The Importance of Endurance in Seasonal Migration

Migration requires a constant supply of energy that animals secure from resourcing their metabolic fuels. Bats are mammals that make their migratory journeys through flight similarly to birds. Since fat has more energy per unit mass, birds use this as their means for enduring long migrations. It was hypothesized that due to evolutionary convergence bats would use fat stores similar to birds as fuel. Hoary bats (Lasiurus cinereus) would accomplish this through their oxidation pathways and fatty acid transport proteins being seasonally upregulated in their flight muscles.

The objective of this paper is to better understand how hoary bats are capable of sustaining a level of endurance that is required during their seasonal migrations. To do so, 8 lactating-female bats and 7 male non-migratory bats were collected between July 20-August 1, 2018 in Saskatchewan, Canada. 15 female and 15 male migratory bats were captured as well using mist nests in Bernalillo County and Manzano Mountains, New Mexico on May 5 – May 17, 2009. The bats were euthanized and their pectoralis muscles were dissected and stored at ?80 °C. After preparing the tissue through RNA isolation and reverse transcription into cDNA, primers were made for these specific target genes: Heart-type fatty acid binding protein (H-FABP), fatty acid transporter and translocase (FABPpm, FAT/CD36). H-FABP would increase in hibernating bats because they rely on stored fats during hibernation. FABPpm and FAT/CD36 would increase in migrating sparrow birds. After the hoary bat sequence was found, PCR was performed.

Birds were used as a model system to make predictions about migratory bats. HOAD, CPT, and CS are oxidative enzymes whose levels vary seasonally in migrating birds. This should lead to an increase of these three enzymes in migrating bats and mRNA expression. Results showed that the target gene H-FABP increased expression 5 times in migrating females, indicating a variation among sex. The female levels may be higher than male bats because extra energy is needed to reach their destination before giving birth or to find appropriate mates. There was a 32% increase in CPT, a 29% increase in CS, and a 53% increase in HOAD levels. Males displayed a greater HOAD presence, but CS and CPT were not affected by sex differences. During spring migration the bats aerobic capacity increased. A higher demand for fatty acids was required due to there being a greater physiological demand on bats around this migratory time.

I found that this paper presented interesting findings but did not include much data on female bats or pups. Lactating females might have a different metabolic rate because of pregnancy and sub-adults could possibly have a faster metabolism, therefore this could lead to inconclusive data being presented. The study found that bats do not increase their fatty acid protein levels like birds. Bats may have other ways they deal with the possible strains that occur during migration. Bats feed and migrate at the same time, unlike birds that fly at night and feed during the day. Since bats do not feed during the day they can shut off their metabolism and save energy. Perhaps the anatomical structure of the bats wings compared to the birds’ makes their overall energy output less. Since bats wings are more flexible they can get better drag and lift than birds and would need to do less work therefore requiring less fat stores. A bigger sample size would be preferred in this study to diminish any outliers, and a greater focus on protein levels not mRNA. Bats are interesting creatures and there is still more to learn on their hormonal control. 

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