Temperature Preference in IAF Hairless and Hartley Guinea Pigs (Cavia porcellus)

Abstract

The Hairless strain of guinea pigs (Cavia porcellus) is the result of a spontaneous recessive mutation first identified at the Institute Armand Frappier (IAF) in 1978. Despite the longstanding availability of this strain, little is known about its thermoregulatory behavior. The aim of this study was to determine temperature preference in Hartley and Hairless guinea pigs by observing each strain in a ring-shaped apparatus containing a nonlinear temperature gradient. Temperatures were maintained by separately controlled heating mats lining the apparatus. Set point temperatures ranged from 24 to 38 °C. Guinea pigs (Hartley female, Hairless female, and Hairless male guinea pigs; n = 8 each group) were placed either singly or in pairs at 1 of the 8 randomized starting points within the apparatus. Subjects were observed for 30 min and coded for location within the temperature gradient by both frequency and duration. When placed singly in the apparatus, all 3 groups spent more time in the 30 °C zones. However, when placed as pairs with a cagemate, Hartley female guinea pigs spent more time in the cooler range of temperatures from 24 to 30 °C, whereas Hairless guinea pigs preferred a range of 30 to 38 °C. These results confirm a temperature preference of 30 ± 2 °C for both Hartley and Hairless guinea pigs when singly housed. However, data from the paired housing condition suggest that context plays an important role in thermoregulatory behavior.

The Hairless strain of guinea pigs (Cavia porcellus) was first discovered at the Institute Armand Frappier (IAF) in 1978. Unlike most other nude rodents, Hairless guinea pigs are euthymic and have a functioning immune system.² This animal model has been used extensively in dermatology research^6,10,18,22 and, more recently, to examine behavioral development.^4,7 Despite the strain's availability for several decades, studies investigating the temperature preference or thermoregulatory behavior of the Hairless strain are unavailable. This lack of information is problematic for behavioral studies, because temperature is a mitigating factor for most mammalian behavior.²³

More importantly, ambient temperatures too divergent from an optimal thermoneutral range can have long-term detrimental effects, particularly during development. For example, pregnant Hartley guinea pigs housed at 15 °C exhibit a greater stress response to handling than do controls housed at typical temperatures (22 °C).¹⁶ In addition, offspring from these cold-condition pregnancies show elevated stress responses 2 mo after birth, demonstrating that the temperature-evoked stress response is transgenerational and possibly persistent.¹⁷ In contrast, outcomes in offspring from elevated ambient temperatures during gestation are not merely persistent but teratogenic. For example, brains from the offspring of Hartley guinea pigs exposed to a 44 °C environment for as little as 1 h while pregnant show signs of apoptosis and arrested mitotic activity for 4 to 8 h after exposure.²⁴ Longer exposure times lead to even more deleterious effects, including abortion and gross malformation.^8,9

Animals naturally seek to avoid extremes in temperature through a process known as behavioral thermoregulation.²³ Several studies have demonstrated that given a choice, Hartley guinea pigs show a preference for an environment of approximately 30.6 °C, which is within the thermoneutral range for guinea pigs (30 to 31 °C).^13,16 Within this range, animals make the most efficient use of energy expenditure, requiring the least amount to maintain body temperature. Outside of this range, lower temperatures cause a rise in metabolic rate, possibly leading to thermal stress, and higher temperatures result in elevated body temperature or even a febrile state.¹⁴

The purpose of the current study was to determine the temperature preference of Hairless guinea pigs as compared with the Hartley strain from which it was derived. This aim was achieved by taking advantage of thermoregulatory behavior and allowing the guinea pigs to freely explore a temperature-gradient environment. Because they lack fur, we hypothesized that Hairless guinea pigs would prefer a slightly warmer temperature (for example, 32 to 38 °C) within the gradient than would Hartley guinea pigs. Although animals typically are tested one at a time in temperature-preference studies, we also observed cagemate pairs in our temperature-gradient experiment. The rationale for this additional study condition was that guinea pigs are naturally social animals. As such, testing solitary subjects is atypical and less likely to evoke the true thermoregulatory behavior of guinea pigs. In addition, guinea pigs housed together tend to spend considerable time in close proximity, often huddling together.¹ We hypothesized that when tested in pairs, the preferences of both strains would shift to slightly cooler temperatures, because of the additional thermoregulation provided by proximity to a cagemate.

