Territorial aggression and hormones during the non-breeding season in a tropical bird

The hormonal control of territorial aggression in male and female vertebrates outside the breeding season is still unresolved. Most vertebrates have regressed gonads when not breeding and do not secrete high levels of sex steroids. However, recent studies implicate estrogens in the regulation of non-breeding territoriality in some bird species. One possible source of steroids during the non-breeding season could be the adrenal glands that are known to produce sex steroid precursors such as dehydroepiandrosterone (DHEA). We studied tropical, year-round territorial spotted antbirds (Hylophylax n. naevioides) and asked (1). whether both males and females are aggressive in the non-breeding season and (2). whether DHEA is detectable in the plasma at that time. We conducted simulated territorial intrusions (STIs) with live decoys to male and female free-living spotted antbirds in central Panama. Non-breeding males and females displayed robust aggressive responses to STIs, and responded more intensely to decoys of their own sex. In both sexes, plasma DHEA concentrations were detectable and higher than levels of testosterone (T) and 17beta-estradiol (E(2)). In males, plasma DHEA concentrations were positively correlated with STI duration. Next, we conducted STIs in captive non-breeding birds. Captive males and females displayed robust aggressive behavior. Plasma DHEA concentrations were detectable in both sexes, whereas T was non-detectable (E(2) was not measured). Plasma DHEA concentrations of males were positively correlated with aggressive vocalizations and appeared to increase with longer STI durations. We conclude that male and female spotted antbirds can produce DHEA during the non-breeding season and DHEA may serve as a precursor of sex steroids for the regulation of year-round territorial behavior in both sexes.     

Spring and autumn territoriality in song sparrows: same behavior, different mechanisms?

Vertebrates show a diverse array of social behaviors associatedwith territoriality. Field and laboratory experiments indicatethat underlying themes—including mechanisms—mayexist. For example in birds, extensive evidence over many decadeshas implicated a role for testosterone in the activation ofterritorial aggression in reproductive contexts. Territorialityat other times of the year appeared to be independent of gonadalhormone control. One obvious question is—why this diversityof control mechanisms for an apparently similar behavior? Controlof testosterone secretion during the breeding season must balancethe need to compete with other males (that tends to increasetestosterone secretion), and the need to provide parental care(that requires lower testosterone concentrations). Regulationof aggressive behaviors by testosterone in the non-breedingseason may incur substantial costs. A series of experimentson the male song sparrow, Melospiza melodia morphna, of westernWashington State have revealed possible mechanisms to avoidthese costs. Song sparrows are sedentary and defend territoriesin both breeding and non-breeding seasons. Dominance interactions,territorial aggression and song during the non-breeding seasonare essentially identical to those during the breeding season.Although in the non-breeding season plasma testosterone andestradiol levels are very low, treatment with an aromatase inhibitordecreases aggression and simultaneous implantation of estradiolrestores territorial behavior. These data suggest that the mechanismby which testosterone regulates territorial behavior at theneural level remains intact throughout the year. How the hormonalmessage to activate such behavior gets to the brain in differentseason does, however, appear to be different.     

Territorial aggression and song of male European robins (Erithacus rubecula) in autumn and spring: effects of antiandrogen treatment

Male robins aggressively defending a feeding territory in winter have low levels of testosterone, while males defending a breeding territory in spring have elevated levels of testosterone. Song is an integrated part of territorial defense during both phases. We investigated whether testosterone is involved in the expression of these behaviors by treating free-living and captive male robins during both phases with the antiandrogen flutamide. Results suggest that, similar to species in which territoriality is restricted to the reproductive phase, aggressive defense of a breeding territory by male robins is facilitated by androgens. Territorial defense during the nonbreeding season, however, does not require androgenic activity. Singing frequency, on the other hand, was not significantly reduced during either phase by flutamide application. Since the quality of male song changes with season we suggest that the incorporation of sexual signals into male song during breeding depends on testosterone, but that the year-round production of territorial "keep out" signals is independent of testosterone.     

