reproduction investment paper

8/8/2019 Reproduction Investment Paper

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Reproductive effort in ninespine stickleback (Pungitius pungitius): evidence of terminal

investment.

J. Gary Palmer

Department of Biology

Northern Michigan University

1401 Presque Isle Ave.

Marquette, MI 49855

[email protected]

Word count: 2270

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Animals that mate more than once in their lifetimes (iteroparity) should optimize reproductive

effort for maximum lifetime reproductive success. Each bout of reproduction comes at the potential

expense of production of future offspring. This is known as the reproductive cost of mating. The other

component of reproductive effort is energetic cost, or the amount of energetic resources an individual

allocates to reproduction. Three separate hypotheses have been proposed as rules governing the

distribution of reproductive effort. This study sought to test which of these best explains the pattern of

reproductive effort in the life history of male ninespine stickleback. In an attempt to control costs of

reproduction among fish of varying ages, individuals were reared in isolation and fed a standardized

amount of food based on their body weight. Mass of sperm produced in individuals either one or three

years old was measured and standardized for body weight. This was used as an indicator of energetic

allocation to reproduction. It was found that on average three-year-old males produced nearly twice as

much sperm relative to their body weight as did one-year-old individuals. Two-year-old males

produced an intermediate amount of sperm. Three-year-old males also scored highest in a measure of

sperm motility and spent the most time fanning eggs in their nest. Results supported the terminal

investment hypothesis, while finding no evidence supporting senescence or the mating strategy-effort

hypothesis.

Keywords: Life history theory, ninespine stickleback, parental investment,Pungitius pungitius,

reproductive effort, senescence, terminal investment.

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There is substantial cost borne by parents when producing offspring. Consequently, heavy

investment in a parent's current brood may come at the expense of future reproduction (Trivers 1972).

This is considered the reproductive cost of breeding. Another factor in cost of breeding which is

suspected to be related with this is the energetic cost of breeding, or the amount of energy a parent

invests in a brood (Clutton-Brock 1984). Energetic cost is much more commonly studied, since it can

be measured more directly and over a shorter time period through quantifying the energy allocated to

production of gametes or measuring weight loss over the breeding season. The reproductive cost is

much more difficult to measure. It requires determining impact on a individual's future survival and any

change caused in future brood sizes, both of which would require considerable effort to make a record

of and be difficult to interpret.

Three hypotheses have been proposed to explain optimal allocation of reproductive effort over

an iteroparous animal's life history. First and perhaps most well known of these is the terminal

investment hypothesis. This trade-off between production of current and future offspring should favor

greater investment in current reproduction as parents age and chance of future reproduction decreases

(Clutton-Brock 1984). As an individual ages the proportion of it's lifetime remaining diminishes, and

consequently future reproductive possibilities decline. The result of this is a lowered reproductive cost

of breeding for older individuals since there are few future broods which could be negatively impacted

by current breeding efforts.

An alternative hypothesis is known as the senescence hypothesis. This states that as an animal

ages, reproductive effort will decrease due to loss of function associated with aging. leading to

decreased performance. This hypothesis refers specifically to the absolute amount of reproductive

effort an individual expends, whereas the terminal investment hypothesis specifies that the costs of

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reproduction must be taken into account. A decrease in ability could cause a decline in absolute output

even though output relative to ability may increase. It is likely that older individuals incur higher costs

of breeding, either due to poorer body condition entering the reproductive season (Mysterud et al.

2001) or decreased ability to defend a territory. These costs must be accounted for or held constant in

order to accurately determine whether a terminal decline in reproductive output is due to increased

costs or decreased investment. Because of this difference, the senescence and terminal investment

hypothesis are not mutually exclusive (Weladji et al. 2002).

A third hypothesis, proposed by Yoccoz et al. (2001), is the mating strategy-effort hypothesis.

This predicts a peak in reproductive effort in individuals just beyond maturity. This is based on the idea

that intrasexual competition plays a large role in reproductive success of males in many species, and

that investment should be highest when there is the greatest chance of success. In red deer it was found

that male reproductive effort increased quickly between maturity and peak age then slowly declined

(Yoccoz et al. 2001). Male reproductive success varies greatly based on number of mates (Trivers

1972) and investment during peak years should lead to greater benefit in this respect than toward the

end of the lifespan, when chance of success in these contests is low (Graves, in press).

