Anthropogenic Activity Influences Animal Biology with the Potential to Produce Reproductive Isolation: A Review
By: Krystal-Amber Rivera
Abstract
In an era of rapidly increasing global change, one prominent factor that can affect animal biology is that of urbanization. With urbanization comes anthropogenic stressors such as environmental contamination, noise pollution, light pollution, and habitat destruction. But even with these stressors, some species survive and even thrive. Anthropogenic activity has been shown to influence animal endocrinology so much that within a few generations individuals among a species adapt and actually can become reproductively isolated. Because few studies have focused on this topic of reproductive isolation due to urbanization, this review attempts to further highlight some instances where researchers could have extended their research to understand how urbanization can encourage reproductive isolation in various organisms. This review also attempts to underline how reproductive isolation, and other factors that result from anthropogenic activity, are not necessarily harmful to all species. I classify anthropogenic stressors into four groups: environmental contamination, noise pollution, light pollution, and habitat destruction. The goal of this review is to encourage further research that will enhance comprehensive knowledge about how humans influence animal endocrinology and behavior. Additionally, it hopes to promote more research on how urbanization contributes to reproductive isolation and possible speciation of organisms. Broader implications of this knowledge will yield a deeper understanding of speciation as a result of urbanization.
Introduction
Recently, much research has focused on the effects of urbanization on the behaviors and physiological processes of organisms. Results yielded show anthropogenic stressors, such as environmental contamination, noise pollution, light pollution, and habitat destruction, are a result of urbanization and have a negative impact on animal behavior and physiological processes (Table 1). Experiments on birds (the most-studied group) indicate the negative effects of anthropogenic noise on avian endocrinology as it relates to reproduction, stress, cognition, and behavior (particularly social, mating, territorial, aggressive, feeding, and parental care behavior). In an experiment by Slabbekoorn & Peet (2003), for instance, anthropogenic noise in the city of Leiden was discovered to negatively affect breeding opportunities and contribute to a decline in Great tit (Parus major) species density and diversity (Slabbekoorn & Peet 2003). Noise-amplitude measurements, taken with a sound-pressure meter, varied evidently between territories and among male great tits. Noisy territories were discovered to be home to great tit males whose songs had a high average minimum frequency, while quiet territories were home to great tit males whose songs reached the lowest frequencies measured for the population. Because of urbanization, physiological processes in the great tits were altered resulting in distinct behaviors. Urbanization, for instance, not only reduced the ability of great tits to conduct song, courting, and mating effectively, but it also reduced breeding opportunities as less great tits reproduced with one another. Importantly, birds are not the only species limited to these results. Other species, such as bats (Chiroptera), whales (Orcinus orca), dolphins (Tursiops), hermit crabs (Paguroidea), and frogs (Anura) are also negatively influenced by noise pollution caused by increased urbanization (Jones 2008, Tyack et al 2003, Wollerman & Wiley 2002).
Similarly, research with marine animals, such as sea turtles (Chelonioidea), reveal the negative effects of anthropogenic light pollution on species’ activity times (eg nocturnal, crepuscular, diurnal), navigation, maturation, reproduction, and nesting (Witherington 1992). Street lights in urban cities were discovered to repeatedly disorient adult loggerhead (Caretta caretta) and green turtle (Chelonia mydas) sea turtles during seaward migration. Through experiments in which a portion of a nesting beach remained dark, or was illuminated with white, mercury vapor (MV) or yellow, low pressure sodium vapor (LPS) luminaires of equal luminance, it was shown that loggerhead and green turtle nesting behavior were influenced by the amount of light that was artificially presented. This revealed alterations in physiological processes that controlled their nesting behavior as a result of forced behavior alterations. Some turtles were misdirected by lighted luminaires (primarily MV) following seaward migration after nesting attempts, which increased chance of predation and decreased survival rate of both adults and offspring (Witherington 1992). Turtles are not the only species limited to these results. Fish (Antigonia), birds (Aves), and moths (Apamea) are also negatively influenced by light pollution caused by increased urbanization (Davies et al 2014, Stracey et al 2014, van Geffen et al 2014).
