I spent my morning going to presentations in the Hormones, Brain and Behavior section, which is the area that I studied for my doctoral degree before I switched fields to evolution and phylogenetics. Some of the presentations I saw included;
- Hau. Evidence from studies in temperate-zone male vertebrates has accumulated that the endocrine regulation of aggressive territorial behavior differs seasonally. During the breeding season a combination of androgenic and estrogenic mechanisms appear to regulate male aggressive behavior, while during the non-breeding season either only estrogenic or non-sex steroidal mechanisms control male aggression. However, research on non-migratory neotropical spotted antbirds, Hylophylax n. naevioides, shows that this is not necessarily always true. This species holds and defends a multi-purpose territory together with a long-term partner, suggesting that testosterone could be involved in regulating territorial aggression at all times of year. Hau experimentally tested this notion of year round territorial aggression in both male and female free-living spotted antbirds. These birds responded to simulated territorial intrusion (STIs) with a live male decoy by displaying aggressive behavior of similar quality and quantity during the non-breeding season, as well as in the early and the middle portions of the breeding season. Circulating concentrations of testosterone after STIs in both sexes were low and were seasonally constant, but both sexes had increased plasma dehydroepiandrosterone (DHEA) concentrations during the non-breeding season. When an androgen receptor “blocking agent”, or antagonist (Flutamide), was given to these birds and an aromatase inhibitor (ATD, which prevents the production of estrogens, another potential source of aggressive behavior) during the non-breeding season did not reduce aggressive behavior in captive spotted antbirds, while in a previous study a similar treatment during the breeding season did reduce aggressive behavior. These data reveal that aggressive behavior can be regulated by different endocrine mechanisms in different seasons, even in sedentary species that remain on the same territory with the same partner for many years. The specific endocrine mechanisms that regulate aggressive territorial behavior in spotted antbirds during the non-breeding season remain to be determined, but may include low concentrations of estrogens.
- Hahn. Crossbills are thought to be capable of reproducing at any time of the year, or being capable of maintaining a reproductive state indefinitely, when their food supply permits them to do so. However, at least the smallest North American form of the Crossbill, type III, displays a fundamentally seasonal reproductive schedule, with autumn gonadal regression even if food supply remains at the late summer annual peak. However, it has been unclear whether this reproductive seasonality is unique to this highly specialized form or if it is a general feature of all crossbills types. Hahn presented evidence from several other crossbill forms that indicate that the autumn collapse of the gonads along with the coincident occurrence of the annual plumage molt, is a general feature of all North American crossbills. Hahn showed that the gonads of free-living birds regress during September or October even when the food supply remains high enough to support subsequent winter breeding. This suggests autumn refractoriness is subject to environmental cues. Experiments have already ruled out absolute photorefractoriness typical of other temperate zone songbirds, leaving relative photorefractoriness or some kind of food refractoriness as possible explanations for the decline in reproductive competence in autumn. Viewed in a comparative context, this is highly unusual and likely represents an adaptive specialization to facilitate a flexible breeding schedule. Comparative experiments will help clarify the extent to which crossbills’ reproductive patterns are the result of evolutionary specializations, or of conditionally flexible responses of phylogenetically conserved cue response traits.
- Perfito. Zebra finches, Taeniopygia guttata, are thought to maintain their reproductive systems in a near-ready state so they can respond quickly to environmental conditions favorable for breeding. This strategy differs markedly from that used by obligately photoperiodic species in which changing photoperiod is used as a cue to modulate gonadotropin-releasing hormone (GnRH) release and thus reproductive activation on a seasonal basis. The recent discovery of another neuropeptide, gonadotropin inhibitory hormone (GnIH), and its subsequent characterization in mammals, provides a completely new avenue for investigating reproductive systems. GnIH acts in opposition to GnRH in birds, inhibiting gonadotropin synthesis and release by the anterior pituitary gland in vitro and in vivo. Perfito’s study characterized the immunoreactivity of GnIH and GnRH in zebra finches captured in the field under two very different environmental and breeding conditions: one from a seasonal population during springtime breeding conditions and another from a population in a more unpredictable habitat during non-breeding conditions and while experiencing drought. Based on previous findings, Perfito hypothesized that ir-GnRH cell number and size would be similar between populations, but that ir-GnIH cells would be larger and more numerous in the non-breeding drought population. The differential regulation of these two important reproductive hormones might help to explain the physiological underpinnings of opportunism.
