Liomys salvini

Liomys salvini (Heteromyidae, Rodentia, Mammalia) species page.
24 October 2012

Citation: Janzen, D.H. and Hallwachs, W. 2012. Liomys salvini (Heteromyidae, Rodentia, Mammalia) species page. Area de Conservacion Guanacaste, Costa Rica.

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Figure 2. Adult 45 g female Liomys salvini on the hand of Winnie Hallwachs (Bosque San Emilio, dry forest, Sector Santa Rosa, Area de Conservacion Guanacaste; 27 July 1980). This exceptionally tame wild mouse displayed no fear when handled, but others require some restraining to handle.Figure 2. Adult 45 g female Liomys salvini on the hand of Winnie Hallwachs (Bosque San Emilio, dry forest, Sector Santa Rosa, Area de Conservacion Guanacaste; 27 July 1980). This exceptionally tame wild mouse displayed no fear when handled, but others require some restraining to handle.

Figure 1. Adult 60 g male Liomys salvini (“spiny pocket mouse” Heteromyidae) removing seeds from a fallen (offered by the tree) ripe fruit of a 30 m tall tree Hymenaea courbaril (“guapinol”, Fabaceae). The very hard fruit has been chewed open by Cuniculus paca (“tepiscuintle” or “paca”, Cuniculidae); two tepiscuintle incisor marks are obvious on the end of the fruit (Bosque San Emilio, dry forest, Sector Santa Rosa, Area de Conservacion Guanacaste; 14 June 1986).Figure 1. Adult 60 g male Liomys salvini (“spiny pocket mouse” Heteromyidae) removing seeds from a fallen (offered by the tree) ripe fruit of a 30 m tall tree Hymenaea courbaril (“guapinol”, Fabaceae). The very hard fruit has been chewed open by Cuniculus paca (“tepiscuintle” or “paca”, Cuniculidae); two tepiscuintle incisor marks are obvious on the end of the fruit (Bosque San Emilio, dry forest, Sector Santa Rosa, Area de Conservacion Guanacaste; 14 June 1986).

 

I. Identification.

Liomys salvini (Heteromyidae, Rodentia, Mammalia), or spiny pocket mouse, is the only member of the mouse family Heteromyidae that occurs in the ACG lowland dry forest (and in lowland dry forest elsewhere in Costa Rica, for that matter). The three other heteromyids in Costa Rica are the long and well known Heteromys desmarestianus of lowland ACG and Costa Rican rain forest (Fleming 1983), Heteromys oresterus of a few high elevation sites in the Cordillera de Talamanca, and the newly described Heteromys nubicolens known from just three localities (Monteverde, Rincon de la Vieja, y Volcan Cacao; Anderson and Timm 2006), rendering it endemic to the Cordillera Guanacaste and Cordillera Tilaran. Parenthetically, the ACG populations, and therefore this new species, of high elevation Heteromys was discovered by Marian Klaus when she was conducting a rodent inventory of the rain and cloud forests of ACG in 1998-1999. Adults of all ACG Heteromys are much larger than Liomys salvini, weighing twice or more as much, so they will not be confused with the dry forest Liomys. The two genera undoubtedly can be captured in the same ACG forest intergrade between dry forest and rain forest, or at least could be before most of it was deforested. 

Heteromyidae, and therefore Liomys, can be distinguished from any other small ACG dry forest rodent (e.g., Sigmodon hispidus, Ototylomys phyllotis, Oryzomys palustris, Oryzomys fulvescens, Oryzomys alfaroi complex, Reithrodontomys, Nyctomys sumichrasti) by having fur-lined cheek pouches. The pouches are used for carrying seeds, insect pupae, and at times their own very young offspring, and evolutionarily formed by in-pocketing of outer body skin, which is the origin of the hair. Liomys salvini is brownish gray on its back and upper sides, and cream white below, while Heteromys desmarestianus and Heteromys nubicolens are gray on the back and sides, and white underneath, but with black forelegs. The face of Liomys is brownish gray with black eyes, while the face of Heteromys is grayish black with a light ring bordering the black eyes. 

II. Geographic and microgeographic distribution. 

International: 
Liomys salvini ranges from the dry Pacific coastal lowlands of Oaxaca, Mexico to Costa Rica's dry forests (Fleming 1983). The northern margin of its distribution adjoins the southern distributions of three other species of Liomys (Liomys pictus, Liomys irroratus, and Liomys spectabilis) that occupy Guatemala and Mexico and even as far north as Texas (Brown and Genoways 1993). 