Materials and Methods

Subjects.

A total of 24 guinea pigs (Cavia porcellus) were used in this study. IAF Hairless adult female (n = 8) and male (n = 8) guinea pigs were obtained from Charles River Laboratory (Kingston, NY). Adult female NIH multicolored Hartley guinea pigs (n = 8) were acquired from Elm Hill Labs (Chelmsford, MA). Guinea pigs were guaranteed to be pathogen-free from the vendors, and no additional surveillance was performed in the vivarium other than weekly weighing and daily monitoring for healthy appearance. Guinea pigs were 8 to 10 mo old at the time of testing and had been exposed to the temperature conditions in the viviarum (Wright State University, Dayton, OH) for at least 6 mo. At the time of testing, female Hairless guinea pigs weighed 955.18 ± 41.47 g (mean ± 1 SD), and males weighed 1071.31 ± 71.82 g; female Hartley guinea pigs weighed 1041.56 ± 61.28 g. Although the 2 strains were maintained in separate vivaria, both were housed in standard polycarbonate laboratory housing (76 cm × 56 cm) with a cagemate from the same strain. Food (Teklad 7006 Guinea Pig Diet, Harlan, Indianapolis, IN) and water were provided free choice. Housing areas were on a 12:12-h light:dark cycle (lights on, 0800). Cages were lined with paper bedding (TEK-Fresh, Harlan) and maintained in humidity-controlled rooms (46% to 56%). To ensure sufficient warmth for the Hairless strain, the room temperature was increased to 24 °C,³ as compared with the standard room temperature of 23 °C maintained for the Hartley strain. In addition, the home cages of Hairless guinea pigs contained a tunnel blanket (length, 36 cm; diameter, 30 cm) constructed from synthetic fleece (Polartec, Monsanto, Creve Couer, MO). The tunnel blanket provided a burrow-like environment that the guinea pigs could use to obtain additional warmth. Hartley guinea pigs were given a section of PVC tubing to act as a home-cage burrow.

In addition to the home-cage supplementation, guinea pigs were exposed to an enrichment program. For approximately 1 h daily on each weekday (Monday through Friday), 8 to 10 same-sex guinea pigs were placed in an open-field arena (140 cm × 70 cm) that was lined with synthetic fleece (Monsanto). Wire spheres containing Western timothy hay (Pheleum pratense L.) were located at 2 ends of the arena, and water was available. Guinea pigs had unlimited access to both the hay and water throughout the time spent in the open-field arena. This enrichment program has been demonstrated to provide increased social interaction and exercise for both Hartley and Hairless guinea pigs.⁷

At all times, guinea pigs were cared for in accordance with the Guide for the Care and Use of Laboratory Animals.¹⁵ All procedures, husbandry, and care involving the guinea pigs in this study were reviewed and approved by the Wright State University Animal Care and Use Committee.

Temperature-gradient apparatus.

The experimental apparatus (Figure 1) was constructed of molded plastic in the shape of a ring (diameter, 92 cm; circumference, 2.9 m). This design was chosen to eliminate corners in which guinea pigs might choose to burrow or huddle regardless of the local ambient temperature. In addition, the round design affords no directional cues for test subjects. To further reduce the influence of external cues, the apparatus was placed in the center of a 20-m² room and tightly enclosed with a ring of curtains (height, 2 m) as a blind. The apparatus was positioned in the room in such a way that only blank ceiling was visible from the apparatus.

Figure 1.

Schematic of the temperature-gradient apparatus. The apparatus was divided into 8 equal zones. Each zone was lined with a heating mat (dotted lines), which covered the apparatus floor (16 cm wide) and both inner and outer walls (height, 23 cm) in a continuous U-shaped pattern. Zones were fitted with a trapezoidal acrylic lid, placed directly over each heating mat. Having individual lids enabled the placement of subjects in the apparatus with minimal heat loss. Above the mat at the top of the outer wall, a temperature probe (T) recorded the air temperature for each of the 8 zones. Individual temperature controllers (not shown) were set to the temperature specified for the zone. For example, the controller for zone 34a was set to 34 °C. These controllers turned the heating mats on or off according to the reading of the temperature probe (within 0.5 °C). This arrangement of equally spaced controllers created a temperature gradient that ranged from 24 °C (room temperature) to 38 °C (guinea pig body temperature). In addition, the apparatus was lined with a fitted synthetic polar fleece that was changed for each subject.

