Usova (2010) Registration of differences between fast‑growing and long‑lived frogs

{ "translated_text": "Usova E. E. Skeletochronological study of lifespan and growth dynamics of representatives of the Pelophylax esculentus complex: registration of differences between fast‑growing and long‑lived frogs // Biodiversity and Sustainable Development. Abstracts of the International Scientific‑Practical Conference – Simferopol: KNC, 2010. – pp. 121–124.\nUDC (57.022+57.032):591.84:591.158.1:597.851\nUsova E. E., junior research associate\nDepartment of Zoology and Animal Ecology, V.N. Karazin Kharkiv National University\ne.e.usova @ gmail.com\nSKELETOCHRONOLOGICAL STUDY OF LIFESPAN AND GROWTH DYNAMICS OF REPRESENTATIVES OF PELOPHYLAX ESCULENTUS COMPLEX: REGISTRATION OF DIFFERENCES BETWEEN FAST‑GROWING AND LONG‑LIVED FROGS\nSkeletochronological investigation of age and growth rates of poikilothermic vertebrates allows reliable determination of demographic characteristics of their populations, which are difficult or impossible to study by other methods. This method relies on three circumstances confirmed by numerous observations. We will present them sequentially, indicating how they are reflected in the methodology we used.\n1. Growth of poikilothermic vertebrates living in a seasonal climate is uneven. Tubular bones increase in width by forming new outer layers. During periods of intensive growth, layers appear that look light on stained sections, whereas during growth pauses (overwintering or other adverse conditions) dark lines of annulation are formed. Counting the annulation lines corresponding to overwintering allows determination of the individual’s age (Smirina, 1983; other works).\nFor age determination we used the third phalanx of the longest toe of the hind limb of the frogs. The phalanx was cleared of soft tissue, decalcified in HNO₃, frozen in an ice block and transverse sections of 20–22 µm thickness were obtained from the middle part of the diaphysis, near the entry of the blood vessel into the bone cavity, using a freezing microtome. Sections were mounted on glass, stained with Ehrlich’s hematoxylin, washed, transferred to glycerin and photographed as temporary mounts. Bone structure analysis and annulation line counting were performed on digital microphotographs on a PC.\n2. In several studies (Marunouchi et al., 2000; Roitberg, Smirina, 2006; Zamaletdinov, Faizullin, 2008), as well as in our research (Usova, Shabanov, 2009), it has been shown that the increase in phalanx thickness is isomorphic, i.e., it occurs with preservation of proportions. The transverse width of each annulation line indicates the body length of the individual at the time the line was formed. Knowing the body length and the transverse width of the phalanx at the time of study, one can calculate the dynamics of body‑length change.\nTo obtain such a growth‑rate estimate, measurements of the maximum and minimum transverse dimensions of the figure bounded by each annulation line were made on finger‑section photographs. Sizes measured on the microphotograph were converted to micrometres by comparing the studied photographs with a photograph of a stage micrometer. Because during frog growth the relationship l/L = li/Li holds, where l is the phalanx transverse width and L is the body length at the time of study, and li is the transverse width of the figure bounded by the annulation line and Li is the body length in the i‑th year of life, we used the formula Li = L × li / l for retrospective calculation of body length.\n3. During external, periosteal bone growth, bone resorption occurs from the endosteal cavity side. The consequence of this process is the destruction of the earliest annulation lines. After resorption ceases, bone grows inward, forming endosteal annulation lines.\nWhen determining frog age from phalanx mounts we applied a correction for bone resorption. After studying bone growth in young frogs, we established the possible diameter of the first annulation lines. Using the transverse width of the endosteal cavity, the first preserved annulation line, and the remnants of destroyed annulation lines, we determined how many lines had been resorbed. Endosteal annulation lines were not counted. In a typical adult frog, 2, occasionally 3, annulation lines are resorbed. In some cases precise age determination is impossible. When age could be reliably determined it was recorded, for example, as “5+” (five overwinterings plus a certain period after the fifth overwintering); when ambiguous, as, for instance, “3–4+” (three or four overwinterings, depending on the number of resorbed annulation lines).\nCurrently we have determined the age and growth dynamics of 157 green frogs (representatives of the Pelophylax esculentus complex) from the Kharkiv region. Their ages range from 0+ to 10+. Pond frogs, P. lessonae, are smaller in body size than P. ridibundus and various hybrid forms, P. esculentus. When comparing relative growth in each year of life, defined as pi = (Li – Li‑1) / Li‑1, we did not register a significant difference between representatives of different forms. Further we compared frogs by their growth in the 4th year of life. This value can be obtained for most individuals, as the 3rd and 4th annulation lines are usually not affected by resorption. Most of the studied frogs at this age are still sexually immature.\nFrom the examined frogs two groups were distinguished. The “young” group included frogs that had experienced no more than four overwinterings (81 individuals in total), and the “old” group comprised individuals “6+” and older (36 frogs). Both groups contained representatives of all studied frog forms and differed only insignificantly in composition. Among these frogs the relative growth in the 4th year of life could be determined for 45 “young” and 28 “old” individuals.\nSince the calculation of mean values for data representing proportions (and relative growth in % is a proportion of the 3rd‑year size added in the 4th year), we performed Fisher’s angular transformation (φ = 2 × arcsin(√p), where p is the growth expressed as a fraction of one, and φ is the angular value in radians). After obtaining mean growth values and their standard deviations we transformed these values back to proportions (percentages): p = (sin(φ/2))². Thus we established that the average growth of “young” frogs in the 4th year of life is 34 % (standard deviation = 7 %, minimum = 5 %, maximum = 81 %), whereas for “old” frogs it is on average 20 % (standard deviation = 2 %, minimum = 17 %, maximum = 56 %). The significance of differences between the examined frog groups by the Mann‑Whitney criterion is quite high: 0.03.\nThe apparent paradox of this result is that among the “young” frogs there are both individuals that grew at rates characteristic of the “old” group and those that grew faster. However, among the “old” frogs there are almost no individuals that grew rapidly in the 4th year; most individuals that passed the 6‑year threshold grew relatively slowly. Earlier (Usova, 2008), analyzing age and length of relatively large and relatively old frogs, we found that the fastest‑growing ones are not the longest‑living. The data presented here confirm this conclusion based on growth‑rate analyses.\nWhy do frogs that grew quickly in the 4th year most often not survive to the 6th? Data to answer this question are still insufficient. It may be related to the implementation of different ontogenetic strategies (Maro, Shabanova, Shabanov, 2008). An alternative (or additional) explanation could be that rapid growth of sexually immature frogs reduces their resistance to adverse environmental factors, increasing mortality risk (but possibly increasing offspring numbers during the first spawning of those individuals that do survive). To provide a robust explanation of the registered phenomenon, it is necessary to determine the fecundity of frogs with different growth rates, which we plan to undertake in further studies.\nThe author sincerely thanks A.V. Korshunov for assistance in material identification and D.A. Shabanov for participation in result interpretation. The work was supported by the Fundamental Research Fund of Kharkiv National University named after V.N. Karazin and a joint grant of the Ukrainian Science Foundation and the Russian Foundation for Basic Research (RFBR)." }