Costa Rica: 
Liomys salvini occurs or ocurred in all Costa Rican dry forests below about 1200-1500 m elevation (Fleming 1983). Populations have survived at low and irregular densities in early secondary succession and on wooded margins of roads and pastures in many parts of Guanacaste Province and Puntarenas Province that were once covered with dry forest (and presumably had a continuous population of L. salvini). L. salvini is/was also present in some of the small pockets of relatively intact dry forest that remain in Costa Rica, such as those that were scattered over the area of today’s Area de Conservacion Guanacaste at the time of decreeing Santa Rosa National Park in 1971 (today’s Sector Santa Rosa). 

 

Area de Conservacion Guanacaste: 
Today Liomys salvini occurs in all ACG dry forest of all ages, from the margins of the coastal mangroves to the lower slopes of the volcanos (300-1000 m). However, the upper elevation distribution may well be artificially generated by the massive deforestation of some of these slopes, which in turn dries and warms them, allowing lowland dry forest species to extend their populations upwards (i.e., as occurs with coyotes, white-tailed deer, ctenosaurs and cotton rats (Sigmodon hispidus)). The regenerating forest is richer in dry forest species of plants than was the evergreen forest that originally covered the sites. It appears that dry and deciduous forest is more friendly to Liomys salvini than is evergreen wet forest, though the detailed boundary and its intersection with Heteromys demarestianus and Heteromys nubicola populations has never been documented. 
When an old pasture (generally not occupied or used by Liomys) is regenerating its dry forest, it takes at least 20 years until the trees are old enough to be producing any serious quantity of the species of seeds that are desired by Liomys. 


L. salvini is generally absent from pastures and other areas covered with herbaceous plants (which is to say, areas that produce quite small seeds and have little or no seed bank in the litter), except in years when there is a superabundant peak in the population (Fig. 4, 5, and see Janzen 1986). In these years there are L. salvini crossing all kinds of dryland habitats in their desperate search to locate habitat that is not already "owned" by other L. salvini. 

L. salvini does not occur on the Islas Murciélago at the extreme western end of the ACG. This may be due to its elimination by predation by the introduced rats (Rattus norvegicus) which thoroughly occupy the islands. On the other hand, it could be that these islands do not have sufficient area (that to say, seed and insect pupae resources) to maintain a population of L. salvini. Certainly the very large grassy and herbaceous areas of the islands, due to the previous annual fires, are not suitable habitat for L. salvini. Today the islands are very impoverished in those woody plant species that produce seeds that are quality food for L. salvini, and I suspect that the elimination of Rattus would not result in a self-sustaining population of L. salvini being able to survive on the islands.

III. Natural history. 

Liomys salvini is a specialist at searching for, caching, and eating mature seeds of woody plants (and some herbs such as forest grasses) in ACG dry forest. IThe mice supplement this diet with moth pupae (and perhaps other dormant insects) found in the litter in both the rainy season and dry season. Seeds of the introduced/naturalized guanacaste tree (Enterolobium cyclocarpum; Fig. 3) offer a specific example (Janzen 1982a, 1982e, 1986). When the (re-introduced from Europe) range horses eat guanacaste fruits (as they would have in the Pleistocene, Janzen and Martin 1982), which they do very eagerly, the ungerminated seeds take 1 week to 6 months to pass through the horse (Janzen 1981a,b). These seeds are eagerly and thoroughly collected from horse dung by the mice, and more reluctantly from cattle dung (Janzen 1982d, 1986). 

Figure 4. Newly fallen ripe guanacaste tree fruits, ready for consumption by the now largely missing Pleistocene megafauna (Janzen and Martin 1986). If not eaten by horses (one species of megafauna brought back by the Spaniards, these fruits are gnawed open by L. salvini mice (Fig. 5), and the seeds taken back (Fig. 6) to the L. salvini underground tunnels and seed caches, and later eaten after partial germination (Fig. 7) (15 March 1976, Sector Santa Rosa, ACG). Figure 4. Newly fallen ripe guanacaste tree fruits, ready for consumption by the now largely missing Pleistocene megafauna (Janzen and Martin 1986). If not eaten by horses (one species of megafauna brought back by the Spaniards, these fruits are gnawed open by L. salvini mice (Fig. 5), and the seeds taken back (Fig. 6) to the L. salvini underground tunnels and seed caches, and later eaten after partial germination (Fig. 7) (15 March 1976, Sector Santa Rosa, ACG).