Pair territoriality in the longnose filefish,oxymonacanthus longirostris

The longnose filefish,Oxymonacanthus longirostris, usually lives in heterosexual pairs, the male and female swimming together and sharing the same territory. Pair territoriality in the species was examined in detail in relation to sexual differences in territorial defense activities. Rigorous pair territoriality was maintained only during the breeding season, although pairs used their home ranges exclusively to a certain extent, during the non-breeding season. The frequency of aggression against other conspecific pairs in the breeding season was higher than in the non-breeding season. Agonistic interactions appear to be over both mates and food resources, the strict pair territoriality in the breeding season possibly being due to mutual mate guarding. In intraspecific aggressive interactions, males usually led their partner females when attacking intruders. The feeding frequency of males was much lower than that of females in the breeding season. Mate removal experiments indicated that females could not defend their original territories solitarily and their feeding frequency decreased. Conversely, males could defend territories solitarily without a decrease in feeding frequency. These results suggest that males contribute most to the defense of the pair territory, with females benefiting from territorial pair-swimming with their partner males.     

Oestrogen regulates male aggression in the non-breeding season

Extensive research has focused on territorial aggression during the breeding season and the roles of circulating testosterone (T) and its conversion to 17beta-oestradiol (E2) in the brain. However, many species also defend territories in the non-breeding season, when circulating T-levels are low. The endocrine control of non-breeding territoriality is poorly understood. The male song sparrow of Washington State is highly territorial year-round, but plasma T is basal in the non-breeding season (autumn and winter). Castration has no effect on aggression in autumn, suggesting that autumnal territoriality is independent of gonadal hormones. However, non-gonadal sex steroids may regulate winter territoriality (e.g. oestrogen synthesis by brain aromatase). In this field experiment, we treated wild non-breeding male song sparrows with a specific aromatase inhibitor (fadrozole, FAD) using micro-osmotic pumps. FAD greatly reduced several aggressive behaviours. The effects of FAD were reversed by E2 replacement. Treatment did not affect body condition or plasma corticosterone, suggesting that all subjects were healthy These data indicate that E2 regulates male aggression in the non-breeding season and challenge the common belief that aggression in the non-breeding season is independent of sex steroids. More generally, these results raise fundamental questions about how sexual and/or aggressive behaviours are maintained in a variety of model vertebrate species despite low circulating levels of sex steroids or despite castration. Such non-classical endocrine mechanisms may be common among vertebrates and play an important role in the regulation of behaviour.     

Territorial aggression does not feed back on testosterone in a multiple-brooded songbird species with breeding and non-breeding season territoriality,the European stonechat

Testosterone mediates reproductive behaviours in male vertebrates. For example, breeding season territoriality depends on testosterone in many species of birds and in some, territorial interactions feed back on testosterone concentrations. However, the degree to which territorial behaviour and testosterone are associated differs even between species with seemingly similar life histories, especially between species that also defend territories outside the breeding season. Here, we investigate the link between territorial behaviour and testosterone in European stonechats. Previous studies found that territorial aggression in stonechats depends on testosterone in a breeding, but not in a non-breeding context. We investigated whether stonechats show a rise in testosterone during simulated territorial intrusions (STI) during the breeding season. Post-capture testosterone concentrations of males caught after an STI were not higher than those of males caught in a control situation regardless of breeding stage. However, most of the males would have been able to mount a testosterone response because the same individuals that did not increase testosterone during the STI showed a substantial increase in testosterone after injections of gonadotropin-releasing hormone (GnRH). GnRH-induced and post-capture testosterone concentrations were positively correlated and both decreased with successive breeding stages. Further, territory owners with a short latency to attack the decoy expressed higher post-capture testosterone concentrations than males with a longer latency to attack the decoy. Thus, there is no evidence for behavioural feedback on testosterone concentrations during male-male interactions in stonechats. In combination with previous studies our data suggest that testosterone functions as an on/off switch of high intensity territorial aggression during the breeding season in stonechats. The among-species variation in the androgen control of territorial behaviour may be only partly a result of environmental differences. Instead, potential differences in how territoriality evolved in different species may have influenced whether and how a reproductive hormone such as testosterone was co-opted into the mechanistic control of territorial behaviour.     