- Strand. In adult male songbirds, increases in circulating testosterone (T) levels and singing behavior induce the growth of specific brain regions that control singing behavior (song control regions; SCRs). However, in some species, this growth occurs early in the breeding season, before significant changes in singing behavior occur and before maximal T levels are reached. Furthermore, birds in captivity have shown maximal growth of one SCR, the HVC, after one week of T treatment. The growth of the HVC is partially due to an increase in neuron number. Strand investigated the timing of the effect of T on the growth of the SCRs and singing behavior in adult male House Finches, Carpodacus mexicanusand found that the effects of T on SCR growth independently of increases in singing behavior, which may help to identify the mechanisms of T alone on SCR growth.
- Addis. Previous work has shown that birds breeding in northern temperate regions adjust production of testosterone in response to the stage of the breeding cycle and in some cases following social interactions. In contrast, studies on tropical breeding birds have suggested they regulate testosterone production in different ways. For example, the rufous-collared sparrow, Zonotrichia capensis, in Ecuador shows a pattern of plasma testosterone levels that are different from its northern congeners and does not increase testosterone, but does exhibit aggressive behavior, in response to simulated territorial intrusions (STI). Building on previous work, Addis investigated whether season has an effect on the social modulation of testosterone and behavior in two populations of rufous-collared sparrows in Costa Rica. Because pairs in both of these populations of birds in Costa Rica breed asynchronously, Addis chose to investigate social modulation of testosterone and behavior at three distinct times of year: the dry season, late March, the transition to the rainy season, early May, and the rainy season, early July. Addis conducted simulated territorial intrusions and collected baseline samples during these three periods. Season had a significant effect on testosterone levels and there was an interaction between season and the simulated territoria intrusion treatment, but no increase of testosterone following STI. Addis explores implications for control of breeding seasons in the tropics.
- Davis. The Puget Sound white-crowned sparrow, Zonotrichia l. pugetensis, has a historic breeding distribution limited to the Pacific coast of Oregon and Washington. In contrast, Gambel’s white-crowned sparrows, Z. l. gambelii, breed predominantly in Alaska and northern Canada south to several high altitude sites in the northern Cascade Mountains of Washington State. Previous studies have shown that gambelii at northern and southern parts of their range express a highly modulated stress response during the breeding season, such that subsequent corticosterone increase is higher during the territory and mate selection, and lower during parental care. This may be an adaptation to the short-breeding season and high cost/benefit ratio associated with extreme environments, allowing flexibility in coping behavior early in the breeding season while making birds relatively resistant to stress-induced reduction in parental behaviors. Historic accounts have shown little or no overlap in the breeding territories of gambelii and pugetensis. However, climate change and the human disturbance of previously forested areas appear to have provided an opportunity for pugetensis to colonize alpine habitats, thus bringing these two subspecies into contact. This range expansion into alpine habitat also presents pugetensis with new challenges, including extremes of weather, limited food sources, and a shortened optimal breeding period. In addition, several of these recent high-altitude populations display gambelii-like morphological and behavioral traits, indicating potential hybridization between the two subspecies.
- O’Brien. Gonadotropin inhibitory hormone (GnIH) and Kisspeptin (KiSS) are two recently discovered hypothalamic neuropeptides found in a variety of vertebrates. In white-crowned sparrows, Zonotrichia leucophrys (WCSP), GnIH rapidly inhibits plasma LH in castrates, as well as in intact, gonadotropin-releasing hormone-treated birds, and in free-living WCSP with naturally high plasma LH. In contrast, KiSS dose-dependently increases plasma LH in captive, cannulated WCSP. GnIH shows seasonal changes in neuronal attributes implying changes in storage and expression of this peptide. Thus, we have evidence for potential roles for GnIH and KiSS in the timing of breeding. Subspecies of WCSP exhibit different lengths of breeding seasons. Z. l. gambelii are arctic breeding birds that must precisely time the onset and cessation of breeding in order to maximize fitness in harsh arctic environments while Z. l. nuttalli are non-migrating sparrows that enjoy a longer breeding season on the coastal chaparral of northern California. Z. l. pugetensis is intermediate in breeding season length, and is expanding its range from coastal to high-altitude mountain sites. GnIH and KiSS peptides may play an important part in the regulation of these variable breeding seasons seen within these subspecies. Furthermore, studying subspecies with different breeding season lengths may illuminate the different ecological factors and reproductive mechanisms that influence seasonal breeding.