mammals

Polar bears’ fasting period increases as sea ice continues to melt

polar bear. Image by Elizabeth Labunski. Retrieved from https://digitalmedia.fws.gov/digital/collection/natdiglib/id/2761/rec/31

Polar bears (Ursus maritimus) have been experiencing the effects of climate change particularly hard.  Polar bears rely on spring feeding in order to build up fat reserves for the summer-fall fasting period, and as seal pups are birthed, it provides ample opportunity for polar bears to do so.  However, as the global temperature continues to increase, the abundance of sea ice declines.  While polar bears are strong swimmers, they rely on sea ice to use as platforms while they hunt seals.  Without the sea ice, their ability to effectively and efficiently hunt is diminished.  This results in polar bears having less fat storage going into the summer-fall fasting period in which food sources become even more scarce as the bears lose access to marine animals.  Polar bears don’t den during this time, so this period is often referred to as “walking hibernation”.  As the sea ice returns in the winter months, polar bears are once again able to access marine animals for food.  

 

Polar bears have shown increasing signs of fasting over the years.  Between 1985 and 2006, the percent of polar bears in a fasting state in April grew over 300% (Cherry et al., 2009).  The spring months are a time in which polar bears should be feasting to increase fat storages for the coming fasting period.  120-day fasts are typical for male and non-pregnant female polar bears during these summer-fall months (Robbins et al., 2012).  However, this period is predicted to increase to 180 days as temperatures continue to rise.  Over a 180-day fasting period, adult males experience a 28% mortality rate, increased from 3% during a 120-day fast.  

 

This increase in fasting time is even more concerning for pregnant females.  Since pregnant female polar bears den on land during the winter, they will have to go up to 8 months without food.  During the summer-fall fasting period, daily mass loss, energy expenses, and loss of lean mass is much higher than in hibernating bears.  By increasing this fasting period due to climate change, polar bears will go into the winter with less mass than usual. Since heavier females are more likely to produce larger cubs, and thus increase the probability of cub survival, pregnant females want to go into winter with as much fat storage as possible.  It is estimated that pregnant females would need more than 34% body fat leading into the summer-fall fasting period in order to successfully reproduce during the winter (Robbins et al., 2012).

polar bear with cub. Image by Scott Schliebe. Retrieved from https://digitalmedia.fws.gov/digital/collection/natdiglib/id/10931/rec/5

 

As temperatures continue to increase and sea ice continues to melt, the future for polar bears is tenuous.  For pregnant females in particular, an abundant spring is of extreme importance for survival of the summer-fall fasting.  As this fasting period increases due to sea ice melting earlier in the season, more and more polar bears won’t make it through to the winter.  Additionally, pregnant females won’t be able to make it through the fasting with enough fat reserves left to successfully reproduce.  Populations will continue to decrease as reproduction rates fall, making it of the utmost importance to ensure enough sea ice for an abundant spring feast. 

 

 

 

References:

Cherry, S.G., Derocher, A.E., Stirling, I. et al. Fasting physiology of polar bears in relation to environmental change and breeding behavior in the Beaufort Sea. Polar Biol 32, 383–391 (2009). https://doi.org/10.1007/s00300-008-0530-0

Labunski, Elizabeth 2008, Polar bear, U.S. Fish and Wildlife Service, accessed Feb 12, 2024, <https://digitalmedia.fws.gov/digital/collection/natdiglib/id/2761/rec/31>

Robbins, C. T., Lopez-Alfaro, C., Rode, K. D., Tøien, Ø., & Nelson, O. L. (2012). Hibernation and seasonal fasting in bears: The energetic costs and consequences for polar bears. Journal of Mammalogy, 93(6), 1493–1503. https://doi.org/10.1644/11-mamm-a-406.1

Schliebe, Scott 2010, Polar bear with cub, U.S. Fish and Wildlife Service, accessed Feb 12, 2024, <https://digitalmedia.fws.gov/digital/collection/natdiglib/id/10931/rec/5>
Wiig Ø, Aars J, Born EW. Effects of Climate Change on Polar Bears. Science Progress. 2008;91(2):151-173. doi:10.3184/003685008X324506

Problems of Gene Flow for Grey Wolf Populations on Isle Royale National Park

In Isle Royale National Park (Michigan), there is an established population of both gray wolves and moose.  It is believed that the wolves arrived on Isle Royale sometime during the 1940s when an ice bridge formed, allowing for wolves to cross over the water to the island.  Ever since the wolves arrived, they have been an important predator on the island, helping to control the local moose population.  The wolves do this by preying on ill or injured moose.  Recently, the wolves have faced the threat of extinction in Isle Royale National Park due to inbreeding (National Park Service, 2022).