Despite the decades of research on the negative effects of urbanization on animals, few experiments have extended their research to understand how urbanization has encouraged reproductive isolation, the inability of a species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers or differences. In other words, urbanization as the unique contribution of a major source of reproductive isolation has often been overlooked. This is important to note because urbanization is becoming one of the major factors that contribute to the inability of a species to breed successfully with related species. I have suggested that Zoologist Ernst Mayr’s classification of the mechanisms of reproductive isolation that fall in two broad categories, those that act before fertilization (pre-zygotic), such as temporal, behavioral, mechanical, and gametic isolation, and those that act after (post-zygotic), such as zygote mortality and hybrid sterility, can be further extended to reveal that urbanization can be a kind of pre-zygotic mechanism of reproductive isolation (Nosil 2008). In other words, urbanization can act as an isolating barrier that prevents fertilization. This means that individuals might mate, but because of urbanization their gametes never form an embryo. Thus, individuals never get to the mating part because of temporal (inability to reproduce due to mating seasons being different times of the year), mechanical (incompatibility of sexual organs), behavioral (mating ritual behaviors differ), and gametic (sperm and egg are not compatible) isolation as a result of urbanization and human influence (Table 2). In fact, urbanization is becoming the major contributor to the geographical, behavioral, physiological, and genetic barriers that result in reproductive isolation of organisms.
Of the few experiments, one experiment by Mockford and Marshall (2009) most clearly shows how urbanization encourages reproductive isolation in various organisms. After studying great tits (Parus major), their results revealed how male great tits respond more strongly to songs from territories with noise levels similar to their own. This suggests how urbanization is influencing reproductive isolation between urban and rural great tit species as a difference in the behavioral plasticity of both rural and urban birds develop. As urban male great tits continue to seek territories that optimize perception of their song, they become more and more distant from those rural great tits and in turn become more dissimilar from the rural species as well. This dispersal shows the possibility of reproductive isolation among the species as more great tits are dispersing to areas with different background noise levels from that which they have experienced previously. Eventually, with continuous noise pollution, the great tits from each environment (urban versus rural) will never get to mate because of temporal, mechanical, behavioral, and gametic isolation. The great tits will even be at a competitive disadvantage in territorial disputes whenever they interact with one another mainly because their vocal communication and the hormones that produce them become more and more distinct. A weaker response to songs may allow neighboring males to intrude on a new arrival’s territory. Alternatively, a reduced response to intruders may decrease the frequency of territorial disputes and permit larger, or overlapping, territories, which could encourage population divergence and ultimately speciation (Mockford & Marshall 2009).
Correspondingly, in an experiment by Wandeler et al., (2003) done in the city of Zurich, Switzerland, the red fox (Vulpes vulpes) experiences genetic differentiation between the urban and rural populations. The significant genetic differentiation between the urban and rural fox populations that was observed could have been caused by different mechanisms. Reproductive isolation may have evolved rapidly due to divergent selection systems and adaptation. However, little is known about how quickly reproductive isolation may evolve in more recent habitats (Wandeler et al 2003). Because little is known about reproductive isolation as a result of urbanization, the majority of experiments on reproductive isolation have concealed the reality that not all organisms are negatively influenced by anthropogenic activities. However, despite this, there are indeed many organisms, especially those isolated within urban environments that are in fact synanthropes, which benefit from anthropogenic activities and its factors.
This review, in turn, attempts to further highlight a few of the many instances where researchers could have extended their research to understand how urbanization has encouraged reproductive isolation in various organisms. It also attempts to underline how reproductive isolation and other anthropogenic factors influence animals, and are not necessarily harmful to all species. For the sake of simplification, anthropogenic stressors will be classified into four groups: environmental contamination, noise pollution, light pollution, and habitat destruction. This organization of anthropogenic stressors will help guide the review by suggesting the natural experiments that have examined such pollution and how these influence animals. In addition, this organization will help direct how each anthropogenic stressor, as a result of urbanization, could encourage reproductive isolation and influence endocrinology alterations in various organisms.