Steroid hormone interrelationships with territorial aggression in an Arctic-breeding songbird,Gambel's white-crowned sparrow,Zonotrichia leucophrys gambelii

The breeding season is very brief for arctic-breeding passerines, and any interruptions of parental care by aggressive interactions over territory may reduce reproductive success. We tested both the "testosterone insensitivity" and "corticosterone insensitivity" hypotheses in the arctic-breeding Gambel's white-crowned sparrow, Zonotrichia leucophrys gambelii. Additionally, we tested whether simulated territorial intrusions (STIs), known to stimulate increases in luteinizing hormone (LH) and testosterone (T) in mid-latitude breeding Z. l. pugetensis, would also be effective in either the early or late phases of the brief breeding season of Z. l. gambelii. Plasma levels of T and LH were high early in the breeding season and declined as egg laying began. Exposure of free-living males to 10 min of STI significantly increased LH but not T secretion. Nonetheless, the pituitary-gonadal axis is sensitive as jugular injection of gonadotrophin-releasing hormone increased plasma T at 10 min relative to saline-challenged controls. T implants failed to increase territorial aggression following STI during incubation. These data are consistent with the T insensitivity hypothesis and contrast sharply with the response of the southerly breeding subspecies, Z. l. pugetensis, in which the territorial response to T administration is retained throughout its relatively long breeding season. However, corticosterone implants during the incubation period decreased territorial aggression during STI. This responsiveness to corticosterone is not consistent with the corticosterone insensitivity hypothesis of stress modulation. Z. l. gambelii retain sensitivity to corticosterone levels that may occur naturally in response to environmental perturbations resulting in suppression of territorial behavior.     

Non-migratory stonechats show seasonal changes in the hormonal regulation of non-seasonal territorial aggression

In many birds and mammals, male territorial aggression is modulated by elevated circulating concentrations of the steroid hormone testosterone (T) during the breeding season. However, many species are territorial also during the non-breeding season, when plasma T levels are basal. The endocrine control of non-breeding territorial aggression differs considerably between species, and previous studies on wintering birds suggest differences between migratory and resident species. We investigated the endocrine modulation of territorial aggression during the breeding and non-breeding season in a resident population of European stonechats (Saxicola torquata rubicola). We recorded the aggressive response to a simulated territorial intrusion in spring and winter. Then, we compared the territorial aggression between seasons and in an experiment in which we blocked the androgenic and estrogenic action of T. We found no difference in the aggressive response between the breeding and the non-breeding season. However, similarly to what is found in migratory stonechats, the hormonal treatment decreased aggressive behaviors in resident males in the breeding season, whereas no effects were recorded in the non-breeding season. When we compared the aggressive responses of untreated birds with those obtained from migratory populations in a previous study, we found that territorial aggression of resident males was lower than that of migratory males during the breeding season. Our results show that in a resident population of stonechats T and/or its metabolites control territorial aggression in the breeding but not in the non-breeding season. In addition, our study supports the hypothesis that migratory status does modulate the intensity of aggressive behavior.     

The influence of testosterone on territorial defence and parental behavior in male free-living rufous whistlers, Pachycephala rufiventris

We studied a population of rufous whistlers, Pachycephala rufiventris, throughout a single breeding season in central New South Wales, Australia. We evaluated the relation between plasma testosterone (T) and reproductive behaviors using both simulated territorial intrusions (STIs) and subcutaneous T implants. We compared circulating T values to aggression levels of males (using STI) during pair bond and territory establishment and again during incubation. Although plasma T levels were significantly lower in the latter period, male responsiveness to STI, in terms of proximity to decoy, call rate, and number of attacks on the decoy, was indistinguishable between the two breeding stages. T levels of males exposed to STI were not different from the levels of unexposed free-living males at the same breeding stage. The effect of exogenous T on parental behavior was examined by comparing duration of incubation bouts of males and their mates prior to and after T treatment. T males significantly reduced the amount of time they incubated following implantation, whereas Control males maintained their incubation effort. After cessation of breeding activities, T males displayed significantly higher call rates due to increased use of the primary intersexual advertisement call in this species. The reduction of incubation behavior following T implantation emphasises the functional significance of the rapid decline in T in free-living males during incubation. The results from both experiments suggest that intersexual advertisement, rather than territorial aggression, may be dependent on high T levels in this species.     