Fig. 3. Adult guanacaste tree (Enterolobium cyclocarpum, Fabaceae), leafless in the dry season. Note the Land Rover and people in lower left corner for scale. This tree could be as young as 30 years old and as old as 150 years. 

This individual is leafless at this time in the dry season, but has a very large crop of nearly mature fruits which darken the upper branch tips in this photograph. These fruits are nearly ready to fall to the ground and be eaten by the megafauna that would originally have consumed them and been seed dispersers (Janzen 1986, Janzen and Martin 1982). In ACG forest with few megafauna present (tapirs, peccaries, horses and cattle in the old days),the seeds are thoroughly harvested and preyed upon by Liomys salvini, either by gnawing them out of the fruits or by harvesting them from the dung of seed dispersers (March 1972, Bebidero, Guanacaste, Costa Rica).Fig. 3. Adult guanacaste tree (Enterolobium cyclocarpum, Fabaceae), leafless in the dry season. Note the Land Rover and people in lower left corner for scale. This tree could be as young as 30 years old and as old as 150 years. 

This individual is leafless at this time in the dry season, but has a very large crop of nearly mature fruits which darken the upper branch tips in this photograph. These fruits are nearly ready to fall to the ground and be eaten by the megafauna that would originally have consumed them and been seed dispersers (Janzen 1986, Janzen and Martin 1982). In ACG forest with few megafauna present (tapirs, peccaries, horses and cattle in the old days),the seeds are thoroughly harvested and preyed upon by Liomys salvini, either by gnawing them out of the fruits or by harvesting them from the dung of seed dispersers (March 1972, Bebidero, Guanacaste, Costa Rica).

Figure 8. Fruits and seeds of Lonchocarpus felipei (Fabaceae). These soft fruits can fall by the thousands on the dry forest litter, and none of them are ever harvested by Liomys mice. When captive mice are given only these seeds for their food, they literally starve to death without doing anything to the seeds except to cut them open to see if there is something edible inside; they eat none of the seed contents. While the seeds are chemically protected from some insects by specific toxic secondary compounds, they are protected from seed predation by the mice by the mix of different compounds in the seed (Janzen et al 1990) (14 March 1981, Bosque San Emilio, Sector Santa Rosa, ACG).Figure 8. Fruits and seeds of Lonchocarpus felipei (Fabaceae). These soft fruits can fall by the thousands on the dry forest litter, and none of them are ever harvested by Liomys mice. When captive mice are given only these seeds for their food, they literally starve to death without doing anything to the seeds except to cut them open to see if there is something edible inside; they eat none of the seed contents. While the seeds are chemically protected from some insects by specific toxic secondary compounds, they are protected from seed predation by the mice by the mix of different compounds in the seed (Janzen et al 1990) (14 March 1981, Bosque San Emilio, Sector Santa Rosa, ACG).

Figure 7. On the right, two hard dormant seeds of Enterolobium cyclocarpum that have been notched by Liomys. On the left, one of these seeds after being on moist ground in the tunnel for several days, enough to begin germination. The germinating seeds are a much superior diet for the mice, and in captivity, when offered a choice, the germinating seeds are always eaten long before starvation forces the mouse to eat hard dormant Enterolobium seeds (7 July 1979, Sector Santa Rosa, ACG). Figure 7. On the right, two hard dormant seeds of Enterolobium cyclocarpum that have been notched by Liomys. On the left, one of these seeds after being on moist ground in the tunnel for several days, enough to begin germination. The germinating seeds are a much superior diet for the mice, and in captivity, when offered a choice, the germinating seeds are always eaten long before starvation forces the mouse to eat hard dormant Enterolobium seeds (7 July 1979, Sector Santa Rosa, ACG).

 

In ACG dry forest, in the evening after dark, the Liomys leave their below-ground tunnels where they nest and store food, and search for seeds in the litter, in fallen fruits, and in the dung of other animals (currasows, coatimundis, horses, ctenosaurs, etc., Janzen 1986). The seeds that are acceptable are stuffed into their cheek pouches with their front legs, and carried to their underground storage rooms (small rooms 0.2 to 1 liter in volume off the sides of their tunnels) to be eaten later. Some species of seeds are eaten at the moment of encountering them or stored, depending on how hungry the mouse. Other species of seeds are rejected because of the presence of defensive chemicals (Fig. 8). No seed is so hard that the mice cannot open them (however, the fruits of guapinol, Hymenaea courbaril, are so hard and large in diameter that L. salvini cannot get to their seeds unless some other animal such as an agouti or tepiscuintle opens them) (Fig. 1).