I hope to encourage further research that will enhance comprehensive knowledge about how humans play a role on the reproductive isolation and possible speciation of organisms. Additionally, I hope to promote research on the positive effects of anthropogenic activities — not to disregard the fact that anthropogenic stressors are becoming increasingly harmful to organisms, but to suggest areas in need of future research, such as distinguishing between those organisms that thrive and those that suffer as a result of anthropogenic activities in urban cities. Accordingly, conversationalist can channel their energy to those organisms that are most vulnerable in order to contribute to the study of urban ecosystems and their effects on organisms, which can more broadly contribute to a better understanding of evolution. Furthermore, this review can more specifically encourage research on urban versus rural organisms that can help biologists understand the possibilities of rural and urban organisms becoming reproductively isolated and creating new species as a result of anthropogenic activity from urban environments. This will provide a deeper understanding of whether speciation, as a result of urbanization, is beneficial or harmful and how we can further encourage or discourage it.
Noise pollution
Noise pollution has been revealed to pose serious risks to many organisms. In a recent experiment, Lengagne (2008) investigated the impact of traffic noise on acoustic communication in European tree frogs (Hyla arborea) through an experimental approach using noise playback. Through his investigation, he revealed how traffic noise in Lyone, France, triggered a decrease in male frog calling activity, with males being more affected when noise amplitude was increased, as a result of higher urbanization. In response to noise playback, males were not able to adjust their temporal or frequency call structures to increase efficiency of the information transfer. Thus, communication between the male frogs was harmed. Males were only weakly affected by noise pollution when calling in a chorus situation as opposed to alone. This was probably because males calling in chorus versus alone employed a strong influence on sensitiveness to noise. In other words, because chorus calling covered traffic noise, it weakly affected the tree frogs as a group. However, when the frogs made calls on their own, traffic noise greatly affected them individually. This could have been a result of each frog’s call being masked by the loud traffic (Lengagne 2008).
Lengagne reveals how urbanization influences European tree frogs as it relates to behavior (particularly social behavior). He manages to investigate the ways that tree frogs react to noise pollution. What he does not elucidate, however, is what noise pollution does specifically to tree frogs that change their hormones to make them behave differently. For example, it is possible that noise pollution decreased the nurohypophyseal hormone arginine vasotocin (AVT) in European tree frogs. AVT is an amphibian hormone related to mammalian hormones oxytocin and vasopressin that has been suggested to affect mating behaviors such as mating calls. In male frogs, AVT influences the transmission of the call, while in female frogs the response to calls is brought on by AVT. Thus, it is most likely that the decrease in European male tree frog calling activity and their inability to adjust their temporal or frequency call structures to increase efficiency of the information transfer in Legagne’s experiment could have been a result of decreased AVT due to excessive noise pollution. This would explain how male and female frogs miscommunicate as a result of noise pollution.
This is further supported by Mahoney’s experiment on a similar species, grey tree frogs (Hyla versicolor) and their hormonal regulation of calling performance. In the experiment, AVT was injected into grey tree frogs and recorded. The calling rates of the AVT injected frogs and the control frogs were then compared revealing that AVT significantly increased the calling rates in frogs sixty and ninety minutes after injection. This increased both the amount of time males spent transmitting the calls and the amount of time females spent locating the source of the calls, affecting the chances that mating occurred (Mahoney 1995). Thus, Mahoney’s experiment explains why Legagne’s European tree frogs behaved as they did when presented with noise playback. She presents the experiment with further research to be conducted and reveals how hormones, like AVT, can regulate calling and how such hormones can be altered as a result of noise pollution, influencing the social, mating, reproduction, and stress behaviors of frogs.
In this way, urban frogs’ calling could become increasingly distinct from rural frogs’ calling, as a result of loud urban traffic noise, discouraging mating and reproduction. Loud traffic could, in addition, eventually create stress and a habituation among the frogs that could harm the species by encouraging a reduction in aggressive callings that could help individuals secure themselves and their mates. It is possible that these amphibians could become reproductively isolated if environmental noise pollution continues to be emitted into other environments. With tree frogs losing communication and attraction to one another as a result of the excessive noise and hormonal alterations, the species could eventually diverge. Those species closest to one another could become one distinct species from those farther away because they would be the only ones to understand their particular behaviors, such as mating calls, considering that auditory signals travel long distances, but as they travel through forests, they degrade and attenuate.