Territoriality and testosterone in an equatorial population of rufous-collared sparrows, Zonotrichia capensis

Territorial aggression, displayed by male vertebrates in a reproductive context, is generally thought to be mediated by testosterone. The challenge hypothesis predicts that in socially monogamous species, territorial challenges should induce an increase in plasma testosterone concentrations, which will enhance aggressive behaviour and territory defence. This hypothesis is based on northern latitude birds and needs to be tested in tropical birds before it can be universally accepted. We tested the challenge hypothesis in an equatorial population of rufous-collared sparrows in Papallacta, Ecuador. This population shows an extended breeding period during which males aggressively guard territories. During the early breeding season, males were challenged with conspecific or heterospecific simulated territorial intrusions (STIs) lasting 10 min. Conspecific-challenged males responded more aggressively than heterospecific-challenged males. However, there was no increase in plasma testosterone in response to the conspecific STI. During the breeding season, males were challenged with conspecific STIs lasting 0, 10 or 30 min. Males behaved aggressively regardless of STI duration, and did not differ in plasma testosterone or luteinizing hormone concentrations. During the breeding season, males were implanted with testosterone-filled or empty silastic tubes and subsequently challenged with a conspecific STI. Testosterone implants significantly raised plasma testosterone concentrations, but testosterone-implanted males were not more aggressive than blank-implanted controls. Combined, these findings suggest that testosterone concentrations above breeding baseline are not related to territorial aggression in this population and therefore do not support the challenge hypothesis.     

Acute and chronic effects of an aromatase inhibitor on territorial aggression in breeding and nonbreeding male song sparrows

Many studies have demonstrated that male aggression is regulated by testosterone. The conversion of testosterone to estradiol by brain aromatase is also known to regulate male aggression in the breeding season. Male song sparrows (Melospiza melodia morphna) are territorial not only in the breeding season, but also in the nonbreeding season, when plasma testosterone and estradiol levels are basal. Castration has no effect on nonbreeding aggression. In contrast, chronic (10 day) aromatase inhibitor (fadrozole) treatment decreases nonbreeding aggression, indicating a role for estrogens. Here, we show that acute (1 day) fadrozole treatment decreases nonbreeding territoriality, suggesting relatively rapid estrogen effects. In spring, fadrozole decreases brain aromatase activity, but acute and chronic fadrozole treatments do not significantly decrease aggression, although trends for some behaviors approach significance. In gonadally intact birds, fadrozole may be less effective at reducing aggression in the spring. This might occur because fadrozole causes a large increase in plasma testosterone in intact breeding males. Alternatively, estradiol may be more important for territoriality in winter than spring. We hypothesize that sex steroids regulate male aggression in spring and winter, but the endocrine mechanisms vary seasonally.     

Testosterone Affects Song Modulation during Simulated Territorial Intrusions in Male Black Redstarts (Phoenicurus ochruros)