One problem encountered in past tests of these hypotheses is that parental investment is

specifically defined as effort relative to costs, and costs may differ among individuals. For example, in

species with indeterminate growth such as many fishes, older individuals will be larger than young

ones and may be able to make a much larger absolute investment in reproduction without necessarily

investing more relative to the costs. This is further complicated by the tendency for larger individuals to

be dominant, giving them greater access to resources.

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This study attempts to control for these factors by raising individuals from eggs under

controlled laboratory settings in order to maintain constant resource availability and by measuring

sperm production relative to the weight of each individual. This should eliminate biases found in

natural settings that favor older individuals and may exaggerate any evidence of terminal investment. It

is hypothesized that when energetic costs are held constant, relative investment will be found to

increase with age.

METHODS

To test these hypotheses data on sperm production was collected from male ninespine

stickleback (Pungitius pungitius), an iteroparous fish which can be easily bred in the laboratory. It has

been shown in male stickleback that individuals vary resource allocation based on reproductive

strategy, with production of sperm being a useful indicator of allocation (de Fraipoint et al. 1993). The

individuals used in this experiment were descended from a population of ninespine stickleback

captured in Lake Superior near Marquette, Michigan. A total of 50 individuals (25 male, 25 female)

were captured during September 2003 and placed in a large aquarium to establish a large freely

breeding population. A proportion of haphazardly selected offspring were reared in isolation, and

month and year of birth was recorded for each individual. This was done to control for differences in

energetic costs and availability of resources to an individual based on competitive interactions which

may change over the course of an individuals life in addition to facilitating separation of individuals

into age classes. These individuals were weighed bimonthly after reaching maturity and given food

daily in proportion to their weight. This was done to ensure all individuals had equal energetic

resources, controlled for differences in body mass. Individual fish were raised in 5 gallon aquaria

maintained at 20 C with a 12:12 light:dark cycle.

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Subjects for the experiment were chosen haphazardly from the available stock of healthy

individuals. Subjects were taken from three distinct age classes; one year old plus or minus 2 months

(n=20), two years old plus or minus 2 months (n=20), and three years old plus or minus 2 months

(n=20). To assess body condition a standard curve was created using data from all available males

raised in isolation (n=79). Body length was plotted against mass and a least squares linear regression

line was drawn. Individuals whose mass differed from the expected according to this curve by more

than 20% were not included in this experiment.

To measure sperm production the collection procedure from de Fraipoint et al. (1993) was used.

Each male was placed upside-down in a slit cut into a wet sponge and pressure was gently applied to

the abdomen. This resulted in secretion of milt and glue, which were easily distinguished and

separated. Sperm was collected from each individual three times, each time separated by 48 hours. The

masses recorded from these three collections were averaged for each individual then divided by body

mass and multiplied by 100 in order to obtain a standardized index of sperm production relative to

body mass. There was no apparent change in mass of sperm over the three collections for each

individual.

In addition to quantity, sperm quality was also assessed. Milt from the second sample taken

from each male was observed and assigned an ordinal motility score using a procedure similar to that

found in de Fraipoint et al. (1993). Each male's second sample was used in order to control for any

differences in motility that may occur as a function of time since last ejaculation. Briefly, sperm were

collected using a capillary tube then diluted in 5ml distilled water. One drop of this solution was then

placed on a slide and observed under a microscope. Because the value recorded reflects a proportion of

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sperm which are motile, it was not crucial to carefully control the volumes placed on the microscope

slides. Sperm motility values were assigned as follows: 0 if no motility is observed, 1 if most

spermatozoa are immotile, 2 if most spermatozoa are vibrating, 3 if all spermatozoa are vibrating, 4 if

most spermatozoa were motile with flagellar movements, and 5 if all spermatozoa were motile with

flagellar movements (Guest et al. 1976).

The third portion of the experiment involved observation of egg fanning behavior displayed by

males. After sperm quantity and motility had been obsesses each of the males included in the study was

allowed to mate. Males were given 10g (wet weight) ofSpyrogyra algae with which to construct a nest.

Mates were gravid females, all two years old plus or minus four months. Females were introduced into

the aquarium of each male 48 hours after the third sperm sample was obtained and allowed to remain

until eggs were deposited, usually occurring within 90 minutes. For the next seven days a group of

student volunteers observed each male for a 15 minute period between 1400 and 1600 hours each day.