The apparatus was covered with 8 overlapping heating mats (Figure 1, dotted lines; width, 30 cm; length, 60 cm) to create 8 temperature zones. The mats were placed in U-shaped patterns so that each one covered a continuous section of the outer wall (height, 23 cm), apparatus floor (width, 16 cm), and inner wall (height, 23 cm). Eight clear acrylic panels, each cut precisely to the trapezoidal shape of a zone, were placed on top of the walls directly over each heating mat. These acrylic panels allowed unobstructed visibility of guinea pig behavior during the experiments and prevented the subjects from exploring outside the apparatus. In addition, the individual lids over the zones enabled the placement of subjects into the apparatus with minimal heat loss or change in temperature.

To regulate each zone, a temperature probe was fixed at the top of the outer wall over the center of each of the 8 heating mats. These individual temperature controllers turned each of the heating mats on and off according to the set-point temperature of the controllers. Controllers (catalog no. 11-463-47A, Fisher Scientific, Waltham, MA) were programmed to regulate the temperature of each zone to within 0.5 °C of the set point. The set points ranged from room temperature for the Hairless strain (24 °C) to body temperature (38 °C). The temperature set points were arranged in 2 to 4 °C increments across the zones to create a gradient of temperatures (Figure 1).

The apparatus was lined with synthetic fleece sewn to the exact shape of the ring dimensions. A fresh liner was used each time a guinea pig was placed in the apparatus. Liners were reused on successive days after laundering with enzymatic detergent (Free and Clear, Seventh Generation, Burlington, VT) and visual inspection under UV light (UVR-9000, BAYCO Products, Wylie, TX). This process ensured the lack of residual bodily fluids that might influence place preference through olfactory cues.

Data collection and analysis.

Data were collected across a 4-d period for each of 3 different experiments: 1) apparatus control; 2) single subject; and 3) cagemate pair. The same subjects were used in all 3 experiments, which were run in the order listed. The purpose of the apparatus-control experiment was to determine whether the guinea pigs displayed a preference regarding location within the apparatus irrespective of temperature. For this experiment, the heating mats were left in place but turned off, and all zones in the apparatus were room temperature (24 °C). Each guinea pig in a group was randomly assigned to 1 of the 8 zones for initial placement. In the single-subject experiment, the zone temperatures were set and maintained as depicted in Figure 1; guinea pigs again were randomly assigned to a starting zone. The cagemate-pair experiment used the same temperature set points as the single-subject experiment. However because guinea pigs are social creatures that spend much of their time huddled with cagemates, guinea pigs were placed as pairs within the apparatus in this third experiment, to ascertain if different temperature zones would be traversed than those observed in the first 2 experiments. Pairs were cagemates that had also served as subjects in the first 2 experiments. Consequently, all guinea pigs were equally familiar with the apparatus. Each subject was tested: 1) once in the apparatus-control study; 2) once in the single-subject study; and 3) twice in the cagemate-pair study. Cagemate pairs were run twice, once on each of 2 d, because cagemates served as subjects on one run, and subjects served as cagemates on the other. The initial starting zone for the subject guinea pig was randomized, with the cagemate placed in the zone opposite from that of the subject. For example, when the subject was placed in zone 34a, the cagemate was placed in zone 26b.

For all experiments, data were collected during the light phase of the light:dark cycle. Zone temperatures were recorded just prior to subject placement in the apparatus. Guinea pigs were transferred from the adjacent colony room (4 to 5 m away) in an opaque container (15 cm × 25 cm × 15 cm) immediately before being placed in the apparatus. Sequestered transfer was preformed to mask any room or orientation cues. Upon entering the experimental room, the container with the guinea pig was placed next to the apparatus inside the curtain barrier. The experimenter then reached between the curtains and placed the guinea pig in the randomly assigned zone.