In another experiment, by Williams, et al., (2006), killer whales (Orcinus orca) in Johnstone Strait, British Columbia, Canada, reduced time spent feeding and the time spent rubbing their bodies on smooth pebble-beaches (for comfort) in the presence of noisy boats over a six year period. These lost feeding opportunities and short-term (uncomfortable) behavioral responses as a result of masking effects of boat noise, and interruption of feeding periods or replacement of feeding activity with boat-avoidance activities, were believed to decrease whale energy intake, which could have long-term population effects if the population were food-limited (Williams et al 2006). Williams et al. reveals how urbanization influences killer whales as it relates to behavior (particularly feeding behavior). He investigates the ways that killer whales react to noise pollution over many years. However, he fails to expose what noise pollution does specifically to killer whales that change their hormones to make them behave differently.
As the vessel impact hypothesis argues, high numbers of vessels in close proximity to the whales cause disturbance via psychological stress and/or weakened foraging ability (Ayres et al 2012). With this in mind, it is interesting to note the possibility of noise pollution influencing specific hormones such as the glucocorticoid hormone (GC), which regulates the metabolism of glucose and immunology and rises in response to nutritional and psychological stress, and the thyroid hormone (T3), which is responsible for the regulation of metabolism and declines in response to nutritional stress, but does not diverge in response to psychological stress (Charmandari, Evangelia, et al 2005). This may explain the change in feeding behavior that Williams et al. reveals as a result of noisy ships and boats. It is very likely that as a result of noise pollution coming from ships, killer whales experience much stress, which raises their glucocorticoid levels. Similarly a lack of nutrition, as a result of an undesire to eat because of stress, raises glucocorticoid levels, but decreases thyroid levels. Thus, the whales lose energy, which could potentially be physically and behaviorally harmful.
This is further supported by Ayres et, al., (2012), and her experiment on endangered southern resident killer whales of British Columbia, Canada and Washington, U.S.A. In the experiment, a combination of fecal thyroid (T3) and glucocorticoid (GC) hormone was measured to assess the effects of boat noise and human activity on the killer whales ‘stress levels and behavior. The GC and T3 measures supported the inadequate prey hypothesis in which human activity has restricted prey availability to whales, which has influenced their stress levels and behaviors dramatically. In particular, GC concentrations in killer whales increased with a decline in their dominant prey, Chinook salmon (Oncorhynchus tshawyscha), and an incline in vessels, as a result of human interference. Whereas, T3 concentrations decreased with such a decline in prey availability. (Ayres et al 2012).
In this way, killer whales could become increasingly distinct. It is likely that these aquatic mammals may become reproductively isolated if environmental noise pollution and growing urbanization continues to be emitted into aquatic environments. As Williams et al. fail to address, constant reduced energy and time spent feeding as a result of loud boats could affect the way specific whales behave, which may alienate them from other whales that do not experience boat noise and intrusion. Thus, whether the whales adapt and evolve with the noise pollution or not defines whether the whales will become reproductively isolated or die off. Knowing this, future research should consider extending on the knowledge gained from Williams et al. to better understand how urbanization encourages reproductive isolation in whales.
Light pollution
Light pollution has also been revealed to pose serious risks to many organisms. In a natural experiment performed at Arlington County, by Miller (2006), proliferation of artificial nocturnal light was believed to strongly affect singing behavior of American Robins (Turdus migratorius) at a population level. Results revealed robin populations in areas with large amounts of artificial light frequently to begin their morning chorus during the night, which is unusual because wild Robins typically begin their chorus during twilight. These results suggest that as the human population and urbanization increases, so does the level of artificial light in the region, and that artificial nocturnal light has now reached a level at which it strongly affects robin singing behavior. As Miller suggests, loud noise could potentially be another variable affecting robin singing behavior, as this appears to increase their alertness and avoidance behaviors. Differences in food availability among lit sites might also influence song initiation times of American Robins. Thus, both light and noise pollution could harm robins if energy demands or predation risk are preeminent (Miller 2006).