In addition to seeds, the mice search for and eat moth pupae, and sometimes insects, that they find in the litter and upper soil layer. There is no evidence that they climb trees for any reason, but in human habitations they learn to climb on furniture and counter tops in search of food. 

L. salvini searches for seeds in the litter of the forest understory at any time during the night (not in the daytime), but every hour spent searching in the litter is another hour exposed to various predators. The mice are very disinclined to search for seeds when there is a full moon (lighting the forest understory), and when the litter is very dry, which makes search for seeds very noisy. It is likely that a given mouse leaves its tunnels to search in the litter every 1-3 nights. When it finds a large source of seeds, such as a newly fallen fruit or seed crop in the litter, it can spend many hours transporting seeds from the source to its tunnels. 

A mouse may, for example, eat 3-7 newly germinated guanacaste tree seeds per night. While a mouse may survive for months in the laboratory on a pure diet of certain species of seeds (for example, Sesbania emercus, Guazuma ulmifolia, Enterolobium cyclocarpum, Apeiba tibourbou, Malvaviscus arboreus, Cochlospermum vitifolium, Bursera simaruba), in nature a mouse's diet is much more mixed (Janzen 1982a,b,c, 1986). It is quite noticeable that in captivity L. salvini will die of hunger before eating the species of seeds that it rejects in the forest (e.g., Pithecellobium platylobum, Lonchocarpus felipei (Fig. 8), Swietenia macrophylla) (Janzen et al 1990). Enterolobium cyclocarpum seeds are a much better diet when germinated than when hard (dormant), and the mice cut the hard seed coat (Fig. 7) and leave the seeds on moist soil in their tunnels to germinate (Fig. 7) before eating them. They are, in effect, making bean sprouts.

Outstanding physical traits.

Liomys salvini weighs 7-13 g when it leaves its mother's nest (fledges) or is expelled by her, and grows to weigh 35-45 g (female; 55 g when very pregnant) or 45-75 g (male). While this mouse normally walks on all four feet, when it is in a hurry or is startled it runs by jumps and may jump as far as 1.5 m in one leap. It is likely that this ability to jump is the antecedent for the great jumping ability of its evolutionary descendent, the kangaroo rat (Dipodomys spp.) of northern Mexico and the southwestern USA. Dipodomys is evolutionarily derived from a heteromyid such as Liomys (Genoways and Brown 1993). 

The hairs on the back of Liomys salvini are of two kinds: thin and "normal", or thickened into a flat weak spine, as if they were evolving in the direction of the ACG’s prehensile-tailed porcupine’s spines (Sphigurrus mexicanus). These "protospines" seem to make the mice much more slippery and hard to hold on to than if they were normal hair, because the approaching hand (or predator) cannot pass them to grab the skin and body. Their stiff sharp points also probably resist an approaching mouth or claw.

Reproduction.

When the population density of Liomys salvini is moderate to high (50-400 mice per 4.5 hectares of forest study plot), the general pattern is that the adult females give birth to a 3-4 member litter between mid-January and mid-February (i.e., the first two months of the dry season, and the dry season is the time of the maximum number of seeds in the litter, both because they are not escaping the mice by germinating and many species of dry forest trees mature their fruits in the dry season (Janzen 1967). These are weaned and out of the nest (whether pushed by the mother or gone on their own is not known, but it is probably the former) by March-April. Adult females often produce a second litter in May-June (beginning of the rainy season). During the remainder of the rainy season (when seed density is rapidly declining owing to germination and trees being non-reproductive), the males have shrunken epididymis and only partly descended testes, and pregnant and lactating females seem to be almost entirely absent. However, when the density of Liomys is very low (e.g., 1-25 mice per 4.5 hectare forest study plot), there are some reproducing females in the rainy season, as evidenced by 3-5 month old mice captured in January. This pulsed, episodic reproductive behavior results in the “average” female usually not reproducing in the first calendar year (February-December) of her life, then 1 or 2 times in the second year, and (if she lives this long) again 1-2 times in her third calendar year. For males, the pattern is roughly the same. Just as only some of the “young of the year” females reproduce for their first time in May-June, and the same for males, Liomys are both not in continuous reproduction and must survive year-long-periods between birth and insemination events; this is very different from the other small rodents in ACG dry forest, which appear to reproduce throughout the year. I feel that one of the consequences of this reproductive pattern is that Liomys individuals are more predator-wary than are other species of small rodents, leading among other things to both strong territoriality and a more detailed and intimate knowledge of the specific details of that territory (multiple nest entrances, obstacles, distances, etc.). From a practical census standpoint, this means that by trapping the mice in January they are caught at their lowest density and in May they are at their highest density (Fig. 15), with the numbers gradually falling from June through December as predators take their toll.