While in the temperature-gradient apparatus, the guinea pigs were monitored through a remote-controlled digital camcorder (3MOS HD, Panasonic, Osaka, Japan) placed above the apparatus. Guinea pigs were videotaped for 30 min in the apparatus and then returned to the colony room inside the covered container. Video files were transferred to a computer and rendered for scoring into MPEG2 format by using Premier Pro CS4 software (Adobe, San Jose, CA).

Coding for the location of the guinea pigs by zone within the apparatus was achieved through play back of the video files by using the JWatcher event recorder.⁵ Although coders were easily able to determine the zone boundaries created by the acrylic covers, they were blinded to the actual designation and temperature of the zones. Guinea pigs were coded as being in a particular zone when the center of the body first moved into that zone, regardless of whether other body parts (for example, head, limbs) were in an adjacent zone. This system of coding allowed both frequencies and durations of time (for example, mean duration, total time) to be calculated for each zone. In the experiment involving cagemate pairs, each guinea pig was scored on a separate pass. The files were then combined for subsequent analysis of frequencies and durations, including quantification of when cagemates shared adjacent zones with the subject. Adjacent was defined as when the cagemate was in the same zone, or one to either side of the subject. For example, when the subject guinea pig was in zone 30a (Figure 1), the cagemate was considered to be adjacent when located in zone 26a, 30a, or 34a.

Data for each experiment were analyzed separately with repeated-measures ANOVA by using Pillai–Bartlett trace multivariate test criteria and Statistica software (version 10, StatSoft, Tulsa, OK). Zones were treated as within-subject repeated measures, with the 3 guinea pig groups (male Hairless, female Hairless, female Hartley) and pairs (subject and cagemate) treated as between-subjects factors. For all analyses, differences were considered significant when the P value was less than 0.05, and effect sizes were calculated by using η_p². Posthoc pairwise comparisons were performed by using the Fisher protected least-square difference only on results with statistically significant overall analyses.

Results

Zone temperatures recorded just prior to the placement of each test subject remained within 1 °C of the designated set point temperatures (Table 1) and did not vary significantly between single-subject and cagemate-pair experiments (F_{1, 46} = 1.400, P = 0.235, η_p² = 0.031). Although a main effect of zone was detected (F_{7, 40} = 1590.601, P < 0.001, η_p² = 0.979), there was no interaction with experiment (F_{7, 322} = 1.400, P = 0.213, η_p² = 0.029).

Table 1.

Mean apparatus temperatures and set points during experiments

Zone	Set point (°C)	Mean temperature (°C), 95% confidence interval
		Single subject	Cagemate pair
24	24	23.995	23.683
		23.766–24.226	23.453–23.914
26a	26	26.408	26.775
		26.136–26.680	26.503–27.047
30a	30	30.229	30.050
		29.934–30.525	29.755–30.345
34a	34	34.521	34.621
		34.135–34.906	34.235–35.006
38	38	38.425	38.717
		38.023–38.827	38.315–39.119
34b	34	34.379	34.521
		34.050–34.709	34.191–34.850
30b	30	30.067	30.296
		29.683–30.450	29.913–30.679
26b	26	26.200	26.563
		26.004–26.396	26.367–26.758

^*see Figure 1 for physical arrangement of zones

Experiment 1: apparatus control.

After placement, guinea pigs actively explored the apparatus, visiting all zones repeatedly (Figure 2 A). No significant difference in the frequency of zone crossing was detected across the 8 zones (F_{7, 15} = 1.842, P = 0.152, η_p² = 0.073) nor was there a zone × group interaction (F_{14, 32} = 0.765, P = 0.696, η_p² = 0.136). There was, however, a main effect of group (F_{2, 21} = 5.407, P = 0.013, η_p² = 0.340), with Hairless females making more frequent visits to most zones than did the Hartley females or Hairless males. Posthoc pairwise comparisons confirmed differences between Hairless and Hartley females for all 8 zones (P < 0.05) and significantly elevated frequencies for Hairless females compared with males for all zones except 26a, 30a, and 34a (P < 0.05).