Time spent fanning eggs was carefully observed and recorded as a proportion of the entire 15 minute

observation period.

RESULTS

All data analysis was performed using SPSS 16.0 for Windows. The data collected show a

significant correlation between length and body mass of a fish (figure 1) (Pearson correlation r= 0.996,

P< 0.001). This relationship was used to judge the body conditions of individuals used in the study,

with those falling below the regression line considered in poor condition, and those above in good

condition. Age was also found to be significantly correlated with body mass (Pearson correlation r=

0.985,P< 0.001). Body mass also correlated significantly with mass of sperm produced (Pearson

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correlation r= 0.954,P< 0.001). This supports the assumption that older males will be larger and

produce more sperm, therefore cost of sperm production must be standardized by body size.

Sperm mass index data for individuals of all three age classes were assumed to be normally

distributed according to a Shapiro-Wilk test (one year: T20 = 0.925,P=0.124; two year: T20=0.952,

P=0.406, three year: T20 = 0.948, P= 0.340). Data for sperm mass were also assumed to have equal

variance (Levene's Test for Homogeneity of VarianceF= 0.673,P= 0.514). Using one-way ANOVA a

significant difference was found in in the mean sperm production index among the age classes (F2,59 =

196.029, p < 0.001). A Tukey's HSD post-hoc test showed that sperm mass was significantly higher for

second-year than first-year males (P


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among the age classes (F2,59=198.834,P


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It would be of interest to test these hypotheses through measurement of sperm production in

other species, particularly ones in which this is the only means of male parental investment.

Stickleback males provide a great deal more investment than just gametes, they construct and defend a

nest as well as fanning eggs during brooding. It is possible due to the relatively low cost of sperm

production that this is only a minor component of a male stickleback's parental investment and different

effects would be seen if other measures of parental investment were used.

References

Clutton-Brock, T. H. 1984. Reproductive effort and terminal investment in iteroparous animals.

American Naturalist, 123, 212-229.

De Fraipont, M., Fitzgerald, G. J., & Guderly, H. 1993. Age-related differences in reproductive

tactics in three-spined stickleback, Gasterosteus aculeatus. Animal Behaviour, 46, 961-968

Graves, B. M. 2008.Ritualized Fighting and Life History Evolution in Human Males.In press.

Mysterud, A., Yoccoz, N.G., Stenseth, N.C., & Langvatn, R. 2001. Effects of age, sex, and density

on body weight of Norweigan red deer: evidence of senescence. Proceedings of the Royal Society

of London B, 268, 911-919.

Guest, W.C., Avault, J.W., & Roussel, J.D. 1976. A spermatology study of channel catfish,Ictalurus

punctatus. Transactions of the American Fisheries Society, 105, 463-468.

Trivers, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the

Descent of Man 1871-1971. (Ed. by B. Campbell), pp. 136-179. Chicago, Illinois: Aldine.

Weladji, R. B., Mysterud, A., Holand, ., & Lenvik, D. 2002. Age-related reproductive effort in

reindeer (Rangifer tarandus): evidence of senescence. Oecologia, 131, 79-82

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Yoccoz, N. G., Mysterud, A., Langvatn, R., & Stenseth, N. C. 2002. Age- and density-dependent

reproductive effort in male red deer.Proceedings of the Royal Society of London B, 269, 1523-

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Figure 1: Length vs. body mass of male stickleback. Males of three different ages were measured and

weighed to construct a standard curve in order to assess body condition. Data from all individuals

available (n=79) are included.

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Figure 2: Average sperm production by age. The sperm production index, calculated for each individual

by dividing mass of sperm by body mass, was averaged for each age group. The data show that on

average this index was twice as high for three-year-old individuals as one-year-olds.

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Age vs. average sperm production index

Age of individuals (years)

Averagesperm

productionin

dexvalue

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Figure 3: Average sperm motility score by age. Sperm motility was ranked on a scale of 0-5, with 0

representing no motility observed and 5 representing all spermatozoa motile with flagellar movement

(Guest et al. 1976).

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Age vs. average sperm motility score


Averagesperm

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Figure 4: Average proportion of time fanning eggs. The proportion of time during a 15 minute

observation window that a male spent fanning eggs was recorded daily for seven days immediately

following fertilization of eggs by the male.

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0.5

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Age vs. proportion of time spent fanning


Proportionoft i

mespentfan

ningeggs

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