Population:

Through capturing Liomys salvini with Sherman live traps (Fig. 9, 10, 11), marking them with tattoos (Fig. 12,13), and releasing them for recapture later, it has been possible to monitor their density and individual locations in two 4.5 ha plots for 30 years in the Bosque Humedo and Bosque San Emilio, Sector Santa Rosa (Fig. 14). The results (Fig. 15) show clearly that the density of L. salvini fluctuated very strongly and with a peak-to-peak cycle of 4-5 years in the beginning, but then leveled off at a very low density (but notably not extinguished) since then. 

These population fluctuations during the first 10 years of monitoring provokes many thoughts as to how it would affect both the many animals that eat Liomys and the many seeds that Liomys eats, to say nothing of the ease of noticing and
 studying these mice.

Figure 9. Sherman live trap at station D12 in Bosque San Emilio (90-year-old secondary dry forest), Sector Santa Rosa, ACG. This is full dry season (20 March 1982) and the full sun exposure of the trap emphasizes why the traps need to be checked in the first two hours of daylight. The same traps have been used, without washing, for 30 years. They are only removed temporarily when they are the container in which the live mouse is brought to a central lab for weighing, sexing, tattooing, etc. before being released that morning in exactly the same place. A trapping session is conducted by a portion of the ACG/GDFCF parataxonomists (Fig. 10) by setting 529 traps at pre-established stations at 10 m intervals in the mid-late afternoon in one plot, and checking all 529 traps the next morning. The next night this is repeated in the other plot, and so on until each plot has been trapped for 7 nights (every other night for 14 nights). Such a 7-night session occurs during the first 2/3 of January and the last 1/2 of May each year.Figure 9. Sherman live trap at station D12 in Bosque San Emilio (90-year-old secondary dry forest), Sector Santa Rosa, ACG. This is full dry season (20 March 1982) and the full sun exposure of the trap emphasizes why the traps need to be checked in the first two hours of daylight. The same traps have been used, without washing, for 30 years. They are only removed temporarily when they are the container in which the live mouse is brought to a central lab for weighing, sexing, tattooing, etc. before being released that morning in exactly the same place. A trapping session is conducted by a portion of the ACG/GDFCF parataxonomists (Fig. 10) by setting 529 traps at pre-established stations at 10 m intervals in the mid-late afternoon in one plot, and checking all 529 traps the next morning. The next night this is repeated in the other plot, and so on until each plot has been trapped for 7 nights (every other night for 14 nights). Such a 7-night session occurs during the first 2/3 of January and the last 1/2 of May each year. Liomys-trFigure 10. The Liomys trapping session crew, morning of 10 January 1991, Bosque Humedo, Sector Santa Rosa, ACG. From left to right, Osvaldo Espinoza, Roster Moraga, Guillermo Pereira, and Manuel Pereira. Each of the 35 Sherman live traps has a mouse in it, is brought to the processing center, and then is released the same morning exactly where it was trapped.apping-session-crew-morning-of-10-January-1991Liomys-trFigure 10. The Liomys trapping session crew, morning of 10 January 1991, Bosque Humedo, Sector Santa Rosa, ACG. From left to right, Osvaldo Espinoza, Roster Moraga, Guillermo Pereira, and Manuel Pereira. Each of the 35 Sherman live traps has a mouse in it, is brought to the processing center, and then is released the same morning exactly where it was trapped.apping-session-crew-morning-of-10-January-1991
Figure 11. Adult male 60 g Liomys salvini inspecting the entrance of a Sherman live trap (Fig. 9) in Bosque San Emilio dry forest, Sector Santa Rosa (15 June 1986). The trap is baited with one highly desirable germinating Enterolobium seed at the entrance and three germinating Enterolobium seeds in the rear of the trap. If a mouse is caught, it usually eats these seeds during its night of living in the trap. If there is no mouse, the seeds are dumped into a bucket and used again the next night of trapping in the other cenus plot (which avoids a massive food subsidy for the mice. Because the seeds are not adorned with a sticky bait such as oatmeal and peanut butter, they are as clean after a trapping session as before. Between the biannual trapping sessions, each trap is perched 40 cm above the ground on the iron stake marking its location, so that an unfortunate (ground-bound) Liomys does not force its way into the (death) trap, as happens if the traps are left on the ground, because of the residual bait odor. Liomys really likes germinating Enterolobium seeds.Figure 11. Adult male 60 g Liomys salvini inspecting the entrance of a Sherman live trap (Fig. 9) in Bosque San Emilio dry forest, Sector Santa Rosa (15 June 1986). The trap is baited with one highly desirable germinating Enterolobium seed at the entrance and three germinating Enterolobium seeds in the rear of the trap. If a mouse is caught, it usually eats these seeds during its night of living in the trap. If there is no mouse, the