Through his experiment, Miller reveals how urbanization influences American robins as it relates to behavior (particularly social and feeding behavior). He manages to investigate the ways that robins react to light and noise pollution by recording song initiation times. What he does not explicate, however, is what light and noise pollution do specifically to robins that change their hormones to make them behave differently. For example, it is possible that light pollution can influence testosterone levels enough to change avian seasonal cycles, reproductive physiology, and overall avian behavior, like the distinct behavior revealed by Miller. This is further supported by Dominoni et, al. (2013), and their experiment on free-living European blackbirds (Turdus merula). After measuring light intensity at night in an urban environment, Dominoni et al., revealed birds exposed to the most light at night developed moulted earlier and even developed their reproductive system up to one month earlier than birds kept under darker nights. Despite Dominoni having done his research on a different species of bird from Miller, results remained very similar in that avian behavior and endocrinology between the population are changing slowly, but drastically, as a result of light pollution caused by urbanization (Dominoni et al 2013).
It is likely that Miller’s experiment could have been further extended to understand how light and noise pollution could influence reproductive isolation in American Robins. With more brightly lit areas, robins waste more energy staying awake. In addition, they use more energy keeping away from predators that may also be awake as a result of brightly lit areas that imitate daylight hours. Alternatively, light pollution might increase robin productivity if robins can forage or seek mates during longer periods each day. Either way, these changes in behavior, as a result of light and noise pollution, can encourage reproductive isolation in robins. It is possible that, both light and noise pollution could influence urban robins so much that rural robins will not be able to communicate with them sufficiently. For example, if rural and urban robins were placed in the same environment after several years, they might not mate with one another because of the distinct behaviors that they had gained as a result of their environment. In other words, because urban robins become accustom to singing at night and eating at night because if previously lit cities, their habits will not coincide with these rural robins whose signing and eating patterns are done at a different time (during the day). Human noise and light would basically act as allopatric barrier for rural and urban robins and their social, mating, territorial, aggressive, feeding, and parental care behaviors. Additionally, with birds exposed to light at night developing their reproductive system up to one month earlier, and also moulting earlier than birds kept under dark nights, it is likely that only the birds that behave this way at these times will be able to reproduce over a few generations as a result of temporal isolation. Thus, mechanical and gametic isolation will eventually follow leading to possible speciation.
In another experiment, by Stone, et al. (2009), bats (Chiroptera) were examined to demonstrate how light pollution may have significant negative impacts upon the selection of flight routes by bats. Making them ideal subjects for testing the effects of light pollution considering that they are nocturnal, Stone et al., revealed how bat activity was reduced dramatically and the inception of commuting behavior was delayed in the presence of lighting, with no evidence of habituation (Stone et al 2009). Through his experiment, Stone reveals how urbanization influences bats as it relates to behavior (particularly cognition and feeding behavior). They manage to investigate the ways that bats react to light and noise pollution by monitoring their flight routes. What they do not expand on, however, is what light and noise pollution do specifically to bats that change their hormones to make them behave differently. For example, it is possible that light pollution can negatively influence growth and cognition in bats, which explains reduced bat activity when exposed to light.
This is further supported by Boldogh et, al. (2007), and their experiment on bats and the prolong duration of emergence and growth as a result of excess light. Boldogh et, al. reveals how bright lighting not only prolongs the surfacing of bats and negatively influences their cognition, but also stunts their growth when they are juveniles. While studying bats in illuminated and non-illuminated buildings, Boldogh et al., finds that juveniles are considerably smaller in illuminated buildings. As Boldogh et, al., reveals, when living in illuminated buildings, juvenile bats grow shorter forearms and have a smaller body mass, indicating that light pollution influences the way that juvenile bats grow and how they are stunted by too much illumination. (Boldogh et al 2007). It is likely that Stone et al. and their experiment could have been further extended to understand how light and noise pollution could influence reproductive isolation in bats. The fact that bats exposed to city lights and bats in the wild act so distinct, there is a chance that they may not encounter each other as they take different routes, or they may not be attracted to one another because of behavioral or phenotypic differences. Thus reproduction between only similar behaving bats will occur and reproductive isolation will be exposed.