Figure 2.

Frequency and total time (mean ± SEM) for the apparatus-control experiment. (A) Frequency of zone crossings in the room temperature (24 °C) apparatus for Hartley female and both male and female Hairless guinea pigs. Hairless females had significantly higher frequencies of zone crossings than Hartley females for all zones (P < 0.05). Asterisks denote the zones for which Hairless females had higher crossing rates than did males (P < 0.05). (B) Total time that guinea pigs spent in each zone of the room temperature apparatus during the 30-min observation period. There was no difference in time spent per zone for any of the 3 guinea pig groups.

In addition, guinea pigs spent similar amounts of time in each zone. (Figure 2 B). Analysis of the total time spent in each zone (Figure 2 B) revealed no significant effects of zone (F_{7, 15} = 1.842, P = 0.152, η_p² = 0.070), group (F_{2, 21} =1.617, P = 0.222, η_p² = 0.133), or zone × group interaction (F_{14, 32} = 0.811, P = 0.652, η_p² = 0.094).

Experiment 2: single subject.

Similar to the findings from the apparatus-control experiment, guinea pigs placed in the temperature gradient as single subjects actively explored the environment, visiting all zones frequently (Figure 3 A). A main effect of zone was revealed (F_{7, 15} = 5.342, P = 0.003, η_p² = 0.191). However, no differences in rates of exploration were detected between the 3 groups (F_{2, 21} = 2.394, P = 0.116, η_p² = 0.186), nor was there a zone × group interaction (F_{14, 32} = 0.778, P = 0.684, η_p² = 0.055). Posthoc analyses determined that most differences were between the coolest zone (24 °C) and the warmest 2 zones, 34b and 38 (P < 0.05).

Figure 3.

Frequency, duration, and total time per zone (mean ± SEM) for the single-subject experiment. Although guinea pigs actively explored all zones within the temperature-gradient apparatus, they spent significantly longer times in the 30 °C zones. (A) Frequency of crossings per zone for the 3 guinea pig groups. (B) Duration within each of the 8 zones. Significantly longer durations were detected for Hartley female guinea pigs in zone 30a (30 °C), for Hairless females in zone 30b (30 °C), and for males in both of the zones maintained at 30 °C (P < 0.05). (C) Total time spent by the guinea pig groups in each of the temperature zones. Guinea pigs spent more than half the 30-min observation period in one of the 30 °C zones. Similar to the duration results, Hartley female guinea pigs spent significantly more time in zone 30a, Hairless females spent more time in 30b, and Hairless males spent increased time in both of the 30 °C zones (P < 0.05).

The average time spent in the 30 °C zones was significantly greater than those in other temperature zones (Figure 3 B). Analyses revealed only a main effect of zone (F_{7, 15} = 6.968, P < 0.001, η_p² = 0.174), with no effect of group (F_{2, 21} = 0.662, P = 0.526, η_p² = 0.059) or zone × group interaction (F_{14, 32} = 0.610, P = 0.837, η_p² = 0.073). Posthoc pairwise comparisons confirmed increased mean durations in the 30a zone compared with zones 24, 26a, 26b, and 34a (P < 0.05) for Hartley female guinea pigs and in the 30b zone compared with all other zones (P < 0.05) for Hairless females. Elevated rates for Hairless male guinea pigs were confirmed for both zones 30a and 30b relative to other zones (P < 0.05).

Similar findings were obtained for the total time spent in each zone (Figure 3 C). Significant effects were revealed for zone (F_{7, 15} = 31.043, P < 0.001, η_p² = 0.215) and group (F_{2, 21} = 14.250, P < 0.001, η_p² = 0.576) but not zone × group (F_{14, 32} = 1.045, P = 0.438, η_p² = 0.303). Posthoc pairwise comparisons confirmed elevated mean durations for Hartley female guinea pigs in the 30a zone compared with zones 24, 26a, 26b, and 34a (P < 0.05) and for Hairless females in the 30b zone compared with all other zones (P < 0.05). Elevated rates for Hairless males were confirmed as well, but for both zones 30a and 30b as compared with other temperature zones (P < 0.05).

