NAME: Sexual Activity and the Lifespan of Male Fruitflies

NAME: Sexual activity and the lifespan of male fruitflies

TYPE: Designed (almost factorial) experiment

SIZE: 125 observations, 5 variables

DESCRIPTIVE ABSTRACT:

A cost of increased reproduction in terms of reduced longevity has been

shown for female fruitflies, but not for males. The flies used were an

outbred stock. Sexual activity was manipulated by supplying individual

males with one or eight receptive virgin females per day. The

longevity of these males was compared with that of two control types.

The first control consisted of two sets of individual males kept with

one or eight newly inseminated females. Newly inseminated females will

not usually remate for at least two days, and thus served as a control

for any effect of competition with the male for food or space. The

second control was a set of individual males kept with no females.

There were 25 males in each of the five groups, which were treated

identically in number of anaesthetizations (using CO2) and provision of

fresh food medium.

SOURCE:

Figure 2 in the article "Sexual Activity and the Lifespan of Male

Fruitflies" by Linda Partridge and Marion Farquhar. _Nature_, 294,

580-581, 1981.

VARIABLE DESCRIPTIONS:

Columns Variable Description

------

1- 2 ID Serial No. (1-25) within each group of 25

(the order in which data points were abstracted)

4 PARTNERS Number of companions (0, 1 or 8)

6 TYPE Type of companion

0: newly pregnant female

1: virgin female

9: not applicable (when PARTNERS=0)

8- 9 LONGEVITY Lifespan, in days

11-14 THORAX Length of thorax, in mm (x.xx)

16-17 SLEEP Percentage of each day spent sleeping

SPECIAL NOTES:

`Compliance' of the males in the two experimental groups was documented

as follows: On two days per week throughout the life of each

experimental male, the females that had been supplied as virgins to

that male were kept and examined for fertile eggs. The insemination

rate declined from approximately 7 females/day at age one week to just

under 2/day at age eight weeks in the males supplied with eight virgin

females per day, and from just under 1/day at age one week to

approximately 0.6/day at age eight weeks in the males supplied with one

virgin female per day. These `compliance' data were not supplied for

individual males, but the authors say that "There were no significant

differences between the individual males within each experimental

group."

STORY BEHIND THE DATA:

James Hanley found this dataset in _Nature_ and was attracted by the

way the raw data were presented in classical analysis of covariance

style in Figure 2. He read the data points from the graphs and brought

them to the attention of a colleague with whom he was teaching the

applied statistics course. Dr. Liddell thought that with only three

explanatory variables (THORAX, plus PARTNERS and TYPE to describe the

five groups), it would not be challenging enough as a data-analysis

project. He suggested adding another variable. James Hanley added

SLEEP, a variable not mentioned in the published article. Teachers can

divulge the details at the end of the data-analysis project.)

Further discussion of the background and pedagogical use of this

dataset can be found in Hanley (1983) and in Hanley and Shapiro

(1994).

PEDAGOGICAL NOTES:

This has been the most successful and the most memorable dataset we

have used in an "applications of statistics" course, which we have

taught for ten years. The most common analysis techniques have been

analysis of variance, classical analysis of covariance, and multiple

regression. Because the variable THORAX is so strong (it explains

about 1/3 of the variance in LONGEVITY), it is important to consider it

to increase the precision of between-group contrasts. When students

first check and find that the distributions of thorax length, and in

particular, the mean thorax length, are very similar in the different

groups, many of them are willing to say (in epidemiological

terminology) that THORAX is not a confounding variable, and that it can

be omitted from the analysis.

There is usually lively discussion about the primary contrast. The

five groups and their special structure allow opportunities for

students to understand and verbalize what we mean by the term

"statistical interaction."

There is also much debate as to whether one should take the SLEEP

variable into account. Some students say that it is an `intermediate'

variable. Some students formally test the mean level of SLEEP across

groups, find one pair where there is a statistically significant

difference, and want to treat it as a confounding variable. A few

students muse about how it was measured.

There is heteroscedasticity in the LONGEVITY variable.

One very observant student (now a professor) argued that THORAX cannot

be used as a predictor or explanatory variable for the LONGEVITY

outcome since fruitflies who die young may not be fully grown, i.e., it

is also an intermediate variable. One Ph.D. student who had studied

entomology assured us that fruitflies do not grow longer after birth;

therefore, the THORAX length is not time-dependent!

Curiously, the dataset has seldom been analyzed using techniques from

survival analysis. The fact that there are no censored observations is

not really an excuse, and one could easily devise a way to introduce

censoring of LONGEVITY.

REFERENCES:

Hanley, J. A. (1983), "Appropriate Uses of Multivariate Analysis,"

_Annual Review of Public Health_, 4, 155-180.

Hanley, J. A., and Shapiro, S. H. (1994), "Sexual Activity and the

Lifespan of Male Fruitflies: A Dataset That Gets Attention," _Journal

of Statistics Education_, Volume 2, Number 1.

SUBMITTED BY:

James A. Hanley and Stanley H. Shapiro

Department of Epidemiology and Biostatistics

McGillUniversity

1020 Pine Avenue West

Montreal, Quebec, H3A 1A2

Canada

tel: +1 (514) 398-6270 (JH)

+1 (514) 398-6272 (SS)

fax: +1 (514) 398-4503