus) breeding on a land-fast ice substrate

The reproductive behavior of male gray seals (Halichoerus grypus) breeding on land-fast ice at Amet Island, Nova Scotia, was studied. Data on energy expenditure (rate of mass loss over time) were collected. The average time budget of males at Amet Island was comparable to that of land-breeding males. The behavior of males showed seasonal changes, with a decrease in the proportion of time spent in the water and an increase in agonistic behavior during the peak mating period. The estimated amount of body mass lost over the season ranged between 25.6 and 77.1 kg, and the estimated percent of initial body mass lost ranged between 7.7 and 26.5% (n=10). The maximum number of observed copulations for an individual male was nine. Only 15 out of 42 males observed during 1992 and 1993 were seen copulating. The number of observed copulations per male was strongly correlated with success in remaining close to, or attending, females (r=0.91, P<0.001, n=42). The mean duration of attendance was 4.5 ± 5.54 days (n=42). Large size was not an important factor in determining attendance success, but reproductive effort (the estimated proportion of body mass lost over the season) and success in agonistic interactions with other males were both correlated with male success.     

Group size in wedge-capped capuchin monkeys Cebus olivaceus and the reproductive success of males and females

Summary The effect of variation in group size on age-specific survivorship and fecundity rates were examined in a population of wedge-capped capuchin monkeys Cebus olivaceus during a 10 year study. Life tables were constructed separately for four large (15 individuals) and four small groups (<15 individuals). Female reproductive success, and its relative contribution to population growth, was much higher in large groups, primarily through higher age-specific fecundity. Age-specific survivorship was similar in groups of different sizes. The reproductive success of the single breeding male in a group was much higher in large than small groups. Compared to small groups, breeding males in large groups had a longer breeding tenure, and access to greater numbers of reproductive females with a higher average fecundity. Differences in female reproductive success apparently resulted from variation in access to monopolizable fruit trees. Large groups predictably displaced small groups during intergroup encounters. Group rank depended on the number of males resident in groups. The large number of non-breeding males in large groups results from their longer average residency time. I explain the longer residency of males in large groups by the higher average reproductive success of breeding males in these groups.     

Age,size, dominance and reproduction among male kudus: mating enhancement by attrition of rivals

Summary Size dimorphism with males larger than females has been related to the benefits for males of enhanced dominance and hence reproductive success. However, mating gains must outweigh the fitness costs of deferred reproduction and the mortality associated with further growth. The relationships between male age, size and reproduction were assessed for greater kudus (Tragelaphus strepsiceros) in the Kruger National Park in South Africa. Individually identifiable animals were monitored over 10 years, with detailed observations made during six breeding seasons. In the non-breeding season males formed loose all-male groups. Horn grappling and low intensity agonistic interactions fostered dominance rankings. Dominance was age-graded, until males reached full weight at 6 years of age. Males aged 6 and 7 years monopolized courtship and mating, but 5-year-old males secured about 10% of mating opportunities. Few males survived beyond 7 years. Male mortality rate rose steeply with age, so that the functional sex ratio of fertile females per mature male was about 14:1. During the breeding season many female groups remained unattended by a mature male. Reproductive sorting among males occurred largely through variation in survival to full size and maturity. Increased size enhances fighting success and hence dominance. Further growth ceases when the functional sex ratio exceeds the number of mating opportunities that males can effectively achieve during a breeding season. Predation amplifies the mortality cost of continued growth. In the absence of large predators, male-male interactions may be atypically exaggerated.     

Reproductive strategy and singing activity: blue tit and great tit compared

The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.     

The cost of success: reproductive effort in male southern elephant seals (<Emphasis Type="Italic">Mirounga leonina</Emphasis>)

Reproductive effort is a key parameter of life history because it measures the resources allocated to reproduction at the expense of growth and maintenance. Male reproductive effort always had a minor role with respect to female effort both in the development of theories and in field research. Elephant seals are an ideal subject for reproductive effort studies because they fast during the breeding season, splitting the phase of energy acquisition from the phase of energy use for breeding. In this paper, we present results on male reproductive effort (weight loss estimated by photogrammetry) in southern elephant seals (Mirounga leonina), the most dimorphic and polygynous of all mammal species. We show that total reproductive effort increases with age, with no sign of late decrease or senescence. Male reproductive effort in this species depends mostly on behavioral factors, i.e., the success in competition with other males, and the intensity of interaction with females. A large effort results in large gains in both mating success and fertilizations. The large reproductive success that a few males are able to achieve come at a big cost in terms of energy expenditure, but this cost does not seem to affect the likelihood of survival to the following breeding season.     

