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Other Anatomical and Physiological Adaptations
|Kangaroo Rat Conservation|
The reproductive capabilities and patterns of Dipodomys have been shown to be dependent on the varying availabilities of water. As with all animals, reproduction places considerable strain on the energetic resources of Kangaroo rats. Since water is a particularly scarce and variable resource for Dipodomys, these rodents have have synchronized their reproductive cycles to respond to rainfall. Nearly all reprodcutive organs of the kangaroo rats, including testis length, testis weight, sperm production in males, and number of embryos, embryo size, ovary size, number of corpora lutea (Figure 1), and number of placental scars have been shown to fluctuate seasonally (Hoditschek & Best, 1983). These peak reproductive capabilities correlate strongly with the amount of rainfall during each month, number of corpora lutea was even found to be correlated to the specific day of the month that receieved the most rainfall. (Hoditschek & Best, 1983). These data show that Kangaroo rats have evolved to be able to maximize their reproductive success in such arid climates.
Figure 1. Number of Corpora Lutea found in female Kangaroo Rats at each month. The Figure shows that reproductive capabilities widely fluctuated throughout the year, likely do to water availability.
Adapted from (Hoditschek & Best, 1983)
In order for Kangaroo rats to survive in xeric desert environments with little primary production, their anatomies, physiologies, and behaviors have had to evolve from their more mesic adapted ancestors. Whereas ancestral heteromyids subsisted on mainly succulent vegetation and free water sources for their energetic requirements, Kangaroo rats evolved due to changing environmental pressures to survive on a different diet. This diet of granivory relies on mainly dry grains and seeds as a single resource packet for the animals, providing nutritional energy and resulting water for the Kangaroo rats.(Macmillen & Hinds, 1983). The consumption of carbohydrate rich seeds and grains is also beneficial in maximizing metabolic water production and in minimizing nitrogenous wastes produced, which require water to be eliminated.(Table 1) Kangaroo rats have accordingly been shown to select grains and seeds high in carbohydrates and low in proteins when under water stress. (Frank, 1988).
Table 1. Maximum water (in percent mass) that can be obtained from nutrients. The table shows that carbohydrates can produce much more metabolic water for a Kangaroo rat than proteins.
Adapted from (Howell and Gersh, 1935)
In order to feed on these grains and seeds, Kangaroo rats have developed cheek pouches that open externally which allow them to consume and store these grains more effectively. Also, the development of bipedality in Perognathus is thought to enhance locomotory efficiency, thereby allowing the rodents to cover more distance while foraging for increasingly sparse food supplies.