The Southern Grasshopper Mouse, Onychomys torridus, is notorious for preying upon the Arizona Bark Scorpion, Centruroides sculpturatus. With only about a four-centimeter difference between the two, O. torridus effectively immobilizes and consumes the scorpion. In order to effectively do this, O. torridus begins by biting the tail and then the frontal area of the cranium, followed by crushing the thorax region (Horner et al. 1965).   

During this interaction, Onychomys are able to withstand scorpion stings. Onychomys have demonstrated high resiliency to the lethal venom from Centruroides that most mammals do not have. Out of 3 O. torridus populations, the individuals co-existing with Centruroides exilicauda had the highest venom resistance, while those in parapatric co-occurrence had the lowest (Rowe and Rowe 2008). The O. leucogaster population according to Rowe and Rowe (2008) are geographically isolated from all scorpion species. Frequency of exposure to Centruroides has a significant relationship to venom resistance, hence why resistance in O. leucogaster was significantly less than that of O. torridus. The populations of Onychomys that frequently interact with Centruroides have developed defense mechanisms against these neurotoxins as a result of coevolution (Rowe and Rowe 2008).

Scorpions and orthopterans make up a majority of O. torridus’s food choices, constituting about 88.87% of their diet (Bailey and Sperry 1929). Of this 88.87%, orthopterans such as crickets and locusts make up 59% of this value (Bailey and Sperry 1929). In a study by Horner et al. (1965), they found that scorpions were present in 36.7% of scat samples. In this sample population, scorpions were the most frequently consumed, and orthopterans were found to be the second most prevalent (Horner et al. 1965). When scorpions and orthopterans are unavailable, O. torridus consume plant materials and seeds, making up 11.13% of their diet (Bailey and Sperry 1929).

Onychomys torridus will consume other species of rodents, generally as a result of interspecies antagonism. Specifically, they consume species such as harvest mice (Reithorodontomys), kangaroo rats (Dipodomys), and pocket mice (Perognathus), which only comprises 3.09% of their diet (Bailey and Sperry 1929).  Notably, O. torridus displayed increased predatory behavior towards laboratory mice when experiencing starvation (Bailey and Sperry 1929).

To immobilize rodents, O. torridus will bite their cervical spine. In all of their prey, O. torridus will eat corpses from the front side of their bodies to the rear (Horner et al. 1964). When eating, the only parts of the carcasses that O. torridus tend to disregard include the hind appendages, wings, skins, and tails (Horner et al.  1964). To detect prey, O. torridus relies on sight, hearing and olfactory senses. 

Most members of Myomorpha have a bilocular stomach, which is separated into a left non-glandular portion and a moderately glandular right portion (Horner et al. 1964). In contrast, the stomach of Onychomys is completely non-glandular. Both the fundic and cardiac glands are located along the greater curvature of the stomach in a pouch-like structure (Horner et al. 1964). The purpose of the pouch placement is to protect the glands’ tissues from the coarseness of chitin material. Kohl et al. (2022) have found that chitinase, chitin disaccharide deacetylase, and chitosanase are present in the microbiome of O. torridus. These enzymes allow the ingested chitin to catabolize in their body, which is suspected to ease the digestion process (Kohl et al. 2022).

Reproduction 

In contrast to most cricetid rodents, O. torridus displays biparental care. Following the birth of their young, the female becomes the more dominant member of the pair. Both the mother and the father help pups with food, grooming, and protection (McCarty et al. 1977).

The average gestation period lasts for around 29 days, and O. torridus have litter sizes ranging from 1-5 pups (McCarty and Southwick 1977). The pre-weaning period in O. torridus lasts for 25 days. In the natural environment, Horner and Taylor (1968) estimate that O. torridus will at least live for a period of up to 9-12 months. Notably, O. torridus have lived for as long as 3 years under laboratory conditions.

Horner and Taylor (1968) observed that sexual maturity in O. torridus corresponds to foot growth. When the length of an individual’s hindfoot reaches roughly 18mm, they have reached sexual maturity. In O. torridus, reaching the state of sexual maturation can begin in as little as 6 weeks (Horner and Taylor 1968).

In males, the onset of spermatogenesis occurs around 19 days of age. Males will experience rapid growth in testis size, which is synchronous with minor swelling in the scrotal region that occurs around the 24th day (Horner and Taylor 1968). Scrotum size increases until the individual’s caudal epididymis is thoroughly developed. The caudal epididymis is responsible for sperm transportation. On the other hand, females only undergo one external alteration in their genitalia. Around the 44th day, the vagina will perforate. Perforation of the vagina allows for copulation to occur and is generally indicative of the female beginning her estrous cycle (Horner and Taylor 1968).

Onychomys torridus use their olfactory senses to identify sexes during courtship. When the male has interest, he will begin by chasing the female. As this process continues, both sexes will chirp and call. Notably, there is no sexual dimorphism in their vocalizations. Breeding season occurs throughout March to August, but the onset of breeding tends to be delayed in laboratory conditions (Horner and Taylor 1968).