The annual reproductive cycles of Uca annulipes,Portunus pelagicus and Metapenaeus affinis (Decapoda: Crustacea) from the South-west coast of India

Employing the gonad index method, the reproductive cycles of three decapod crustaceans, Uca annulipes (Latreille), Portunus pelagicus (Linnaeus) and Metapenaeus affinis (Milne-Edwards) have been studied. In these crustaceans breeding is not continuous all the year round, but extends over several months of the year with distinct peak periods of gonadal activity. The male and female reproductive cycles are not concurrent. The peak of the reproductive cycle of males occurs slightly earlier in the breeding season than that of females. These studies indicate the possibility of production of successive broods of eggs during the same breeding season. In these species, the low saline conditions of the monsoon period are unfavourable for breeding. The medium and high saline conditions during the post-monsoon and pre-monsoon months, respectively, with plenty of planktonic food for the larvae, seem to be the favourable periods for breeding activity.     

Dynamics of space use and male vigour amongst wood mice,Apodemus sylvaticus,in the cereal ecosystem

The spatial organisation of male and female wood mice,Apodemus sylvaticus, was investigated in a large-scale radio-tracking study on arable farmland near Oxford, United Kingdom, during the breeding season. Both sexes had significantly larger home ranges in the breeding season than at other times, and the breeding season home ranges of male (X = 1.44 ha) were significantly larger than those of females (X = 0.49 ha). Home range overlap was significantly greater between males, and between males and females, than it was between females. Overlap between males tended to be greatest in heavily utilised areas. Except during sexual consortship, there was minimal evidence of dynamic interaction among individuals. Home range sizes of breeding males varied widely, as did their body weights. There was no relationship between male body weight and home range size or any other movement parameter. However, males with the largest home ranges had the highest scores on all other movement parameters, indicating that they expended more energy in movement. These more vigorous males had access to the home ranges of more females than did males with small home ranges.     

Reproductive effort in Adélie penguins

Summary We estimated reproductive effort (energy expenditures for reproduction, as opposed to maintenance) in Adélie penguins breeding at Palmer Station, Antarctica. Data on body composition changes and metabolic rate were obtained using isotopic methods. Adelie breeding behavior consists of an initial courtship stage (during which both sexes fast), incubation, the guard stage (when chicks are 1 to 18–28 days old), and the creche stage (from the end of guarding until chicks are 28–45 days old). Both males and females lost considerable mass during the initial stages of the reproductive season, but males fasted longer and lost more mass. Mass losses of both sexes consisted of 66% depot fat and 34% lean tissue. Mass and body composition remained constant once birds resumed feeding. The metabolic expenditure for the foraging necessary to accumulate the mass lost while fasting — one component of reproductive effort —was about 63 MJ in males and 39 MJ in females. Field metabolic rates (FMR) were low during courtship and while incubating, increasing more than 2-fold when birds resumed foraging. Although mean FMR increased between incubation and the creche stage, differences between stages were small and not significant. We used FMR data and an energy balance model to estimate the cost of feeding chicks. Results suggest a maintenance FMR of about 2.7 × basal metabolism (BMR), increasing to 3.4–3.6 × BMR during the creche stage. The reproductive effort (as metabolic expenditures) associated with feeding chicks is 31 MJ (males) to 36 MJ (females). Cumulative reproductive effort is 94 MJ in males and 75 MJ in females, or 5.3–6.2% of the annual energy budget. The reproductive effort devoted to chick care does not appear to be constrained by physiological or time limitations. Instead, selection to reduce the risk of predation may prevent the evolution of increased parental care. Correspondence to: M.A. Chappell     

Seasonal variation in male-female competition,cooperation and selfish hoarding in a monogamous songbird

Both cooperation and conflict between the sexes are commonplace in monogamous mating systems. However, little is known about how cooperation and competition varies seasonally in monogamous species that maintain permanent territories. We presented territorial pairs of male and female New Zealand robins (Petroica australis) with a large supply of insect prey at monthly intervals for 2 years. Behavioural observations after food presentation were then made to quantify seasonal and sexual differences in aggressive interactions over prey, prey acquisition rates, mate provisioning, offspring provisioning, selfish food hoarding and cache retrieval. Data were used to evaluate sex-specific behavioural strategies of mediating competition for food. Results showed that males aggressively excluded females from experimental food sources year-round. Females only accessed food sources when males left them unattended. Consequently, females acquired fewer prey than males. After controlling for differences in prey acquisition, both sexes consumed similar amounts of prey in the non-breeding season. Even though males aggressively excluded females from accessing food sources directly, males fed large amounts of prey to females during the breeding season. Both sexes provisioned young at similar rates. Males cached less prey than females in the breeding season but more prey than females in the non-breeding season. Females showed similar caching intensities year-round. Although males tried to defend their hoards, females frequently retrieved male-made caches. Overall, results showed that although New Zealand robins cooperate to raise offspring during the breeding season, conflict between the sexes occurs year-round. Males and females display different behavioural strategies to gain access to experimental food sources, which appear to lessen male–female competition for food and evenly distribute food resources between the sexes.     