Experiment 3: cagemate pairs.

Similar to findings from both experiments 1 and 2, guinea pigs actively explored the apparatus in the cagemate experiment. Overall 3-way ANOVA for activity by zone revealed elevated frequency rates for Hartley female guinea pigs, as compared with both Hairless groups, when placed in the temperature apparatus with a cagemate (Figure 4). Although a significant main effect of group emerged (F_{2, 42} = 19.514, P < 0.001, η_p² = 0.482), no significant differences were detected for zone (F_{7, 36} = 1.443, P = 0.219, η_p² = 0.009), pair (F_{1, 42} = 0.047, P = 0.830, η_p² = 0.001) zone × group (F_{14, 74} = 1.726, P = 0.068, η_p² = 0.087), zone × pair (F_{7, 36} = 0.680, P = 0.688, η_p² = 0.013) or zone × group × pair (F_{14, 74} = 1.193, P = 0.299, η_p² = 0.028).

Figure 4.

Zone frequency (mean ± SEM) for the cagemate-pair experiment. Bars depict the mean activity level in each of the temperature zones for subjects (white) and cagemates (black). Increased frequencies of zone crossings were observed for Hartley female guinea pigs (A, P < 0.05) relative to Hairless females (B) and males (C). No differences were detected between subjects and cagemates for any guinea pig group in any zone.

Quantification of mean durations (Figure 5 A through C) revealed main effects of zone (F_{7, 36} = 2.657, P = 0.025, η_p² = 0.059) and group (F_{2, 42} = 3.276, P = 0.048, η_p² = 0.135) but not pair (F_{1, 42} = 0.039, P = 0.844, η_p² = 0.135). Similarly, no interaction was detected for zone × group, (F_{14, 74} = 1.650, P = 0.086, η_p² = 0.067), zone × pair (F_{7, 36} = 0.722, P = 0.654, η_p² = 0.010), or zone × group × pair (F_{14, 74} = 0.533, P = 0.906, η_p² = 0.019). Posthoc comparisons revealed no differences in mean duration in a zone between Hartley pairs (Figure 5 A) but elevated mean durations were detected for Hairless female guinea pigs in zone 30a (P < 0.05), with subject means further increased over those of cagemates (Figure 5 B). For the Hairless male guinea pigs, significant elevation in zone duration were detected for cagemates only relative to subject means (Figure 5 C). Elevations in mean duration were detected for zone 30a relative to 38; for zone 30b relative to 26a, 34a, and 38; and for zone 34b relative to both 34a and 38 (P < 0.05).

Figure 5.

Duration and total time per zone for the cagemate-pair experiment. (A–C) Duration and (D–F) total time of subjects (white) and cagemates (black) in the 3 guinea pig groups: Hartley female guinea pigs (A and D), Hairless females (B and E), and Hairless males (C and F). There was no difference in duration or total time between the Hartley subject and cagemate for any zone (A and D). Although there was no difference in duration for any zone (A), Hartley females spent a greater amount of the total time in zone temperatures of 24 to 30 °C (P < 0.05) compared with the warmest 3 zones (D). In contrast, Hairless females stayed for longer periods in one of the 30 °C zones (B, P < 0.05) and spent more time in the warmer zones (E, P < 0.05). Similarly, Hairless males stayed for longer periods in the 30 °C zones, with longer mean durations for cagemates (C, P < 0.05) and greater total time per zone for subjects (F, P < 0.05). In addition, cagemates showed increased durations (C) and total time (F) in one of the 34 °C zones and greater total time in one of the 26 °C zones (F, P < 0.05).