There have been fluctuations in the wolf and moose populations in Isle Royale for many years. Some factors include the available amount of food resources and weather changes.  However, a common pattern that has been seen is that when the wolf populations spike, the moose populations decrease and vice versa.  This can be seen in the following graph below.  From 2011 to 2014, the wolves had reached a population size that was unsustainable.  Around 2011, the wolf population decreased, and the moose population increased. (National Park Service, 2021).

(National Park Service, 2021)

If there is low genetic variation within a population, then a species risks extinction, therefore lowering its fitness.  The wolves on Isle Royale are facing this problem, likely due to climate change not allowing for the ice bridges to form anymore.  With no ice bridges forming, the amount of gene flow that occurs in the populations has decreased.  For example, in 1997, a wolf known as M93 crossed the ice bridge to Isle Royale, creating gene flow within the population.  By 2008, 59.4 % of the wolf population descended from wolf M93.  Once this ice bridge was no longer forming, the gene flow in the population decreased, causing increases in inbreeding.  By 2012, there were only 9 wolves left, and 5 of them, through genetic testing, were found to be full siblings (Hedrick, 2014).

To save the wolf populations, a technique that can be used is called genetic rescue.  Genetic rescue allows members from outside the struggling population to be introduced either naturally or by humans to add more genetic variation into the population (Hedrick, 2014).  In June 2018, the National Park Service decided to do this to help control the moose populations.  If the wolves are not present on Isle Royale, the moose population will negatively impact the island’s vegetation.  The plan is for the National Park Service to introduce a total of 20 to 30 wolves onto the island (National Park Service, 2018).

Citations

Hedrick PW, Peterson RO, Vucetich LM, Adams JR, Vucetich JA et al. (2014) Genetic rescue in Isle Royale wolves: genetic analysis and the collapse of the population. Conservation Genetics 15: 1111–1121. https://link.springer.com/article/10.1007/s10592-014-0604-1

National Park Service. 2018 Press Release National Park Service Releases Record of Decision to Introduce Wolves at Isle Royale National Park. Version 2018.6. https://www.nps.gov/isro/learn/news/press-release-national-park-service-releases-record-of-decision-to-introduce-wolves-at-isle-royale-national-park.htm. (date last accessed 12 February 2024).

National Park Service. 2021 Wolf & Moose Populations. Version 2021.5. https://www.nps.gov/isro/learn/nature/wolf-moose-populations.htm.  (date last accessed 12 February 2024).

National Park Service. 2022 Wolves. Version 2022.1. https://www.nps.gov/isro/learn/nature/wolves.htm.  (date last accessed 12 February 2024).

Big City Bats

An LED Floodlight, Image by Leon Brooks. Retrieved from https://pixnio.com/objects/electronics-devices/electric-lights-pictures/stadium-lights

Throughout the world, more and more people are living in urban areas. We’re called by the glimmering lights, the whirlwind of activity, the sights and sounds of the big city. Some of these same factors that draw people to cities are what drive other animals away–or, in many cases, cause harm to the animals who stay.

For bats, who are most active at night, urban light pollution can pose some major issues (Laforge et al., 2019; Langley, 2019; Seewagen and Adams, 2021). Artificial light can be disorienting for nocturnal animals, disrupting their circadian cycles and changing their typical activity levels throughout the day (Seewagen and Adams, 2021). Further, high levels of light leave species that are used to the cover of darkness vulnerable to predators (Cravens and Boyles, 2018; Laforge et al., 2019). Still, some bats–individuals or entire species–choose to spend more time foraging in lighter areas because UV lights attract their preferred food choice: insects (Cravens and Boyles, 2018; Langley, 2019; Seewagen and Adams, 2021).