Although it has been suggested that testosterone plays an important role in resource allocation for competitive behavior, details of the interplay between testosterone, territorial aggression and signal plasticity are largely unknown. Therefore, we investigated if testosterone acts specifically on signals that communicate the motivation or ability of individuals to engage in competitive situations in a natural context. We studied the black redstart, a territorial songbird species, during two different life-cycle stages, the early breeding phase in spring and the non-breeding phase in fall. Male territory holders were implanted with the androgen receptor blocker flutamide (Flut) and the aromatase inhibitor letrozole (Let) to inhibit the action of testosterone and its estrogenic metabolites. Controls received a placebo treatment. Three days after implantation birds were challenged with a simulated territorial intrusion (STI). Song was recorded before, during and after the challenge. In spring, both treatment groups increased the number of elements sung in parts of their song in response to the STI. However, Flut/Let-implanted males reacted to the STI with a decreased maximum acoustic frequency of one song part, while placebo-implanted males did not. Instead, placebo-implanted males sang the atonal part of their song with a broader frequency range. Furthermore, placebo-, but not Flut/Let-implanted males, sang shorter songs with shorter pauses between parts in the STIs. During simulated intrusions in fall, when testosterone levels are naturally low in this species, males of both treatment groups sang similar to Flut/Let-implanted males during breeding. The results suggest that song sung during a territorial encounter is of higher competitive value than song sung in an undisturbed situation and may, therefore, convey information about the motivation or quality of the territory holder. We conclude that testosterone facilitates context-dependent changes in song structures that may be honest signals of male quality in black redstarts.     

Temporal Patterns of Territorial Behavior and Circulating Testosterone in the Lapland Longspur and Other Arctic Passerines

We studied territorial aggression in relation to circulatingtestosterone levels in free-living birds of four species innorthern Alaska. The Lapland longspur, Calcarius lapponicus,is an abundant breeding passerine on the arctic tundra. Unlikemany passerines at lower latitudes, male Lapland longspurs donot defend a "multiple-purpose territory" that serves to providenest sites, food and shelter. Rather, after arrival on the breedinggrounds, they perform aerial display flights over a looselydenned "nest area" for a very brief period of two days or so,showing tolerance of other males. This song display may be involvedin courtship. During this phase, male longspurs show a briefand pronounced peak in circulating testosterone levels, andare not aggressive toward simulated territorial intrusions (STIs).Males then "guard" their sexually receptive mates for aboutten days, during which they are highly aggressive toward STIs,but do not sing as much. During the next phase, incubation,the males become very tolerant of conspecific males. Their circulatingtestosterone levels decline to baseline levels, and they generallydo not sing or display aggression in response to STIs. Threeother passerines, the white-crowned sparrow, Zonotrichia leucophrysgambelii, American tree sparrow, Spizella arborea, and savannahsparrow, Passerculus sandwichensis, show patterns of territorialaggression typical of species studied at lower latitudes. Welldefinedterritories are defended for several weeks, during which thereis a prolonged peak in plasma concentrations of testosterone.These three species continue to sing and display aggressioneven late in the season, unlike the longspurs. The peak of testosteronein the longspurs occurs simultaneously with the peak in songdisplay, while in mid-latitude species it occurs with the peakin reproductive aggression. These data suggest that the interrelationshipof testosterone and aggression in Lapland longspurs may be differentfrom that of passerines with multiple-purpose territories, andmay be related to the constraints of breeding in the open arctictundra.     

Seasonal variation in male testosterone levels in a tropical bird with year‐round territoriality

Testosterone is important in mediating investment in competing activities such as territoriality, parental care, and maintenance behavior. Most studies of testosterone function have focused on temperate species and less is known about the role of testosterone in territoriality or variation in mating systems of tropical species. Results of studies of tropical species with year‐round territoriality indicate that territorial aggression during the non‐breeding season is maintained with low levels of testosterone, and increased levels of testosterone in males during the breeding season may increase mating opportunities or aid in competition for mates. We studied seasonal variation in testosterone levels of male Red‐throated Ant‐tanagers (Habia fuscicauda), a socially monogamous species with year‐round territoriality and with high levels of extra‐pair matings (41% of young), to determine if testosterone levels increased during the breeding season. We captured males during the non‐breeding and breeding seasons and collected blood samples for hormone analysis. We found that mean testosterone concentrations were low during the non‐breeding season (0.18 ± 0.05 [SD] ng/ml, range = 0.11–0.31 ng/ml), and significantly higher during the breeding season (2.37 ± 2.47 ng/ml, range = 0.14–6.28 ng/ml). Testosterone levels of breeding males were not related to aggression levels as measured by attack rates toward a stuffed decoy or singing rates during simulated territorial intrusions. These results suggest that the higher testosterone levels of breeding male Red‐throated Ant‐tanagers may be important in an extra‐pair mating context, possibly in display behavior or mate attraction, but additional study is needed to clarify the role of testosterone during the breeding season.     