A different pattern emerged for the total amount of time each group of guinea pigs spent in the various zones (Figure 5, D through F), as evidenced by a main effect of zone (F_{7, 36} = 5.179, P < 0.001, η_p² = 0.065) and a zone × group interaction (F_{14, 74} = 2.171, P = 0.017, η_p² = 0.096). Specifically, Hartley female guinea pigs and their cagemates spent more time in zones 30 °C and cooler (Figure 5 D) than in the 3 warmest zones (P < 0.05). In contrast, the Hairless female guinea pigs spent significantly more time in zones 26 °C and warmer (Figure 5 E) than in cooler zones, with the greatest amount of time in zone 30a (P < 0.05). Male Hairless subjects spent more time in the 30 °C zones, whereas cagemates spent significantly more time in zones 26b and 34b compared with zones 34a and 38. The remaining analyses revealed no main effect of pair (F_{1, 42} = 2,547, P = 0.118, η_p² = 0.057) or interaction of group × pair (F_{2, 42} = 0.002, P = 0.998, η_p² < 0.001), zone × pair (F_{7, 36} = 0.560, P = 0.783, η_p² = 0.016), or zone × group × pair (F_{14, 74} = 0.761, P = 0.706, η_p² = 0.032).

To better quantify the spatial relationship between subject and cagemate, the frequency and total time that paired guinea pigs spent in proximity to one another were analyzed. Proximity was defined as when the cagemate was in the same or an adjacent zone as the subject. For example, when the subject was in zone 34a, the cagemate was adjacent when it was in zone 30a, 34a, or 38 (Figure 1). The adjacent analysis for zone frequency revealed a main effect of group (F_{2, 21} = 11.392, P < 0.001, η_p² = 0.520), with increased rates of proximity for Hartley female guinea pigs compared with the other groups, but no effect of zone (F_{7, 147} = 0.828, P = 0.565, η_p² = 0.038) or zone × group interaction (F_{14, 147} = 1.608, P = 0.083, η_p² = 0.133; Figure 6 A). Posthoc analyses confirmed significantly increased rates of proximity for Hartley female guinea pigs, relative to both other groups, at all zones except 30a, 30b, and 38 (P < 0.05).

Figure 6.

Frequency and total time for proximity between subject and cagemate. (A) Frequency and (B) total time per zone for the 3 guinea pig groups when the cagemate was in proximity to the subject. Proximity was defined as when the cagemate was in the same zone as or an adjacent zone to that of the subject. For example, when the subject was in zone 30a, the cagemate was proximate when it was in zone 26a, 30a, or 34a (Figure 1). Hartley female cagemates were frequently adjacent to the subject in most temperature zones (A, P < 0.05). No differences in frequency of proximity were observed for Hairless females or males. In contrast, Hairless males spent a greater amount of time in proximity for the 30 °C zones relative to zones 26a and warmer (P < 0.05).

Similar to those for zone frequency, analyses of the total time animals spent adjacent in each zone revealed main effects of group (F_{2, 21} = 3.695, P = 0.042, η_p² = 0.260) and zone (F_{7, 15} = 3.535, P = 0.019, η_p² = 0.259) but no zone × group interaction (F_{14, 32} = 1.509, P = 0.164, η_p² = 0.109; Figure 6 B). However, in this analysis of total time adjacent, the main effects of group and zone occurred in Hairless male guinea pigs instead of Hartley females, as occurred for the frequency analysis. Posthoc comparisons confirmed that Hairless males spent increased time in zones 30a and 30b compared with zones 26b and higher.

Discussion

Guinea pigs from all groups actively explored the apparatus, crossing frequently to all zones. In the apparatus-control experiment, neither the frequency of crossing nor total time differed between zones (Figure 2), suggesting that the guinea pigs had no particular preference for place within the apparatus. Similarly, the frequency of exploration in the single-subject and cagemate-pair experiments did not differ across the various zones (Figures 3 A and 4). However, female guinea pigs of both strains showed increased zone crossings compared with male Hairless guinea pigs, and Hairless females showed higher rates of zone crossings in the apparatus control than did Hartley females (Figure 2 A). In comparison, Hartley female guinea pigs had higher frequencies of zone crossings in the cagemate-pair experiment than did the other groups (Figure 4). These increased rates for the Hartley strain may reflect higher rates of social interaction between the cagemate pairs. In a prior study examining the behavioral differences of the 2 guinea pigs strains, we found increased levels of social interaction between Hartley compared with Hairless female guinea pigs in an open-field environment.⁷ Consequently, the variation noted in the current study might similarly reveal social differences between these 2 strains. Further support for this conclusion is found in the increased frequency of proximity for Hartley cagemates relative to those for both Hairless male and female guinea pigs (Figure 6 A).

