Individual, Sexual, Seasonal, and Temporal Variation in the Amount of Sagebrush Lizard Scent Marks

1. What they did

Many animals leave scent marks, but most studies of how much scent animals leave, and when, have been done on rodents. The authors studied the sagebrush lizard, a small, territorial lizard of the western United States. Like many lizards, they have femoral pores, glands that release a chemical secretion. The authors asked whether the amount of secretion a lizard leaves on its favourite perch tells others who it is (which individual, which sex), or rather how it is doing (body size, condition and breeding state).

• The lizards. 82 males and 49 females caught near Wrightwood in southern California in May 2002 and May 2003 were kept in a laboratory colony at Indiana University. Each lived alone in an aquarium with a brick basking perch under a heat lamp. One group had been in captivity for 17 months, the other for 5 months, at the time of the study.
• Collecting the marks. A white paper towel was taped to the top of each brick and changed every day. Under a black light, femoral pore spots glow (they reflect ultraviolet light) while fecal spots do not, so the two could be told apart. Every spot was counted and its diameter measured to the nearest 0.5 mm, and the total area per lizard per day was calculated.
• Two seasons. Data were collected daily for 35 days before hibernation (October to November 2003, a season when both sexes defend territories) and for 21 days after hibernation (January to February 2004, the early mating season).
• Analysis. They tested which factors explained the number and size of femoral pore spots: individual, sex, season, body size, weight and time in captivity. They checked whether the same lizards marked heavily in both seasons, and looked for a rhythm in marking over the 56 days.

2. What they found

How much they marked

• Every lizard left femoral pore spots on at least one day before hibernation, but 10 males and 5 females left none after hibernation.
• Lizards left 0 to 11 femoral pore spots a day, and most marked the brick on fewer than 30% of days. Most spots were under 3 mm² in area, a few up to 18 mm². Fecal spots were sometimes larger, up to 40 mm².

What explained the amount

• Condition mattered more than identity or sex. In the mating season, lizards left about 20% fewer femoral pore spots than in the territorial season, though the spots were slightly larger.
• Longer lizards (snout-to-vent length) left fewer, but not smaller, spots. Weight did not predict marking.
• Time in captivity had a strong effect. The text reports that lizards held for more than a year left fewer but larger spots than those held for 5 months; the table’s coding suggests the opposite direction (see keywords).
• Surprisingly, males and females did not differ, even though males have visibly larger femoral pores, especially in the mating season.
• Lizards that left more fecal spots also left more femoral pore spots.

Not a signature of the individual

• Lizards that marked a lot in the territorial season were not the same ones that marked a lot in the mating season. Only fecal spot size was linked across seasons, and that seemed to be driven by an outlier.

A weekly rhythm

• Averaged over all lizards, the number of spots rose and fell on a roughly weekly cycle, for both femoral pore and fecal spots, with the fecal pattern weaker.
• Lizards seemed more in step with each other after hibernation, when the number of femoral pore spots and fecal spots on each day were also more closely linked.

3. What they concluded

• The amount of scent a lizard leaves on one perch carries little information about which individual or which sex it is. That information is more likely carried by the chemical make-up of the secretion, which other studies show differs between individuals, sexes, populations and species.
• Instead, the amount reflects body size, condition and breeding state, and so may help other lizards make decisions about mates and territory.
• The roughly weekly cycle suggests marks are topped up regularly, so how fresh a mark is may tell others how active the resident is. The authors note that lizards might even see the marks from a distance, as well as smell them.
• Breeding males are known to produce more secretion, yet here they left less on one perch. The authors suggest that in the wild, extra secretion in the mating season is used to mark more places (males defend several perches then), rather than marking one place more heavily.
• Playback experiments and studies of how fast marks fade are needed to test whether lizards actually read this information.

6. Other technologies, skills or innovations

• Paper towels changed daily on a single preferred perch, giving a simple, repeatable daily record of scent marks and droppings.
• Black (ultraviolet) light to make femoral pore secretions visible and separate them from fecal spots.
• Time series analysis to find a rhythm in marking behaviour.