Environmental contamination
Environmental pollutants, such as metals, pesticides, and other organics, have been revealed to pose serious risks to many organisms. In an experiment by Smith and Weis (1997), where Mummichog (Fundulus heteroclitus) predatory behavior were videotaped in minnow traps from both Piles Creek and an unpolluted reference site near Tuckerton on the Great Bay estuary north of Atlantic City, New Jersey, exposure of uncontaminated fish to conditions similar to those of polluted creeks caused both a reduction in prey capture rate and an increase in brain mercury to levels similar to those of fish natural to polluted creeks. Exposure to toxins basically eradicated the performance of behaviors, such as predator avoidance, that are essential to fitness and survival of Mummichogs in natural ecosystems. In this way, polluted fish maintained in the laboratory for extended periods failed to show either an increase in prey capture rate or a decrease in their levels of brain mercury. Additionally, fish from the polluted environment suffered significantly greater mortality in the presence of a predator, the blue crab (Callinectes sapidus), than fish from the unpolluted environment. This revealed possible disruption of sensory, hormonal, neurological, and metabolic systems as a result of environmental pollutants from urban cities (Smith & Weis 1997).
Through the experiment, Smith and Weis reveal how urbanization influences Mummichog fish as it relates to behavior (particularly cognition and feeding behavior). They managed to investigate the ways that the fish react to pollution by monitoring their swimming routes and prey capturing. However, it would have been more informative to know about the actual alteration of mechanisms altered by the pollution. For example, perhaps pollution in the water affected activation of the thyroid hormones of the Mummichog. The behavioral dysfunction of the fish may thus have been associated with thyroid impairment due to endocrine disruption. This inclination is further supported by Zhou, et al. (2000). In their study, they compared thyroid histology and thyroid hormones in the two populations and determined experimentally whether the polluted environment could alter thyroid hormone levels. Fish in contaminated water had greater follicle cell heights, larger thyroid follicles, and contained higher plasma thyroxine (T4) levels than fish in uncontaminated water. However, there were no significant differences in either plasma or tissue triodothyronine (T3). Fish in contaminated water held in simulated uncontaminated environments had higher plasma T4 and lower plasma T3 than field-sampled fish. Fish held in clean water had lower plasma T4 and T3 than field-sampled fish. Moreover, there was no significant difference in tissue T3 content (Zhou et al 2000).
It is likely that these fish may become reproductively isolated if environmental pollutants continue to be emitted into aquatic environments. As Smith and Weis (1997) fail to address, excessive pollutants could alter the lateral line system, or mechanoreceptors/neuromats that provide hydrodynamic information about water velocity, water pressure, acceleration, body position, and movement. Thus, environmental pollutants may act as a barrier to species of fish, discouraging many from mating and from finding food, which could lead to reproductive isolation. Knowing this, future research should consider extending on the knowledge gained from Smith and Weis to better understand how urbanization encourages reproductive isolation.
In another experiment, Gomot (1998) demonstrated the effects of pollutants, such as Cd2+, on reproduction and development in L. stagnalisa snails. This highly toxic metal was revealed to stunt various stages of reproduction (number of egg masses, number of eggs, embryo development, and hatching) in snails, including egg production and hatching. The experiment revealed that embryo development was the most sensitive stage depending on the Cd2+concentration. At the highest concentration studied, the eggs were blocked in the first cleavage stage, whereas during the next highest concentration of pollutant, development of the eggs was ceased at various stages of embryogenesis (pre-hatching, veliger, cleavage, and gastrula) depending on their position in the egg masses. At the least concentration of pollutant, growth and development was reduced and hatching occurred 5 to 15 days later than in the controls (controls hatched 12 to 13 days after laying) (Gomot 1998).