Our guinea pigs had very different distributions for time spent across various zones in the single-subject compared with cagemate-pair experiments. When placed individually in the temperature gradient, subjects from all 3 groups spent greater durations and longer total time in the 30 °C zones. This finding is similar to prior studies of behavioral thermoregulation for the Hartley strain.^13,16 However, when placed within the apparatus with a cagemate, guinea pigs from all 3 groups spent more time across a much wider range of zones (Figure 5). This result differed by strain, with Hartley females spending more time in the cooler zones of 24 to 30 °C (Figure 5 D) and both male and female Hairless guinea pigs spending more time in the warmer zones at 30 to 38 °C (Figure 5 E and F).

The greater distribution across zones between the subjects in the cagemate-pair experiment was not likely the result of space constraint, because neither of the female groups spent significant time in proximity. Only the Hairless male guinea pigs showed elevated times in proximity to one another and even then only in the 30 °C zones (Figure 6 B). This finding of limited time spent in proximity stands in sharp contrast to typically observed, social behavior of guinea pigs^1,7,19,20 and likely results from placement in a novel environment (that is, different from the homecage). Such a conclusion raises the question of whether temperature-gradient testing outside the homecage environment is generalizable beyond the apparatus used. Additional studies comparing homecage temperature preferences with those observed in an apparatus may help answer this question.

In addition, the atypical apparatus used in the current study raises the question of whether a testing period of 30 min is sufficient to determine temperature preference. A prior study comparing the behavior of Hairless and Hartley guinea pigs found that most differences compared with homecage behavior were observed in the first 15 min after placement in a familiar apparatus.⁷ These findings suggest that the 30-min observation period used in the current study may have been sufficient for the guinea pigs to acclimate to the apparatus. However additional testing with longer observation times would provide confirmation of the temperature preferences reported in this study.

Taken together, the data do not fully support our 2 hypotheses in that the Hairless strain did not seek a warmer zone than did the Hartley strain and, when placed with a cagemate, the Hairless guinea pigs actually spent more time in warmer zones instead of demonstrating the shift to cooler temperatures that we predicted. Only the data from the Hartley strain cagemates supported our hypothesis of a shift to cooler temperature zones (Figure 5 D through F). Regardless, the Hairless strain's preference for the same ambient temperature as Hartley guinea pigs, a strain with much more thermoregulatory resources in the form of fur insulation, requires additional discussion.

Three explanations for this singularity in preference are possible. The first is that the Hairless strain has had insufficient evolutionary time to develop new thermoregulatory adaptations to being hairless and therefore seeks the same thermoneutral zone temperature as does the Hartley strain.²³ This explanation might suggest that Hairless guinea pigs are incapable of adapting its thermoregulatory behavior to a new environment and thus simply selects a preprogrammed temperature. Alternatively, Hairless guinea pigs may have adapted a higher metabolism and therefore do not require a warmer ambient environment to maintain body temperature. For both these explanations, studies recording physiologic responses of Hairless guinea pigs to different ambient temperatures may help answer these questions.

Finally, there may be a factor involving discrimination within the temperature gradient. Because the temperature difference between the zones surrounding 30a and 30b was 4 °C (Figure 1), guinea pigs may have been seeking temperatures slightly above or below the set point for the zone. For example, within the 30 °C zones, the ambient temperature likely ranged from 28 to 32 °C. Within such a wide range, the Hairless guinea pigs could have been seeking a slightly higher temperature within the zone compared with that preferred by the Hartley strain. Future studies that use a narrower range of temperatures (for example, 28 to 32 °C) across the entire apparatus may reveal differences not detected in the current set of experiments.

Furthermore, caution should be used in extrapolating our current results to ambient temperatures in other contexts such as housing. Studies with other rodents have demonstrated that the temperature an animal prefers may not be the optimal housing temperature.^11,12,21 As an initial study, however, the experiments presented here provide much needed data on the temperature preference of Hairless guinea pigs. In addition, this paper reports for the first time the thermoregulatory behavior of cagemate pairs for both laboratory strains. Results from this study also highlight the role that context may play in thermoregulatory behaviors.