Testosterone stimulates reproductive behavior during autumn in mockingbirds (Mimus polyglottos)

Mockingbirds normally secrete little or no testosterone during the period of autumnal territoriality. To determine the behavioral effects of exogenously administered testosterone, 20-mm lengths of Silastic tubing filled with crystalline testosterone were implanted into free-living resident mockingbirds during the autumn. Control residents were given sealed empty implants. Focal animal sampling showed that T-implanted males sang significantly more than controls. Perhaps as a consequence, a significantly greater percentage of the T-implanted males acquired mates. Though nest building does not naturally occur in autumn, T-implanted males also showed significantly more nest building than control males. However, T-implanted males only built if there was a female in the territory, suggesting a synergy between the presence of testosterone and social cues provided by the female. Examination of the effects of testosterone on territorial aggression showed that despite the high levels of territorial activity common in this species in autumn, territorial fights were unaffected by the presence of testosterone. One aggressive call, known to function in fall territorial defense, was significantly decreased in T-implanted versus control males. The presence of fall testosterone appears to stimulate a number of reproductive activities in mockingbirds, leaving autumnal aggressive interactions either unchanged or decreased. We discuss the application of these data to the effects of testosterone on the mockingbird's reproductive behavior during the breeding season.     

Simulating winning in the wild--the behavioral and hormonal response of black redstarts to single and repeated territorial challenges of high and low intensity

In many vertebrates testosterone increases during aggressive interactions and the surges in this hormone may be responsible for the winner effect. So far studies on this relationship have been done in captivity only, because simulating a winning situation for a territory owner in the field is difficult. However, an increasing number of studies show that territorial aggression is not necessarily accompanied by elevated testosterone after a single simulated territorial intrusion (STI) and therefore it has been proposed that STIs may even create a losing experience. We examined whether free-living male black redstarts (Phoenicurus ochruros) show changes in androgens, corticosterone and behavior following repeated STIs of high or low intensity and in contrast to being challenged only once. Repeated intrusions had no influence on androgen and corticosterone levels regardless of intrusion intensity. In contrast, the behavioral response changed over days depending on the intensity of the intrusion. Only birds challenged with high-level intruders approached the decoy significantly faster during the third intrusion than during the first one, stayed closer to the decoy, and sang more songs than males challenged with low-level intruders. Thus, although black redstarts reacted differently to STIs varying in frequency and intensity, these behavioral differences were not reflected in androgen or corticosterone levels. Our data show that it is unlikely that STIs induce a losing experience. Furthermore, they indicate that a hormonal effect of winning an encounter may not be universal in vertebrates and may depend on the ecological or life-history context.     

Simulating winning in the wild — The behavioral and hormonal response of black redstarts to single and repeated territorial challenges of high and low intensity

In many vertebrates testosterone increases during aggressive interactions and the surges in this hormone may be responsible for the winner effect. So far studies on this relationship have been done in captivity only, because simulating a winning situation for a territory owner in the field is difficult. However, an increasing number of studies show that territorial aggression is not necessarily accompanied by elevated testosterone after a single simulated territorial intrusion (STI) and therefore it has been proposed that STIs may even create a losing experience. We examined whether free-living male black redstarts (Phoenicurus ochruros) show changes in androgens, corticosterone and behavior following repeated STIs of high or low intensity and in contrast to being challenged only once. Repeated intrusions had no influence on androgen and corticosterone levels regardless of intrusion intensity. In contrast, the behavioral response changed over days depending on the intensity of the intrusion. Only birds challenged with high-level intruders approached the decoy significantly faster during the third intrusion than during the first one, stayed closer to the decoy, and sang more songs than males challenged with low-level intruders. Thus, although black redstarts reacted differently to STIs varying in frequency and intensity, these behavioral differences were not reflected in androgen or corticosterone levels. Our data show that it is unlikely that STIs induce a losing experience. Furthermore, they indicate that a hormonal effect of winning an encounter may not be universal in vertebrates and may depend on the ecological or life-history context.     