It is likely that these snails may become reproductively isolated if environmental pollutants continue to be emitted into their environments. As Gomot fails to suggest, constant prevention of egg hatchling, and the members of two different species from producing offspring, promote reproductive isolation between species that are not contaminated and species that are. Therefore, if one of these contaminated snails were allowed to reproduce with one other, their biology will be so different as a result of the varied amounts of chemicals in their bodies that they will not be able to reproduce together at all. Only those from the same environment may be able to reproduce together as they would gain similar evolutionary changes
Habitat destruction
Habitat destruction poses serious risks to many organisms. In an experiment by McLellan and Shackleton (1988) urbanization yielded both beneficial and harmful effects on bears. As McLellan and Shackleton reveal, habitats near roads may have been relatively safe for vulnerable classes of grizzly bears (Ursus arctos horribilis). Females with cubs generally avoiding adult grizzly males who sometimes kill cubs and yearling, for example, benefit from habitats near roads since adult males used habitats near roads less, especially during daylight (McLellan & Shackleton 1988). Through their experiment, McLellan and Shackleton reveal however how urbanization influences the biology of bears as it relates to behavior (particularly social and territorial behavior). They essentially explain how different the bears act, particularly in reference to their feeding behaviors, when in comparison to those in more natural habitats. Hamilton (2007) supports how unusual bears act in urban environments and how their hormones are altered through one of his experiments that measure cortisol levels in Canadian grizzly bears. Results reveal how anthropogenic activity, such as habitat alterations, extraction, and destruction, influences bears and raise their stress levels (Hamilton 2007).
It is likely that these bears may become reproductively isolated if habitats are further altered. The reason for this notion is that as more and more bears are segregated amongst urban and rural areas; the more they become accustomed to such environments and have a change in their behavior and physiology. Over many years, this change may be so large that the bears will become reproductively isolated and perhaps form a new species of bears that follows the optimal foraging theory (Stephens and Krebs 1986). This theory assumes natural selection favors feeding behavior that maximizes fitness. Thus, those bears that survive the best, near roads and urban cities as a result of excess food and protection from other bear species, will potentially have greater fitness.
In another experiment, by Symons (1974), territorial behaviors of young Atlantic salmon (Salmo salar) were revealed to decrease as a result human-made environmental alterations thereby increasing their vulnerability to predation. As Symons describes, forest spraying with insecticides can alter the food eaten by young salmon and sometimes reduces its quantity, thus increasing the chances that a young salmon will desert its territory (Symons 1974). Through his experiment, Symons reveals how urbanization influences the endocrinology of salmon as it relates to behavior (particularly social and territorial behavior). What he does not say, however, is the fact that by influencing the social and territorial behavior of salmon, urbanization also influences stress, aggressive, and feeding behavior. It is likely that salmon may become reproductively isolated if habitat destruction continues to occur. The more salmon that become vulnerable to predation, the greater the chance there is for that species to either die out or be selected to resist predation. If these “urban” salmon evolve, these will become better fit than “rural” salmon and could eventually replace them or become reproductively isolated from them.
Conclusion
The fastest evidence of reproductive isolation in the wild reported so far — and controversially discussed — was believed to have occurred after 13 generations in introduced salmon/sockeye salmon (Oncorhynchus nerka) (Hendry et al 2000). This discovery led to the conclusion that the observed differentiation is a consequence of adaptation in divergent habitats. In this way, my review expands on the notion that forces of urbanization should be more highly considered when examining the mechanisms of reproductive isolation. However organisms adapt to urban cities, whether it is by evolving to select different mating location, mating time, or mating rituals, this adaption ultimately alters animal behavior, which promotes internal barriers to gene flow, and thus, speciation.
Through a review of the change in the endocrinology of animals in their natural environments, as a result of anthropogenic stressors, such as environmental contamination, noise pollution, light pollution, and habitat destruction. Here, I bring attention to how temporal and behavioral isolation, and courtship rituals (breeding calls, mating dances), as affected by the urban environment, need to be further studied to elucidate how humans play a role in the barriers that maintain the integrity of a species over time, reducing or directly impeding gene flow between individuals of different species, and allowing the conservation of each species’ characteristics. In this way, future researchers can better understand the various ways that urbanization influences animal endocrinology, reproduction, stress, cognition, and behavior. Instead of accepting the notion that all anthropogenic stressors are harmful, scientists are to look at this notion in a new light. Human influence may be promoting the evolution of certain characteristics. Whether this phenomenon is harmful or beneficial is another topic to examine. Accordingly, conversationalists can channel their conservation energy to those organisms that are most vulnerable.
Experiments on Anthropogenic Stressors and Animal Biology
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