Endocrine influences on parental care during a short breeding season: testosterone and male parental care in Lapland longspurs (Calcarius lapponicus)

In males of socially monogamous birds, plasma testosterone (T) typically declines to low levels during the parental phase. Studies on multiple-brooded species indicate that high T may be incompatible with high-quality paternal care. The length of the breeding season may affect the costs and benefits of high T and its effect on paternal care. We studied the effect of experimentally elevated T on paternal care in a single-brooded species with a short breeding season, the Lapland longspur (Calcarius lapponicus). We monitored T levels and parental behavior in 16 males with subcutaneous T implants, 14 males with empty implants, and 14 unimplanted males. We videotaped nests when nestlings were 2–3 days old and again at 4–5 days. T males with 2- to 3-day-old young visited nests and fed young less often than control males, and the mates of the T males compensated with elevated visits and feedings. However, when nestlings were 4–5 days old, T males visited their nests at normal rates – though feeding movements remained below normal – and T females visited and fed at normal rates. Nestling mass and nest success were similar in both groups. Overall, high T suppresses paternal care in Lapland longspur males. The partial improvement of paternal care when nestlings are older, despite high T, may be related to the short 6-week breeding season of this arctic species, and the consequently reduced benefits of sexual behavior late in the breeding season. Received: 2 February 1998 / Accepted after revision: 2 November 1998     

Determinants of chorus tenure in barking treefrogs (Hyla gratiosa)

In all species of anuran amphibians studied to date, male mating success is positively correlated with male chorus tenure (the number of nights that a male is present in breeding aggregations), yet males spend only a small portion of the total breeding season in choruses. Three hypotheses might explain abbreviated chorus tenure in anuran amphibians: (1) mortality, (2) energy limitations, and (3) movement among choruses. I tested these hypotheses in a study of the barking treefrog (Hyla gratiosa), a species with very abbreviated chorus tenure. Mortality accounted for the short tenures of some males: an estimated 20% of males died while calling in the chorus. Energy limitations were an important determinant of chorus tenure. Males lost weight over successive nights in the chorus, and males with longer chorus tenures were in better condition on their first night in the chorus, lost condition more slowly, and were in poorer condition on their last night in the chorus than were males with shorter chorus tenures. Males that I fed crickets as they left the chorus returned sooner and for more nights than did control (unfed) males. Movement between ponds did not explain abbreviated chorus tenure: less than 16% of the males called at more than one pond, and these males increased their chorus tenures by a median of only 2.5 nights by calling in another chorus. Thus, abbreviated chorus tenure in H. gratiosa appears to be primarily the result of energy limitations, with mortality playing a secondary role.     

Paternity analysis shows experience,not age,enhances mating success in an aquatically mating pinniped,the Weddell seal (<Emphasis Type="Italic">Leptonychotes weddellii</Emphasis>)

For polygynous mammals with no paternal care, the number of offspring sired is often the sole measure of male reproductive success. The potential for polygyny is highest when resources or other environmental factors such as restricted breeding sites force females to aggregate. In these circumstances, males compete intensely for females and mating success may vary greatly among males, further intensifying selection for those traits that confer an advantage in reproduction. Hence, determinants of male success in competition for females are likely to be under strong sexual selection. Paternity analysis was used in conjunction with measures of age, site fidelity, and behavior during the breeding season to assess variance in male breeding success in Weddell seals (Leptonychotes weddellii) breeding at Turtle Rock, McMurdo Sound (77.727S, 166.85E) between 1997 and 2000. Paternity could be assigned to 177 pups at relaxed or 80% confidence level or 111 pups at strict or 95% confidence levels. Weddell seals at Turtle Rock show a modest degree of polygyny with the greatest number of pups sired by any individual male in a single season equalling 5 or ∼10% of the pups born. Over four consecutive years, most (89.2%) males sired at least one pup. In a generalized linear model (GLM), age and the age first seen at the study site as an adult were unrelated to mating success, but adult experience, either site-specific or elsewhere in McMurdo Sound, over the reproductive life span of males explained nearly 40% of variance in total mating success with 80% confidence and 24% of variance at 95% confidence. While learning where females are likely to be may enhance male reproductive success, aquatic mating reduces the ability of males to monopolize females, and thereby increases equity in mating success.     

Temporary alteration of local social structure in a threatened population of Cuban iguanas (Cyclura nubila)

In small, insular populations, behavioral patterns that lead to increased variance in individual reproductive success can accelerate loss of genetic variation. Over a 1-year period, we documented behavior and hormone levels in a breeding group of adult Cuban iguanas (Cyclura nubila) at Guantánamo Bay. Male dominance was associated with body and head size, display behavior, testosterone levels, home-range size, and proximity to females. Based on their success in agonistic encounters, we ranked males in a linear dominance hierarchy. During the subsequent breeding season, we conducted a removal experiment in which the five highest-ranking males were temporarily relocated from the study site. Although we were unable to assess reproductive success directly, previously lower-ranking males assumed control of vacated territories, won more fights, and increased their proximity to females in the absence of the dominant males. When it results in greater mating opportunities for otherwise socially suppressed individuals, temporary alteration of local social structure may help limit erosion of genetic variation in small, insular populations. Electronic Publication