seeds are dumped into a bucket and used again the next night of trapping in the other cenus plot (which avoids a massive food subsidy for the mice. Because the seeds are not adorned with a sticky bait such as oatmeal and peanut butter, they are as clean after a trapping session as before. Between the biannual trapping sessions, each trap is perched 40 cm above the ground on the iron stake marking its location, so that an unfortunate (ground-bound) Liomys does not force its way into the (death) trap, as happens if the traps are left on the ground, because of the residual bait odor. Liomys really likes germinating Enterolobium seeds. Figure 12. An adult female Liomys being tattooed on its inner thigh with a tattoo parlor tattoo gun (17 August 1973, processing center in Area Administrativa, Sector Santa Rosa, ACG). The ink is black India Ink and is injected by 4 tiny needles that vibrate in and out of the barrel of the tattoo gun (see Fig. 13).Figure 12. An adult female Liomys being tattooed on its inner thigh with a tattoo parlor tattoo gun (17 August 1973, processing center in Area Administrativa, Sector Santa Rosa, ACG). The ink is black India Ink and is injected by 4 tiny needles that vibrate in and out of the barrel of the tattoo gun (see Fig. 13).
 Figure 13. The female Liomys in Figure 12 after the excess ink has been wiped off. These tattoo marks will be distinctive and not fade for her entire 1-3 year life. This is the number 3, with two marks above the thigh bone and one below it. Each hind leg has 3 potential digits and each ear area 2 potential digits, and this 5-digit combination allows for many thousands of combinations. Numbers are not re-used until 5 years have passed. Figure 13. The female Liomys in Figure 12 after the excess ink has been wiped off. These tattoo marks will be distinctive and not fade for her entire 1-3 year life. This is the number 3, with two marks above the thigh bone and one below it. Each hind leg has 3 potential digits and each ear area 2 potential digits, and this 5-digit combination allows for many thousands of combinations. Numbers are not re-used until 5 years have passed.  Figure 14. Aerial photograph from 1969 of the two Liomys 4.5 ha study plots, Bosque Humedo on the left and Bosque San Emilio on the right. The forest patch containing Bosque San Emilio was a banana plantation in 1920, where as the plot in Bosque Humedo occupies a 22 ha patch of old-growth forest (minus its large mahoganies, Swietenia macrophylla, cut in 1942-43). The road bordering the lower side of the elongate forest patch is today the road from the “Intersection” (the Y at the right hand end of the forest patch) in the vicinity of the Area Administrativa of ACG out to the Interamerican Highway. The light non-forest areas were covered in 1969 with pasture grasses, and were not Liomys habitat.Figure 14. Aerial photograph from 1969 of the two Liomys 4.5 ha study plots, Bosque Humedo on the left and Bosque San Emilio on the right. The forest patch containing Bosque San Emilio was a banana plantation in 1920, where as the plot in Bosque Humedo occupies a 22 ha patch of old-growth forest (minus its large mahoganies, Swietenia macrophylla, cut in 1942-43). The road bordering the lower side of the elongate forest patch is today the road from the “Intersection” (the Y at the right hand end of the forest patch) in the vicinity of the Area Administrativa of ACG out to the Interamerican Highway. The light non-forest areas were covered in 1969 with pasture grasses, and were not Liomys habitat.
 Figure 15. Numbers of Liomys mice per 4.5 ha census plot from January 1983 to January 1994. After 1994, the population has dropped to 1- 25 mice per study plot and has stayed at that low level for 18 years. The open circles are the Bosque Humedo (old-growth forest) and the filled circles are Bosque San Emilio (80-90-year-old regenerating forest). Each peak is a late May (beginning rainy season trapping session. Given the high desirability of the germinating Enterolobium seeds as bait, and the frequent recaptures during each trapping session, it is believed that essentially all of the mice in or circulating through the census plot were captured in each 7-night trapping session. Both the extreme fluctuations and the persistence of the Liomys population at very low density (there is even one Bosque Humedo trapping session with no mice captured.Figure 15. Numbers of Liomys mice per 4.5 ha census plot from January 1983 to January 1994. After 1994, the population has dropped to 1- 25 mice per study plot and has stayed at that low level for 18 years. The open circles are the Bosque Humedo (old-growth forest) and the filled circles are Bosque San Emilio (80-90-year-old regenerating forest). Each peak is a late May (beginning rainy season trapping session. Given the high desirability of the germinating Enterolobium seeds as bait, and the frequent recaptures during each trapping session, it is believed that essentially all of the mice in or circulating through the census plot were captured in each 7-night trapping session. Both the extreme fluctuations and the persistence of the Liomys population at very low density (there is even one Bosque Humedo trapping session with no mice captured.