So how does the presence of light impact the bats who choose to stay?

Seewagen and Adams (2021) found that several species of bats reacted to the presence of LED floodlights by greatly reducing their foraging activity. Migratory tree bats tended to not be impacted by light or even be attracted to it, while nonmigratory species tended to avoid light areas (Seewagen and Adams, 2021). Little brown bats (Myotis lucifugus) have been found to have difficulty avoiding obstacles in highly-illuminated areas, indicating that light may have some impact on their sensory systems, including systems involved in orientation (Seewagen and Adams, 2021).

Little Brown Bat, Image by Moriarty Marvin, USFWS. Retrieved from https://pixnio.com/fauna-animals/bats-pictures/myotis-lucifugus-little-brown-bat

Cravens and Boyles (2018) focused on the differences between levels of the blood metabolite beta-hydroxybutyrate in bats found in lit and unlit conditions. Beta-hydroxybutyrate is a fasting metabolite, and is used for energy in metabolic processes when triglyceride fat storage is low. Interestingly, this is not the exact case in bats: beta-hydroxybutyrate levels spike after feeding–so they are correlated with periods of intense exercise (Cravens and Boyles, 2018). For red bats (Lasiurus borealis), beta-hydroxybutyrate levels were highest just after sunset in highly-lit sites, while the opposite was true in dark sites (Cravens and Boyles, 2018). Cravens and Boyles (2018) suggest that red bats have altered their foraging activity to prey on insects at lit sites just after sunset, while other bats may forage throughout the night. Also, red bats captured at lights late at night had low levels of these blood metabolites, suggesting that they were able to gain more energy from well-lit areas with less work (Cravens and Boyles, 2018).

Despite this positive impact on foraging in some bat species, high levels of light pollution can negatively impact bat diversity and species abundance in urban areas (Laforge et al., 2019; Langley, 2019; Seewagen and Adams, 2021). Seewagen and Adams (2021) were only able to detect little brown bats on 14% of light nights, while they were able to hear little brown bats calling on 65% of dark nights. Laforge et al. (2019) had similar results, with artificial light being a significant predictor of bat presence and activity, as well as their ability to move through their landscapes.

While artificial nighttime light can provide light-tolerant bats with novel foraging opportunities, finding ways to mitigate the impacts of nighttime light can greatly improve bat biodiversity. The most obvious solution is to turn out the lights–reducing nighttime light can help bats expand their species range by providing opportunities to safely move between habitat patches (Laforge et al., 2019). With less light, bats will be better able to avoid predators, and their sensory systems and circadian cycles may fall back in line. However, reducing light is not enough to truly improve bats’ habitat quality (Laforge et al., 2019)–and it is not the only option. Increasing urban greenspace can provide bats with shelter from predators in the form of tree canopy cover (Langley, 2019). Bats may even be able to strike a balance–enjoying the benefits of insects attracted to UV lights while remaining safely covered (Langley, 2019).

Whatever the solution may be, one thing is for sure: providing safe habitats for bats is necessary for supporting pollination, controlling insect populations, and preserving biodiversity in a changing world.

Works Cited

Cravens, ZM & Boyles, JG (2019) Illuminating the physiological implications of artificial light on an insectivorous bat community. Oecologia 189:69-77. doi: 10.1007/s00442-018-4300-6

Laforge, A, Pauwels, K, Faure, B, Bas, Y, Kerbiriou, C, Fonderflick, J, & Besnard, A (2019) Reducing light pollution improves connectivity for bats in urban landscapes. Landsc Ecol 34:793-809. doi: 1o.1007/s10980-019-00803-0

Langley, L (2019, April 17) Light pollution hurts urban bats. Trees can help. National Geographic. https://www.nationalgeographic.com/animals/article/bats-appreciation-day-cities-urban-lights

Seewagen CL & Adams, AM (2021) Turning to the dark side: LED light at night alters the activity and species composition of a foraging bat assemblage in the northeastern United States. Ecol Evol 11(10):5635-5645

Image 1 source: https://pixnio.com/objects/electronics-devices/electric-lights-pictures/stadium-lights

Image 2 source: https://pixnio.com/fauna-animals/bats-pictures/myotis-lucifugus-little-brown-bat