Effects of corticosterone on territorial behavior of free-living male song sparrows Melospiza melodia

A group of 10 territorial male song sparrows, Melospiza melodia, were given subcutaneous implants of corticosterone in Silastic tubing. A second group of 10 territorial males were given empty implants as controls. After 18-24 hr all males were then subjected to a simulated territorial intrusion (STI) by placing a caged male song sparrow in the center of the subject's territory, and playing tape recorded songs through a speaker placed alongside. Significantly fewer males with corticosterone implants responded to STI than to controls, and the latency to respond was longer. Of the 3 experimental males that did respond to STI, all had a lower frequency of songs and did not approach the simulated intruder as closely as controls. Many males were captured 2-7 days after implantation and blood samples collected for measurement of circulating hormone levels. As expected, plasma levels of corticosterone were high in the group given corticosterone implants. However, plasma levels of luteinizing hormone (LH) were not affected by treatment with corticosterone, and although circulating levels of testosterone were depressed slightly compared with controls, they were within the normal range for territorial and breeding males. There were no differences in body mass despite greatly increased fat depots in males treated with corticosterone. These data suggest that high levels of corticosterone, similar to those measured during stressful episodes both in the laboratory and field, may suppress territorial behavior independently of the adenohypophysial-gonad axis. Since plasma levels of LH and testosterone are not depressed markedly, thus maintaining the gonads in a near functional state, renesting can begin as soon as environmental conditions ameliorate. Such mechanisms could potentially increase the probability of raising viable young after unpredictable, severe weather resulted in failure of the previous breeding attempt.     

Testosterone in tropical birds: effects of environmental and social factors

Previous investigations suggest that male tropical birds have lower plasma testosterone concentrations than northern latitude species. To test whether this generalization is valid, we analyzed all currently available plasma testosterone data of tropical birds. We focused on peak breeding testosterone levels using phylogenetic and conventional statistics. Explanatory variables considered were social mating system, type of territoriality, breeding season length, and altitude. On average, tropical birds had lower mean peak testosterone levels than northern temperate birds. However, in several tropical species, testosterone levels were well within the range of northern latitude birds. Without controlling for phylogeny, breeding season length, type of territoriality, and altitude explained a significant proportion of the variance in testosterone levels. The shorter the breeding season, the higher the testosterone levels. Tropical birds that defend a breeding season territory had higher testosterone levels than birds that were year-round territorial or colonial, and testosterone levels were positively correlated with altitude. When controlling for phylogeny, only breeding season length predicted testosterone levels. In conclusion, we propose to refine previous notions of low plasma testosterone levels in tropical birds: short breeding seasons and perhaps environmental conditions at high altitudes precipitate conditions under which high testosterone levels are beneficial in the tropics.     

The Behavioral Ecology of Libellula luctuosa (Burmeister) (Anisoptera: Libellulidae): I. Temporal Changes in the Population Density and the Effects on Male Territorial Behavior

A single population of a common pond dragonfly, Libellula luctuosa, was studied at a site where the density of males increased dramatically during the breeding season. Early in the summer one active male was found on each territory on the pond. Satellite males were only occasionally found on the territories. Later in the season the number of males per territory increased so that two or more males simultaneously defended on many of the territories, and several satellite males occupied each of the territories. The number and rate of female visitations per day did not change over the summer. These factors resulted in a change in the operational sex ratio with variations in male density. Male behavior was also altered with increasing population density. As male density increased, males were less likely to be seen perching on their territories and more likely to be seen performing aggressive acts such as chasing nearby territorial males and chasing intruders. At high male density, the duration of territorial behaviors was shorter than at low male density. Thus, the percent of a time budget spent in any one activity did not change despite the change in number of males present. Male activity in L. luctuosa is not strictly determined by the opportunity for aggression. Costs of aggression associated with territoriality are minimized by maintaining flexible territorial behaviors.