Parasites and Predators.

Liomys salvini is prey for many species of nocturnal predaceous vertebrates (snakes (Fig. 16-17), cats, owls, skunks, possums, coyotes, gray fox) that live in ACG dry forest. It seems that the ups and downs in the population in Fig. 15 are generated by the interaction between the mice and these predators. When the density is very low, the predators lack one of their principal foods and generally do not breed, and their density falls. Then, the mouse population grows very rapidly, thereby generating lots of food for the predators, which in turn have a bout of reproduction. This increased population of predators then lowers the mouse population again. In the 1983-1987 peak this process was cleanly executed, while in the 1990-1994 boom-bust cycle the relationship was less clear, probably due to the six successive years of excessively low rainfall that occurred during this period. 

Based on early survey of their parasites, it is known that Liomys salvini in the ACG supports a rich fauna of mostly undescribed tapeworms, nematodes, moths (Davis et al 1986), pseudoscorpions, lice, mites and ticks. It does not, however, have any fleas.

Figure 16. A 50-gram Boa constrictor (Boidae) after swallowing a 65-gram adult male Liomys salvini. The mouse was in a cage for a feeding experiment, and the Boa entered through the mesh, but was unable to exit the cage because of the mouse inside it (2 July 1980, Area Administrativa, Sector Santa Rosa, ACG). Figure 16. A 50-gram Boa constrictor (Boidae) after swallowing a 65-gram adult male Liomys salvini. The mouse was in a cage for a feeding experiment, and the Boa entered through the mesh, but was unable to exit the cage because of the mouse inside it (2 July 1980, Area Administrativa, Sector Santa Rosa, ACG). Figure 17. Since Liomys mice are nocturnal foragers for seeds and moth pupae on the forest floor, that is when and where most predation takes place. However, diurnal foragers that can seek in the tunnels may prey on them in the daytime as well. Here a Conophis lineatus (Colubridae) snake - called “guarda camino” in Spanish - flees from me with a Liomys half swallowed, a mouse that it has just caught in the middle of the day. The snake had gone down the burrow of the mouse, and I happened to be standing there when the mouse popped out of the burrow and fled across the litter, and hid partly under a fallen leaf. The snake emerged from the burrow, elevated its head, saw the mouse, bit it and hung on. With 1-2 minutes the mouse was immobilized by the snake’s venom (Conophis is a rear-fanged snake, with an anticoagulant venom and is only mildly obnoxious to a large vertebrate like you). The snake moved its grip to the head, and began to swallow it when I appeared with my camera. It then elevated its head and the 35 g mouse and fled into dense brush to continue its meal in peace.Figure 17. Since Liomys mice are nocturnal foragers for seeds and moth pupae on the forest floor, that is when and where most predation takes place. However, diurnal foragers that can seek in the tunnels may prey on them in the daytime as well. Here a Conophis lineatus (Colubridae) snake - called “guarda camino” in Spanish - flees from me with a Liomys half swallowed, a mouse that it has just caught in the middle of the day. The snake had gone down the burrow of the mouse, and I happened to be standing there when the mouse popped out of the burrow and fled across the litter, and hid partly under a fallen leaf. The snake emerged from the burrow, elevated its head, saw the mouse, bit it and hung on. With 1-2 minutes the mouse was immobilized by the snake’s venom (Conophis is a rear-fanged snake, with an anticoagulant venom and is only mildly obnoxious to a large vertebrate like you). The snake moved its grip to the head, and began to swallow it when I appeared with my camera. It then elevated its head and the 35 g mouse and fled into dense brush to continue its meal in peace.

IV. How to find a Liomys salvini.

When the density of Liomys is very low, as it has been since 1996-1997 there is no reliable way to “find” a Liomys other than setting a very large number of Sherman live traps. When the density is high, setting as few as 10 Sherman live traps in Sector Santa Rosa dry forest more than 50 years old will usually catch a Liomys during the first night of trapping. When densities are high, it is also relatively easy to meet a free-foraging Liomys in the forest at night by just walking quietly in the forest and searching with a headlamp. Try to catch it by hand and you will discover how incredibly rapidly a Liomys can avoid a predator. A relatively easy way to see one is to ask about the detailed live-trapping schedule in January or May, and it can be arranged for you to meet at least a few of them (to meet both sexes may require several nights of our trapping in Bosque San Emilio). Alternatively, even with very low density, we occasionally have 1-2 of them foraging on the floor of our very open house in Area Administrativa of Sector Santa Elena, and we can tell you by e-mail if there is a regular visitor at any given time.

Literature cited:

Anderson, R. P. and Timm, R. M. 2006. A new montane species of spiny pocket mouse (Rodentia: Heteromyidae: Heteromys) from northwestern Costa Rica. American Museum Novitates, No. 3509, 38 pp.

Brown, J. H. and Genoways, H. H.,eds. 1993. The biology of the Heteromyidae. Special Publication No. 10, The American Society of Mammalogists, 715 pp.

Davis, D. R., Clayton, D. H., Janzen, D. H., and Brooke, A. P. 1986. Neotropical Tineidae, II: biological notes and descriptions of two new moths phoretic on spiny pocket mice in Costa Rica (Lepidoptera: Tineoidea). Proceedings of the Entomological Society of Washington 88:98-109.

Fleming, T. 1983. Heteromys desmarestianus (Raton Semiespinosa, Spiny Pocket Mouse). Costa Rican Natural History, pp. 474-475, ed. D. H. Janzen, University of Chicago Press, Chicago.

Janzen, D. H. 1967. Synchronization of sexual reproduction of trees with the dry season in Central America. Evolution 21:620-637.

Janzen, D. H. 1981a. Enterolobium cyclocarpum seed passage rate and survival in horses, Costa Rican Pleistocene seed dispersal agents. Ecology 62:593-601.

Janzen, D. H. 1981b. Guanacaste tree seed-swallowing by Costa Rican range horses. Ecology 62:587-592.

Janzen, D. H. 1982a. Seed removal from fallen guanacaste fruits (Enterolobium cyclocarpum) by spiny pocket mice (Liomys salvini). Brenesia 19/20:425-429.

Janzen, D. H. 1982b. Fruit traits, and seed consumption by rodents, of Crescentia alata (Bignoniaceae) in Santa Rosa National Park, Costa Rica. American Journal of Botany 69:1258-1268.

Janzen, D. H. 1982c. Natural history of guacimo fruits (Sterculiaceae: Guazuma ulmifolia) with respect to consumption by large mammals. American Journal of Botany 69:1240-1250.

Janzen, D. H. 1982d. Attraction of Liomys mice to horse dung and the extinction of this response. Animal Behaviour 30:483-489. 

Janzen, D. H. 1982e. Removal of seeds from horse dung by tropical rodents: influence of habitat and amount of dung. Ecology 63:1887-1900 

Janzen, D. H. 1986. Mice, big mammals, and seeds: it matters who defecates what where. In Frugivores and seed dispersal, A. Estrada and T. H. Fleming, eds., Dr. W. Junk Publishers, Dordrecht, Holland, pp. 251-271. 

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Janzen, D. H., Fellows, L. E., and Waterman, P. G. 1990. What protects Lonchocarpus (Leguminosae) seeds in a Costa Rican dry forest? Biotropica 22:272-285. 



 Janzen, D. H. and P. S. Martin 1982. Neotropical anachronisms: the fruits the gomphotheres ate. Science 215:19-27.